review solid phase extraction of trace elements - … · 1178 v. camel / spectrochimica acta part b...

Spectrochimica Acta Part B 58(2003) 1177–1233

0584-8547/03/$ - see front matter 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0584-8547Ž03.00072-7

Review

Solid phase extraction of trace elements

Valerie Camel*´

Institut National Agronomique Paris-Grignon, Analytical Chemistry Laboratory, 16 Rue Claude Bernard,Paris Cedex 05 75231, France

Received 2 December 2002; accepted 26 March 2003

Abbreviations: AAS, Atomic absorption spectrometry; ACN, Acetonitrile; AES, Atomic emission spectrometry; APDC,Ammonium pyrrolidine dithiocarbamate; BAS, Biswl-hydroxy-9,10-anthraquinone-2-methylxsulfide; 5-BrPADAP 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol; BSQ, 8-(Benzenesulfonamido)quinoline; 18C6, 18-Crown-6; CA, Chromotropic acid; CTA,Cetyltrimethylammonium; CV-AAS, Cold vapour atomic absorption spectrometry; DAD, Diode array detector; DDQ, 7-Dodecenyl-8-quinolinol; DDTC, Diethyldithiocarbamate; DDTP, 0,0-Diethyl-dithiophosphate; DEBT,N,N9-Diethyl-N9-benzoylthiourea; DESe,Diethyl selenide; DMBS, Dimethylglyoxal bis(4-phenyl-3-thiosemicarbazone); DMDSe, Dimethyl diselenide; DMG, Dimethylglyox-ime; DMSe, Dimethyl selenide; DPC, Diphenylcarbazide; DPCO, Diphenylcarbazone; DPD,N,N-Dimethyl-p-phenylenediamine;DPTH, l,5-bis(di-2-pyridyl) methylene dithiocarbohydrazide; DVB-VP, Divinylbenzene-vinylpyrrolidone; DZ, Dithizone; DzS,Dithizone sulfonic acid; ECD, Electron capture detection; EDTA, Ethylene diamine tetraacetic acid; ERT, Eriochrome black-T; ET-AAS, Electrothermal atomic absorption spectrometry; Et, Ethyl; EtOH, Ethanol; F-AAS, Flame atomic absorption spectrometry;FI, Flow injection; FPD, Flame photometric detection; FZ, Ferrozine; GC, Gas chromatography; GCB, Graphitized carbon black;HDEHP, Bis(2-ethylhexyl) hydrogen phosphate; HMDC, Hexamethylenedithiocarbamate; H MEHP, 2-Ethylhexyl dihydrogen2

phosphate; 8-HQ, 8-Hydroxyquinoline; 8-HQ-5-SA, 8-Hydroxyquinoline-5-sulfonic acid; HT18C6, Hexathia-18-crown-6;HT18C6TO, Hexathia-18-crown-6-tetraone; IBMK, Isobutyl methyl ketone; ICP, Inductively coupled plasma; IDA, Iminodiacetate;IP, Ion-pair; KR, Knotted reactor; LC, Liquid chromatography; LLE, Liquid–liquid extraction; LOD, Limit of detection; MBT, 2-Mercaptobenzothiazole; Me, Methyl; MeOH, Methanol; MPSP, 3-Methyl-l-phenyl-4-stearoyl-5-pyrazolone; MPT, Microwave plasmatorch; MS, Mass spectrometry; NCH, Neocuproine; NDSA, 2-Naphthol-3,6-disulfonic acid; NN, 1-Nitroso-2-naphthol; ODETA, 4-(N-octyl)diethylenetriamine; PA, Polyacrylate; PAA, Piconilic acid amide; PADMAP 2-(2-pyridylazo)-5-dimethylaminophenol;PAN, 1-(2-pyridylazo)2-naphthol; PaPhA, Poly(aminophosphonic acid); PAR, 4-(2-pyridylazo)resorcinol; PC, Pyrocatechol;PDATA, Propylenediaminetetraacetic acid; PDT, 3-(2-Pyridyl)-5,6-diphenyl-l,2,4-triazine; PDTC, Poly(dithiocarbamate); PE,Polyethylene; PGC, Porous graphitized carbon; Ph, Phenyl; PipDTC, Piperidine dithiocarbamate; PS-DVB, Polystyrene-divinylben-zene; PTFE, Polytetrafluoroethylene; PUF, Polyurethane foam; PV, Pyrocatechol violet; ROMP, Ring-opening metathesis polymer-isation; SA, Salicylic acid; SDS, Sodium dodecylsulfate; SFE, Supercritical fluid extraction; SGBM, Silica gel bound macrocycles;SPE, Solid phase extraction; SPS, Solid-phase spectrophotometry; TAN, 1-(2-tiazolylazo)-2-naphthol; TBP, Tri-n-butyl phosphate;T BPP, Tetra-(4-bromophenyl)-porphyrin; TBT, Tributyltin; TCPP, Carboxyphenylporphyrin; THF, Tetrahydrofuran; TOPO, Tri-n-4

octylphosphine oxide; TPhT, Triphenyltin; TS, Methylthiosalicylate; TSA, Thiosalicylic acid; TTA, 2-Thenoyltrifluoroacetone; UV,Ultraviolet; XO, Xylenol orange.

*Corresponding author. Tel.:q33-1-44-08-17-25; fax:q33-1-44-08-16-53.E-mail address: [email protected](V. Camel).

1178 V. Camel / Spectrochimica Acta Part B 58 (2003) 1177–1233

1. Introduction

Despite the selectivity and sensitivity of analyt-ical techniques such as atomic absorption spec-trometry, there is a crucial need for thepreconcentration of trace elements before theiranalysis due to their frequent low concentrationsin numerous samples(especially water samples).Additionally, since high levels of non-toxic com-ponents usually accompany analytes, a clean-upstep is often required. Liquid–liquid extraction isa classical method for preconcentrating metal ionsandyor matrix removal. Solid phase extraction(SPE) is another approach that offers a number ofimportant benefits. It reduces solvent usage andexposure, disposal costs and extraction time forsample preparation. Consequently, in recent yearsSPE has been successfully used for the separationand sensitive determination of metal ions, mainlyin water samples. After outlining the theory of thistechnique, guidelines are given for the develop-ment of SPE-based methods for preconcentrationof many trace elements. Finally, examples of appli-cations are presented.

2. Theory

The principle of SPE is similar to that of liquid–liquid extraction (LLE), involving a partitioningof solutes between two phases. However, insteadof two immiscible liquid phases, as in LLE, SPEinvolves partitioning between a liquid(samplematrix) and a solid(sorbent) phase. This sampletreatment technique enables the concentration andpurification of analytes from solution by sorptionon a solid sorbent. The basic approach involvespassing the liquid sample through a column, acartridge, a tube or a disk containing an adsorbentthat retains the analytes. After all of the samplehas been passed through the sorbent, retainedanalytes are subsequently recovered upon elutionwith an appropriate solvent. The first experimentalapplications of SPE started fifty years agow1,2x.However, its growing development as an alterna-tive approach to liquid–liquid extraction for sam-ple preparation started only in the mid-1970s. Ithas been extensively used in the past fifteen yearsfor the preconcentration of organic micropollu-

tants, especially pesticides, in water samplesw3x.However, numerous studies have also shown thegreat potential of this technique for speciationstudies.

2.1. Presentation of the technique

2.1.1. Basic principlesAn SPE method always consists of three to four

successive steps, as illustrated in Fig. 1. First, thesolid sorbent should be conditioned using anappropriate solvent, followed by the same solventas the sample solvent. This step is crucial, as itenables the wetting of the packing material andthe solvation of the functional groups. In addition,it removes possible impurities initially containedin the sorbent or the packaging. Also, this stepremoves the air present in the column and fills thevoid volume with solvent. The nature of theconditioning solvent depends on the nature of thesolid sorbent. Typically, for reversed phase sorbent(such as octadecyl-bonded silica), methanol is{

frequently used, followed with water or aqueousbuffer whose pH and ionic strength are similar tothat of the sample. Care must be taken not toallow the solid sorbent to dry between the condi-tioning and the sample treatment steps, otherwisethe analytes will not be efficiently retained andpoor recoveries will be obtained. If the sorbentdries for more than several minutes, it must bereconditioned.The second step is the percolation of the sample

through the solid sorbent. Depending on the systemused, volumes can range from 1 ml to 1 l. Thesample may be applied to the column by gravity,pumping, aspirated by vacuum or by an automatedsystem. The sample flow-rate through the sorbentshould be low enough to enable efficient retentionof the analytes, and high enough to avoid excessiveduration. During this step, the analytes are concen-trated on the sorbent. Even though matrix com-ponents may also be retained by the solid sorbent,some of them pass through, thus enabling somepurification (matrix separation) of the sample.The third step(which is optional) may be the

washing of the solid sorbent with an appropriatesolvent, having a low elution strength, to eliminatematrix components that have been retained by the

1179V. Camel / Spectrochimica Acta Part B 58 (2003) 1177–1233

Fig. 1. SPE operation steps.

solid sorbent, without displacing the analytes. Adrying step may also be advisable, especially foraqueous matrices, to remove traces of water fromthe solid sorbent. This will eliminate the presenceof water in the final extract, which, in some cases,may hinder the subsequent concentration of theextract andyor the analysis.The final step consists in the elution of the

analytes of interest by an appropriate solvent,without removing retained matrix components. Thesolvent volume should be adjusted so that quanti-tative recovery of the analytes is achieved withsubsequent low dilution. In addition, the flow-rateshould be correctly adjusted to ensure efficientelution. It is often recommended that the solventvolume be fractionated into two aliquots, andbefore the elution to let the solvent soak the solidsorbent.

2.1.2. Retention of trace elements on the sorbentAdsorption of trace elements on the solid sor-

bent is required for preconcentration(see Fig. 2).The mechanism of retention depends on the natureof the sorbent, and may include simple adsorption,chelation or ion-exchange. Also, for trace elemen-ts, ion-pair solid phase extraction may be used.

2.1.2.1. Adsorption. Trace elements are usuallyadsorbed on solid phases through van der Waalsforces or hydrophobic interaction. Hydrophobicinteraction occurs when the solid sorbent is highlynon-polar (reversed phase). The most commonsorbent of this type is octadecyl-bonded silica(C -silica). More recently, reversed polymeric18

phases have appeared, especially the styrene-divi-nylbenzene copolymer that provides additionalp-p interaction whenp-electrons are present in theanalyte w4x. Elution is usually performed withorganic solvents, such as methanol or acetonitrile.Such interactions are usually preferred with on-line systems, as they are not too strong and thusthey can be rapidly disrupted. However, becausemost trace element species are ionic, they will notbe retained by such sorbents.

2.1.2.2. Chelation. Several functional group atomsare capable of chelating trace elements. The atomsmost frequently used are nitrogen(e.g. N presentin amines, azo groups, amides, nitriles), oxygen(e.g. O present in carboxylic, hydroxyl, phenolic,ether, carbonyl, phosphoryl groups) and sulfur(e.g. S present in thiols, thiocarbamates, thioeth-ers). The nature of the functional group will givean idea of the selectivity of the ligand towards


Fig. 2. Interactions occurring at the surface of the solid sorbent.F, functional group; TE, trace element; MS, matrix solvent; MI,matrix ions; ES, elution solvent.

trace elements. In practice, inorganic cations maybe divided into 3 groups:

– group I-‘hard’ cations: these preferentially reactvia electrostatic interactions(due to a gain inentropy caused by changes in orientation ofhydration water molecules); this group includesalkaline and alkaline-earth metals(Ca ,2q

Mg , Na ) that form rather weak outer-sphere2q 2q

complexes with only hard oxygen ligands.– group II-‘borderline’ cations: these have anintermediate character; this group containsFe , Co Ni Cu Zn Pb Mn . They2q 2q 2q 2q 2q 2q 2q

possess affinity for both hard and soft ligands.– group III-‘soft’ cations: these tend to formcovalent bonds. Hence, Cd and Hg possess2q 2q

strong affinity for intermediate(N) and soft(S)ligands.

For soft metals, the following order of donoratom affinity is observed: 0-N-S. A reversedorder is observed for hard cations. For a bidentateligand, affinity for a soft metal increases with theoverall softness of the donor atoms:(0, 0)-(0,N)-(N, N)-(N, S). The order is reversed forhard metals. In general, the competition for a givenligand essentially involves Group I and Group IImetals for O sites, and metals of Group II andGroup III for N and S sites. The competitionbetween metals of Group I and Group III is weak.Chelating agents may be directly added to the

sample for chelating trace elements, the chelatesbeing further retained on an appropriate sorbent.An alternative is to introduce the functional che-lating group into the sorbent. For that purpose,three different means are available:(1) the synthe-sis of new sorbents containing such groups(newsorbents); (2) the chemical bonding of suchgroups on existing sorbents(functionalized sor-bents); and(3) the physical binding of the groupson the sorbent by impregnating the solid matrixwith a solution containing the chelating ligand(impregnated, coated or loaded sorbents). Thelatter remains the most simple to be used inpractice. Its main drawback is the possible flushof the chelating agent out of the solid sorbentduring sample percolation or elution that reducesthe lifetime of the impregnated sorbent.


Different ligands immobilized on a variety ofsolid matrices have been successfully used for thepreconcentration, separation and determination oftrace metal ions. Chelating agents with an hydro-phobic group are retained on hydrophobic sorbents(such as C -silica). Similarly, ion-exchange resins18

are treated with chelating agents containing an ion-exchange group, such as a sulfonic acid derivativeof dithizone (i.e. diphenylthiocarbazone) (DzS),5-sulfo-8-quinolinol, 5-sulfosalicylic acid, thiosal-icylic acid, chromotropic acid, or carboxyphenyl-porphyrin(TCPP) w5–8x.

Binding of metal ions to the chelate functionalityis dependent on several factors:(1) nature, chargeand size of the metal ion;(2) nature of the donoratoms present in the ligand;(3) buffering condi-tions which favor certain metal extraction andbinding to active donor or groups; and(4) natureof the solid support(e.g. degree of cross-linkagefor a polymer). In some cases, the behavior ofimmobilized chelating sorbents towards metal pre-concentration may be predicted using the knownvalues of the formation constants of the metalswith the investigated chelating agentw9x. However,the presence of the solid sorbent may also have aneffect and lead to the formation of a complex witha different stoichiometry than the one observed ina homogeneous reactionw10,11x. In fact, severalcharacteristics of the sorbent should be taken intoaccount, namely the number of active groupsavailable in the resin phasew7,10x, the length ofthe spacer arm between the resin and the boundligand w12x, and the pore dimensions of the resinw13x.

2.1.2.3. Ion-pairing. When a non-polar sorbent isto be used, an ion-pair reagent(IP) can be addedto the sorbentw14x. Such reagents contain a non-polar portion(such as a long aliphatic hydrocar-bonated chain) and a polar portion(such as anacid or a base). Typical ion-pair reagents arequaternary ammonium salts and sodium dodecyl-sulfate(SDS) w15,16x. The non-polar portion inter-acts with the reversed-phase non-polar sorbent,while the polar portion forms an ion-pair with theionic species present in the matrix(that could beeither free metallic species in solution orcomplexes).

2.1.2.4. Ion exchange. Ion-exchange sorbents usu-ally contain cationic or anionic functional groupsthat can exchange the associated couter-ion. Strongand weak sites refer to the fact that strong sitesare always present as ion-exchange sites at anypH, while weak sites are only ion-exchange sitesat pH values greater or less than the pK . Stronga

sites are sulfonic acid groups(cation-exchange)and quaternary amines(anion-exchange), whileweak sites consist of carboxylic acid groups(cat-ion-exchange) or primary, secondary and tertiaryamines (anion-exchange). These groups can bechemically bound to silica gel or polymers(usuallya styrene-divinylbenzene copolymer), the latterallowing a wider pH range.An ion-exchanger may be characterized by its

capacity, resulting from the effective number offunctional active groups per unit of mass of thematerial. The theoretical value depends upon thenature of the material and the form of the resin.However, in the column operation mode, the oper-ational capacity is usually lower than the theoret-ical one, as it depends on several experimentalfactors, such as flow-rate, temperature, particlesize and concentration of the feed solution. As amatter of fact, retention on ion-exchangers dependson the distribution ratio of the ion on the resin,the stability constants of the complexes in solution,the exchange kinetics and the presence of othercompeting ions. Even though ion-exchangersrecover hydrated ions, charged complexes and ionscomplexed by labile ligands, they are of limiteduse in practice for preconcentration of trace ele-ments due to their lack of selectivity and theirretention of major ionsw17x. Yet, for some partic-ular applications they may be a valuable tool.Hence, iron speciation was possible through selec-tive retention of the negative Fe(III )-ferron com-plex on an anion-exchangerw18x. Seleniumspeciation was also feasible by selectively elutingSe(IV) and Se(VI) retained on a anion-exchangerw19x.

2.1.3. Elution of trace elements from the sorbentThe same kind of interactions usually occur

during the elution step. This time, the type ofsolvent must be correctly chosen to ensure strongeraffinity of the trace element for the solvent, to


Fig. 3. Disposable sorbent containers.

ensure disruption of its interaction with the sorbent(as illustrated in Fig. 2). Thus, if retention on thesorbent is due to chelation, the solvent couldcontain a chelating reagent that rapidly forms astronger complex with the trace metal. Elutionmay also be achieved using an acid that willdisrupt the chelate and displace the free traceelement. Similarly, if retention is due to ionexchange, its pH dependence enables the use ofeluents with different pH to be used, such as acids.Of prime importance is to selectively elute only

the target species. So, if they are more stronglyretained on the sorbent than the interferent com-pounds, a washing step with a solvent of moderateelution strength is highly advisable before elutionof the target species with the appropriate solvent.

2.2. Operation

The sorbent may be packaged in different for-mats: filled micro-columns, cartridges, syringe bar-rels and discsw2,20,21x. The disposable sorbentcontainers are illustrated in Fig. 3.

2.2.1. Micro-columnsThe use of a micro-column is a common pro-

cedure for extraction of trace elements from vari-ous samples. It affords the opportunity of packingthe column with the desired sorbent, so that abroader choice than the commercially disposable

containers is available. In addition, the size of thecolumn (i.e. the sorbent weight) may be adaptedto the sample volume. In particular, it allows largersample volumes, thus enabling the preconcentra-tion of metal ions at very low concentration levels.However, such columns must be reused, so thatcareful blank washings should be conducted toavoid cross-contamination. In addition, columnswith a narrow internal diameter limit usable flow-rates to a range 1–10 mlymin that necessitateslong trace-enrichment times for large sample vol-umesw22x.

As will be discussed later, micro-columns arefrequently used in systems affording the on-linecoupling of SPE to analytical techniques. However,in that case, the size of the column is limited toachieve acceptable analytical performance.

2.2.2. Disposable cartridges and syringe barrelsNowadays, the most frequently used design in

off-line SPE is the cartridge or the syringe barrel.They are usually made of polypropylene or poly-ethylene and filled with packing material havingdifferent functional groups. The solid sorbent iscontained between two 20mm polypropylene frits(in some cases they may be made of glass). Theyafford great selectivity due to the broad types ofsorbents contained in commercially available sys-tems with different column volume available. Inaddition, their disposable character prevents pos-sible cross contamination.Cartridges vary from as little as 100 mg to 1 g

or more. Syringe barrels range in size from 1 to25 ml and packing weights from 50 mg to 10 g.Solvent reservoirs may be used at the top of thesyringe barrels to increase the total volume(50–100 ml). The barrel of the syringe terminates in amale Luer tip, which is the standard fitting to beused with various SPE vacuum manifolds availa-ble. For cartridges, both a female and male Luertips are present, to enable use of either a positiveor negative pressure.The major disadvantages of cartridges and

syringe barrels are slow sample-processing ratesand a low tolerance to blockage by particles andadsorbed matrix components, due to their smallcross-sectional area. Channeling reduces the capac-ity of the cartridge to retain analytes and results


in contamination of the isolated analytes withimpurities originating from the manufacturing andpacking process. Such contaminants were evidentfor C -silica cartridges, while less contaminants18

were observed with C -silica disksw23,24x.18

2.2.3. DisksThe use of flat disks with a high cross-sectional

area may largely prevent all the problems encoun-tered with columns, cartridges and tubesw21x. Thepacking material is usually embedded in an inertmatrix of polytetrafluoroethylene(PTFE) microfi-brils, with a typical composition of 90% wywsorbent and 10% wyw PTFE fibers w25x. Othertypes of disks use a glass-fibre matrix to hold thesorbent particles, in order to enable a higher flow-rate. The disks are available in different diametersfrom 4 to 90 mm, the size most frequently usedbeing 47 mm. They are designed to be used inconjunction with a filtration apparatus connectedto a water aspiratorw25x. In order to removepotential interferences and to ensure optimalextraction of the analyte of interest, disk cleaningand conditioning should be done before its use.Due to a lower void volume and a higher surface

area associated with small particles as comparedto cartridges, partitioning of the analytes isfavored. Hence, a smaller mass of sorbent isrequired to process a similar volume of sample.Disks thus present the advantage of reducing sol-vent volumes for both the conditioning and elutionsteps. Additionally, the decreased back-pressureencountered with these devices enables the use ofhigh flow-rates, and their wide bed minimizes thechance of plugging. In addition, new technologyfor embedding the stationary phase prevents chan-neling and improves mass transfer. As classicaldisks are dedicated to the SPE of large-volumesamples, new systems have very recently emergedthat enable the use of disks for small-volumesamples: the extraction disk cartridge(the disk isplaced in a syringe-barrel format), and the 96-wellmicrotiter plate configurationw20,21,26x. Such sys-tems are primarily dedicated to biological samples.One of the drawbacks of using disks is the

decrease in the breakthrough volume(which is thevolume that can be percolated without analytelosses). In addition, disks have lower capacity than

cartridges, so that for real samples(e.g. highcontent of natural organic matter in river water)incomplete retention of the target metal speciesmay result w27x. As a consequence, disks arerecommended when there is a strong interactionbetween the analyte and the sorbent.

2.3. Advantages of the technique

Classical liquid–liquid extractions of trace ele-ments are usually time-consuming and labor-inten-sive. In addition, they require strict control ofextraction conditions, such as temperature, pH andionic strength. For all these reasons, several pro-cedures tend to be replaced by SPE methods. Thistechnique is attractive as it reduces consumptionof and exposure to solvents, their disposal costsand extraction timew28x. It also allows the achieve-ment of high recoveriesw29x, along with possibleelevated enrichment factors. However, as differentresults between synthetic and real samples may beobservedw30x, recoveries should be estimated inboth cases as far as possible. In addition, SPE canbe interfaced on-line with analytical techniques,such as liquid chromatography(LC) or atomicabsorption spectrometry(AAS). Its application forpreconcentration of trace metals from differentsamples is also very convenient due to sorption oftarget species on the solid surface in a more stablechemical form than in solution. Finally, SPEaffords a broader range of applications than LLEdue to the large choice of solid sorbents.

2.3.1. PreconcentrationLLE requires the use of large volumes of high-

purity solvent, thereby affording limited precon-centration factors. The use of SPE enables thesimultaneous preconcentration of trace elementsand removal of interferences, and reduces theusage of organic solvents that are often toxic andmay cause contamination. Upon elution of theretained compounds by a volume smaller than thesample volume, concentration of the extract canbe easily achieved. Hence, concentration factorsof up to 1000 may be attained.

2.3.2. Preservation and storage of the speciesSPE allows on-site pre-treatment, followed by

simple storage and transportation of the pre-treated


samples with stability of the retained metallicspecies for several daysw21,24,31,32x. This pointis crucial for the determination of trace elements,as the transport of the sample to the laboratoryand its storage until analysis may induce problems,especially changes in the speciation. In addition,the space occupied by the solid sorbents is minimaland avoids storage of bulky containers and themanpower required to handle them.

2.3.3. High selectivitySPE offers the opportunity of selectively extract-

ing and preconcentrating only the trace elementsof interest, thereby avoiding the presence of majorions. This is crucial in some cases, such as withspectrophotometric detection, since the determina-tion of heavy metals in surface waters may neces-sitate the removal of non-toxic metals, such as Feor Zn, when they occur at high concentrationsw33x. It may also be possible to selectively retainsome particular species of a metal, thereby ena-bling speciation. For example, salen I modifiedC -silica is quite selective towards Cu(II) w34x,18

while chemical binding of formylsalicylic acid onamino-silica gel affords selectivity towards Fe(III )w35x. This high selectivity may also be used toremove substances present in the sample that mayhinder metal determination, such as lipid sub-stances in the case of biological samplesw36x.

2.3.4. Automation and possible on-line coupling toanalysis techniquesSPE can be easily automated, and several com-

mercially available systems have been recentlyreviewedw26x. Home-made systems have also beenreportedw37x. In addition, SPE can be coupled on-line to analysis techniques. On-line proceduresavoid sample manipulation between preconcentra-tion and analysis steps, so that analyte losses andrisk of contamination are minimized, allowinghigher reproducibility w38x. In addition, all thesample volume is further analyzed, which enablessmaller sample volume to be used. However, inthe case of complex samples, off-line SPE shouldbe preferred due to its greater flexibility, and theopportunity to analyze the same extract usingvarious techniques.

2.3.4.1. On-line coupling to liquid chromatogra-phy. On-line systems mainly use a micro-column.The sorbent is chosen not only for its efficiencyin trapping analytes, but also for its compatibilitywith the stationary phase packed into the chromat-ographic column. Indeed, it is highly recommend-ed to use the same packing in the precolumn andthe chromatographic column to prevent losses inefficacy upon analysis. For the case of two differ-ent sorbents being used, the retention of the anal-ytes in the precolumn should be lower than in theanalytical column to ensure band refocusing at thehead of the chromatographic column. On-line sys-tems with several detectors have been reported,such as ultraviolet(UV) detectorw39x or induc-tively coupled plasma mass spectrometer(ICP–MS) w40x, with detection limits in the 0.05–50mgyl range. Detection limits as low as 0.5 ngylcould even be achieved by detection at the maxi-mum absorption wavelength using a photodiodearray UV detectorw41x. Additional coupling maybe feasible, such as the on-line coupling of super-critical fluid extraction (SFE) with an on-lineSPE–LC systemw42x. The coupling of SPE to LCvia flow injection has also been reported usingcold vapour AAS (CV-AAS) as the detection,enabling enrichment factors approximately 850w43x.

2.3.4.2. On-line coupling to atomic absorptionspectrometry. Olsen et al. w44x and Fang et al.w45,46x were the first to describe an on-line flow-injection (FI) sorbent extraction preconcentrationsystem for flame AAS(F-AAS) using micro-columns packed with a cation-exchanger. Later,they also proposed a system for on-line flow-injection sorbent extraction preconcentration withelectrothermal vaporization AAS(ET-AAS) usinglead as a model trace elementw47x. Since then,numerous papers reported FI with on-line precon-centration followed by AAS, as exemplified bydetermination of Cu, Cr(VI) or Pb w48–50x.Selected applications are reported in Table 1. Thesorbent should provide for rapid sorption anddesorption of the analytes to be used in FI systemsw65x. In addition, it should be provided for a highselectivity. In practice, C -silica is very frequently18

used as organic solvents(such as methanol) can

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Table 1Applications of SPE to FI on-line preconcentration systems

Matrix Trace Chelating Sorbent Eluent Analysis methodRecovery Loading PreconcentrationLOD Sampling Ref.elements agent (%) time factor (ngyl) frequency

added (volume) (h )y1

Inorganic sorbentsSea and waste Cd None DPTH- HNO –3 ICP–AES 97.5–104 1 min 86 1100 40 w51x

waters functionalized- HClSiO2

Sea and waste Cd None TS- Thiourea ICP–AES 95.2– 2 min 62 4300 24 w51xwaters functionalized- 2.5% in 103.3

SiO2 HNO3

Sea waters Fe None 8-HQ- HCl Spectro- 106.3 2 min — 0.016 nM — w52xfunctionalized- photometrySiO2

Geological Ag None MBT- Thiourea F-AAS 93.5–101 1 min — 660 60 w53xsample, Cu functionalized-metal, Pb SiO2

nitrateCertified ore Ag, Au, None Amidino- Thiourea F-AAS 98.7–101.4 1 min — 1100–17 000 — w54x

samples, Pd thioureido- (4.5 ml)Ni alloy, SiO2

anode slime,electrolyticsolution

Fish, human MeHg, APDC C -silica18 MeOH- LC–CV-AAS 92–106 20 min 750–950 5.5–10.4 2.3 w43xurine EtHg, ACN- (58.5 ml)

PhHg, waterHg(II)

Sea water MeHg, DDTC C -silica18 EtOH CV-AAS 85–107.5 (25 ml) 500 16 — w55xHg(II)

Certified sea Cu, Cd APDC C -silica18 MeOH ET-AAS 86.7– 104 s 25–100 6.5–1.26 — w56xwaters 106.5 (0.5 ml)

Certified sea Cu, Cd, Co 1,10- C -silica18 EtOH F-AAS 88.9– 30 s 22–32 300–6000 90 w57xwater, Phenan- 100.5mussel, throlinegeologicalsamples

Certified Cd PAR or C -silica18 MeOH ET-AAS 82.5– 110 s 25–50 1.7–4 9–12 w58xsea waters PADMAP 111.2 (0.5 ml)

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Table 1(Continued)



Sea water, Cr(III ), DDTC C -silica18 MeOH F-AAS 95–105 60–300 s 90–500 20 30 w59xindustrial Cr(VI),effluents Cr(total)

Sea, river Cr(VI), DDTC C -silica18 EtOH ET-AAS 101–105 1 min 12 1600– 22 w60xwaters Cr(total) (3 ml) 1800

Certified low Co NN C -silica18 Acidified F-AAS 98–102 30 s 17.2 3200 90 w61xalloy steel, EtOH (3.25 ml)mussel,tomatoe leaves

Certified Pb DDTC C -silica18 IBMK F-AAS 99.2– 2–10 min 60–189 3000 24 w62xbiological, 137.9vegetablesamples

Standard Pb DDTP C -silica18 EtOH F-AAS — 2.5–75 14–1000 300–3000 — w63xsolutions min

(10–150ml)

Certified citrus MeHg, DDTP C -silica18 EtOH CV-AAS 99.6– 4.5 min 20 10 12 w64xleaves, Hg(II) 112.5 (23.85 ml)marinesediment

Standard Cu, Pb DDTC, 8-C -silica18 MeOH F-AAS — 80 s 14–60 4000– — w65xsolutions quinolinol (4 ml) 10 000

or PARSea water Cu None TAN-loaded- HCl ET-AAS 93.2–99.6 1 min 33 5 35 w66x

C -silica18 (3 ml)Pharmaceutical Zn None TAN-loaded- HCl Spectro- 89.8–107.8 — — 10 000 45 w67x

preparations C -silica18 photometrySea water Fe(II), None FZ-loaded- MeOH Spectro- — 2–20 min 6–60 0.1– — w68x

Fe(III ) C -silica18 photometry (4–40 ml) 0.3 nMRiver, tap, rain Al, Bi, None ZrO2 HNO3 ICP–AES 95.4–99 33 min 100 6–90 — w69x

waters Cd, Co, (100 ml)Cr, Cu,Fe, Ga,In, Mn,Mo, Ni,

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Pb, Tl,V, Sb,Sn, Zn

Waters Cr(VI) None Acidic Al O2 3 NHq4 ICP–AES — (2 ml) 10 200 — w70x

Lake, river, Cr(III ), None Acidic Al O2 3 HNO or3 F-AAS 90–106 35 s 25 800–1000 55 w71xtap waters Cr(VI) NHq

4

Sewage waters Cr(III ), None Al O2 3 HNO or3 F-AAS 86–117 — — 42 000– — w72xCr(VI) NHq

4 81 000Urine Cr(III ) None Basic Al O2 3 HNO3 ICP–AES )93 (10 ml) 50 50 — w73x

Organic sorbentsTap, river, Cu APDC PTFE turnings IBMK F-AAS 94–102 1 min 340 50 40 w48x

coastal waters (12 ml)Tap, river, Cr(VI) APDC PTFE turnings IBMK F-AAS 95.5– 3 min 80 800 18 w49x

coastal, 100.5 (37.8 ml)industrialwaste waters

Tap, river and Pb APDC PTFE turnings IBMK F-AAS 95–102 3 min 330 800 15 w50xcoastal (39 ml)waters,marinesediment,fish andmussel tissues

Drinking, sea Cr(VI) APDC PTFE(KR) EtOH ET-AAS 105 1 min 19 4.2 21.2 w74xwaters (5 ml)

Certified natural Cr(VI) APDC PTFE(KR) EtOH ET-AAS — 1 min 16.3 16 16.7 w75xwater, sea (5 ml)water

Certified Cd, Co, Cu, Dithizone PTFE(KR) IBMK F-AAS 95.3– 1 min 23.4–69.3 1060– 18 w76xhuman air, pig Zn 108.4 (5 ml) 2560liver, seaprawn

Certified Cd DDTC PTFE(KR) IBMK F-AAS 97.9–110 50 s 30 100 55 w77xhumain hair (4.2 ml)and ricepowder

Certified natural Cr(VI) APDC PTFE beads EtOH ET-AAS 104–108 1 min 30.1 8.8 16.7 w75xwater, sea (5 ml)water

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Standard Pb DDTP PUF EtOH F-AAS — 2.5–75 14–1000 300–3000 — w63xsolutions min

(10–150ml)

Certified Cu None PAN-coated- HCl- F-AAS 99.6–103(2.5– 29.1–296.1 60–600 — w32xseawaters XAD-4 EtOH 25 ml)

Standard Cd, Zn None BSQ-loaded- HCl Fluorimetry — 20 s 10 1600– — w78xsamples XAD-7 (0.1 ml) 1900

Alloys, ores Pd, Pt, Rh ODETA Highly cross- HCl- F-AAS 77.8–103.6 1 min — 3000– 30 w79xlinked EtOH 8000polystyrene

Tap water Cu, Cd, — IDA-Novarose HCl ICP–AES — 10 min 500–1000 — — w80xNi

River, ground Cu, Cd, Sulfa- Lewatit HCl Spectro- 80–120 50 min 50 2000– — w33xwaters Pb sarzene TP807’84 photometry (100 ml) 5500

River, mineral Cr(III ) None PAPhA HCl F-AAS 97–01 90 s 35 200 30 w81xand tap (6.6 ml)waters

Estuarine Cu, Cd, None Toyopearl AF-HNO3 ICP–MS 87–10 1 min — 1.4–86 — w82xwaters Ni, Chelate 650 (1 ml)

Zn, Mn MSynthetic sea Cd, Cu, None Cation HNO3 F-AAS — — 50–105 30–200 60 w45x

water Pb, Zn exchangerNatural waters Cr(VI), DPC Cation HNO -3 Spectro- 93–08 — — 8.9–15.2 — w83x

Cr(total) exchanger acetone photometryCertified pig Cd, Ni, Pb None Cation HCl ICP–MS 96.7–03.7 2 min 10 1000–4000 90 w84x

kidney, rye exchanger (7.8 ml)grass, riceflour, tomatoleaves

Ground Cu Batho- Anion HNO3 Spectro- — 10 min — 80 — w85xwater cuproine exchanger photometry (8.3 ml)

Water, vegeta- Cd None Anion HNO3 Spectro- 94–104 90 s — 230 20 w86xble samples exchanger photometry (5 ml)

Standard Pb DDTP Activated EtOH F-AAS — 2.5–75 14–1000 300–3000 — w63xsolutions carbon min

(10–150ml)

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Steels, Al solutions Bi DDTP Activated EtOH ET-AAS 87–104.3 4 min 14 48 7 w87xcarbon (10 ml)

Tap, mineral, Cu APDC Activated IBMK F-AAS 88–113 2 min 100 600 17 w88xwell, river, carbon (4.5 ml)swimmingpool waters

Silicon, tap Fe None Activated HCl MPT–AES 97.4–105 1 min 4.3–6.4 1000–36 000 — w89xwater carbon (1.2 ml)

Rock, copper Pd None Activated Thiourea F-AAS 103–107 3 min 145 300 15–20w90xore carbon (15.6 ml)


be used as eluting solvents leading to a highsensitivity in flame AAS. Complexing reagentsare, therefore, added for efficient retention of tracemetals. Their choice is based on their fast reactionwith metals, such as diethyldithiocarbamate(DDTC) and ammonium pyrrolidine dithiocarba-mate (APDC) w56,59,60,62,91x. In addition, bothreagents are water soluble and do not adsorb onC -silica so that it does not overload with the18

reagent itself. However, these reagents lack selec-tivity, so that other reagents have been used forparticular applications, like 1-10-phenanthrolinew57x, 4-(2-pyridylazo)resorcinol (PAR) or 2-(2-pyridylazo)-5-dimethylaminophenol (PADMAP)w58x, 0,0-diethyl-dithiophosphate(DDTP) w63x, l-nitroso-2-naphthol(NN) w61x. l-(2-tiazolylazo)-2-naphthol (TAN) w66x. The microcolumn can beinserted into the tip of the PTFE capillary in theautosampler arm of a graphite furnace atomicabsorption spectrometerw56x.Even though C -silica has been the most fre-18

quently used sorbent for FI preconcentration, othersorbents were found satisfactory for some appli-cations as shown in Table 1, such as functionalizedsilica w51–53x, aluminaw70–73x, activated carbonw63,87–90x, polyurethane foam(PUF) w63,92x, orPTFE turningsw48–50x. A particular knotted reac-tor (KR) has been recently developed, whichconsists of a long tube properly knotted usuallymade of PTFE. The trace element species areadsorbed on the inner wall of the tubing asindicated by scanning electron microscopyw77x.This reactor allows higher sample loading volumesthan micro-columns due to its lower back-pressure.In addition, the inner wall may be precoated witha hydrophobic ligand for subsequent retention oftrace elementsw93x. However, on the other hand,lower enrichment factors are attained as comparedto micro-columnsw75x. For that reason, in case oftrace metals, micro-columns are usually preferredfor the achievement of low levels of determination.

2.3.4.3. On-line coupling to ICP–AES or ICP–MS. The first report of FI on-line preconcentrationcoupled to ICP-atomic emission spectrometry(AES) appeared nearly twenty years agow94x.Since then, several studies have used this couplingwith different sorbents such as ZrO or function-2

alized silica gel for examplew51,69x. Similarly,numerous studies have reported on-line couplingto ICP–MS, as noted earlierw95x. A few examplesare given in Table 1w82,84x.

2.3.4.4. On-line coupling to spectrophotometry.Spectrophotometry offers the advantage of requir-ing inexpensive and very common instrumentation.In addition, by choosing a non-selective chromo-genic reagent, multi-metal determinations may bepossiblew33x. Its coupling to FI analysis is wellsuited for monitoring purposes and a few studiespresent such systems as indicated in Table 1w33,52,68,86x. Solid-phase spectrophotometry(SPS) has also been reported with FI systems dueto its simplicity and low detection limits. The solidsorbent is packed in either commercially availableor customized flow cells. With such systems theretained analytes are periodically removed fromthe flow cell using an acid or a complexingsolution w67,83,85x.On-line FI sorbent extraction procedures have

several advantages over the corresponding off-linemethods: higher sample throughput(increased by1 to 2 orders of magnitude), lower consumptionof sample and reagent(also reduced by 1 to 2orders of magnitude), better precision(with rela-tive standard deviations approx. 1–2%), lower riskof loss or contamination and easy automation.However, the FI method by using column extrac-tion has some disadvantages. In particular, it maysuffer from insufficient adsorption on the resin andclogging of the column when insoluble ligands areusedw96x.

3. Step-by-step method development guide

Development of an SPE method can be consid-ered as a two-step procedure. First, the mostappropriate sorbent for the application should bechosen(the following is intended to help the readerin choosing a solid sorbent for trace elementdetermination). Optimization of the most influen-tial parameters should then be undertaken. Obvi-ously, optimization should initially be performedusing spiked synthetic solutions, but it must befollowed by the use of certified reference materialsor spiked real samples, as matrix components


(such as ligands or other ions) may change thetrace element retention on the sorbent, therebydecreasing recoveries of the target species.

3.1. Selection of solid sorbent

Solid sorbents may be hydrophobic or polar. Itis common to call reversed-phase sorbents thepacking materials that are more hydrophobic thanthe sample, which are frequently used with aque-ous samples. On the other hand, normal-phasesorbents refer to materials more polar than thesample and they are used when the sample is anorganic solvent containing the target compounds.When hydrophobic supports are used, retention ofionic metal species will require the formation ofhydrophobic complexes. This can be achievedthrough addition of the proper reagent to thesample or thorough immobilization of the reagenton the hydrophobic solid sorbent. Addition ofreagent to the sample is appropriate for the fixationof unstable metal species(such as Cu(I) andFe(II)) to maintain speciation, while immobiliza-tion offers the convenience of having a preparedcartridge or disk before analysis. Immobilizationmay also provide a significant development inspeciation analysis, because metal equilibrium inthe sample may not be affected by reaction on thecartridge.The nature and properties of the sorbent are of

prime importance for effective retention of metallicspecies. Careful choice of the sorbent is thuscrucial to the development of SPE methodology.In practice, the main requirements for a solidsorbent are:(1) the possibility to extract a largenumber of trace elements over a wide pH range(along with selectivity towards major ions); (2)the fast and quantitative sorption and elution;(3)a high capacity;(4) regenerability; and(5) acces-sibility. In particular, sorbents that allow fast reac-tion rates are preferred to achieve faster extractionas well as higher loading capacities. Hence, sor-bents based on hydrophilic macroporous polymersand cellulose or on fibrous materials provide excel-lent kinetic propertiesw97x.The broad variety of sorbents available explains

one of the most powerful aspects of SPE, whichis selectivity. Sorbents can be mainly categorized

as organic based ones(natural polymers, as wellas synthetic polymers) and inorganic based ones(silica gel SiO , alumina A1 O , magnesia MgO2 2 3

and other oxide species). Immobilization of organ-ic compounds on the surface of the solid supportis usually aimed at modifying the surface withcertain target functional groups for a higher selec-tivity of the extraction. The selectivity of themodified solid phases towards certain metal ionsis attributed to several well-known factors, such asthe size of the organic compound used to modifythe sorbent, the activity of the loaded surfacegroups, and the type of the interacting functionalgroup. However, the selective extraction of a singletrace element from other interfering ion(s) repre-sents a direct challenge for finding a suitable phasecapable of exhibiting a sufficient affinity to selec-tively bind that metal ion. For particular applica-tions, the combination of two sorbents may thusbe advisable. As an example, the passage of watersamples through two successive chelating resinsenabled the determination of trace and majorelementsw98x. Similarly, the combination of ananion and a cation-exchange resin enabled thespeciation of Cu and Mn in milk samplesw99x.

3.1.1. Inorganic based sorbentsInorganic based sorbents are mainly made of

silica gel even though other inorganic oxides maybe used, as discussed later(cf. Fig. 4). Silica gelbased sorbents present the advantages of mechan-ical, thermal and chemical stability under variousconditions. They frequently offer a high selectivitytowards a given metal ion. However, all silica-based sorbents suffer from different chemical lim-itations, namely the presence of residual surfacesilanol groups(even after an end-capping treat-ment) and a narrow pH stability range. Applica-tions of such sorbents to off-line SPE are presentedin Tables 2 and 3.

3.1.1.1. Silica gel. Silica gel can be used as a verysuccessful adsorbing agent, as it does not swell orstrain, has good mechanical strength and canundergo heat treatment. In addition, chelatingagents can be easily loaded on silica gel with highstability, or be bound chemically to the support,affording a higher stability.


Fig. 4. Sorbents based on inorganic supports.

The surface of silica gel is characterized by thepresence of silanol groups, which are known to beweak ion-exchangers, causing low interaction,binding and extraction of ionic speciesw131x. Inparticular, silica gel presents high sorption capacityfor metal ions, such as Cu, Ni, Co, Zn or Few132x.Retention is highly dependent on sample pH withquantitative retention requiring pH values over7.5–8, as under acidic conditions silanol groupsare protonated and the ion-exchange capacity ofthe silica gel is greatly reduced or even reducedto zero at low pHs. In addition, this sorbent has avery low selectivity, and is prone to hydrolysis atbasic pH. Consequently, modification of the silicagel surface has been performed to obtain solidsorbents with greater selectivity. Two approachesare used for loading the surface with specificorganic compounds, chemical immobilization andphysical adsorption. In the first case, a chemicalbond is formed between the silica gel surfacegroups and those of the organic compound(func-tionalized sorbent). In the second approach, theorganic compound is directly adsorbed on thesilanol groups of the silica gel surface(impregnat-ed or loaded sorbent), either by passing the reagentsolution through a column packed with the adsor-

bent, or by soaking the adsorbent in the reagentsolution.Impregnating reagents are ion-exchangers or

chelating compounds. Numerous reagents havebeen investigated for impregnation of silica gel asa means of increasing retention capacity and selec-tivity of the sorbent for trace elements, namelythionalide (2-mercapto-N-2-naphthylacetamide)w101,102x, 2-mercaptobenzothiazole (MBT)w133x, NN w103x, 8-hydroxyquinoline (8-HQ)w134,135x, 3-methyl-l-phenyl-4-stearoyl-5-pyrazo-lone (MPSP) w100x, salicylaldoximew132x, dime-thylglyoxime (DMG) w13x, Aliquat 336 (methyl-tricaprylammonium chloride) and Calcon(hydro-phobic sodium sulfonate) w136x. Examples ofapplications are given in Table 2. Increased stabil-ity of the sorbent is obtained by the chemicalbinding of chelating functional groups on silicagel w104x. Applications to the determination oftrace elements have been reported for more thantwenty years with several functional groups, suchas amines, dithiocarbamates, iminodithiocarba-mates or dithioacetalsw13,105,106,137,138x. Care-ful choice of the bound chelating groups enablesspeciation studies. Hence, dithizone-functionalizedsilica gel was reported selective towards Hg(II)

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Table 2Applications of off-line SPE to water samples using inorganic supports

Matrix Trace Sorbent Operation Experimental Analysis Recovery Adsorptive Preconcentration LOD Ref.elements conditions method (%) capacity factor (mgyl)

SilicaTap water CuyCoyNi MPSP-loaded- Glass Sample F-AAS 94.6–101 43y45y49 40 60y40y70 w100x

SiO2 column pH: 4.5 mmolygElution:HCl 1 M

Sea water Pd Thionalide- Glass Sample pH: 4 F-AAS 83–99 0.8 mgyg 3200 0.03 w101xloaded-SiO2 column Washing

(1 cm elution:i.d) thiourea 0.2

MqHCl 0.1 MSea water As(III ) Thionalide- Glass Sample pH: 7 Spectro- 92–95 5.6mmolyg — 0.12 w102x

loaded-SiO2 column Washing photometry(1 cm elution:i.d) NaBoratey

NaOHyIodine(pH 10)

River and sea Co NN-loaded- Glass Sample pH: 3.5g-Emission 96–98 0.03 10–100 — w103xwater SiO2 column Washing mmolyg

elution:aceticacid

Spiked tap Hg(II) Dithizone- Column Elution: CV-AAS 99–99.5 300mmolyg 200 3.96 w104xwater functionalized- HCl 10 M

SiO2

Tap and sea Hg(II) Dithioacetals- Column Elution: water CV-AAS 91–100 917–1100 5 — w105xwaters functionalized- mmolyg

SiO2

Spiked tap Hg(II) Dithiocarbamates- Glass Elution: water CV-AAS 88–100 0.6–0.983 — — w106xand sea functionalized- column mmolygwaters SiO2

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Matrix Trace Sorbent Operation Experimental Analysis Recovery Adsorptive Preconcentration LOD Ref.elements conditions method (%) capacity factor (mgyl)

Sea water VyCoyNiy 8-HQ- Column Washing ICP–MS 85–116 — 10 0.00037– w107xGayYyMoy functionalized- (6 mm i.d, sample 2200 ngylCdyCryPry fluorinated 30 mm pH: 5 WashingNdySmyEu metal bed elution: HNO3yGdyTbyDy alkoxide height) 0.5 MyMoyEry glass backflushTmyYbyLuyWyU

Other oxidesRain, river, Cr(III )yCr TiO2 Glass Sample pH: ET-AAS 78.4–99.2 8125y6983 100 0.030y0.024 w108x

sea, tap (VI) column 2 or 8 Elution: mgygwaters (1 cm HNO 0.53

i.d) or 1 MNatural, waste, CdyCoyCu TiO2 Glass Sample pH: 8 F-AAS 89–100 5000 300 0.01– w109x

sea waters yFeyMnyNi column Elution: mgyg 0.04yPb (1 cm HNO 1 M3

i.d) andyor EDTA0.1 M

Tap, ground Cr(III )y Neutral Column Sample pH: ET-AAS 99–100 — 25 0.01 w110xwaters Cr(VI) Al O2 3 (1 cm 6.5–7 Elution:

i.d) NH 1 Mq3

HNO 4 M3

Tap, ground Se(IV)y Acidic Teflon Sample pH: ET-AAS 90–98 23.2y2.0 16y100 0.049y w111xwaters Se(VI) Al O2 3 column 2–8 Elution: mgyg 0.80

(1 cm NH 0.13

i.d) M and 4 M

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Table 3Applications of off-line SPE to water samples using C -silica based supports18

Matrix Trace Reagent Operation Experimental Analysis Recovery Adsorptive Preconcentration LOD Ref.elements conditions method (%) capacity factor (mgyl)

No reagentSea water TBT None Cartridge Conditioning GC–ECD 93.5– — 1000 — w24x

or sample drying 111.5Empore elution:disk acidified(25 mm) ethyl acetate

Spiked sea TPhT None Bond Elut Conditioning: Fluorimetry 81–89 — 100 — w112xwaters cartridge MeOHqNaCl

SampleWashingAir dryingElution:10 M FlOHy4

in MeOHbackflush

Sea water TPhT None Bond-Elut Washing Fluorescence — — 250 — w113xcartridge sample (after addition(40 mm) Washing air of flavonol

drying elution: to the eluate)MeOH

Addition of the reagent to the sampleSea water SeySb APDC Glass Sample ET-AAS 94–97 — 40–75 0.007y0.05 w114x

column pH: 1.2(1.4 cm Washingi.d) elution:

MeOHSea water CdyZnyCuy 8-HQ Glass Sample ET-AAS 67–108 — 50–100 — w115x

MnyFeyNiy column pH: 8.9Co (1.4 cm Washing

i.d) (waterqoxine)elution:MeOH

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Loaded reagent onto the sorbentRiver water Cu Neocuproine Empore Washing Spectro- 99.7– 940mg 50–100 0.12 w116x

disks conditioning: photometry 102.6 Cu2q

(47 mm) MeOH Sample (454 nm)pH: 5.0 Drying

Elution:isopentylalcohol

CRM water Cu(I) Bathocuproine Bond Elut Conditioning: Spectro- — — 20–40 0.40–3.8w27x(SLRS-3), cartridge MeOHqwater photometrylake, Empore Sample pH: 4.3(484 nm)river, disk Elution:drinking MeOH-waters water 90:10

(vyv)Tap waters, Fe Bathophenan- Empore Washing Spectro- — — 0.080 w117x

well water throline disks activation: photometry(47 mm) MeOHqwater (533 nm)

SamplepH: 4–7Elution:EtOHqNaClO4

Synthetic MeHgy Dithizone Sep Pak Sample pH: LC–DAD 95–104 200 0.14y w31xseawaters PhHgy cartridge 4qEDTA 0.16y

Hg(II) 0.001 0.14M WashingElution: MeOH

Rain, lake, MonoBTy Tropolone Sep-Pak Conditioning: Ethylation-GC– — — — — w118xriver DiBTyTBT cartridge MeOH sample FPDwaters yMonoPhT pH: 2–3 Air

yDiPhTyTP drying Elution:hT diethyl ether

Tap, well and Cu Quinone Empore Washing F-AAS 98.4–102 360mg 400 0.2 w119xriver waters derivative disks Conditioning: Cu2q

(47 mm) buffer samplepH: 7.0DryingElution: HNO3

0.1 M

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Tap, rain, Cu(II) Schiff’s base Empore Washing AAS — 396mg )500 0.004 w34xsnow and (salen I) disk sample Cu2q

sea waters (47 mm) pH: 5.5–6Air dryingelution: HNO30.1 M

Synthetic and Pb(II) Schiff’s base Empore Washing F-AAS 97.1– 700 50 16.7 w120xspring waters disk sample 100.2 mgydisk

pH: 2–8 Airdrying Elution:HNO O.5 M3

River water Pb(II) BAS Empore Washing F-AAS — 476mg G300 0.050 w121xdisk sample Pb2q

(47 mm) pH: 2–7 Airdrying elution:acetic acid 1 M

Sea waters Fe(II) Ferrozine Sep Pak Conditioning: Spectro- 91 — 40 0.6 w122xcartridge MeOHqwater photometry nmolyl

sample pH: (562 nm)6.8–8.3WashingElution: MeOH

Rain, sea Fe(II) Ferrozine Sep Pak Conditioning: LC–UV 92–99 — 100–500 0.1 nmolyl w123xwaters cartridge MeOHqwater (254 nm)

SampleWashingElution: MeOH

Certified sea CuyCd APDC Teflon Conditioning: ET-AAS 95.8– — 25–50 0.0024y w124xwaters cartridge MeOHqwater 103.3 0.00018(NASS-2 and (0.94 mm Sample: 6–8SLEW-1) i.d) Air drying

Elution:MeOH

Tap and U(IV) TOPO Empore Washing Spectro- 85 4033 8 0.1 w125xSpring disk conditioning photometry mgydiskwaters (47 mm sample elution:

i.d) MeOH

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Sea waters LayCeyPry HDEHPy Sep Pak Sample ICP–MS 88.8–99.8 — 200–1000 — w126xNdySmyEuy H ME2 cartridge pH:3–3.5GdyTby HP WashingDyyHoy elution:EryTmy HCl 6 MYbyLu

Spiked natural Bi Cyanex 301 Cartridge Conditioning: ET-AAS 98.5–100 — 10–100 105 w127xwaters (0.5 g) HCl 0.1 M

SamplepH: 1Air dryingElution:HNO 3 M3

Sea, well Be Quinalizarin Sep pak Washing F-AAS 98–101 200mg 200 0.2 w128xand tap cartridge conditioningwaters sample

pH: 6–6.6Elution:HNO 0.5 M3

Tap, spring Agq HT18C6 Empore Conditioning: AAS 996– 210mg 200 0.050 w129xwaters disk MeOHqwater 100.3 Agydisk

Sample elution:Na S O 0.1 M2 2 3

Tap, river, Hg(II) HT18C6TO Empore Washing CV-AAS 97.1– 241mg 50 0.006 w130xwell and disk sample 101.3 Hg2qspring waters (47 mm pH:-7 Air

i.d) dryingElution:HBr 1 M


w104x, even though dithizone was reported to reactwith many trace elementsw139x. Similarly, purpu-rogallin-bound silica gel enabled selective extrac-tion of Fe(III ) w140x. Simultaneous retention oftrace elements is possible by choosing a non-selective chelating group, such asN-propylsalicy-laldimine w141x or Bismuthol I (2,5-dimercapto-l,3,4-thiadiazole) w142x. Acidic groups can also beused for further chelation of trace elements, suchas phosphonic acidw143x and calixarene tetrahy-droxamic acid w144x. Alternatively, macrocyclesmay be bound to silica(SGBM) w145x, such as18-crown-6(18C6) w146x.

It must be kept in mind that despite chemicalbonding of functional groups on the silica gelsurface, free silanol groups still remainw143x.Their number can be minimised by end-cappingthe sorbent, but some will still be present. As aconsequence, they will participate in the retentionof trace elements somewhat, especially at pHsabove their pK (ionized form).a

3.1.1.2. C -bonded silica gel. Despite the large18

variety of bonded phases available, octadecyl-bonded silica has currently become the most pop-ular phase used. Numerous applications report theuse of C -silica, as indicated by the studies18

reported for water samples in Table 3. In particular,organometallic compounds(e.g. tributyltin (TBT),triphenyltin (TPhT), alkylselenides) can beretained on this sorbent due to possible hydropho-bic interactionw19,24,112,113x. Bare C -silica can18

also retain a fraction of inorganic trace elements,probably due to the presence of silanol groups onits surfacew34x. However, in practice, due to itshydrophobic character, C -silica is not well suited18

for retention of trace element species, as the latterare often polar or ionic. Retention on C -silica18

may be improved by addition of a ligand reagentto the sample before its percolation through thesorbent. The hydrophobic part of the ligand willthus have hydrophobic interaction with the C -18

silica and be retained on the sorbent, while thefunctional group of the ligand will ensure chelationof the trace element. Among reagents, one can cite8-HQ w115x, APDC w114x, 1,10-phenanthrolinew18x, or bathocuproinew27x.

An alternative approach is to form the complexby passing the sample through a C -silica con-18

taining the immobilized reagent. Octadecyl bondedsilica, modified by suitable ligands has been suc-cessfully used for the separation and sensitivedetermination of metal ions. Examples are givenin Table 3. The careful choice of the ligand mayadd selectivity to the extraction step, favoringspeciation. For example, salen I-modified C -18

silica was found selective for Cu(II) w34x, whileimpregnation of C -silica with neocuproine was18

suitable for Cu(I) w116x. C -silica coated with bis18 {

wl-hydroxy-9,10-anthraquinone-2-methylxsulfide(BAS) was preferred for Pb(II) retention w121x,while coating withN,N9-diethyl-N9-benzoylthiourea(DEBT) was recommended for Pdw147x. Macro-cycles may also be loaded on C -silica and effi-18

ciently used for the retention of trace metals, suchas hexathia-18-crown-6(HT18C6) w129x or calix-arene hydroxamatew144x.For some particular applications, mixed ligand

complexes may be used to ensure synergisticadsorption of the metal complex on the solidsorbent. Thus, while Cu(II) ions cannot complexwith neutral tri-n-butyl phosphate(TBP) mole-cules adsorbed on C -silica, the form of Cu(TTA)18

complex (TTA being 2-thenoyltrifluoroacetone)was retained at approximately 80%w148x. Alter-natively, in some cases, loading the chelating agenton C -silica instead of C -silica may give better8 18

results as observed for the retention of bismuth onoxinate-loaded reversed phasew149x.

Despite their broad application to trace elementpreconcentration, bonded silica phases(either C -18

silica or functionalized-silica gel) present thedrawback of a limited range of pH that can beused, as in acidic(below 2 to 4) and basic(above8) pHs hydrolysis may occur, which changes theinteractions that occur between the sorbent and thetrace elements. As a consequence, polymeric sor-bents may be preferred.

3.1.1.3. Other inorganic oxides. Apart from silicaother inorganic oxides have been tested for theadsorption of trace elements as shown in Tables 1and 2. Whereas SiO , due to its acidic properties,2

is expected to adsorb only cations, basic oxides(such as magnesia MgO) should adsorb only


Fig. 5. Sorbents based on organic supports.

anions. As a matter of fact adsorption of ions onoxide surfaces is believed to proceed with partici-pation of hydroxyl groups. These groups are neg-atively charged (deprotonated) under basicconditions, thereby retaining cations and positivelycharged (protonated) under acidic conditions,thereby retaining anions. Consequently, on ampho-teric oxides(namely titania TiO , alumina Al O ,2 2 3

zirconia ZrO), cations are adsorbed under basic2

conditions(pH above the isoelectric point of theoxide which was reported to be 6.2 for TiO2w109x) while anions are adsorbed under acidicconditions(pH below the isoelectric point of theoxide). For example, chromium speciation may beachieved by careful adjustment of the sample pH:pH 2 and 7 for retention of Cr(VI) (anionic) andCr(III ) (cationic), respectively, on acidic aluminaw70,71x; pH 2 and 8 for retention of Cr(VI) andCr(III ) on titania, respectivelyw108x. The concur-rent adsorption of H is responsible for theq

absence of retention of cationic species at verylow pHs. However, changing the sample pH mayaffect speciation and should be avoided as far aspossible. So it may be preferred to find a suitablesorbent for retaining the targeted species withsubsequent selective elution for further speciation

studies. With regards to chromium speciation, neu-tral alumina has been used for that purposew110,150x.The preparation technique is of prime impor-

tancew109x, as the adsorption properties of manyoxides strongly depend on the characteristics ofthe solid, namely crystal structure, morphology,defects, specific surface area, hydroxyl coverage,surface impurities and modifiers. Thus, the coatingof acidic alumina with an anionic surfactantallowed the selective retention of Cr(VI) in veryacidic solutionsw151x. Adsorption on inorganicoxides may also be influenced by the presence ofsalts in the matrix. In particular, high concentra-tions of phosphates and sulfates may decreasetrace element retention on titaniaw152x. On theopposite, major cations(Na , K , Ca , Mg )q q 2q 2q

are weakly adsorbed on titaniaw152,153x.

3.1.2. Organic based sorbentsOrganic based sorbents may be divided into

polymeric and non-polymeric sorbents, as shownin Fig. 5. Polymeric sorbents have been, by far,the most used for trace element preconcentrationhaving the advantage over bonded silica in thatthey can be used over the entire pH range. Their


disadvantage is that the conditioning step is moretime consuming as they require extensive cleaningbefore use. Comprehensive reviews on polymericphases have been publishedw97,154x. Masque et´al. published an extensive review on sorbents usedfor the SPE of polar organic micropollutants fromnatural watersw155x. The purpose of this sectionis to summarize the most frequently used organicbased sorbents for trace elements, as well as themore recently reported ones.In most applications, new sorbents have been

synthesized by chemically bonding chelatinggroups to polymeric cross-linked chains and char-acterizing their ability to selectively adsorb traceelements. Most of the chelating groups reportedhave low water solubility to avoid their leachingfrom the sorbent, as most applications deal withaqueous samples. At the same time, a too hydro-phobic group will hinder wettability of the sorbentby the aqueous sample, resulting in poor retentionefficiency. A compromise is thus necessary. Inaddition to the functional group, the efficiency ofpolymeric sorbents depends on various physico-chemical parameters, such as particle size, surfacearea, pore diameter, pore volume, degree of cross-linking and particle size distribution.

3.1.2.1. Polystyrene-divinylbenzcne based sor-bents. Macroporous hydrophobic resins of theAmberlite XAD series are good supports for devel-oping chelating matrices. Amberlite XAD-1,XAD-2, XAD-4 and XAD-16 are polystyrene-divinylbenzene (PS-DVB) resins with a highhydrophobic character and no ion-exchange capac-ity. In addition to the hydrophobic interaction thatalso occurs with C -silica, such sorbents allow18

p–p interactions with aromatic analytes.Due to the hydrophobic character of PS-DVB,

retention of trace elements on such sorbentsrequires the addition of a ligand to the sample.Inorganic ligands may be usedw156x, but organicligands are preferred, such as APDCw157,158x, 8-HQ w159,160x, or diphenylcarbazide(DPC) w161x.Alternatively, ligands may be attached to the PS-DVB by physical adsorption such as dithizonew162x, PDT (3-(2-pyridyl)-5,6-diphenyl-l,2,4-tria-zine) w4x, tropolonew163,164x, l-(2-pyridylazo)2-naphthol (PAN) w32,165x, DDQ (7-dodecenyl-

8-quinolinol) w166x, APDC w167x or 5-BrPA-DAP(2-(5-bromo-2-pyridylazo)-5-(diethylamino) phe-nol) w168x. Macrocycles can also be adsorbed,such as calixarene hydroxamatew144x. However,in practice, the resins prepared by impregnation ofthe ligand are difficult to reuse, due to partialleaching of the ligand(thus resulting in poorrepeatability). To overcome this problem, the resinmay be chemically functionalized. Several chemi-cal modifications of PS-DVB have recently beenreviewedw169x, but only a few are commerciallyavailable. The ligands are generally coupled to amethylene or an azo spacer on the matrix. Amongligands, one can cite Alizarin Red-Sw170x, sali-{

cylic acid (SA) w171x, thiosalicylic acid (TSA)w172x, pyrocatechol violet(PV) w173x, chromo-tropic acid (CA) w174x, pyrocatechol (PC)w175,176x, Tiron (disodium salt of l,2-dihydroxy-benzene-3,5-disulfonic acid) w177x, quinalizarin(1,2,5,8-tetrahydroxy-anthraquinone) w22x, bicinewN,N-bis (2-hydroxy-ethyl) glycinex w178x, andpoly(dithiocarbamate) (PDTC) w179x.Of great interest are also the sulfonated PS-

DVB resins, as they show excellent hydrophilicityand high extraction efficiencies for polar organiccompoundsw154,155x. In the case of rapid sulfo-nation under mild conditions, a mixed-mode reten-tion can be observed: adsorption of neutralcompounds on the polymeric resin, and cationexchange of ionic species on sulfonate groupsw180x. The use of a particular sulfonated PS-DVBresin has been recently reported to enable chro-mium speciationw181x. 2-Naphthol-3,6-disulfonicacid (NDSA) has been coupled to the PS-DVBthrough an azo function. In that way, the formationof an azo cation at very low pHs enabled retentionof the anionic Cr(VI), whereas the sulfonate groupenabled retention of Cr(III ) in neutral and basicmediaw181x. For particular applications, trimethy-lammonium functionalized PS-DVB may also beused with anion-exchange properties.As summarized in Table 4, which presents

selected applications of polymeric sorbents for thepreconcentration of trace elements from watersamples, even though PS-DVB has been probablythe most widely used of polymers, others alsohave been successfully used as detailed below.

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Table 4Applications of off-line SPE using polymeric sorbents to water samples

Matrix Trace Sorbent Operation Experimental Analysis Recovery Adsorptive PreconcentrationLOD Ref.elements conditions method (%) capacity factor (mgyl)

Adsorptive resinsTap water CdyCuy XAD-2 Polypropylene Conditioning ICP–AES 82.3– — 100 — w160x

MnyNiy column sample 97.2PbyZnq pH: 8–98-HQ Elution:

HCl 2 MTap water Cr(VI), XAD-16 Glass column Washing F-AAS 97.3–99.0 0.4 5.25 45 w161x

total (1 cm i.d) sample mgygCrqDPC pH: 1

Elution:H SO 0.052 4

MyMeOHDrinking and BiyCdyCoy Chromosorb- Glass column Washing F-AAS 95–110 — 300 0.10–11 w158xsea waters CuyFeyNiy 102 (0.9 cm i.d) conditioning

PbqAPD sample pH: 6C Elution:

acetoneTap, mineral, Coq8-HQ Chromosorb- Column Washing ET-AAS 95.2–99.2 — 80 0.0134 w182xriver 105 (4 mm i.d) samplewaters pH: 8

Elution:EtOHyHNO 2 M3

Tape, lake, Cr(III ) Cellulose Syringe barrel Purification ET-AAS 98–99.3 — 100 0.0018 w183xwaste samplewaters pH: 11

Elution:HCl 2 M

Chelating resinsWaters TBT Tropolone- Glass column Sampleq ET-AAS 104 — 80 0.0144 w164x

loaded- (1.5 cm i.d) 0.8%XAD-2 H SO2 4

WashingElution:IBMK

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Sea water CoyCuy PDT-loaded- Column Washing F-AAS 96.3– — — — w4xFeyNiy XAD-2 (0.9 cm i.d) elution: 103.5Zn MeOH in

Soxhletapparatus

Well water, CuyCdy Quinalizarin- Glass column Washing F-AAS 91–98 3.15y 100y50y40y50y 2.0y1.3y5.0y w22xriver water CoyPby functionalized- (1 cm i.d) sample 1.70y 100y65y65 15.0y1.0y1.6

ZnyMn XAD-2 pH: 5–7 1.62yElution: 5.28yHNO 4 M3 1.42y

0.94y2.19 mgyg

Well waters ZnyCdy PV- Glass column Washing F-AAS 98 1410y 60y50y — w173xPbyNi functionalized- (1 cm i.d) sample 1270y 23y18

XAD-2 pH: 3–7 620yElution: 1360HNO 4 M3 mgyg

Well waters ZnyPb Salicylic acid- Glass column Washing F-AAS 98–100 1146y461 180y140 — w171xfunctionalized- (1 cm i.d) sample mgygXAD-2 pH: 5.0

Elution:HCl 1M-2–4 M

Well waters ZnyCdy Alizarin Red-S- Glass column Washing F-AAS 95–100 511y124y 40 10 w170xPbyNi functionalized- (1 cm i.d) sample 306y124

XAD-2 pH: 4–6 mgygElution:HNO3

1–4 M orHCl 4 M

River waters CuyCdyCo Tiron- Glass column Washing F-AAS 91–99 — 25–200 0.5–24 w177xyNiyPbyZny functionalized- (1 cm i.d) sampleMnyFe XAD-2 pH: 4–7.5

Elution:HNO 4 M3

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River waters CdyCoyCu CA- Glass column Washing F-AAS 95–100 9.35y 100–200 — w174xyNiyFeyZn functionalized- (1 cm i.d) sample 3.84y

XAD-2 pH: 4–7 8.50yElution: 3.24yHNO or3 6.07yHCl 2 M 9.65

mgygRiver and tap Pb CA- Glass column Washing F-AAS 97 186.3 200 4.06 w176x

waters functionalized- (1 cm i.d) sample mmolygXAD-2 pH: 3–8

Elution:HNO3

2–10 MRiver and tap CdyCoyCu PC- Glass column Washing F-AAS — 0.023– 80–200 — w175xwaters yFeyNiyZn functionalized- (1 cm i.d) sample 0.092

XAD-2 pH: 3–6.5 mmolygElution:HNO 2 M3

River and tap Pb PC- Glass column Washing F-AAS 94 104.7 100 3.80 w176xwaters functionalized- (1 cm i.d) sample mmolyg

XAD-2 pH: 5–7.5Elution:HNO 1 M3

River and tap Pb TSA- Glass column Washing F-AAS 93 89.3 100 4.87 w176xwaters functionalized- (1 cm i.d) sample mmolyg

XAD-2 pH: 4Elution:HNO3

0.5–2 MTap, river CdyCoyCuy TSA- Glass column Washing F-AAS 92–98 197.5y 180–400 0.48y0.20y w172x

waters FeyNiyZn functionalized- (1 cm i.d) sample 106.9y 4.05y0.98yXAD-2 pH: 3.5–7 214.0y 1.28y3.94

Washing 66.2yelution: 309.9yHNO 2 M3 47.4

mmolyg

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Artificial sea CdyCuyMny APDC-loaded- Glass column Sample ICP–AES 98.2–99.6 9.47y 180y230y120y 0.1y0.4y0.3y w167xwater, NiyPbyZn XAD-4 (0.9 cm i.d) pH: 5.0 11.08y 130y160y215 0.4y0.6y0.5natural Washing 8.62ywaters elution: 7.21y

HNO 4 M3 10.25y10.62mgyg

Artificial sea CdyCuy pipDTC-loaded- Glass column Sample ICP–AES 97.6–99.1 9.18y 150y200y140y 0.7y1.0y0.8y w167xwater, MnyNiy XAD-4 (0.9 cm i.d) pH: 5.0 10.76y 120y150y200 0.9y1.7y1.2natural PbyZn Washing 8.17ywaters elution: 7.46y

HNO 4 M3 9.86y10.28mgyg

Sea water AgyAl y DDQ-loaded- Teflon column Washing F-AAS or ET- 73–107 0.55 62.5 0.00016–0.3w166xBiyCdy XAD-4 (8 mm i.d) sample AAS mmolygCuyFey pH: 8GayMny WashingNiyPbyTi elution:

HCl 2 MBackflush

Tap water CuyMnyZn Calixarene Plastic Sample F-AAS — — 25 — w144xTetrahydroxa- cartridge pH: 8.5mate- (0.9 cm i.d) Elution:loaded- acidifiedXAD-4 water

(pH 2.0)Tap and Mn PDTC- Glass column Conditioning F-AAS 97.2 9.1 20 0.5 w179x

mineral functionalized- (1 cm i.d) sample mmolygwaters XAD-4 pH: 10

Elution:HNO 8 M3

Spiked CoyCuy Bicine- Glass column Conditioning ET-AAS 97.6–99.1 0.32–0.44 40–50 — w178xsolutions FeyHgy functionalized- sample mmolyg

NiyPbyZn XAD-4 pH: 5.5–7Elution:HCl 1 M

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Waste waters Cr(III )yCr NDSA- Glass column Sample pH: F-AAS 85.9–96.1 0.40y1.18 20 — w181x(VI) functionalized- (1 cm i.d) 1.5 or 6 mmolyg

PS-DVB Elution:HCl 4 M

River water Hg(II) PAA- Glass column Conditioning Spectro- 96 0.6 30 — w184xfunctionalized- (1 cm i.d) sample photometry mmolygPS-DVB pH: 5.4

Elution:H SO 2 M2 4

River and Pb XO-loaded- Glass column Washing F-AAS 91 16.9 100 2.44 w176xtap XAD-7 (1 cm i.d) Sample mmolygwaters pH: 5

Elution:HNO 1 M3

River waters CdyCoyCu XO-loaded- Glass column Sample F-AAS 96–100 1.6–2.6 10–200 9y24y6y w185xyFeyNiyZn XAD-7 pH: 4–5 mgyg 6y3y21

Elution:HCl 1 or 2 M

River and Cr(III ) 8-HQ- Glass column Washing ICP–MS 98–105 41.7 5 0.06 w186xreservoir functionalized- (4 mm i.d) sample mmolygwaters polyacrylonitrile pH: 6

WashingElution:HCl 2 MyHNO 0.1 M3

Aqueous AuyPty Aminothiourea- Glass column Washing ICP–AES 97–99 2.80y 6–65 — w187xsample PdyIr functionalized- (0.5 cm i.d) sample 1.75yfrom a non- polyacrylonitrile pH: 2 1.56yferrous metal Washing 1.15smelter elution: mmolyg

HCl 4 MyCS(NH ) 3%2 2

Sea water BeyBiy Aminophosphonic- Glass column Sample ICP–MS 93–104 0.83–74.1 200 0.002–0.601w188xCoyGay dithiocarbamate- (4 mm i.d) pH: 6 mmolyg ngylAgyPby functionalized- WashingCdyCuy polyacrylonitrile elution:MnyIn HCl 2 M

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River, lake Hg(II)y Dithiocarbamate- Column Washing CV-AAS 91–95 — 667 0.0002 w189xand rain MeHg functionalized- (1.5 cm i.d) Samplewaters polyvinyle pH: 1–11

Elution:thiourea5% in HCl

Sea water CdyCuy Chelamine Column Washing ET-AAS 91–102 1 200 0.0023– w190xMnyNiy sample mmolyg 0.033PbyZn pH: 6.5

Washingelution:HNO 2 M3

Sea water CdyCoy Chelex-100 Column Sample ET-AAS or )98 — 50 0.001–0.1 w191xCuyMny pH: 6.5 F-AASNiyPbyZn Washing

elution:HNO 2 M3

River water, CdyPbyZn Amberlite Glass column Washing F-AAS 63–104 1.06y 10 — w192xseawater IRC-718 (0.5 cm i.d) conditioning 0.096y

sample 1.77washing mmolygelution:HNO3

Ground and Se(IV)ySe Amberlite Glass column Washing LC–ICP– 93–97 — 55 0.010 w193xfresh (VI)y IRA-743 (1 cm i.d) conditioning MSwaters SeCyst sample

elution:HClO 14

Mqwater

Ion exchangersWaste water Cr Anion Cartridge Activation: Spectro- 80–98.8 — — 8 w194x

exchanger (0.5 g) MeOHq photometry(SAX) buffer (544 nm) after

Sample reaction DPCpH: 4.5Elution:Na SO2 4

0.5 M

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Sea, river, Se(IV)y Anion Cartridge Conditioning GC–MS 91–99 — 40–500 1600y w19xtap Se(VI) exchanger Sample 1400waters (SAX) pH: 7

Elution:HCOOH 1MqHCl 3 M

River, spring, Be Anion Column Conditioning: F-AAS 95–102.5 — 125 0.045 w195xwaste exchanger (1 cm i.d) HClqH Oq2

waters NaOHSamplepH: 6–8Elution:HCl 1.5 M


3.1.2.2. Divinylbenzene-vinylpyrrolidone copoly-mers. Sorbents made of divinylbenzene-vinylpyr-rolidone (DVB-VP) copolymers have recentlybeen developed, such as Oasis HLBw154x. ThehydrophilicN-vinylpyrrolidone affords good wett-{

ability of the resin, while the hydrophobic divinyl-benzene provides reversed-phase retention ofanalytes. This sorbent has been successfullyapplied to the determination of polar organic com-pounds in water samples. It is more convenient touse, compared to classical sorbents, as it can dryout during the extraction procedure without reduc-ing its ability to retain analytes. In addition, it isstable over the entire pH range. However, untilnow, no application related to the preconcentrationof trace elements has been reported. Similarly, theuse of Oasis MCX, a sulfonated divinylbenzene-vinylpyrrolidone copolymerw154x, may be usefulfor the retention of trace element species, as thissorbent combines the properties of the previoussorbent with those of a strong cation-exchanger.Application to the preconcentration of triphenyltinhas been reportedw36x.

3.1.2.3. Polyacrylate polymers. Amberlite XAD-7and XAD-8 are ethylene-dimethacrylate resins.They are non-aromatic in character and possessvery low ion-exchange capacity. Due to the polar-ity of acrylates, such resins enable the recovery ofpolar compounds. However, this polarity is quitemoderate so that most of the time reagents areadded to increase retention. Direct addition to thesample is sometimes performed. As an example,Cu(II) forms a complex with 8-hydroxyquinoline-5-sulfonic acid(8-HQ-5-SA), which can be furtherretained on Amberlite XAD-8 as an ion-pair withcetyltrimethylammonium(CTA) w15x. Yet, most ofthe time chelating reagents have been loaded onsuch resins, mainly Amberlite XAD-7, to increasetheir retention capacity for trace elements andyortheir selectivity such as 8-(benzenesulfonami-do)quinoline (BSQ) w78x, xylenol orange(XO)w176,185x, 5-BrPADAP w168x, or dimethylgly-oxal bis(4-phenyl-3-thiosemicarbazone) (DMBS)w196x. Such loaded sorbents are stable for severalmonths and can be reused. For a higher stability,chemical binding of the chelating group may beperformed, as reported for dithizone on

poly(ethylene glycol dimethacrylate-hydroxyethyl-methacrylate) microbeadsw197x.

3.1.2.4. Polyurethane polymers. Due to its sorptioncapacity for several trace elements polyurethanefoam has been tested for use in SPE. Most of thetime complexing reagents are added to enhancethe sorption capacity. Hence, PUF coated withDMG, NN, DDTC or hexamethylenedithiocarba-mate (HMDC) was found efficient in retainingtrace elementsw198–200x. The chelating reagentcan also be directly added to the sample, and themetal chelates further retained on PUF, as observedwith thiocyanate complexesw92,201,202x andDDTP complexesw63x. Very recently, the immo-bilization of an enzyme(alkaline phosphatase) hasbeen reported on PUF with further application asan enzymatic procedure for Pb(II) determinationw203x.

3.1.2.5. Polyethylene polymers. Polyethylene isalso attractive for SPE of trace elements as thissupport adsorbs several metal complexed withhydrophobic ligands. Additionally, the adsorbedcomplexes can be eluted with a small volume oforganic solvents permitting high enrichment fac-tors. Polyethylene can also be used in stronglyacidic and basic media and for that reason, it hasbeen used as a sorbent for the retention of chro-mium in an acidic medium after the addition ofDPC w204x.

3.1.2.6. Polytetrafluoroethylene polymers. PTFEcan retain trace elements after addition of a che-lating reagent to the sample such as APDC, DDTCor dithizone(DZ) w48–50,74,76x. PTFE may alsobe precoated with a suitable ligand, like 2-methyl-8-hydroxyquinolinew93x. The sorbent can be usedas PTFE turningsw48–50x, PTFE beadsw75x, oras a PTFE tubing in a knotted reactorw74,76x.

3.1.2.7. Polystyrene polymers. Polystyrene poly-mers may be an interesting alternative to commonsorbents(namely Amberlites XAD-2 and XAD-8,C -silica) when they have a hyper cross-link18

structure. The addition of a reagent to the sampleis required to form complexes that are furtherretained on the hydrophobic sorbentw79x.


3.1.2.8. Polyamide polymers. Polyamide polymershave been used for the retention of rare earthelementsw205x. A chelating reagent was added tothe sample for complexing the trace elements. Thisreagent Thorin (o-w3,6-disulfo-2-hydroxy-l-naphthylazoxbenzenearsonic acid) was chosen toenable interaction with the sorbent through electro-static forces and non-hydrophobic interaction.

3.1.2.9. Iminodiacetate-type chelating resins. Pol-ymeric resins containing iminodiacetate groupsw–CH –N(CH COO ) x as active sites(IDA res-y

2 2 2

ins) have been widely used for the retention oftrace elements. They have been synthesized bybonding the iminodiacetate functional groups toseveral polymeric sorbents, such as polystyrene(Chelex-100) w10,29,30,191,206–209x or a highlycrosslinked agarose gel(IDA-Novarose) w80x. Thespacer arm length was found to have an effect onthe formation of metal complex species in thechelating resinw12x.A major drawback of such sorbents is that, due

to the weak acid character of the functional group,the degree of protonation will critically affect theability of the resin to retain metal cations. Hence,for Chelex-100, protonation of the carboxylatesand the donor N atom are reported to be completeat pH 2.21, while a completely deprotonated formis reached at pH 12.30. Also, such sorbents arenon-selective, so that trace element retention maybe reduced due to retention of major ions(namelyCa(II) and Mg(II)) w99,191x. Besides, the presenceof ligands in the sample may prevent trace elementretention on the sorbent due to their complexationas observed in real waters due to the presence oforganic matterw80,210x.

3.1.2.10. Propylenediaminetetraacetate-type che-lating resins. The synthesis of a fine-particlemacroporous polymer-based propylenediaminete-traacetic acid(PDATA) type resin has been recent-ly reportedw211x. The structure of this sorbent isvery similar to that of ethylene diamine tetraaceticacid (EDTA) with a spacer arm enabling theretention of several trace elements upon chelation.

3.1.2.11. Polyacrylonitrile based resins. Polyacry-lonitrile fibers have been functionalized to obtain

ion-exchange chelating sorbents with aminophos-phonic, dithiocarbamate or aminothiourea groupsw187,188x. However, as such synthesis are time-consuming an alternative is to coat the polyacry-lonitrile fiber with a proper reagent for furthertrace element retention such as 8-HQw186,212x.

3.1.2.12. Ring-opening metathesis polymerisation-based polymers. A high-capacity carboxylic acid-functionalized resin has been prepared usingring-opening metathesis polymerisation(ROMP)w213x. Electron microscopy revealed that theobtained material consists of irregularly shaped,agglomerated particles having a non-porous struc-ture with diameter and specific surface areadependent on the polymerisation sequence and thestoichiometries. This material was pH stable andcould be reused. The presence of the carboxylicgroups confers an excellent hydrophilic characterto the sorbent(ensuring a high wettability of thesorbent by water), while the polyunsaturation ofthe carrier chain, as well as the entire backboneprovides for a significant reversed-phase character.The carboxylic acid groups provide weak coordi-nation sites enabling the retention of rare earthelements. Similarly, dipyridyl amide-functionalizedresins have been reported to allow the extractionof ‘soft’ metals such as Pd(II) and Hg(II) w214x.

3.1.2.13. Carbon sorbents. Activated carbon isprepared by low-temperature oxidation of vegeta-ble charcoals. Due to their large surface areas(300–1000 myg), these sorbents are well-recog-2

nized for their very strong sorption both for traceorganic compounds and trace elements. There isevidence of two types of adsorption sites onactivated carbons:(1) graphite-like basal planesthat enable adsorption through van der Waalsforces, especiallyp-electron interactions, and(2)polar groups like carbonyls, hydroxyls and carbox-yls, that may interact via ionic interaction ofhydrogen bondingw215x. Consequently, trace ele-ments may be directly adsorbed on activated car-bon w19,216x. Metal chelates may also be retainedon this sorbent after addition of a proper chelatingagent to the samplew17x such as amino acidsw217x, dithizone w218x, APDC w88,219x, PANw220x, 8-HQ w221x, cupferronw221x, Bismuthiol II


(3-phenyl-5-mercapto-l,3,4-thiadiazole-2(3H)-thi-one) w222x, or DDTP w63,87x. The ligand shouldbe chosen to avoid a strong interaction with theactivated carbon otherwise complete dissociationof the metal chelate would be observedw215x.

The main drawback when using activated car-bons is their heterogeneous surface with activefunctional groups that often lead to low reproduc-ibility. In addition, these sorbents are very reactiveand can act as catalysts for oxidation and otherchemical reactions. Fortunately, along with thedevelopment of polymer materials and bondedphases, a new generation of carbon sorbentsappeared in the 1970s and 1980s with a morehomogeneous structure and more reproducibleproperties. Graphitized carbon blacks(GCB) areobtained from heating carbon blacks at 2700–30008C in an inert atmospherew155x. They are non-specific and non-porous sorbents(surface areaapprox. 100 myg), and are considered to be both2

reversed-phase sorbents and anion-exchangers dueto the presence of positively charged chemicalheterogeneities on their surface. Such sorbentshave been extensively used in the past few yearsfor the SPE of polar organic pollutants from watersamplesw155x, but their use for trace elements isstill rare w223x. Their main drawbacks are possibleirreversible retention of analytes, which may beovercome by elution in the backflush mode, andpoor mechanical stability. Porous graphitized car-bon (PGC) is a more stable carbon based sorbentthan GCB as the graphite is immobilized on asilica substrate. So this sorbent should be suitablefor trace element retention even though until nowits applications have been limited to trace organiccompounds.

3.1.2.14. Cellulose. Cellulose was found effectivein retaining trace elements present in water sam-ples either directly or upon addition of a chelatingagent to the samplew183,224x. In particular, theselective retention of Cr(III ) was reported therebyenabling chromium speciationw183x. This sorbentmay also be functionalized to increase the SPEselectivity. Thus, selenium speciation has beenreported on cellulose functionalized with quater-nary amine due to the selective elution of the

retained Se(IV) and Se(VI) species using nitricacid at two different concentrationsw225x.

3.1.2.15. Naphthalene based sorbents. Retentionof trace elements on microcrystalline naphthaleneis also feasible, either after addition of a ligand tothe samplew226x, or after functionalization of thesolid to ensure better adsorption characteristicstowards trace elementsw227,228x. However, theuse of this solid support is rather uncommon. Inaddition, until now it has been reserved to batchexperiments.

3.2. Influential parameters

The main experimental variables that affectanalyte recovery by SPE have been extensivelyreported by Poole et al. w2,229x. They are brieflydiscussed below and illustrated with reportedapplications.

3.2.1. Conditioning parameters

3.2.1.1. Washing step. A washing step is highlyrecommended, especially when ultratraces of ele-ments are to be determined. Thus, blank extractscontaining trace levels of Zn, Cu and Fe weresuspected to be due to contaminants from C -18

silica w115x.

3.2.1.2. Conditioning solvent. Even though somesorbents have been used without a conditioningstep this is not recommended. This step will atleast remove possible remaining contaminants andair from the sorbent bed. Additionally, in somecases, this step is crucial for successful retentionof the analytes. The nature of the conditioningsolvent must be appropriate to the nature of thesolid sorbent to ensure good wettability of thefunctional groups. As an example with hydropho-bic supports such as C -silica or PS-DVB, quite18

polar organic solvents such as methanol should beused. The sorbent should further be conditionedby a solvent whose nature is similar to that of thesample. Thus, for aqueous samples, the solventwill be water with a pH and ionic strength similarto that of the sample.


Fig. 6. Typical representation of the breakthrough curve(i.e.concentration of the analyte at the outlet of the SPE system vs.sample volume percolated through the system). V is the break-B

through volume,V the chromatographic elution volume andR

V the sample volume corresponding to the retention of theC

maximum amount of analyte,C is the initial analyte concen-0

tration in the sample.

3.2.2. Loading parameters

3.2.2.1. Sample volume to be percolated. Animportant parameter to control in SPE is thebreakthrough volume, which is the maximum sam-ple volume that should be percolated through agiven mass of sorbent after which analytes start toelute from the sorbent resulting in non-quantitativerecoveries(Fig. 6). The breakthrough volume isstrongly correlated to the chromatographic reten-tion of the analyte on the same sorbent anddepends on the nature of both the sorbent and thetrace element, as well as on the mass of sorbentconsidered and the analyte concentration in thesamplew3x. In addition, it depends on the sorbentcontainers, as disks usually offer higher break-through volumes than cartridges. This volume maybe determined experimentally or estimated usingseveral methodsw229x. For that purpose the natureof the sample has to be taken into account, as thepossible presence of ligands may dramaticallyreduce the breakthrough volumew230x.

3.2.2.2. Sample flow-rate. The sample flow-rateshould be optimized to ensure quantitative reten-tion along with minimization of the time requiredfor sample processing. This parameter may have adirect effect on the breakthrough volume, andelevated flow-rates may reduce the breakthroughvolume w4,229x. As a rule, cartridges and columns

require lower maximum flow-rates than disks rang-ing typically from 0.5 to 5 mlymin. This valuemay be increased by a factor of 10 using disks.

3.2.2.3. Sample pH. Sample pH is of prime impor-tance for efficient retention of the trace elementson the sorbent. Its influence strongly depends onthe nature of the sorbent used. Careful optimizationof this parameter is thus crucial to ensure quanti-tative retention of the trace elements and in somecases selective retention. In particular with ion-exchangers, correct adjustment of sample pH isrequired to ensure preconcentration. Thus, in thecase of cationic-exchangers, low pH usually resultsin poor extraction due to competition betweenprotons and cationic species for retention on thesorbent.When retention of trace elements is based on

chelation (either in the sample or on the solidsorbent), the sample pH is also a very importantfactor as most chelating ligands are conjugatedbases of weak acid groups and accordingly, theyhave a very strong affinity for hydrogen ions. ThepH will determine the values of the conditionalstability constants of the metal complexes. Bycontrast, pH may have no influence with somenon-ionizable organic ligandsw130x.For inorganic oxides, pH is also of prime impor-

tance. In particular, on amphoteric oxides such asTiO or Al O , cations are adsorbed at elevated2 2 3

pHs due to the deprotonation of functional groups,whereas anion retention requires acidic conditionsfor the protonation of functional groups.

3.2.2.4. Sample matrix. The presence of ligands inthe sample matrix may affect trace element reten-tion when stable complexes are formed in thesample with these ligands, as trace elements areless available for further retention. Thus, if metalsare present in the sample as strong complexes,they may not dissociate resulting in no retentionof the free metal on the sorbent. As an example,reduction in the retention of Cu(II) on AmberliteCG50 occurs in the presence of ligands such asglycine w230x. In the case of real samples, thepresence of natural organic matter is of greatconcern as it may complex trace elements asobserved for Cu(II) w231,232x. Yet, in some cases


the presence of ligands may be a valuable tool foradding selectivity to the SPE step. This requiresthat the added ligands be correctly chosen tocomplex only the elements that are not of interest,so that they are not retained on the sorbentw31x.The presence of ions other than the target ones

in the sample may also cause problems during theSPE step. In particular, due to their usually highlevels (e.g. Ca(II)), they may hinder the precon-centration step by overloading the sorbent or causeinterferences during spectrophotometric analysis.Therefore, their influence should be studied beforevalidating a SPE method. Sometimes the additionof a proper masking agent(such as EDTA, thioureaor ethanolamine for example) may prevent theformation of interferences due to ions present inthe samplew128x.Finally, the ionic strength of the sample is

another parameter to control for an efficient SPE,as it may influence the retention of trace elements,and thus the value of the breakthrough volume fora given sorbentw122,195x.

3.2.3. Elution parameters

3.2.3.1. Nature of the solvent. The nature of theelution solvent is of prime importance and shouldoptimally meet three criteria: efficiency, selectivityand compatibility, as discussed below. In addition,it may be desirable to recover the analytes in asmall volume of solvent to ensure a significantenrichment factor. The eluent may be an organicsolvent (when reversed-phase sorbents are used),an acid(usually with ion-exchangers), or a com-plexing agent.Firstly, the eluting solvent should be carefully

chosen to ensure efficient recovery of the retainedtarget species and quantitative recovery as far aspossible. As an example among several solventstested for the elution of TPhT from C -silica18

(namely a Triton X-100 surfactant aqueous solu-tion, acetonitrile, tetrahydrofuran(THF), metha-nol–water 80:20, methanol), only methanolenabled the achievement of acceptable recoveries(approx. 85%) w112x.A further characteristic of the elution solvent

arises with the possibility of introducing selectivity.Using a solvent with a low or moderate eluting

power, the less retained analytes can be recoveredwithout eluting the strongly retained compounds.Thus, if the elements of interest are those thatremain on the sorbent another elution step with amore eluent solvent will ensure their quantitativerecovery. In that way interferent analytes wereremoved during the first eluting step(also calledwashing step). On the opposite, if the compoundsof interest are the less retained on the sorbent theirelution with a low or moderate eluting solventensures their selective recovery, as the interferentcompounds will remain on the sorbent due tostronger interactions with the solid support. Insome cases, this selectivity may authorized speci-ation. For example, 1 M HCOOH removed onlySe(IV) from an anion-exchange resin, leavingSe(VI) retained on the sorbent, which was furthereluted using 2 M HClw19x.Finally, the elution solvent should be compatible

with the analysis technique. In particular, whenusing both flame and electrothermal AAS, HNO3

should be preferred to other acids(namelyH SO , HCl), as nitrate ion is a more acceptable2 4

matrix w128x.

3.2.3.2. Solvent pH. As retention of trace elementson solid sorbents is usually pH-dependent, carefulchoice of the elution solvent pH may enhanceselectivity in the SPE procedure. As an example,once retained on eriochrome black-T(ERT)-func-tionalized-silica gel, Mg(II) could be eluted firstat a pH approximately 4, while increasing the pHto 5–6 was required for eluting Zn(II) w9x.

3.2.3.3. Elution mode. Most of the time, for prac-tical reasons, sample loading and elution steps areperformed in a similar manner. However, to avoidirreversible adsorption and ensure quantitativerecoveries, elution in the backflush mode is rec-ommended in some cases. This means that theeluent is pumped through the sorbent in the oppo-site direction to that of the sample during thepreconcentration step. This is especially crucialwhen carbon-based sorbents have to be used dueto possible irreversible adsorption of the analytes.

3.2.3.4. Solvent flow-rate. The flow-rate of theelution solvent should be high enough to avoid


excessive duration, and low enough to ensurequantitative recovery of the target species. Typicalflow-rates are in the range of 0.5 to 5 mlymin forcartridges and of 1 to 20 mlymin for disks w34x.As a rule, the higher the flow-rate, the larger thesolvent volume required for complete elutionw119,121,129,130x.

3.2.3.5. Solvent volume. Similar to the break-through volume, the elution volume may be deter-mined either experimentally or estimatedtheoretically w229x. Minimum elution volume fora cartridge is defined as 2 bed volumes of elutionsolvent. Bed volume is typically 120mly100 mgof sorbent. For classical disks, the minimum sol-vent volume required is approximately 10mlymgof sorbentw20x. Consequently, larger elution vol-umes would be required for disks. The elutionvolume can usually be reduced by increasing theconcentration of the eluting solvent(e.g. acid).However, in this case, problems with subsequentanalysis may be encountered(e.g. F-AAS). Alter-natively, the use of micro-sized disks may allowreduced solvent volumew20x.

The elution step should enable sufficient timeand elution volume to permit the metallic speciesto diffuse out of the solid sorbent pores. As a rule,2 elution cycles are usually recommended as com-pared to a single step(e.g. 2=5 ml elution shouldbe preferred to a single 10 ml elution). Soakingtime is also critical and 2 to 5 min soak is mostof the time allowed before each elution.

4. Applications of SPE to the determination ofselected trace elements

4.1. Cadmium

Cadmium is known to be a highly toxic tracemetal. Owing to its very low concentrations in theenvironment, a preconcentration is usually requiredfor its determination. This can be performed on ananion-exchange resin, after reaction of Cd(II) withchloride and the formation of the anionic 2yCdCl4complex w86x. Yet, Cd(II) retention generallyoccurs through chelation, either by adding a che-lating agent to the sample, by impregnation of thesorbent, or by the synthesis of new chelating resins.

Hence, a FI preconcentration on-line with F-AAShas been reported for the determination of Cd(II)w77x. Cadmium complexed with DDTC was sorbedon the inner walls of a PTFE knotted reactor, andfurther on-line eluted with isobutyl methyl ketone(IBMK ) giving a detection limit of 0.1mgyl.However, very acidic pHs(lower than 2) wererequired for optimum sensitivity and collectionefficiency. In addition, DDTC is rather non-selec-tive, so that retention of other trace elements mayoccur. This drawback maybe overcome by addingmasking agents, such as thiourea and ascorbicacidyphenanthroline for copper and iron, respec-tively, or by choosing a more selective reagent,such as DDTPw233x. Still very acidic pHs wererequired for optimum sensitivity and collectionefficiency. APDC offers the advantage of a broaderpH range without decomposition. Hence, Cd(II)complexed with APDC was stable for pHs between4 and 8, and could be efficiently retained on C -18

silica w56,124x. Further elution with methanol andFI on-line ET-AAS analysis enabled detectionlimits of 0.178 ngyl or 1.26 ngyl depending onthe system used. Other chelating reagents may beused, such as PAR or PADMAPw58x, or tetra-(4-bromophenyl)-porphyrin(T BPP)w41x.4

Impregnation of the sorbent with a chelatingreagent has been reported for the preconcentrationof Cd(II). In that case, the choice of a chelatantthat have a high affinity for cadmium is preferredto ensure high selectivity. For example, BSQ hasbeen immobilized on Amberlite XAD-7 and usedin a FI system giving a detection limit of 1.9mgyl w78x. However, Zn(II) was also retained, whileother ions were found to interfere(namely Mg(II),Cu(II), Fe(III )). Amberlite XAD-7 coated withDMBS also enabled the retention of Cd(II) fromneutral medium with simultaneous retention ofPb(II) w196x.

When ICP–AES is used as the analysis tech-nique, the use of organic solvents should beavoided as they may generate strong turbulence inthe ICP. So, new chelating resins were developedfor the Cd(II) preconcentation in a FI–ICP–AESsystem, such as l,5-bis(di-2-pyridyl) methylenedithiocarbohydrazide(DPTH)- or methylthiosali-cylate (TS)-functionalized silica gel leading todetection limits of 1.1 and 4.3mgyl, respectively


w51x. They offer the advantage of having noaffinity for sodium, potassium, calcium and mag-nesium, enabling the Cd(II) analysis in real watersamples. However, other trace elements reducedCd(II) retention on both resins, such as Zn(II) orCu(II). In some cases, the interfering effect ofZn(II) can be avoided by careful adjustment ofthe sample pH, as observed on the LewatitTP807’84 resin that contains a phosphonic deriv-ative as extractantw33x. Nevertheless, other ionswere still co-extracted with Cd(II), namely Cu(II)and Pb(II).

4.2. Chromium

Chromium species enter the environment as aresult of effluent discharge from steel industries,electroplating, tanning industries, oxidative dyeing,chemical industries and cooling water towers. Theymay also enter drinking water supply systems fromthe corrosion inhibitors used in water pipes andcontainers or by contamination of the undergroundwater from sanitary landfill leaching. Therefore, itis of major concern to study the characteristics ofchromium in aquatic systems. Chromium occursmainly in (III ) and (VI) oxidation states. WhileCr(III ) is an essential trace element, Cr(VI) ishighly carcinogenic and mutagenic due to its highoxidative character. So, it is important to developanalytical methods that enable analysis of chro-mium in its different oxidation states. However,sampling and preconcentration steps might disturbthe redox equilibrium between Cr(III ) and Cr(VI),thereby affecting the original speciation state ofthe sample.Most SPE methods are based on the high reac-

tivity of Cr(VI), due to the relatively inert natureof Cr(III ). Many methods are thus based on thedetermination of Cr(VI) and total chromium. FIon-line preconcentration procedures reported forchromium species were exhaustively reviewed upto 1992 by Sperling et al. w71x, and later(until1998) by Prasada Rao et al.w59x.

4.2.1. Cr(VI)The determination of chromium is frequently

achieved by spectrophotometry after derivatisationwith a reagent such as DPC. The reaction of DPC

is very selective for Cr(VI) so it can be performeddirectly without a separation step. The chromateoxidizes DPC to diphenylcarbazone(DPCO) toform a soluble strongly red–violet compound withCr(III ) (Cr(III )-DPCO ). A large excess of3yn q( )

DPC is essential as compounds present in thesample may consume the reagent. The Cr(III )-DPCO complex can be retained on polyethylenepacked in a column and subsequently eluted withmethanol before being analyzed by LCw204x. Thisprocedure was applied to the determination ofchromium in geological samples. The determina-tion of total chromium may also be achieved afterpreliminary oxidation of Cr(III ). Elements posinginterference with this method are mainly Mo(VI),Fe(III ) and V(V), and their separation using SPEwith an anionic-exchanger(SAX) has been per-formed for the subsequent spectrophotometricdetermination of chromiumw194x. The Cr(III )-DPCO complex was also found to be retained oncation-exchange(SCX) membrane disksw234x.The color intensity of the membrane was thencorrelated to the Cr(VI) concentrations by visualanalysis. This simple procedure was found toprovide a highly sensitive semi-quantitative fieldtest for the determination of Cr(VI) in aqueoussamples. A cation-exchange resin was also usedfor the FI on-line preconcentration of the Cr(III )-DPCO complexw83x, which can be retained at pH1 on Amberlite XAD-16 resin and then elutedusing 0.05 M H SO solution in methanolw161x.2 4

In this way, Cr(VI) can be determined in tap watersamples, as total chromium after oxidation ofCr(III ) into Cr(VI) using potassium permanganate.Cr(VI) may also be chelated by DDTC, and

subsequently retained on C -silicaw60x. Total18

chromium may be estimated using the same pro-cedure with prior oxidation of Cr(III ) to Cr(VI).The FI on-line preconcentration systems enableddetection limits of 16 ngyl for Cr(VI) and 18 ngyl for total Cr. Very recently, selective retention ofCr(VI), compared to Cr(III ) was obtained usingPTFE turnings packed in the micro-column of aFI manifold. APDC was used for complexation ofCr(VI) in the samples before preconcentration,and elution was achieved with IBMK before anal-ysis using F-AASw49x. Similarly, after reactionwith APDC, the Cr(VI) complex could be retained


on the PTFE inner wall of a knotted reactor or onPTFE beads packed into a column, and subse-quently eluted with ethanolw74,75x. Also, acidicalumina enables selective retention of Cr(VI) w70xand a micro-column was used in a FI manifold toseparate and preconcentrate Cr(VI) from Cr(III )in water samples. However, it was later reportedthat between pH 3 and 6 Cr(III ) could also beretained on that sorbentw235x.

4.2.2. Cr(III)Very recently, selective preconcentration of

Cr(III ) on a cellulose micro-column has beenreported to be highly dependent on sample pHw183x. Below pH 8, very low retention occurred,while at pH 11 almost quantitative adsorptioncould be achieved. Elution of the retained Cr(III )species with 5 ml of HCl(2 M) enabled itssubsequent determination by ET-AAS with a 1.8ngyl limit of detection. This method also enabledthe determination of total chromium followinginitial reduction of Cr(VI) to Cr(III ) withhydroxylamine.The coating of the positively charged acidic

alumina with an anionic surfactant, SDS, has beenreported to enable selective retention of Cr(VI) instrongly acidic solution, while the cationic Cr(III )remains unabsorbedw151x. Separation of the twospecies could thus be obtained and Cr(III ) deter-mined by ET-AAS. However, this method presentstwo major limitations; no analyte enrichment andthe need to adjust sample pH to 0.6, which mayaffect chromium speciation. In another study, basicalumina has been shown to selectively retainCr(III ) at pH 2–7 w73x, permitting elution withnitric acid.Selective retention of Cr(III ) was also reported

with a macroporous PS-DVB resin(CHP-20P)after complexation in solution with 8-HQ at 858Cw159x. The use of hydroxylamine as a reductantwas found efficient for the reduction of Cr(VI) toCr(III ) without affecting Cr(III ) retention as itdoes not complex this species. 8-HQ-immobilized-polyacrylonitrile fiber has been recently reportedto selectively retain Cr(III ) at pH 6, enabling itspreconcentration from river and reservoir watersamples, while Cr(VI) was unretainedw186x.Selective retention of Cr(III ) (cationic) was also

achieved on a chelating ion-exchange columnpacked with poly(aminophosphonic acid) (PaPhA)resin w81x. By using on-line FI preconcentrationcoupled to F-AAS, a detection limit of 0.2mgylcould be obtained with a sample throughput of 30h .y1

4.2.3. Cr(VI)yCr(III)A procedure based on the reaction of chromium

species with APDC and subsequent retention ofthe complexes on SPE permitted a subsequent on-line LC–UV analysisw39x. The Cr(III ) reacts withthe chelating agent under relatively mild conditionsto give only one product, i.e. triswpyrrolidine-l-dithioato-S,S9xchromium(III ), whereas Cr(VI)reacts to give two products, one being the formercomplex. Thus, the concentration of Cr(VI) needsto be corrected for the Cr(III ) complex. Theautomated SPE system was optimized to yielddetection limits of 0.2mgyl for Cr(III ) and 0.06mgyl for Cr(VI). A FI on-line preconcentrarionprocedure followed by F-AAS detection has beenreported to allow determination of both chromiumspecies in water samples based on selective for-mation of DDTC complexes of Cr(VI) in the pHrange of 1–2, and of Cr(III ) in the 4–9 pH rangew59x. A detection limit of 0.02mgyl could thus beachieved. Cr(III ) could also be retained in thepresence of Mn(II), which enhances the Cr(III )signal w59x.

TiO has been recently reported to be very2

promising for chromium speciationw108x. Indeed,this sorbent can selectively adsorb Cr(III ) orCr(VI) depending on the pH of the sample. At pH2, Cr(VI) is the sole chromium species retained,while at pH 8 it is Cr(III ). Nitric acid (0.5 or 1M) allows quantitative elution of the retainedspecies. Acidic alumina has also been reported toretain both Cr(III ) and Cr(VI) w235x. However,careful adjustment of the pH was of prime impor-tance as below pH 3 and above pH 6 retention ofCr(III ) and Cr(VI), respectively, decreasedw235x.Hence, a FI on-line preconcentration method hasbeen reported on activated acidic alumina withbuffering of the sample before retention either atpH 7 (for Cr(III )) or pH 2 (for Cr(VI)) w71x.Cr(III ) exhibited a typical cationic sorption(itincreased with pH and decreased when competing


cations are present), whereas Cr(VI) exhibited atypical anionic sorption(it decreased with increas-ing pH and in the presence of competing dissolvedanions). Thus, the use of neutral alumina seemsmore appropriate for chromium speciation since itoffers the advantage of requiring no adjustment ofthe water sample pH. Hence, both Cr(III ) andCr(VI) can be quantitatively retained on neutralalumina with speciation being achieved usingselective elution of the species, i.e. 1 M ammoniasolution for Cr(VI) followed by 4 M HNO for3

Cr(III ) w110x. This method affords a 25-fold pre-concentration factor with a limit of detection of10 ngyl. A similar preconcentration on aluminafollowed by selective elution for separation of thetwo chromium species has been reportedw72x.

A new resin that consists of PS-DVB function-alized with NDSA enabled the retention of Cr(VI)at sample pH of 1.5, and of Cr(III ) at pH 6.5w181x. Thus, speciation of chromium was accom-plished by adjustment of the pH along with thepercolation of two sub-samples at the desired pH.The selective retention of both chromium speciescould also be achieved on a polymeric sorbentcontaining aminocarboxylic groups at pH 3 and 7,for Cr(VI) and Cr(III ), respectivelyw236x. Micro-wave heating was found to promote the sorption.Chromium species may also be retained on ananionic-exchanger(SAX) after their complexationwith EDTA w237x. Controlled elution of the anal-ytes with 0.5 M NaCl enables their speciation. Inthis manner, detection limits of 0.4 and 1.1mgylwere obtained for Cr(III ) and Cr(VI), respectively.

4.3. Iron

Iron is widely distributed in nature and is oneof the most important elements in biological sys-tems. Its biological effectiveness is influenced byits chemical properties, such as valence, solubilityand the degree of chelation or complex formation.Several methods have been proposed for the deter-mination of Fe(III ) and Fe(II) species.

4.3.1. Fe(III)Some solid phases have been synthesized to

enable high selectivity towards Fe(III ). Thus, thechemical bonding of formylsalicylic acid on ami-

no-silica gel enabled the preconcentration ofFe(III ) from a mixture containing other traceelements in batch experimentsw35x. The capacityof this sorbent for Fe(III ) was 0.95 mmolyg. Thishigh selectivity was attributed to the presence oftwo chelating oxygen atoms. The selective SPE ofFe(III ) on purpurogallin chemically immobilizedon silica gel has also been reportedw140x. Onceeluted from the sorbent the iron species wereanalyzed using AAS. Due to strong affinity ofFe(III ) to the bound organic compound, as com-pared to Fe(II) (the distribution coefficient,K ,dwas found to be 120 500 for Fe(III ) and 12 700for Fe(II)), speciation of iron could be achievedusing this procedure. Only minor Fe(III ) (2.1%)was found to be reduced to Fe(II) upon interactionwith the sorbent. Using batch experiments, Fe(III )could be selectively extracted from tap water aswell as from a soft drink sample(7-Up).Yamini and Amiri w117x developed an efficient

method for the selective extraction, concentrationand determination of trace amounts of Fe(III ) inaqueous media, enabling determination of iron ina 500 ml water sample in less than 30 min with areproducibility better than 3%. Iron(III ) was ini-tially reduced to Fe(II) by addition of hydroxyla-mine (NH OH) and the bathophenanthroline2

complex was analyzed2qFe(bathophenanthroline)3by the use of C -silica membrane disks and18

spectrophotometry. The ligand was added to thesample before SPE, and the solution heated toenhance formation of the complex. By comparisonwith AAS and ICP emission spectrometry, thismethod offers simplicity and applicability to fielddetermination of iron. However, pure solventseluted only a fraction of the complex and additionof NaClO to the elution solvent was required for4

complete removal of the retained complex, indi-cating that interactions between the complex andthe C -silica are dispersive or ionic. At sample18

pH below 2.5 recovery decreased probably due tocompetition of H with Fe(II) for reaction withq

bathophenanthroline. The influence of several ionswas investigated, most were tolerated at high levelswithout interfering with the determination of iron.However, some species such as Co , Ni ,2q 2q

and especially Cu , interfered. The inter-y 2qVO3

ference effect of Cu is due to the preferential2q


formation of a very stable, almost colorless, com-plex between Cu(I) and bathophenanthroline. Itseffect was eliminated by the addition of excessamounts of thiourea, as a masking agent, whilethe effect of the other ions was eliminated by useof excess amounts of bathophenanthroline. A FIcatalytic spectrophotometric method has also beendeveloped for the shipboard determination of ironin sea water samplesw52x. Retention was achievedusing 8-HQ-functionalized silica gel. After elutionwith HCl mixing with the reagents(H O and2 2

N,N-dimethyl-p-phenylenediamine (DPD)) wasperformed to ensure the formation of the color. Adetection limit of 0.016 nM could thus beachieved.

4.3.2. Fe(II)Iron(II) is thermodynamically unstable in sea

water containing dissolved oxygen due to its rapidoxidation to Fe(III ) (half-life being approx. 4–10min depending on the pH). Yet, the determinationof Fe(II) in sea water is important because of itsrole in the solubility, speciation and biologicalutilization of iron in oceanic surface waters; inaddition Fe(II) may reduce oxygen, thereby pro-ducing radicals in the water.The colorimetric reagent ferrozine(FZ) w3-

(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)l,2,4-tri-azinex forms a stable, colored complex with Fe(II)in a pH range of 4–10, but not with Fe(III ) w238x.Interferences are also possible due to complexationof Cu(I), and to a lesser extent of Co(II) andNi(II) by FZ. Thus, a procedure has been devel-oped enabling the preconcentration of Fe(II) fromsea water, thanks to its retention as the Fe(FZ)3complex on a C -silica cartridge and subsequent18

elution with methanolw122x. Iron was then ana-lyzed directly by spectrophotometry(562 nm).C -silica was loaded with FZ by passing a FZ18

solution through the cartridge(the retention capac-ity of the cartridge for FZ increased with increasingionic strength of the solution). Once the sea waterhas been passed through the cartridge, washingwas performed with 5 ml of 0.1 M NaCl–0.005M NaHCO to remove sea salts and prevent3

precipitation of Mg and Ca in methanol upon2q 2q

elution. Cu(I) interference was minimised by add-ing neocuproine(NCH) to the methanol extract

avoiding the presence of the Cu(FZ) complex.2

However, in this method both the Fe(FZ) complex3

and the excess FZ were eluted and contributed tothe absorbance measurement, which may result inunreliable determination of low Fe(II) concentra-tions. Therefore, this procedure was furtherimproved by performing a chromatographic sepa-ration of the two species after the SPE step. Inaddition, 254 nm was found more suitable fordetection than 562 nm. In this way, ultratraceamounts of Fe(II) could be determined in severalsamples(aerosols, rainwater and sea water) w123x.

A procedure, similar to that reported by King etal. w122x has been used for the on-line FI precon-centration of Fe(II) from sea water samplesw68x.It could be extended to Fe(III ) determination basedon its initial reduction by addition of ascorbic acidto the sample. Copper interference could be sup-pressed by loading the C -silica with a mixture18

of FZ and NCH, but at the same time the columncapacity was lower than when loaded with FZalone.

4.3.3. Fe(III)yFe(II)Most of the methods report the determination of

only one of the two iron species mainly byselective complexation. However, this can cause ashift in the Fe(II)yFe(III ) equilibrium in the solu-tion as a result of redox reactions. To prevent suchproblems, a procedure has been reported thatenables the simultaneous complexation of bothFe(II) and Fe(III ) in the sample followed byretention of the complexes on selective solid sor-bentsw18x. Fe(II) was complexed by addition of1,10-phenanthroline, while Fe(III ) formed a com-plex with ferron(8-hydroxy-7-iodoquinoline-5-sul-fonic acid). The solution was thus passedsuccessively to an anion-exchange resin and areversed-phase sorbent. Since the Fe(III )-ferroncomplex is negatively charged, it was retained bythe first solid phase(for sample pH 3–6), whilethe Fe(II)-phenanthroline was passed through dueto its non-polar character. This complex was thenretained by the second solid sorbent(C -silica).18

This method enabled the determination of bothlabile Fe(II) and Fe(III ) species in wine samples.


4.4. Lead

Lead is a toxic metal, which accumulates in thevital organs of man and animals. Its cumulativepoisoning effects are serious haematological dam-age, anemia, kidney malfunctioning, brain damage,etc. Lead is still emitted into the biosphere inconsiderable amounts owing to its application as afuel additive, mainly as tetraethyllead and tetra-methyllead. It is also present in many industrialstreams. Due to the presence of lead in environ-mental samples at low levels, its separation fromother elements present and the use of a preconcen-tration step prior to lead determination are usuallynecessary. All the studies reported until nowfocused on the preconcentration of inorganic leadPb(II). The performances of Pb determinationusing different sorbent materials have beenreviewed recentlyw50,176x. Some examples aregiven below. Firstly, as a cationic species, Pb(II)can be retained on cation-exchangers, such as basicalumina w239x. However, such sorbents are rathernon-selective. So, other strategies have been usedthat are the retention of Pb complexes on hydro-phobic sorbents, the retention of Pb(II) on sorbentscoated with a chelating reagent and the synthesisof new chelating resins.Lead complexed with DDTP could be retained

on C -silica, activated carbon and PUFw63x. In18

that way analysis through a FI system coupled toF-AAS was performed with limits of detection of0.3, 1.2 and 3mgyl for C -silica, PUF and18

activated carbon, respectively. In another FI systemPb(II) was complexed with APDC retained onPTFE turnings and further eluted with IBMKleading to 0.8mgyl as a limit of detectionw50x.This method enabled the determination of Pb(II)in various environmental and biological samples.Complexes can be formed with DDTC in acidicmedium, further retained on C -silica and eluted18

with IBMK or methanol before F-AAS, enablingthe detection of 3 to 10mgyl w62,233x. Alterna-tively, the sorbent can be impregnated with thechelating reagent. Hence, the SPE of lead on C -18

silica disks has been reported after impregnationof the sorbent with a S-containing Schiff’s base toenable Pb(II) chelation w120x. After elution withnitric acid lead was further analyzed by F-AAS

with a limit of detection of 16.7myl. Retention oflead was quite selective even in the presence ofother ions. The addition to the sample of ammoniaas a masking agent was recommended to suppressthe interfering effect of some cations, namelyCu(II), Zn(II) and Hg(II). Similarly, the coatingof C -silica with BAS was found highly selective18

in retaining Pb(II) with subsequent elution usingacetic acid and F-AAS analysis leading to 50 ngyl as a limit of detectionw121x. Pb(II) could alsobe retained on Amberlite XAD-7 coated withDMBS w196x or XO w176x, Amberlite IRA-904impregnated with TCPPw8x, or PUF coated with2-(2-benzothiazolylazo)-2-p-cresolw240x.

Chelating resins have also been synthetized forthe selective preconcentration of Pb(II). For exam-ple, Amberlite XAD-2 was functionalized throughan azo spacer by several chelating agents, such asCA, PC and TSA, and used for the retention ofPb(II) from water samplesw176x. Similarly, thissorbent functionalized with SA also enabled theretention of Pb(II), along with Zn(II) w171x. Ineach case nitric acid enabled lead desorption.Limits of detection in the 2.4–4.9mgyl range wereobtained. Amberlite XAD-7 functionalized with acrown ether was also found suitable for the pre-concentration of Pb(II) w241x.Very recently, an attractive test procedure has

been reported that enables the determination of aslow as 20 ngyl of Pb(II) w203x. It is based on theimmobilization of an enzyme, alkaline phosphataseon PUF.

4.5. Mercury

The presence of mercury species in aquatic foodchain is of great concern as it is well-known thatinorganic mercury(Hg ) is converted into highly2q

toxic methylmercury(MeHg ) by aquatic organ-q

isms. Due to the presence of mercury in environ-mental samples at low levels, its separation fromother elements present and the use of a pre-concentration step prior to the determination isusually necessary.

4.5.1. Hg(II)Mercury can be preconcentrated from aqueous

samples using a chelating ion-exchange resin con-


taining histidine covalently bound to its carboxylgroupw242x. A chelating PS-DVB-based resin withpicolinic acid amide(PAA) as the functional groupwas also found efficient for Hg(II) retention fromwater samplesw184x. Several functional groupschemically bound to silica gel have also beenreported to afford selective sorbents for preconcen-tration of Hg(II) during adsorption. This was thecase with DZ w104x and dithioacetal derivativesw105x. Another procedure involving the use ofC -silica disks impregnated with hexathia-18-18

crown-6-tetraone(HT18C6TO), was shown toquantitatively extract Hg(II) from natural watersin less than 15 minw130x. Recovery was nearlyindependent of pH(in the range of 1 to 7) asalready reported for the solvent extraction of met-als with crown ethers. Before eluting Hg with 12q

M HBr, a washing step with 1 M HNO was3

recommended to remove small amounts of retainedCu , Zn , Pb and Cd .2q 2q 2q 2q

4.5.2. Hg(II)yorganic HgA chelating resin based on vinyl polymer and

containing dithiocarbamate groups was found effi-cient for retention of inorganic and organic mer-cury from water samples over a broad range ofpHs (1 to 11) w189x. The species were elutedusing an acidic aqueous solution of 5%(wyv)thiourea, enabling a preconcentration factor of 666and the determination of mercuric species with alimit of detection of 0.2 ngyl. The use of anotherchelating resin containing dithiocarbamate groupswas also found effective in retaining inorganic aswell as organic mercury in the pH range 1–4resulting in limit of detection of 0.5 ngyl w243x.The on-line FI preconcentration of mercury species(Hg(II), methylmercury, ethylmercury) on adithiocarbamate resin has also been reportedw244x.Detection limits of 0.05 ngyl and 0.15 ngyl fororganic and inorganic mercury, respectively, couldbe obtained. However, this method involved man-ual steps once the species were eluted with thio-urea, extraction into toluene as the diethyldithiocarbamate complexes, butylation with a Gri-gnard reagent and subsequent gas chromatography(GC) analysis. In addition, preconcentration failedin the presence of high amounts of humic sub-stances in the water samples.

Complexation of mercuric species with APDC,FI on-line preconcentration on C -silica, further18

separation with LC, and analysis by CV-AAS hasbeen reportedw43x. In this manner, Hg species(methylmercury, ethylmercury, phenylmercury andHg(II)) in fish and human urine could be analyzedat the ngyl level. Chelation with DDTP and pre-concentration on C -silica in a FI–CV-AAS sys-18

tem resulted in detection limits of 10 ngyl formethylmercury and Hg(II) with an enrichmentfactor of 20 w64x. Several ligands were tested:DDTC, APDC and DZ(or diphenylthiocarbamate)w55x. Results showed the superiority of carbamatetype reagents for the preconcentration of Hg(II)and methylmercury using this system. WithDDTC, detection limits of 16 ngyl of Hg could beobtained.

4.6. Selenium

Selenium is present in the environment fromboth natural and anthropogenic inputs. This ele-ment has been recognized as an essential nutrient.However, at concentrations higher than 130mgylit becomes toxic. Se(IV) and Se(VI) are thepredominant species in natural waters. Biomethy-lation may also occur, leading to the formation oforganic species such as dimethyl selenide(DMSe),dimethyl diselenide(DMDSe) and diethyl selenide(DESe), and detoxification. Therefore, a reliablespeciation procedure is required to evaluate toxic-ity of samples. An overview of SPE proceduresdeveloped for selenium has been recently givenw193x and some are detailed below. It appears thatmost methods were dedicated to the determinationof inorganic selenium.

4.6.1. Se(IV)APDC has been loaded on C -silica and used18

for the preconcentration of Se from sea water priorto ET-AAS w114x. Speciation of inorganic Se(IV)and Se(VI) was possible since APDC selectivelychelates Se(IV). A reduction of Se(VI) to Se(IV)prior to chelation is required for the determinationof total inorganic Se. However, this method wasnot selective as other trace elements were retainedon the sorbent(such as Bi, Pb, Zn, As, Sn, V).Complexation of Se(IV) could also be obtained


with 3-phenyl-5-mercapto-1,3,4-thiadiazole-2-(3H)-thione (Bismuthiol II), which was thenretained on activated carbon in batch experimentsw222x. Se(IV) could also be retained on Chelex-100 (in the iron form) over a wide pH range(upto 10) w245x.

4.6.2. Se(IV)ySe(VI)Both forms of inorganic selenium can be

retained on acidic alumina and extracted fromnatural waters without any pH adjustmentw111x.Selective elution is achieved using ammonia atdifferent concentrations for eluting Se(VI) andthen Se(IV) enabling speciation of inorganic sele-nium in natural waters. Quantitative recoveriesfrom spiked tap water and ground water wereobtained except for one tap water, where 42 and154% of Se(IV) and Se(VI), respectively, wererecovered possibly due to oxidation of Se(IV) byresidual chlorine. Similar observations were madein another studyw40x.

Dowex-lX8 ion-exchanger enabled preconcen-tration of both Se(IV) and Se(VI) w246x. Thespecies were then separated during elution withtwo different concentrations of HCl. Inorganicselenium species were also retained on severalanion-exchange resins based on either cellulose orPS-DVB copolymerw225x. Despite a lower affinity,the functionalized cellulose sorbent was preferred,as it enabled a better separation of Se(IV) andSe(VI) during elution with different concentrationsof nitric acid. Traces of Se(IV) and Se(VI) couldalso be retained on a molybdate-form anion-exchange resin in batch experimentsw247x, per-mitting speciation of inorganic selenium in naturalwaters except sea water(where foreign ionsinterfered).The coupling of SPE to LC–ICP–MS enabled

the speciation of inorganic selenium in naturalwater samples by ion-pairing(with tetrabutylam-monium phosphate as the reagent) and sorption onC -silica, subsequent elution and separation in the18

chromatographic column by anion-exchangew40x.However, sample volumes were limited to 10 mldue to the limited capacity of the preconcentrationcolumn.

4.6.3. Organic SeTanzer and Heumannw248x developed a method

for the selective determination of acidicyneutraland basic organoselenium species in water samplesbased on the selective retention of the species onAmberlite XAD-2 at different sample pHs(3 and8, respectively). However, this method was foundquestionable by other authors who noted partialretention of Se(IV) on that sorbentw225x.

4.6.4. Se(IV)ySe(VI)yorganic SeSimultaneous preconcentration of inorganic and

organic selenium species is a more difficult task.A combined SPE method has been reported toenable preconcentration of both inorganic andorganic species of seleniumw19x. An anion-exchanger cartridge was placed on the top of aC -silica cartridge so that inorganic selenium was18

retained on the first cartridge, while organic spe-cies were retained on the reversed-phase sorbent.The cartridges were then separated and the specieseluted: 1 M HCOOH for Se(IV), 3 M HCl forSe(VI), CS for organic compounds. In another2

recent procedure, simultaneous determination oforganic (selenocystine) and inorganic selenium(Se(IV) and Se(VI)) species was achieved usingoff-line preconcentration on Amberlite IRA-743followed by separation of the species and analysisusing LC–ICP–MSw193x. No adjustment of thesample pH was required. However, the sorbentwas not selenium selective, and selenomethioninecould not be retained under the conditions devel-oped due to strong competition with hydrogencarbonate.

4.7. Tin

Organotin compounds have found widespreadindustrial applications as biocides, antifoulingpaints, catalysts and polyvinyl chloride stabilizers.So, there are a variety of pathways for their entryinto the environment. Whereas inorganic tin isbasically harmless, some organotin compounds arehighly toxic, especially tri-substituted organotinspecies. Therefore, there is a need for sensitivemethods that enable the determination and speci-


ation of these compounds. Several studies reportthe use of SPE for organotin determinationw249–251x.

4.7.1. ButyltinsTributyltins have been used as insecticides, fun-

gicides, acaricides and preservatives for manydifferent types of materials. In particular, they havebeen used as antifouling paints(as biocides) onships, boats and dock resulting in release of TBTsdirectly into the aquatic environment where theyare non-specific and extremely toxic to non-targetanimal and plants species. C -silica, either in18

cartridges or in PTFE disks, has been foundeffective in preconcentrating TBTs from aqueoussolutionsw24x. Enrichment factors up to 1000 canbe obtained enabling the quantification of TBTs atthe 0.1mgyl level. In addition, TBTs can be storedon such solid supports at room temperature for atleast 1 month. Amberlite XAD-2 impregnated withtropolone has also been reported to retain TBTand dibutyltin, while monobutyltin was notretainedw164x. The addition of 0.8% sulfuric acidto the water sample enabled the selective retentionof only TBT on the resin. This species wassubsequently eluted with IBMK and analyzed byET-AAS enabling quantitative determination ofTBT in water samples with a limit of detection of14.4 ngyl and a preconcentration factor of 80.

4.7.2. PhenyltinsTriphenyltin is retained on C -silica cartridges,18

even though a fraction of the compound cannot berecoveredw112x. Hence, best recoveries(between81 and 89%) were obtained by elution in thebackflush mode with 10 M 3-hydroxyflavone iny4

methanol. Retention of TPhT on other silica-basedsorbents(octyl, phenyl and cyanopropyl) has alsobeen reportedw112x.

4.7.3. ButyltinsyphenyltinsA semi-automated system was reported that

enabled organotin speciationw251x, wherein pre-concentration was performed using a microcolumnof C -silica placed in a FI manifold. Percolation18

of a derivatising reagent(sodium tetraethylborate)through the column enabled derivatisation of theorganotin compounds. Elution of the derivatised

species(monobutyltin, monophenyltin, dibutyltin,diphenyltin, TBT and TPhT) was achieved usingmethanol and their separation and analysis wasperformed using GC–AES. In this manner, withrather small sample volumes(10–50 ml) detectionlimits in the range of 0.10–0.17 ngyl could beobtained. Application of the method was testedwith real river water samples, and results wereconsistent with those obtained using classical liq-uid–liquid extraction. Similarly, in an off-line sys-tem, organotins derivatised by sodiumtetraethylborate could be retained on C -silica18

disks, and further eluted with supercritical CO2

w252x.The use of tropolone-loaded C -silica has also18

been used for the retention of several butyltins andphenyltins(mono-, di-, tri- and some tetra-substi-tuted compounds) w118x. However, selectivitycould not be achieved using SPE only. Organotinswere separated by subsequent GC analysis aftertheir ethylation with Grignard reagent. This meth-od affords a sensitivity of low ngyl in surfacewaters and mgykg in sewage sludges.Speciation of tin has also been reported using

both graphitized carbon black and silica gel assorbentsw223x. Water samples were first passedthrough GCB, allowing the retention of TBT andTPhT, while inorganic tin passed unimpeded andwas analyzed directly. The organic species weresubsequently eluted with a mixture of methanolydichloromethane(4:1), and separated on silica gel.The overall method provided an enrichment factorof up to 80 000, but required the complete evapo-ration of the organotin fraction eluted from GCB,and the dissolution of the solid residue in hexanebefore separation on silica gel.Finally, organotins can be efficiently retained on

strong cation-exchange silica-based bonded phases(Bond-Elut SCX) and strong cation-exchangepolymeric-based phase(Oasis-MCX). The pres-ence of was essential for elution of theqNH4

compounds. Recoveries were lower with the pol-ymeric phase particularly for TPhT, probablybecause of strong interaction of the aromatic ringswith the N-vinylpyrrolidone-divinylbenzene sup-port w36x.


5. Conclusion

The use of SPE procedures has been growing inthe past few years due to their advantages offeredfor trace element determinations, namely conser-vation of species, good preconcentration factors(thus enabling the achievement of very low limitsof detection), ease of automation, and possible on-line coupling to instrumental techniques.Despite the numerous steps and parameters used

to enable efficient extraction and recovery of thetarget analytes, the choice of the solid sorbent isthe most critical step. Among the numerous sor-bents that have been used, it clearly appears thatthe initial use of ion-exchangers is being replacedby more selective supports containing chelatingfunctional groups. Such sorbents are frequentlybased on hydrophobic supports namely C -silica18

or PS-DVB copolymer, the latter affording a broad-er pH range. The simplest procedure consists ofadding the chelating reagent directly to the sample.A more suitable way of proceeding is to load thereagent on the solid sorbent. The coated sorbentthus obtained may usually be used several times,but partial leaching of the chelating agent mayoccur over time during elution of the analytes,{

especially if the eluting solvent is too concentrated.Alternatively, the reagent may also be chemicallybound to the sorbent leading to the synthesis ofnew phases, and thus avoiding any leaching of thereagent. Promising results have also been recentlyobtained with inorganic oxides such as titania andalumina, as retention of some trace elements couldbe achieved with the raw sample(i.e. no reagentaddition nor pH adjustment), thereby avoidingpossible speciation changes in the sample.Examples given in this review for several trace

elements show the high potential of SPE, espe-cially its possible high selectivity(by choosing thenature of the sorbent andyor the chelating agent,as well as the nature of the eluent). In fact, insome cases, differentiation of species may beachieved, thereby offering new opportunities forspeciation. There is thus no doubt that this tech-nique will face a growing interest for trace elementdetermination and speciation in the future, asalready evidenced for organic micropollutant deter-minations in the recent years.

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