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Development of a microwave-assisted extraction method andisotopic validation of mercury species in soils and sediments w

G. M. Mizanur Rahman and H. M. Skip Kingston*

Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, PA 15282, USA.E-mail: [email protected]; Fax: 412 396 4013 or 412 396 5683; Tel: 412 396 5564

Received 26th March 2004, Accepted 7th January 2005First published as an Advance Article on the web 4th February 2005

An efficient and rapid closed vessel microwave-assisted extraction method based on an acidic extractant hasbeen developed to determine inorganic mercury and methylmercury in soils and sediments. Parametersoptimized during this study were nitric acid concentration, amount of sample, extraction temperature andirradiation time. The results suggest that the nitric acid concentration and the irradiation temperature arestatistically signicant both for extraction efficiency and for stability of mercury species. A processed topsoil(Hg o 0.01 ng g 1) spiked with inorganic mercury and methylmercury and SRM 2711 (spiked withmethylmercury) were used during the method development. The sample preparation was optimized in aclosed-vessel system by heating 1.0 g of sample in 10.0 ml of 4.0 mol l 1 HNO 3 for 10 min at 100 1 C withmagnetic stirring. Analyses of the extracts were carried out by using three types of instruments, DirectMercury Analyzer-80 (DMA-80), inductively coupled plasma mass spectrometer (ICP-MS) and high-performance liquid chromatograph coupled with inductively coupled plasma mass spectrometer (HPLC-ICP-MS). The results obtained from each of these detection techniques agreed signicantly at the 95% condencelevel. The method was validated by the analyses of two types of specically prepared reference soil samplesand four certied reference materials (BCR 580, SRM 2704, SRM 2709 and SRM 1941a). The inorganicmercury and methylmercury concentrations found were in good agreement at the 95% condence level withthe certied or made-to value. The method was also validated using EPA Method 6800 as a diagnostictool to check whether interconversion of inorganic mercury to methylmercury or vice versa took place duringor after extraction; the amount of such interconversions was found to be statistically negligible. The methodis in the process of consideration and adopted by the United States Environmental Protection Agency (USEPA) as a primary mercury species extraction protocol from soils and sediments in EPA draft Method 3200.

IntroductionMercury speciation has long been a eld of concern. Suchinterest is mainly due to toxicological impact, ecological pro-blems and biogeochemical cycling of mercury involving dis-tribution, accumulation, transformations and transportpathways in the natural environment. 1 Mercury is a very toxicelement. However, the toxicity of mercury is highly dependenton its chemical form. Methylmercury is one of the most toxicmercury species. To understand the toxicological impact andpathways of mercury species in the environment, the determi-nation of total mercury is frequently not sufficient. Therefore,the assessment of inorganic mercury and methylmercury con-centrations, specically in sediments and soils, is very impor-

tant to the interpretation of biogeochemical cycles of mercuryin aquatic environments. 2

Determination of different mercury species from variouscomplex matrices, e.g. , soils and sediments, is still considereda difficult task due to the frequently very low concentration of methylmercury in soils and sediments (less than 1.5% of thetotal mercury). 3 The quality of the results mainly depends onthe sample pre-treatment stages (sampling, storage and samplepreparation), in spite of signicant improvements in the in-strumentation techniques. The most widely used methods forthe extraction and separation of inorganic and methylmercuryare the Westo o technique 47 (acidic leaching method), alkalinedigestion, 810 steam distillation, 911 solvent extraction, 1214 a

modied Westo o methodology 15 (alkaline based technique),and supercritical uid extraction, 16 followed by one or twoseparation steps. The separation and detection techniquesassociated with these methods include gas chromatography(GC), HPLC coupled with element-selective detection techni-ques, such as ICP-MS, atomic emission spectrometry (AES),atomic absorption spectrometry (AAS), atomic uorescencespectrometry (AFS), or cold vapor atomic absorption spectro-metry (CV-AAS). As all of the aforementioned sample pre-paration methods use either acid or base with organic solvents,and, after extraction, most of them implement sample pre-concentration steps ( e.g. , ethylation or reduction with SnCl 2 orhydride generation with NaBH 4), there is a possibility of interconversion or unidirectional transformation of inorganic

mercury to organic mercury13,1629

or vice versa2932

duringsample storage, shipment, extraction, pre-concentration oranalysis steps. Therefore, the results obtained using theseprocedures frequently introduce positive or negative biasesfor either inorganic mercury or methylmercury, or both. Be-sides such drawbacks, these methods require much solvent,labor and time.

The efficiency of the less solvent- and time-consumingmicrowave-assisted extraction (MAE) technique for samplepreparation in environmental applications has been evaluatedelsewhere in different matrices (soils, sediments, and biologicaltissues) in different applications (total digestion for elementalanalysis, extraction of selected organic compounds), and inspeciation analysis (organotin). Vazquez et al .33,34 used the

focused microwave-assisted extraction (FMAE) technique toextract methylmercury with HCl and toluene, a modiedmethod of Westo o ,4,5 from sediment and biological tissuesamples. Tseng et al .3537 also implemented FMAE for the

w Electronic supplementary information (ESI) available: optimizationof HNO 3 concentration, optimization of sample weight, optimizationof irradiation temperature and optimization of irradiation time. Seehttp://www.rsc.org/suppdata/ja/b4/b404581e/

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extraction of methylmercury, also from sediment and tissuesamples. There are several drawbacks to FMAE: samples mustbe extracted at atmospheric pressure and below the boilingpoint of the solvent; simultaneous extraction of multiplesamples is not possible; it is difficult to preset a constanttemperature prole as this technique only allows control of the applied power which, in turn, is directly dependent on thenumber of samples or the total mass, and, there is a highpossibility of losing the volatile organomercury compoundsduring extraction. However, no one has yet tried the closed-vessel microwave-assisted extraction technique (which is freefrom the aforementioned drawbacks) for mercury speciation insoils or sediments.

Therefore, the purpose of this study was to develop amicrowave-assisted extraction procedure capable of quantita-tive extraction with little or no transformation of inorganicmercury and methylmercury from soils and sediments in aclosed-vessel microwave system, and incorporate it into EPAdraft Method 3200 as an alternative extraction procedure formercury species. Careful optimization of the conditions for themicrowave extraction procedure is required to stabilize themercury species in the microwave eld, prior to speciationanalysis. Essential parameters, such as concentration of theextraction medium, amount of sample, temperature and timeof exposure must be optimized w. The literature 35 suggests thatnitric acid (HNO 3) is a better solvent for microwave-assistedextraction because it introduces little or no interferences to theICP-MS. Therefore, nitric acid has been evaluated as anextraction solvent. The irradiation power, one of the mostuseful parameters for microwave extraction, was not optimizedduring this study due to its dependency on the number of samples or the total mass of the extraction medium.

This paper describes a fast and easy method for the quanti-tation of inorganic mercury and methylmercury using closed-vessel microwave-assisted extraction, followed by separationwith HPLC and detection with ICP-MS. The stability of themercury species in a microwave eld and the optimization of different parameters are also described in detail. The developedmethod was then validated by using different standard refer-ence materials and reference soils obtained from Environmen-tal Resource Associates s . The developed method was alsovalidated using EPA Method 6800 [ Elemental and Speciated Isotope Dilution Mass Spectrometry, (IDMS and SIDMS,respectively )].38 EPA Method 6800 was used as a diagnostictool to check whether any interconversion between inorganicmercury and methylmercury is taking place during or afterextraction. One of the unique applications of SIDMS is to traperrors related to specic portions of a protocol. This isaccomplished by using multiple spikings with multiple iso-tope-labeled species at specic method protocol points. Theerror of the specic steps can be discovered, and their con-tribution to the overall transformation of a species known. To

perform these types of applications, inorganic mercury andmethylmercury labeled with multiple isotopes are required.Although methylmercury labeled with different isotopes hasrecently become commercially available, 3941 the methylmer-cury labeled with a minor mercury isotope was synthesized inthe laboratory.

ExperimentalReagents and chemicals

Analytical grade nitric acid (Fisher Scientic, Pittsburgh, PA,USA) and double de-ionized (DDI) water (18 M O cm 1),prepared from a Barnstead NANOpure Ultrapure Water

System (Dubuque, IA, USA), were used. Different concentra-tions of nitric acid were prepared by diluting an appropriatevolume of nitric acid in DDI water. Reagent grade HCl,H 2 SO 4 , NaCl, CuCl 2 2H 2O, NaOH, acetic acid, acetic

anhydride, Na 2S2 O 3 , toluene, sodium acetate, ammoniumacetate, 2-mercaptoethanol (98%), and optima grade methanolwere obtained from Fisher Scientic (Pittsburgh, PA, USA).Mercaptoacetic acid (97%) was obtained from Aldrich (Mil-waukee, WI, USA). The reagent grade tetramethyltin (98%)was obtained from Alfa Aesar (Ward Hill, MA, USA).

Standard solutions and certied reference materials

A standard stock solution of 1000 mg ml 1 of HgCl2

in 5%HNO 3 and 1000 mg ml

1 of CH 3HgCl in water were commer-cially available from Alfa Aesar (Ward Hill, MA, USA). Allstock solutions were stored in amber glass bottles in a coldroom at 4 1 C. Working standards were prepared daily byproper dilution with DDI water.

201 HgO and 199 HgO were obtained from Isotech Inc. (Mia-misburg, OH, USA). Approximately 100 mg g 1 of stock199 Hg 2 1 spike was prepared by dissolving B 10 mg of 199 HgOin 2 ml concentrated HCl and made up to 90 g with 1% HNO 3 .CH 3

201 Hg 1 was synthesized from 201 HgO using tetramethyl-tin. 39 To prepare 201 HgCl 2 , 6 ml of

201 Hg 2 1 solution (11 mgml 1) was mixed with 2 ml of 6.0 mol l 1 HCl in a 20 ml amberglass vial and stirred for 5 min. A 0.93 mol l 1 methanolicsolution of (CH 3 )4 Sn was prepared by dissolving 0.340 g of (CH 3 )4Sn in 2 ml of methanol. This reagent was quantitativelytransferred into the 201 HgCl 2 solution and the glass vial capwas put back on. The resulting reaction mixture was thenstirred for 1 h in a 60 1 C water bath to complete the synthesis.The reaction mixture was cooled to room temperature andextracted 3 times with toluene (4 3 3 ml). The synthesizedmethylmercury (in toluene) was washed with DDI water 3times (4 3 3 ml). The toluene extract was then extractedtwice with 2.5 ml of 1% Na 2 S2O 3 . All of the extracts werestored in amber glass vials in a cold room at 4 1 C. The nalconcentrations of the stock CH 3

201 Hg 1 in 1% Na 2S2O 3 and199 Hg 2 1 in 1% HNO 3 were determined by reverse isotopedilution calibration. Working standards of CH 3

201 Hg 1 and199 Hg 2 1 were prepared daily by diluting with DDI water.

Caution. Mercury compounds, especially methylmercury,are highly toxic materials. Proper knowledge and safety guide-lines regarding working with mercury compounds are requiredto handle these compounds.

The sulfhydryl cotton ber (SCF) as a solid phase extraction(SPE) medium was prepared according to Han et al .12 Amixture of reagent containing 50 ml mercaptoacetic acid, 35ml acetic anhydride, 16 ml acetic acid, 0.15 ml H 2SO 4 and 5 mlDDI water was prepared in a clean beaker. A 15 g portion of cotton was immersed in this reagent mixture, and kept it in anoven at 40 2 1 C for 4 d. The product was removed from theoven and washed with DDI water on a vacuum lter funneluntil the pH of the washing was neutral. The cotton product(now SCF) was then dried in an oven at 40 2 1 C for 2 d.

SCF Eluent 1 (1 mol l1

HCl 1 mol l1

NaCl) formethylmercury was prepared by diluting 20.7 ml of concen-trated HCl to 200 ml in DDI water, then dissolving 14.6 g of NaCl in the prepared solution and making up to 250 ml withDDI water.

SCF Eluent 2 (6 mol l 1 HCl saturated NaCl 0.1%CuCl 2 2H 2O) for inorganic mercury was prepared by diluting124 ml of concentrated HCl to 200 ml in DDI water, thendissolving 11 g of NaCl and 0.25 g of CuCl 2 2H 2O in theprepared solution and making up to 250 ml with DDI water.

A 0.2 mol l 1 acetate buffer (pH 3.6) was prepared bydissolving 1.14 ml of glacial acetic acid and 0.1148 g of sodiumacetate in 100 ml DDI water.

HPLC speciation mobile phase [30% (v/v) methanol 0.06

mol l1

ammonium acetate 0.005% 2-mercaptoethanol],modied from Wilkens procedure, 42 was prepared by diluting300 ml of methanol, 50 ml of 2-mercaptoethanol and 4.8 gammonium acetate with 700 ml of DDI water.

184 J . A n a l . A t . S p e c t r o m . , 2 0 0 5 , 2 0 , 1 8 3 1 9 1

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NIST SRMs 1941a (Organics in Marine Sediment, 0.5 0.2mg Hg kg 1 ), 2704 (Buffalo River Sediment, 1.44 0.07 mgHg kg 1 ), 2709 (San Joaquin Soil, 1.40 0.08 mg Hg kg 1),and 2711 (Montana Soil, 6.25 0.19 mg Hg kg 1), IRMMreference material BCR 580 (Estuarine Sediment, 132 3 mgHg kg 1 and 75 4 ng CH 3Hg

1 g 1), topsoil (100% processedtopsoil, mercury content, o 0.01 ng Hg g 1) and reference soils(Material-1: 100% processed topsoil, and Material-2: 75%processed topsoil and 25% Ottawa Sand) from EnvironmentalResource Associates s (ERA) (Arvada, CO, USA) were usedfor method development and validation.

Instrumentation

A laboratory microwave system (Ethos 1600) (Milestone,Monroe, CT, USA), equipped with temperature and pressurefeedback control and a magnetic stirring capability, was used inthis study. This device is accurate in sensing temperature within

2.0 1 C of set temperature, and automatically adjusts themicrowave eld output power. This device extracts ten samplessimultaneously. The high pressure closed digestion vessels usedfor extraction are made of high purity TFM (a thermallyresistant form of Teon) and have a capacity of 100 ml.

Caution. Safety guidelines regarding work with microwaveelds in the laboratory must be observed. 43

A ConstaMetric 4100Bio/MS polymeric inert pump (Ther-mo Separation Products, Riviera Beach, FL, USA) and a 5 mmSupelcosil LC-18 HPLC column with a Pelliguard LC-18guard column (Supelco, Bellefonte, PA, USA) were used inthis study to separate inorganic mercury and methylmercury. Asix-port injection valve (Valco Instrument Co. Inc., Houston,TX, USA) was placed between the pump and the column.Because no special interface is required between the LC-18column and the ICP-MS, one outlet of the column is directlyinterfaced with the nebulizer of the ICP-MS using a piece of TFM tubing; the other end is connected to a 50 ml TEFZEL t

sample loop (CETAC Technologies, Omaha, NE, USA). Themobile phase was buffered 30% methanol (refer to StandardSolutions and Certied Reference Materials).

The SPE apparatus used for this study was an SPE manifold(Supelco, Bellefonte, PA, USA) with the custom SCF SPEcolumn prepared according to Han et al .12

An HP 4500 ICP-MS (Agilent Technologies, Palo Alto, CA,USA and Yokogawa Analytical Systems Inc., Tokyo, Japan)was used as the detector for the HPLC system in this study.The HP 4500 ICP-MS was also used for direct determinationof total mercury from sample extracts. The sample deliverysystem consisted of a peristaltic pump and a Scott-type doublepass quartz spray chamber with concentric nebulizer andquartz torch. The instrument was tted with platinum samplerand skimmer cones, and optimized daily using 10 ppb tuningsolution (Agilent Technologies, Palo Alto, CA, USA) contain-

ing Li, Y, Ce and Tl in 30% methanol (for speciation analysis),and in 2% HNO 3 (for total mercury analysis). The timeresolved analysis (TRA) mode was engaged for speciationanalyses, and the spectrum mode was engaged for total mer-cury analyses. ICP-MS operating conditions for total mercuryand speciation measurements are given in Table 1.

A Direct Mercury Analyzer-80 (DMA-80; Milestone, Mon-roe, CT, USA) was used in this study to determine the totalmercury content in each of the extraction and puricationsteps. The operation for the DMA-80 used throughout thiswork was based on the guidelines provided in EPA Method7473 protocol. 44,45 The DMA-80 operates on the basis of thermal decomposition, catalytic reduction, amalgamation,desorption and atomic absorption spectrometry. Prior to

decomposition, the sample is initially dried in an oxygenstream passing through a quartz tube located inside a con-trolled heating coil. The combustion gases are further decom-posed on a catalytic column at 750 1 C and mercury vapour is

collected on a gold amalgamation trap and subsequentlydesorbed for quantitation. Mercury content is determinedusing atomic absorption spectrometry at 254 nm.

Preparation of SCF column, preconcentration and mercury

species separationThe SCF column was prepared in a 5 ml disposable hypoder-mic syringe (Aldrich, Milwaukee, WI, USA). A PTFE frit(Supelco, Bellefonte, PA, USA) was added at the bottom of the syringe, and then a 0.2 g portion of SCF along with 3 mlDDI water was added. A second frit was placed on top of thewater and the syringe was shaken for 23 min to get ahomogenized mixture. Pressure was applied on the top frit tocompact the SCF into a homogeneous disk between the twofrits. The SCF columns were conditioned just before applica-tion by passing 10 ml DDI water, then 10 ml 6 mol l 1 HCland nally 15 ml DDI water with a ow rate of not more than1 ml min 1 .

The pH values of the extracts were adjusted to 6 1 with 10mol l 1 NaOH and they were ltered through a 1 mm lter toretain particles larger than 1 mm. The retained particles wererinsed with 0.1 mol l 1 HCl. The ltered solutions and therinsed solutions were combined. 1 ml of 0.2 mol l 1 acetatebuffer was added to each solution and the pH was re-adjustedto 34 by adding 6 mol l 1 HCl. The solutions were then passedthrough the conditioned SCF column at a ow rate r 1 mlmin 1 . Both mercury species were retained in the SCF medium.

Methylmercury was eluted from the SCF medium by passing8 ml of SCF Eluent 1 through the medium followed by 2 ml of DDI water at a ow rate r 1 ml min 1 . The inorganic mercuryis still retained in the SCF medium.

Inorganic mercury was then eluted from the SCF medium bypassing 8 ml of SCF Eluent 2 through the medium followed

by 2 ml DDI water at a ow rate r 1 ml min1.

Optimized analytical procedure

Samples of approximately 1.0 g of homogenized soil or sedi-ment and 10 ml of 4 mol l 1 HNO 3 were placed in themicrowave extraction vessels. A magnetic stirrer bar was addedto each vessel for thorough mixing of solvent with the sample.Microwave vessels were sealed and irradiated at 100 1 C for10 min with magnetic stirring taking place. A 2 min rampingtime was used to reach the desired temperature of 100 1 C. Aftermicrowave irradiation, the vessels were cooled to room tem-perature, ltered through a 0.22 mm glass ber lter (FisherScientic, Pittsburgh, PA, USA) and stored in the cold room

until analyzed (usually less than 2 d). Blanks were preparedalong with the samples in each batch.To evaluate the stability of mercury species in the microwave

eld, 10 ml nitric acid solutions at different concentrations

Table 1 Operating conditions for ICP-MS

Plasma conditions

Radiofrequency power/W 1450Plasma gas ow rate/l min 1 15.0Auxiliary gas ow rate/l min 1 1.0Measurement parametersAcquisition mode Spectrum a , Time resolved

analysis (TRA) b

Isotope monitored 199

Hg, 200

Hg, 201

Hg, 202

HgNebulizer gas ow rate/l min 1 1.0 a , 0.93 b

Liquid sample ow rate/ml min 1 1.0 ab

Integration time per point/s 0.5Total analysis time/s 65 a , 450 b

Replicates 5 a , 1b

a Total Hg analysis. b Hg speciation analysis.

J . A n a l . A t . S p e c t r o m . , 2 0 0 5 , 2 0 , 1 8 3 1 9 1 185

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were spiked with 100 ml of Hg 21 standard (100 mg Hg 2

1 ml 1

standard) and 100 ml of CH 3Hg1 standard (100 mg CH 3 Hg

1

ml 1 standard) and irradiated at different temperatures and fordifferent irradiation times. The topsoil was spiked with thesame concentrations of inorganic mercury and methylmercury;and the SRM 2711 was spiked only with methylmercury, towhich the extraction solvent was added in order to optimize themicrowave assisted extraction procedure. The samples werethen irradiated by varying both irradiation time and irradia-tion temperature.

Sample preparation for interconversion study

About 1 g of sediment or soil samples along with magneticstirrer bar was placed into microwave vessels. Samples weredouble spiked with known amounts of dilute aqueous solutionsof isotopically enriched CH 3

201 Hg 1 and 199 Hg 2 1 standard asSIDMS internal standard and artifact formation controller, insuch a way that the desired isotope ratio became approxi-mately 1 : 1, and left for 1 h for equilibration. The exactamounts of isotopic spike depend on the levels of inorganicmercury and methylmercury present in the samples and thespike concentration. 10 ml of 4.0 mol l 1 HNO 3 was added in

each vessel and extracted according to the MAE proceduredescribed in Optimized analytical procedure.

Results and discussionStability of mercury species under microwave irradiation

The effects of nitric acid concentration on the stability of mercury species was studied at different HNO 3 concentrations(0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mol l 1 HNO 3 ) at50 1 C for 5 min. A mixed mercury standard (100 mg ml 1) wasused for spiking. 10 ml of extraction solvent for each concen-tration level was measured and dispensed into a series of microwave vessels. 100 ml of the mixed standard was spikedinto each of the vessels and a magnetic stirring bar was added.The samples were irradiated for 5 min and analyzed withHPLC-ICP-MS. It was found that both mercury species werestable at that temperature for 5 min up to 4.0 mol l 1 HNO 3concentrations (Table 2).

The temperature effect on the stability of mercury species wasstudied at different irradiation temperatures (30, 40, 50, 60, 70,80, 90 and 100 1 C) using 4.0 mol l 1 HNO 3 (spiked with 100 mlof mixed mercury standard) for 5 min and analyzed withHPLC-ICP-MS. It was found that both mercury species werestable within this temperature range (Table 2).

The irradiation time effect on the stability of mercury specieswas also studied at different irradiation times (2, 4, 6, 8, 10, 12and 14 min) using 4.0 mol l 1 HNO 3 and irradiated at 100

1 C.The solvent was also spiked with the same concentration of

mixed mercury standard. It was found that both mercuryspecies were stable within the time range studied (Table 2).

After careful evaluation of all data sets (Table 2), it wasconcluded that mercury species are stable in 4 mol l 1 nitricacid at temperatures up to 100 1 C and for at least 14 min of microwave irradiation. Results may vary due to temperatureand time effects, or show different trends with a higher con-centration of nitric acid. During this study, only the extractionsolvent was spiked with mixed mercury standard. Results mayalso vary or show different trends with soil or sedimentsamples. Therefore, the next step was to use soil and sedimentsamples or SRMs to develop a methodology for the micro-wave-assisted extraction of mercury species.

Optimization of HNO 3 concentration

The nitric acid concentration effects on the extraction efficiencyand stability of mercury species in soils and SRM 2711 werestudied. Approximately 0.4 g of each soil sample and SRM2711 were weighed directly in the microwave vessel. Sampleswere left for 1 h to equilibrate, and then 10 ml of extractionsolvent (1.0, 2.5, 4.0, 5.5 and 7.0 mol l 1 HNO 3) was addedinto the microwave vessel. The sample was then extracted at

50 1

C with the following microwave procedure.Step 1: Time 2 min (Ramping to 50 1 C); Temperature 50 1 C; Power 1000 W.

Step 2: Time 5 min (Hold at 50 1 C); Temperature 50 1 C;Power 1000 W.

Note: Automated feedback control was engaged for bothprotocol steps; Venting Time 3 min.

After each extraction cycle was completed, the samples werecooled to room temperature and ltered through 0.22 mm glassber lter and stored in the cold room at 4 1 C until analyzed.The extracts were analyzed with the DMA-80 and the ICP-MSfor total mercury concentration, by direct aspiration in spec-trum mode, and with the HPLC-ICP-MS for inorganic mer-cury and methylmercury concentrations. In order to validatethe quantication ability of the three individual detectiontechniques, the total mercury values for the HPLC-ICP-MStechnique were determined from the summation of inorganicmercury and methylmercury concentration values. Resultsfrom different analysis methods were statistically indistinguish-able at 95% CL (Table 3). It was found that 95 7% to 1078% of total mercury is extractable from spiked topsoil using1.0 to 7.0 mol l 1 HNO 3 (DMA-80). On the other hand, theSRM 2711 extraction efficiency is highly dependent on theconcentration of the solvent used. The extraction efficiencyincreases from 54 8% to 92 5% by increasing the nitricacid concentration from 1.0 mol l 1 to 7.0 mol l 1 (DMA-80).It is evident that the sample matrix inuences the extractionefficiency. In the case of topsoil, the spikes were freshly addedand had very limited time to interact physically and/or chemi-

Table 2 Effects of microwave irradiation on mercury species: the results are expressed in percent recovery

HNO 3 concentration effectsa

(at 50 1 C for 5 min)Temperature effects a

(4.0 mol l 1 HNO 3 and 5 min)Time effects a

(4.0 mol l 1 HNO 3 at 100 1 C)

(mol l 1) Hg 21

CH 3Hg1

(1 C) Hg 21

CH 3Hg1

(min) Hg 21

CH 3Hg1

0.0 101 4 96 4 30 93 6 93 13 2 93 12 96 170.5 97 4 97 4 40 88 6 106 8 4 100 14 103 131.0 90 5 94 8 50 92 14 98 7 6 97 5 107 91.5 90 8 93 11 60 96 11 101 7 8 100 2 91 52.0 92 8 99 8 70 96 14 96 13 10 89 15 100 62.5 91 8 89 8 80 101 10 103 8 12 89 14 99 33.0 93 6 89 7 90 97 4 103 4 14 91 15 89 8

3.5 89 3 95 2 100 103 9 99 84.0 97 6 97 8a Uncertainties are expressed at 95% CL, n 3.


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cally with soil particles, and were easy to extract with solventsat different concentrations. On the other hand, SRM 2711 is anatural soil and inorganic mercury is naturally tightly boundwith the soil particles. Therefore, it was difficult to extract with

less concentrated extraction solvents. However, from the spe-ciation data for concentration effect on extraction efficiencyand stability of mercury species (Table 3), it is observed thatthe percent recovery for inorganic mercury increased from 101

7% at 1.0 mol l 1 HNO 3 to 128 5% at 7.0 mol l1 HNO 3

for spiked topsoil and from 44 8% at 1.0 mol l 1 HNO 3 to147 19% at 7.0 mol l 1 HNO 3 for SRM 2711 (spiked). Onthe other hand, the percent recovery for methylmercury in-creased from 91 6% at 1.0 mol l 1 HNO 3 to 105 5% at 4.0mol l 1 HNO 3 for spiked topsoil and then gradually decreasedto 69 3% at 7.0 mol l 1 HNO 3 . For SRM 2711 (spiked withmethylmercury), the trend was different. In this case thepercent recovery of methylmercury was almost stable up to4.0 mol l 1 HNO 3 and then decreased gradually to 47 17%

at 7.0 mol l1

HNO 3 . The degradation of methylmercury leadsto decreasing recoveries for methylmercury and apparentincreasing recoveries for inorganic mercury. As the methylmer-cury was stable up to 4.0 mol l 1 HNO 3 and degraded at higherconcentrations for both evaluated materials, the 4.0 mol l 1

HNO 3 was used as optimized extraction solvent concentrationthroughout the study.

Optimization of sample weight

The effect of sample weight on the extraction efficiency wasstudied using the same topsoil and SRM 2711. Differentamounts (0.25, 0.50, 0.75, 1.00 and 2.00 g) of topsoil andSRM 2711 were weighed and processed as described in the

Experimental section. Extracts were analyzed using threedifferent instruments. The results are summarized in Table 4and are statistically indistinguishable at 95% CL. From Table4, the total mercury results obtained from spiked topsoil, it is

found that the recovery for 0.25 g of sample was 108 5% andin the range of 90 4% to 97 4% for all the other sampleamounts studied (DMA-80): statistically, there was no signi-cant difference between the last set of results. But, on the otherhand, the recovery from SRM 2711 (spiked with methylmer-cury) was 65 4% for 0.25 g and 59 4% for 0.50 g, whichare statistically indistinguishable at their 95% CL. The percentrecoveries for all the other sample amounts studied were in therange of 51 2% to 57 3%. The speciation data (Table 4)indicates that the sample weight has very little or no effect onthe extraction efficiency at 50 1 C. In case of spiked topsoil, thepercent recovery for inorganic mercury was from 91 3% to102 7% and for methylmercury was from 97 3% to 1067%. But in SRM 2711, the recovery of inorganic mercury was

poor (from 33 2 to 47 3%). On the other hand, therecovery of spiked methylmercury for SRM 2711 was from73 1 to 92 5% and was stable. The robustness of theextraction method is demonstrated in this study by optimizingthe sample amount over one order of magnitude. For the entiresample range tested, statistically identical recoveries wereobtained from 0.25 g to a 2.0 g aliquot of sample. An inter-mediate 1.00 g sample size was used as optimized sampleamount during rest of the evaluations.

Optimization of irradiation temperature

The effect of irradiation temperature on the extraction efficiencyand stability of the mercury species was studied using spiked

Table 3 Percent recovery results for optimization of HNO 3 concentration

HPLC-ICP-MS a (%)

SampleHNO 3 concentration(mol l 1)

DMA-80 a

(total Hg) (%)ICP-MS a

(total Hg) (%) Hg 2 1 CH 3Hg1 Total Hg b

Topsoil 1.0 95 7 98 2 101 7 91 6 96 5(Spiked) 2.5 105 2 101 3 106 1 107 11 106 6

4.0 103 2 95 2 103 1 105 5 104 35.5 100 2 94 2 111 7 87 17 99 97.0 107 8 102 2 128 5 69 3 99 3

SRM 2711 1.0 54 8 57 2 44 8 81 3 63 4(Spiked) 2.5 57 5 61 2 51 7 77 9 64 6

4.0 70 1 68 3 74 8 83 5 79 55.5 74 2 77 2 103 7 59 8 81 57.0 92 5 88 1 147 19 47 17 97 13

a Uncertainties are reported as 95% CL with n 4 (no. of individual sample). b Total Hg average of Hg 2 1 and CH 3Hg1 values from HPLC-ICP-

MS.

Table 4 Percent recovery results for optimization of sample weight

HPLC-ICP-MS a (%)

Sample Sample weight/gDMA-80 a



Topsoil 0.25 108 5 103 1 102 7 97 3 100 4(Spiked) 0.50 97 4 98 1 100 8 106 5 103 5

0.75 92 6 96 1 91 3 98 1 95 21.00 90 4 94 1 92 9 106 7 99 62.00 94 9 98 1 100 9 106 4 103 5

SRM 2711 0.25 65 4 61 1 47 3 74 1 61 2(Spiked) 0.50 59 4 57 1 34 4 85 2 60 2

0.75 57 3 60 1 34 1 92 5 63 3

1.00 51 2 49 1 33 2 73 1 53 12.00 55 2 57 1 34 2 87 2 61 1a Uncertainties are reported as 95% CL with n 4. b Total Hg average of Hg 2 1 and CH 3Hg

1 values from HPLC-ICP-MS.


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topsoil and SRM 2711 (spiked with methylmercury) at differ-ent temperatures (50, 60, 70, 80, 90, 100, 110, 120 and 130 1 C).Samples were processed as described in the Experimentalsection. The extracts were analyzed with three different instru-ments and results were statistically indistinguishable at 95%CL (Table 5). It was found from Table 5 that the recoveries of the total mercury in spiked topsoil were from 90 4% at 50 1 Cto 105 2% at 130 1 C, and are statistically indistinguishable at95% CL. On the other hand, the recoveries for total mercury inSRM 2711 (spiked with methylmercury) increased from 51 2% at 50 1 C to 97 4% at 130 1 C (DMA-80). The speciationdata are shown in Table 5. It was found that the recoveryresults for inorganic mercury in spiked topsoil were from 929% at 50 1 C to 98 10% at 130 1 C, and for methylmercurywere from 106 7% at 50 1 C to 96 4% at 130 1 C, and arestatistically indistinguishable at 95% CL throughout the tem-perature range studied. However, the extraction efficiency of inorganic mercury in SRM 2711 increased from 33 2% at50 1 C to 116 10% at 130 1 C. The recovery for methylmercuryin SRM 2711 (spiked with methylmercury) also increased from73 1% at 50 1 C to 102 7% at 100 1 C, then decreased to 79

11% at 130 1 C. The simultaneously increasing recoveries of Hg 2 1 suggest a degradation of methylmercury at higher tem-perature. As the recovery for both inorganic mercury andmethylmercury was 98 4% and 102 7%, respectively, at100 1 C, this temperature was used as optimum throughout thestudy.

Optimization of irradiation time

The effect of irradiation time on the extraction efficiency andstability of mercury species was studied at different irradiationtimes (5, 10, 15, 20, 25 and 30 min). Samples were processed asdescribed in the Experimental section. The extracts wereanalyzed with same three instruments. The results are summar-ized in Table 6 and are statistically indistinguishable at their95% CL. From the nal mercury results, the recovery of totalmercury in spiked topsoil was from 103 6% at 5 min to107 2% at 30 min, and in SRM 2711 (spiked with methyl-mercury) was from 93 7% at 5 min to 99 5% at 30 min(DMA-80) and were statistically indistinguishable at 95% CLthroughout the studied time periods. The speciation data for

Table 5 Percent recovery results for optimization of irradiation temperature

HPLC-ICP-MS a (%)

SampleIrradiationtemperature/ 1 C

DMA-80 a



Topsoil 50 90 4 94 1 92 9 106 7 99 6(Spiked) 60 98 2 99 2 91 10 98 2 95 5

70 96 1 101 2 91 9 102 4 97 580 97 1 95 2 88 6 99 9 94 590 103 5 97 1 94 8 105 8 100 6100 103 6 97 2 95 2 101 8 98 4110 103 2 101 1 94 4 99 5 97 3120 103 6 100 1 102 10 98 9 100 7130 105 2 97 1 98 10 96 4 97 5

SRM 2711 50 51 2 49 1 33 2 73 1 53 1(Spiked) 60 57 1 56 1 31 3 80 8 56 4

70 64 1 66 2 48 1 95 6 72 380 71 2 72 1 69 5 95 7 82 490 82 4 78 2 76 8 107 9 92 6100 93 7 91 2 98 5 102 7 100 4110 94 7 96 1 109 6 96 7 103 5120 92 4 97 1 112 5 70 7 91 4130 97 4 99 1 116 10 79 11 98 7

a

Uncertainties are reported as 95% CL with n

4. b

Total Hg

average of Hg2 1

and CH 3Hg1

values from HPLC-ICP-MS.

Table 6 Percent recovery results for optimization of irradiation time

HPLC-ICP-MSa

SampleIrradiationtime/min

DMA-80 a



Topsoil 5 103 6 97 2 95 2 101 8 98 4(Spiked) 10 107 7 98 3 91 9 94 8 93 6

15 106 8 102 1 92 9 93 9 93 620 105 5 104 2 94 4 95 11 95 625 104 6 106 2 91 10 101 5 96 630 107 2 104 2 91 10 95 3 93 5

SRM 2711(Spiked)

5 93 7 91 2 98 5 102 7 100 4

10 91 5 99 1 100 9 97 10 99 715 94 5 97 5 93 10 94 4 94 520 95 4 102 2 93 8 93 13 93 8

25 93 5 100 5 94 10 90 11 92 730 99 5 100 2 103 9 88 8 96 6a Uncertainties are reported as 95% CL with n 4. b Total Hg average of Hg 2 1 and CH 3Hg

1 values from HPLC-ICP-MS.


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both spiked blank soil and SRM 2711 (spiked with methylmer-cury) are shown in Table 6. From the speciation data, it wasalso found that the recovery results for inorganic mercury wasfrom 95 2% at 5 min to 91 10% at 30 min, and formethylmercury was from 101 8% at 5 min to 95 3% at 30min in the spiked topsoil with no distinguishable degradationof methylmercury during the studied time period. On the otherhand, the recovery of inorganic mercury was from 98 5% at5 min to 103 9% at 30 min, and for methylmercury was from102 7% at 5 min to 88 8% at 30 min in SRM 2711 (spikedwith methylmercury) and results were stable up to 25 min (9011%), after which degradation of methylmercury is suspected.As a result, the recovery of inorganic mercury increased andmethylmercury recovery decreased, although at 95% CL, thesechanges in recovery were not statistically distinguishable. Inorder to shorten the sample preparation time, it was decided touse 10 min as the optimum time for extraction.

The venting time used throughout this study was 3 min. Thecooling rate of the vessels depends on the make, model and type of both the microwave and the vessels used. Therefore, the recom-mended venting or cooling time used may be more than 3 min.

Validation of the developed and optimized method usingreference soil and SRMs

The microwave-assisted extraction method was validated byusing two different sets of reference soil samples (Lot # 0611-01-02), prepared by Environmental Resource Associates s forSAIC and the United States Environmental Protection Agency(US EPA) from 100% processed topsoil and a mixture of 75%topsoil and 25% Ottawa sand. The preparation of thesereference soil samples is described elsewhere. 46 Both of thesematerials contain Hg 2 1 (inorganic mercury), CH 3Hg

1 (organicmercury) and an equal mixture of Hg 2 1 and CH 3Hg

1 (mixedmercury).

SRM 1941a, SRM 2704, SRM 2709 (certied for totalmercury), and BCR 580 (certied for both total mercury and

methylmercury) were used for method validation. All of thereference soil samples and NIST SRMs were analyzed directly(as solid, without any sample preparation) with the DMA-80using EPA Method 7473 protocol before extraction by themicrowave-assisted extraction method. Results are summar-ized in Table 7. The results obtained for different soil samplesand SRMs from the direct mercury analyses, except for organicmercury in Material-1, are indistinguishable from their corre-sponding certied-value at 95% CL. Samples and NIST SRMswere then processed according to the optimized MAE proce-dure described in the Experimental section. The extracts wereanalyzed with DMA-80 and ICP-MS for total mercury and

with HPLC-ICP-MS for mercury speciation and quantica-tion. As the results obtained from different instrumentalanalyses overlapped at 95% CL and were statistically indis-tinguishable, only the speciation results for different samplesare summarized in Table 7. The results demonstrated that goodrecoveries of methylmercury and inorganic mercury wereachieved for all the studied materials and were in excellentagreement with the certied or known values at 95% CL. The100% recoveries of both mercury species indicates no conver-sion of species during extraction.

The IRMM reference material BCR 580 was also processedaccording to the MAE procedure described in the Experimen-tal section, and extracts were analyzed with DMA-80 and ICP-MS for total mercury measurement (Table 8). Although theamount of methylmercury present in BCR 580 was higher thanthe HPLC-ICP-MS detection limit (5 ng g 1 ), it could not beanalyzed directly using this instrument for speciation. The ratioof methylmercury to inorganic mercury in BCR 580 is 1 :1760. When the HPLC-ICP-MS was used for speciation, themethylmercury peak was overlapped with the inorganic mer-cury peak and could not be quantied. This reveals thelimitations of the HPLC separation technique for extractscontaining analytes at a very high concentration differencewith close retention times. This would not happen if a GC-MSor a GC-ICP-MS were used for speciation and quantication.Therefore, the extracts were speciated rst using SCF SPEmethodology (refer to Preparation of SCF column, preconcen-tration, and mercury species separation), and then analyzedand quantied with HPLC-ICP-MS. The results are summar-ized in Table 8. The amount of methylmercury and inorganicmercury determined by the SCF-SPE-HPLC-ICP-MS proce-dure is 0.073 0.002 mg g 1 and 133 6 mg g 1 , which resultsare in excellent agreement with their corresponding certiedvalues at 95% CL.

From the speciation results, it is observed that the MAEmethod is highly efficient in extracting different mercury spe-cies from the variety of matrices tested during this study with99100% recovery.

Application of EPA Method 6800 in the validation of the currentextraction method under study

EPA Method 6800 38 was applied as a diagnostic tool toidentify analytical biases and species transformations in thedeveloped microwave-assisted extraction (MAE) method.SIDMS was applied as an alternative detection method toidentify the steps that might alter species distribution in theMAE method protocol. Interconversions that occur afterspiking are traceable and can be quantitatively corrected by

Table 7 Comparision of different analysis methods for the validation of the microwave-assisted extraction results. The results are expressed inmg g 1 at 95% CL, n 3

HPLC-ICP-MS

SampleCertied/knownvalue/ mg g 1

Method 7473(direct analysis)/ mg g 1 Hg 2 1 /mg g 1 CH 3Hg

1 /mg g 1 Total Hg/ mg g 1

Material-1Inorganic mercury 4.0 4.08 0.16 4.26 0.17 ND a 4.26 0.17Organic mercury 4.0 3.58 0.27 ND 3.81 0.20 3.81 0.20Mixed mercury 3.0 3.0 5.73 0.58 3.02 0.06 2.66 0.07 5.68 0.09Material-2Inorganic mercury 6.0 6.73 1.04 6.06 0.56 ND 6.06 0.56Organic mercury 6.0 5.44 0.62 ND 5.94 0.52 5.94 0.52Standard Reference MaterialsSRM 1941a 0.5 0.2 0.61 0.02 0.67 0.06 ND 0.67 0.06

SRM 2704 1.44 0.07 1.51 0.05 1.40 0.08 ND 1.40 0.08SRM 2709 1.40 0.08 1.46 0.03 1.38 0.12 ND 1.38 0.12a ND not detectable (lowest measurable mercury 0.5 ng ml 1 and this corresponds to 5 ng g 1 in soil or sediment sample).


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monitoring isotopes in each species. 47 As SIDMS can measurethe concentration of species at the time of spiking, one set of samples was double spiked before extraction and processed asdescribed in the Experimental section. Another set of sampleswas extracted rst according to the MAE procedure and thendouble spiked and stored in a cold room until analyzed withHPLC-ICP-MS. The reference soil sample (Material-1) con-taining mixed mercury was used in this study for SIDMSanalysis. SIDMS calculations for species transformation cor-rections 30 were performed; the results are summarized in Table9. It was observed that the deconvoluted concentrations foreach species obtained from both sets of extractions overlap atthe 95% CL and are statistically indistinguishable. Also, resultsfor both species obtained from SIDMS calculations agree withthe result obtained from method validation. Moreover, there isno statistically signicant distinguishable interconversion usingthe developed MAE method. Correction of conversions, whenthey occurred, was accomplished using EPA Method 6800 anddid not alter the accuracy of the analysis. EPA describes thisdiagnostic and quantitative method as being a legally denitivemethod for such active species. Each species transformationcan be tracked and corrected through this procedure. In thisstudy, this was used to monitor and correct for specic proto-col steps and was found to provide the quality assurance thatwas necessary to evaluate the method under study.

ConclusionsA simple, fast and efficient closed vessel microwave-assistedextraction method for sample preparation and mercury specia-tion in soils and sediments has been developed in which, afterextraction with 4.0 mol l 1 HNO 3 , inorganic and methylmer-cury concentrations were determined by DMA-80, ICP-MSand HPLC-ICP-MS techniques. The optimum conditions formicrowave-assisted extraction of mercury species from soilsand sediments were found to be 1.0 g sample, 10 ml of 4.0 moll 1 HNO 3 and an irradiation time of 10 min at a temperatureof 100 1 C. The recoveries from the matrices analyzed weresimilar and quantitative. The proposed microwave-assisted

extraction method offers the following advantages: (1) a no-table reduction of solvent volume; (2) higher efficiency of extraction achievable under optimized conditions; (3) consid-erable time savings in the procedure of sample preparation; (4)

no statistically signicant distinguishable interconversionswere found for the materials studied; and (5) the possibilityof simultaneously extracting up to ten samples, resulting inincreased sample output compared with conventional extrac-tion techniques. Since the extracts are analyzed with HPLC-ICP-MS for speciation, there is no need for additional steps,such as clean-up or derivatization. However, the HPLC-ICP-MS technique has the limitation that it requires separationand/or preconcentration of mercury species ( e.g. SCF SPE) if methylmercury is to be analyzed in sediment or soils and/or if species concentrations differ by an order of magnitude orgreater. Results obtained in the analyses of two types of specically prepared reference soils and four standard refer-ence materials (soils and sediments) containing inorganic mer-cury and methylmercury in four order of magnitude rangeveried the simplicity, efficiency, precision and accuracy of theproposed microwave-assisted extraction method for mercuryspeciation in soils and sediments. Moreover, the application of the EPA Method 6800 as a diagnostic tool signicantly en-hances the reliability of the proposed microwave-assisted ex-traction method. The MAE method has been chosen by the USEPA as the primary extraction protocol for mercury speciesextraction in EPA draft Method 3200. 48

AcknowledgementsThe authors thank Science Applications International Cor-poration (SAIC), United States Environmental ProtectionAgency (US EPA) for funding and nancial support, as wellas Milestone Inc., Agilent Technologies and Duquesne Uni-versity for instrument and material support. Portions of thispaper methodology are patented and/or patent pending.

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Table 8 Extraction of mercury species from BCR 580

Mercuryspecies

Certiedvalue/ mg g 1

DMA-80/mg g 1

ICP-MS/mg g 1

HPLC-ICP-MS/ mg g 1

Hg 21

NR a NA NA 133 6CH 3Hg

1 0.075 0.004 NA NA 0.073 0.002Total Hg 132 3 131 4 133 4 133 6a NR not reported. NA not applicable.

Table 9 The deconvoluted concentration and transformation of mercury species in Material-1 using SIDMS calculations. Uncertaintiesare expressed at 95% CL with n 3

Deconvoluted concentration Interconversion

Hg 21

/mg g 1

CH 3Hg1

/mg g 1

Hg 21

toCH 3Hg

1 (%)CH 3Hg

1to

Hg 2 1 (%)

DSBE a 3.05 0.12 2.69 0.10 1.3 1.5 0.1 1.4DSAE b 2.94 0.07 2.62 0.09 0.8 1.5 0.7 0.6a Double spiked before extraction. b Double spiked after extraction.


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