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Analytica Chimica Acta 679 (2010) 7–16

Contents lists available at ScienceDirect

Analytica Chimica Acta

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low discharge analysis of nanostructured materials and nanolayers—A review

eatriz Fernández, Rosario Pereiro, Alfredo Sanz-Medel ∗

epartment of Physical and Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Julian Clavería 8, 33006 Oviedo, Spain

r t i c l e i n f o

rticle history:eceived 6 July 2010eceived in revised form 20 August 2010ccepted 24 August 2010vailable online 23 September 2010

a b s t r a c t

Advances in instrumentation and the parallel development of proper analytical methodologies havefuelled an extraordinary growth of analytical applications with glow discharge (GD) techniques. Infact, GDs with detection by optical emission spectrometry (OES) and mass spectrometry (MS) havebecome today, fast, comparatively simple, and reliable tools for materials analytical characterizationat the nanoscale.

A critical description of latest advances and presently available GD-OES and GD-MS instrumentation

eywords:anostructured materialshin filmslow dischargeptical emission spectrometryass spectrometry

(commercial, prototype and laboratory equipments) is carried out here. Analytical strategies developedfor the analysis at the surface and for concentration depth profile analysis of thin and ultrathin layers withGDs are also discussed. Finally, selected representative applications and trends of GD-OES and GD-MStechniques for the nanometer range analysis (e.g. nanolayers, two-dimension nanostructured materialsand molecular depth profiling of polymer-based coatings) are briefly described, confirming the increasing

S and D-MS techniques in the nanotechnology field.© 2010 Elsevier B.V. All rights reserved.

Alfredo Sanz-Medel has been Professor in the Depart-ment of Physical and Analytical Chemistry of OviedoUniversity (Spain) since 1982. He is author or coauthorof around 500 scientific publications in interna-tional journals, several patents and books. His presentresearch interests include new atomic detectors andion sources for ultratrace analysis using plasmas, newmolecular optical sensors particularly those based onthe use of quantum dots, as well as hybrid techniques,coupling a separation unit and an atomic detector,for ultratrace and trace metal speciation to solve bio-logical and environmental problems and speciationfor proteomics, integrating mass spectrometry (MS)

“molecular” (matrix-assisted laser desorption/ionization and electrospray MSn)and “atomic” [inductively coupled plasma (ICP)MS] techniques and introducingthe extensive use of ICP-MS to carry out “heteroatom-tagged proteomics”, bothfor qualitative and quantitative purposes. He has been an editor of Analyticaland Bioanalytical Chemistry since 2002. At Euroanalysis 2007 in Antwerp, hereceived the Robert Kellner Award.

analytical value of GD-OE

Beatriz Fernández was born in 1978 in Oviedo(Asturias, Spain). In 2002 she joined the “AnalyticalSpectrometry” research group of Prof. A. Sanz-Medelat the Dept. of Physical and Analytical Chemistryof the University of Oviedo. In 2006 she obtainedher PhD in Analytical Chemistry (European degree)under the supervision of Dr. Rosario Pereiro and Dr.Nerea Bordel working on the development of quan-tification methodologies based on Glow DischargeSpectrometry. During that time she made severalshort stages in Switzerland at the Swiss Federal Lab-oratories for Materials Testing and Research (EMPA)with Dr. Johann Michler and in USA at Clemson Uni-

versity with Prof. Kenneth Marcus. Her PhD was awarded with ExtraordinaryDoctorate Award (Analytical Chemistry) of the University of Oviedo and withthe 2007 award from the College of Chemists of Asturias and Leon. Afterwards,she spent two years at the IPREM (Multidisciplinary Institute of EnvironmentalScience and Materials) in Pau (France) headed by Prof. Olivier Donard workingwith Dr. Christophe Pecheyran in the field of Laser Ablation ICP-MS in combi-nation with isotope dilution analysis and other solid state techniques such asAtomic Force spectroscopy and XPS. Since September 2008, she has got a “Juande la Cierva” research contract at the University of Oviedo (Spain). She is co-author of more than 30 publications and 2 book chapter and her main scientificinterests are related to optical and mass spectrometry techniques for the direct

analysis of solid materials.

∗ Corresponding author. Tel.: +34 985 10 34 84; fax: +34 985 10 31 25.E-mail address: asm@uniovi.es (A. Sanz-Medel).

003-2670/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2010.08.031

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Rosario Pereiro is an Associate Professor at the Uni-versity of Oviedo (Spain) and she got the accreditationto become Full Professor in Analytical Chemistry. Hercurrent research interests include the developmentof analytical strategies for in-depth profile quanti-tative analysis of thin layers in advanced materialsusing optical emission and mass spectrometry as wellas the investigation of new strategies to improveselectivity and/or sensitivity in molecular lumines-cence by using nanostructured sensing phases (e.g.molecular imprinted polymer, quantum dots). She hasco-authored more than 100 scientific papers and sev-eral patents.

. Introduction

Progress in the development of advanced materials, elec-ronics, information storage devices and new analytical sensors,trongly depends on continued improvements in miniaturiza-ion and nanotechnology. Ultrathin layers (or nanolayers) and

ultilayer systems, which can be considered as one-dimensionanostructures (i.e., the thickness of the layer is between 0.1 and00 nm), are being used in a wide range of applications such asard protective coatings for mechanical tools, optical coatings for

enses and filters, architectural glass panels, and barrier contactsor microelectronics, among many others. Nanolayers are gain-ng increasing interest because they can exhibit important novelr improved physical and/or chemical properties, regarding thosef the corresponding bulk materials [1]. On the other hand, nan-tubes and nanowires are two-dimension nanostructures (e.g. theiameter is in the nanoscale, while its length can be much greater).etallic nanowires, for example, are among the most attractive

anometer-sized materials because of their unique properties thatay lead to applications as interconnectors in nanoelectronic, mag-

etic, chemical or biological sensors, and as biotechnological labels2,3].

Important properties of nanostructured materials depend onheir chemical composition and structure and, therefore, the use ofeliable characterization techniques is of critical importance todayo assist the optimization of the synthesis procedures as well as tossess their quality as a final product. Over the past twenty yearshe increasing interest in the field of nanomaterials has createdhe present need for novel or advanced analytical techniques capa-le of characterizing the new nanostructures and the fabricatedunctionalized nanostructured materials [4,5]. For such purpose,urface techniques with lateral and depth resolution capabilitiesn the nanometer range are increasingly demanded. Auger elec-ron spectroscopy, X-ray photoelectron spectroscopy, secondaryon mass spectrometry (SIMS), glow discharge (GD) with opticalmission (OES) or with mass spectrometry (MS), and laser ablationLA) – inductively coupled plasma (ICP) – MS, are most commonlysed for this purpose [6,7]. However, any individual technique has

ts specific advantages and limitations and, frequently, no one cane solely used for the characterization of all nanostructured mate-ials, being in many cases necessary to resort to a combination ofomplementary techniques (depending on the analytical problemhat needs to be solved).

By now, both GD-OES and GD-MS have demonstrated to beowerful and versatile techniques for depth profiling analysis ofifferent types of materials [8,9]. The GD ensures a high sputter-

ng rate due to the high flux of energetic species. Additionally,he species contributing to sputtering are of relative low energy<50 eV), resulting in a low penetration depth and limiting theurface damage to a very shallow layer of about 2 nm thick,hus allowing for high depth resolution [10]. Other important

ica Acta 679 (2010) 7–16

advantages of GD techniques include easy of use, excellent sen-sitivity, multielemental capability, low matrix effects (and, so,comparatively simple quantification procedures) and high samplethroughput. Moreover, GDs can be used both for the analysis of rel-atively thick films (in the tens of microns range) [11] and for thecharacterization of materials on the nanoscale [12,13].

In this article, the current status of GD plasma-based analyticaltechniques is critically reviewed. GD-OES and GD-MS instrumenta-tion (commercial, prototype and laboratory equipments) is revised,stressing their capabilities and pros and cons. Moreover, method-ologies developed for the analysis at the nanoscale with GDs arediscussed. Finally, representative applications and trends of GD-OES and GD-MS techniques for nanometer scale analysis (e.g.depth profiling analysis of thin films, two-dimension nanostruc-tured materials and polymer-based coatings) are described anddiscussed.

2. Instrumentation available for GD analysis: capabilitiesand limitations

Glow discharge analytical plasmas have gained importance asatomization, excitation and ionization sources, both for opticalemission and mass spectrometry, due to their capability to generateatoms and ions directly sputtered from the solid samples followingan atomic “layer by layer” approach [14,15]. GD traditional instru-ments incorporated a direct current (dc) power supply to get thebreakdown of the discharge gas and to provide the ion and elec-tric currents necessary for successful operation. This requires thesample, normally the cathode, to be a conducting material or atleast capable of incorporation into some conducting matrix (in thislatter case “spatial information” is lost). More recently, radiofre-quency (rf) discharges have attracted great interest because theycan be directly applied not only to electrically conductive samples,but also to those that are non-conductive, whether bulk or layeredmaterials. These advances widely broadened the applicability of GDsources [16].

The interest in pulsed GDs, on the other hand, has steadilyincreased since the publication of the initial works [17–19]. PulsedGDs offer certain advantages with respect to classical steady-stateoperation, such as an additional way of controlling the plasma byselecting the pulse parameters (e.g. pulse and period lengths); forexample, the instantaneous power (responsible for the sputtering,excitation and ionisation yields) can be chosen nearly independentfrom the average power (responsible for thermal stress on the sam-ples) by just varying the duty cycle of the applied pulses. Moreover,different discharge processes take place at different times withina single pulse and this allows selective measurements if a timeresolved acquisition detector is available, making possible to obtainquasi-simultaneous structural, molecular and elemental informa-tion of the analysed sample [20,21].

Belenguer et al. [22] have recently reviewed important aspectsof pulsed GDs. To obtain optimum performance from pulsed dis-charge operation, two instrumentation features become crucial:first, as stated above, a gated detection system is required to per-mit temporal selection of the analytical signal, whose accumulationand averaging creates a high signal-to-noise ratio most favourablefor analytical measurements; second, an adequate programmablepower supply is necessary to ignite and sustain the discharge forthe desired period and of sufficient speed to facilitate a range ofpulse rates.

Although the essential difference between dc and rf pulsed dis-charges is that in pulsed dc the amplitude of the applied voltageis constant during each pulse (while in pulsed rf the amplitudevaries at radiofrequencies during each pulse, e.g. at 13.56 MHz therewould be 13.56 cycles in 1 �s) [23,24], most of the earlier work on

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ig. 1. Part of the A2�+–X2� system of OH, recorded with an epoxy-coated steelheet. A LECO GDS500A spectrometer was used; 4 mm source; 14 mA 1000 V. Notehat there is an imperfect overlap of two CCD detectors in the range 312–314 nm.ource: Ref. [25], with permission of Elsevier.

ulsed GDs have been focused on dc discharges. Studies on pulsed rfischarges are scarce, either using optical emission or mass spectro-etric detection. However, as presented in this review, increasing

fforts are being made during the last years to better understandhe pulsed discharges and develop pulsed rf-GDs. In fact, the pulsedf mode will offer a broader application field than the pulsed dcecause the thermal stress effect, caused by the steady-state oper-tion mode, is more serious in non conductors (which just can benalysed by rf glow discharges). In next sections, we will addresspecifically the latest important developments and instrumenta-ion in GD-OES and GD-MS.

.1. Glow discharge-optical emission spectrometry

GD-OES instruments mainly comprise a GD source, power sup-ly and associated components, one or more optical spectrometers,acuum and gas-handling systems, and electronics and softwareor control and data treatment. Nowadays, several manufactur-rs offer commercial GD-OES instruments, including Horiba Jobinvon (France), Leco Corporation (USA) and Spectruma AnalytikMBH (Germany). The competition among those manufacturingnd vending companies has led to important developments innstrumentation as well as to improved performance levels. Therst commercial rf instrument was the JY5000RF by Horiba Jobinvon, released in 1995. Since then, Leco and Spectruma Ana-

ytik GMBH have marketed several combined dc (steady-state andulsed mode) and rf instruments. More recently, pulsed rf sourcesave been also introduced in systems.

Spectrometers in GD-OES traditionally use polychromators andonochromators equipped with photomultiplier tubes (PMTs) for

ast and sensitive detection. However, the right choice of analyticalines for a given system is crucial and this is usually predeterminednd limited in their number by the instrument manufacturer. Inecent years, instruments using additionally or exclusively charge-oupled device (CCD) detectors became commercially availabley Leco and Spectruma Analytik GMBH. Recently, Bengtson [25]as reported a detailed study using CCD detectors to investigateow molecular emission can affect elemental analysis (of particular

mportance in the in-depth profiling analysis of polymer coatingsnd thin films) in GD-OES. Considering the broad shape of molecu-ar emission bands (see for example Fig. 1), it is readily understoodhat there will be spectral overlap with several atomic analyticalines (e.g. Zn and W atomic lines shown in Fig. 1). From the obser-ations so far, it appears that molecular emission is predominantly

bserved from molecules made up of the light elements H, C, N and(e.g. CO, OH, NH and CH). Thus, elevated background from sucholecular emission at atomic emission lines can easily be observed

n several GD-OES applications such as in very thin films applica-ions (adsorbed gases are frequently released from the GD source

ica Acta 679 (2010) 7–16 9

interior at the discharge ignition) and, of course, in those caseswhere the sample material includes one or several of these ele-ments as majors (polymers, oxides, hydroxides, etc.). Since mostGD-OES systems do not record spectra because they use polychro-mators with fixed wavelength channels, such interference is easilymistaken as real analytical signals. However, such artefacts haveto be identified in order to develop effective corrections to thebackground signals and, so, avoid analytical errors.

Apart from the possible light elements (H, C, N and O) presentas sample constituents, and as a result of the prevacuum technol-ogy required in GD instruments, serious contamination by waterand hydrocarbons can be found in GD sources. Such contaminantsdisturb a fast achievement of the needed balance in the plasmaand, moreover, as stated in the paragraph just above, the exper-imental results could be misinterpreted. Of course, the thinnerthe layers under examination, the more pronounced this negativeeffect. Therefore, an effective reduction of those impurities is an aimhighly pursued for accurate analysis of nanostructured materials[26]. Such disturbing influence of contamination on the GD anal-yses is generally reduced by adopting different strategies (whichcan be combined), including: (i) reduction of gas adsorption andincrease of gas desorption before every measurement (e.g. by veryquick sample change and venting with a clean noble gas at pres-sures slightly higher than the ambient atmospheric pressure andby evacuation with high pumping speed and an increase of thesource temperature, respectively); (ii) pre-sputtering with a pieceof monocrystalline silicon; the sputtering of sacrificial material isundertaken under conditions similar to those used for the analysisof the sample, being in this manner the inner surfaces of the anodecovered with this low-outgassing coating [27]; and (iii) use of a lowenergy plasma to allow for a soft cleaning of the specimen surfaceprior to the analysis [28], mainly removing contaminants from thesurface of the target material. Fig. 2a shows the GD-OES depth pro-file of electropolished aluminium obtained directly and Fig. 2b theprofile for the same sample previously using a soft plasma. As canbe observed, the plasma treatment allows for a significant reduc-tion of the carbon signal and a slight decrease in the hydrogen atthe beginning of the analysis. It should be stressed here the impor-tance of adjusting in an individual manner for specific specimensto prevent material damage. Recently, Hoffmann et al. [29] havereported another strategy based on improving the vacuum systemby optimising the pump line to get an evacuation pressure less than5 × 10−5 hPa. Therefore, at the beginning of the analysis one couldfind information on the state of the vacuum system, and thereby onthe reliability of the further analysis, and the instantaneous stateof the equipment.

Concerning pulsed GD sources, one of the limiting factors ofavailable commercial GD-OES instruments is the impossibilityto perform time-resolved studies of the pulse profile. Aside ofother advantages, the most notable characteristic of pulsed modeincludes the existence of “anomalies” in the temporal emissionprofiles for various transitions [30]. In particular, some analyte tran-sitions exhibit an emission maximum near the start of the dischargepulse (denoted as “prepeak” region), while others show a maxi-mum just after the discharge power is terminated (“afterglow” or“afterpeak”). Therefore, temporally gated separation and detectionof species found in these distinct plasma regimes is required toincrease the utility of pulsed GD devices. Currently, several researchlaboratories are using monochromators with the PMT connectedto a digital oscilloscope to perform time-gated detection measure-ments using pulsed GD-OES. Yan et al. [31] have monitored the

temporal behaviour of Ar and Cu emission lines in a microsecondpulsed dc-GD in order to obtain insight into the excitation andrecombination processes in the “afterglow” region for analytes andfill gases and reported that lines from low energy levels showedsmaller “afterpeak” than higher levels. Additionally, Nelis et al.

10 B. Fernández et al. / Analytica Chim

Fig. 2. GD-OES depth profile of electropolished aluminium obtained at 750 Pa and3ptp

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5 W. From Ref. [28], with permission of the Royal Society of Chemistry. (a) Withoutlasma cleaning. The presence of air-formed oxide film of about 2 nm thickness onhe surface of aluminium was distinguished in the profile. (b) After application oflasma cleaning (plasma cleaning conditions were: 750 Pa, 3 W, 1 min).

32,33] have performed time-resolved measurements of Cu emis-ion lines using pulsed rf-GDs. The effect of a pulsed rf power supplyn the sample sputtering and the subsequent excitation and/or ion-zation in the glow were investigated using different experimentalonditions (several pulse frequency, pulse width, and duty cycles).esults showed that the “prepeak” observed for resonance linesas linked to diffusion processes in the discharge chamber and

educed self-absorption at the beginning of the pulse. Moreover, itas concluded that a minimum of 100–150 �s off-time is needed

at 600 Pa) to remove in time sputtered material from the dischargeolume and, therefore, to warrant that most of the sputtered mate-ial can be measured with minimum self-absorption.

During the last decade, efforts in research laboratories aim-ng to improving analytical performances with GD-OES have beenocussed also in other directions. For example, although the anodemployed in GD sources is commonly made by copper, Efimovat al. [34] have recently investigated the influence of the anodeaterial on the GD performance in terms of crater shapes, sput-

ering rates and emission spectra. The use of anode materialsith different thermal conductivity (e.g. copper alloys, graphite

nd steel) showed a noticeable effect on GD spectra and slight

ifferences in sputtering rates (slightly lower for a bad thermalonducting anode). On the other hand, several approaches haveeen investigated using steady-state rf-GDs for the improved anal-sis of non-conducting samples. Deposition of thin gold layers onon-conductive glasses and alumina substrates have been widely

ica Acta 679 (2010) 7–16

investigated in terms of electrical signals, sputtering rates, ana-lytical emission intensities and emission yields as a function ofthe sample thickness and diameter [35–37]. The deposition of aconductive top layer on an insulator gave rise to an increase ofthe voltage transfer coefficient and, therefore, to an enhancementof the analytical emission signals related to non-conducting sub-strates. However, risk of contamination increases and extra samplepreparation time as well as additional laboratory equipment isrequired. Another way to improve the voltage transfer coefficientthrough a better power deposition into the discharge could bethe use of an external magnetic field. Alberts et al. [37] studiedthe effect of placing an external magnet between the rf sourceand the non-conducting substrate (alumina wafers of 0.5–10 mmthickness). Although higher spatial inhomogeneity in the sputteredcrater shapes could be obtained, higher sputtering rates togetherwith better ionization and excitation efficiencies were observed forall thicknesses and, as a consequence, the presence of a magneticfield could be presented as an attractive option to improve emissionintensities.

2.2. Glow discharge coupled to mass spectrometry detection

Glow discharge sources have been successfully coupled to dif-ferent types of mass spectrometers since the early 1970s, includingquadrupole, sector field (SF), and time of flight mass spectrome-ters (ToF) [15,38]. Although several commercial instruments wereavailable over the years (e.g. TS Sola quadrupole, Kratos, VG Glo-Quad and VG 9000) [39], it should be highlighted that the onlycurrent commercially available GD systems are based on GD-SFMS;Element GD by Thermo Fischer Scientific and, more recently, NuAstrum by Nu Instruments and AutoConcept GD90 from MassSpectrometry Instruments. These equipments provide low limitsof detection (≤ng g−1) and high mass resolving power (>3000),which overcomes many problems related to spectral interferences.However, the GD–SFMS systems have a sequential analyser, whichpresents some restrictions for the accurate and precise analysis offast transient signals (e.g. depth profiling analysis of thin films),being mainly focused their applications towards the elemental andisotopic analysis of conducting materials [40] (collaborative workbetween the Clemson University and Oak Ridge groups on couplingan rf source to a VG 9000 has allowed the direct analysis of insula-tors [41]). The implementation of pulsed GDs for routine analysiswith SFMS is very limited so far. Recently, Voronov et al. [42] havesuccessfully investigated the combination of a pulsed dc-GD withthe Element GD and the application for depth profile analysis wasdemonstrated [43]. Unfortunately, the facts that dc voltage wasused to powering the GD as well as the sequential nature of themass analyser, restrict its application field.

Alternatively, ToF mass spectrometers offer high mass spectraacquisition rate for quasi-simultaneous multi-elemental detectionof fast transient signals, making ToFMS especially suitable for depthprofiling applications and for its coupling to pulsed GD sources[44]. Since commercial GD-ToFMS instruments are not availableyet, several research laboratories have built their own equipmentfor different applications [45–49]. Recently, a new prototype ofrf-GD-ToFMS has been successfully developed within the frame-work of a European Project (STREP-NMP, No. 032202). Direct linksbetween research labs and manufacturers have made possible thedevelopment of an innovative rf-GD ion source, operating both insteady-state and pulsed rf modes, and a novel ToF mass spectrom-eter with high mass resolution capabilities [50]. The developed

rf-GD-ToFMS instrument is able to acquire complete mass spec-tra (1–300 m/z) every 33 �s, follow ultra-fast transient signalsand achieve good limits of detection (in the ng g−1 level), offer-ing rather adequate mass resolving power (>2000). Moreover, ithas been recently shown that good depth resolution [51,52] can

B. Fernández et al. / Analytica Chimica Acta 679 (2010) 7–16 11

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Table 1Isotopic ratios calculated in non-pulsed rf and pulsed rf modes. Source: Ref. [58] withpermission of the Royal Society of Chemistry.

Isotopic ratios

Theoreticalratio

Experimental ratiopulsed rf mode

Experimental rationon-pulsed rf mode

11B+/10B+ 4.03 4.3 6.112C+/13C+ 89.09 82.5 11.224Mg+/25Mg+ 7.90 8.0 –28Si+/29Si+ 19.75 20.8 –28Si+/30Si+ 29.75 28.3 –32S+/33S+ 126.69 142.7 58.732S+/34S+ 22.57 23.5 29.946Ti+/47Ti+ 1.09 1.3 0.948Ti+/46Ti+ 9.26 9.3 1.548Ti+/47Ti+ 10.10 12.3 1.390Zr+/91Zr+ 4.59 4.6 2.3120Sn+/118Sn+ 1.35 1.4 1.4118Sn+/119Sn+ 2.82 3.0 1.9120 + 119 +

GD-ToFMS, both in the positive and negative ion detection modes).

ig. 3. Qualitative depth profile obtained by pulsed rf-GD-ToFMS of a bilayer con-isting on 50 nm of Nb and 5 nm Al on a Si wafer.ource: Ref. [52], with permission of Springer.

e obtained, as shown in Fig. 3 for the qualitative in-depth profileion signal intensity versus sputtering time), obtained with pulsedf-GD-ToFMS, of a bilayer consisting of 50 nm Nb and 5 nm Al onSi wafer [52]. Despite such good features, there are of course

till important improvements to be achieved. Some relevant recentesearch efforts, carried out in GD-MS instrumentation develop-ent, are reviewed below.Driving forces for enhanced source development were the

earch for improved sensitivity and increased depth profiling capa-ilities. The relatively low buffer gas pressure, current and powersed in GD-MS systems limited the sensitivity but also increasedhe sputtering time and hence the analysis time. Several “fast flow”ources have been designed and evaluated along the years [53–55].fast flow GD source, operating in steady-state dc and rf modes, haseen successfully investigated by Pisonero et al. [56]. Crater shapesor depth profiling analyses using the fast flow source coupled to aFMS were evaluated. As expected, the “flow tube” directing the gasow onto the surface of the sample plays a critical role, althoughloss of sensitivity compared with the best bulk analysis condi-

ions was observed under the best conditions for depth profilingintensities at about 1 × 1010 cps were achieved for the matrix ionntensity of a gold layer). More recently, a fast flow GD source for MS

as studied by Voronov et al. [43,57] and work on both steady-statend pulsed dc modes is described.

In a comparison of steady-state and pulsed dc-GD sourcesor ToFMS, Martin et al. [58] demonstrated noticeable sensitivityncreases using the latter operation mode, in spite that the aver-ge power reduction caused a decrease in the measured sputteringates (3–6-fold decreases, depending of the matrix). Pisonero etl. [59] worked with a double pulsed GD-ToFMS system to fur-her improve ion detection and resolution of isobaric interferences.he double pulsed GD provides certain enhanced atomization andonization characteristics as compared with the use of a singleulse. With a proper flow rate and adequate delays between pulses,he second pulse is able to affect the sputtered atoms of the firstulse, producing enhanced ionization. Unfortunately, although theouble pulse reduced somewhat polyatomic interferences, manypectral complexities were still apparent.

Recently, Lobo et al. [60] carried out the comparison of steady-

tate and pulsed rf-GD sources coupled to an orthogonal ToFMS. Aseported for dc-GD sources, the observed sensitivity in the pulsedode was higher than that in the steady-state mode, being the

nhancement factor element dependent. Moreover, it was also pos-ible to use the pulsed GD source time resolution to further separate

Sn / Sn 3.80 4.5 2.7121Sb+/123Sb+ 1.34 1.3 1.4184W+/182W+ 1.17 1.2 1.2186W+/182W+ 1.09 1.1 1.1

analyte ions from polyatomics because they are commonly formedat different temporal and spatial locations [45,60,61]. Such ability todiscriminate polyatomic interferences from analytical ion signals isclearly demonstrated when measuring isotopic ratios: Table 1 col-lects the theoretical isotopic ratios of analytes present in a stainlesssteel, along with the corresponding experimental isotopic ratiosobtained in pulsed and non-pulsed rf modes [60]; as can be seen,the isotopic ratios measured using the pulsed mode showed higheraccuracy than those obtained in non-pulsed mode, thus indicatinglower spectral interference levels. For example, the isotopic ratio12C+/13C+ is deviated 7% from the theoretical value using the pulsedrf mode, while it derivates 87% using the non-pulsed rf mode (dueto the presence of 12C1H+, i.e. the interference not resolved from13C+).

Concerning the new rf-GD-ToFMS prototype [50], severalinstrumental developments have been carried out during the lastyears. For instance, Pisonero et al. [62] have investigated the use of acompact magnetically boosted rf-GD source in terms of sputteringrates, ionization processes and ions transport into the MS. Althoughno significant changes were observed on the sputtering rates andcrater shapes under the influence of a magnetic field, significantimprovements were reported on the analyte ion intensities, whiledecreasing the Ar species ion signals. Another trend is the use ofnegative ions for analytical applications [63,64]. Currently avail-able commercial GD-MS instruments only allow the detection ofpositive ions, in spite of the fact that for specific applications (e.g.halogens or other species with high electron affinity), the negativeion mode may offer greater sensitivity, reduced background andcomplementary structural information for molecular ions. The ana-lytical utility of using negative ions has been explored in a pulsedrf-GD-ToFMS where the mass analyser has bi-polar power suppliesin order to allow either positive or negative ion detection. The anal-ysis of polytetrafluoroethylene (PTFE) has shown that the 19F− ionsexhibited the highest intensity among the negative ions and wereenhanced by at least three orders of magnitude when compared tothe 19F+ ion signals in the positive ion spectrum, where the analytemass is interfered partially with H3O+ [63]. Moreover, negative ionswere also observed for sputtered PTFE molecular fragments. (Fig. 4shows the spectra reported for the analysis of PTFE by pulsed rf-

More recently, this strategy was applied to analyse anodic tan-tala samples containing known in-depth distributions of fluorinespecies [64]. In this case, the detection of Ta in the negative ionmode was achieved by measuring tantalum oxides.

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. Applications of GD as a tool for characterization at theanoscale

Nowadays GD-OES is considered a fast and reliable technique forhe in-depth profile analysis of coatings in the micrometer range,ut it can be used also in the nanometer scale [4,5,8,13] providedhat special care is taken to avoid plasma contamination at theeginning of the analysis. Moreover, by proper selection of exper-

mental conditions, effects such as variations in sputtering ratecross the crater (giving rise to non-desirable crater geometries)nd re-deposition of sputtered material can be minimised. Back in003 Shimizu et al. already illustrated the nanometer depth pro-le capabilities of the rf-GD-OES technique by analysing samplesuch as a thin anodic alumina layer with a chromium delta functionarker layer (approx. 2 nm) buried at a depth of 40 nm [65].In the case of GD-MS, most reported uses were related to sensi-

ive analysis of bulk high purity materials. In recent years, however,uccessful developments have allowed also the in-depth profile

nalysis of coatings and thin films [51,52]. Advantages broughtbout by the use of GD-MS include: (i) the possibility to obtainsotopic information, demonstrated for example by the differenti-tion of 18O- and 16O-rich layers in anodic alumina films [66], (ii) a

ig. 4. Pulsed rf-GD-TOFMS mass spectra of a 0.2 mm thick PTFE polymer taken from the–500 m/z. Note the high F− signal, and the absence of argon ions in the negative ion speource: Ref. [63] with permission of Royal Society of Chemistry.

ica Acta 679 (2010) 7–16

lack of molecular broad bands, which would affect the backgroundor interfere the analyte signals, (iii) the possibility to obtain, rela-tively easy, molecular information and, (iv) the ability to providea complete mass spectrum in a very short period of time using amass analyser such as the ToF (this facilitates detection of all sam-ple constituents, including those unexpected, at each sample depthdesired).

Nowadays GDs are used for depth profile analysis in manydifferent fields. In fact, GDs are implemented as routine tech-nique for quality control in many industries (steel, aluminium,car-manufacturing, etc.) and as a valuable tool in materials science.GDs are being used to reveal processes at the surface (e.g. passi-vation on highly corrosion-resistant stainless steel [67]), as wellas to understand the behaviour (tribological properties, corrosion,diffusion processes, etc.) of surface treatments such as physical orchemical vapour deposition or ion implantation [68,69]. Moreover,GDs have demonstrated their capabilities to assist the improvedsynthesis of specialized materials, including glass coatings [51,70],

biomedical implants [71], photocatalyzers [72], thin films for thephotovoltaic [73] and microelectronic industries [74], etc.

In fact, the number of applications already published is so highthat, of course, we do not intend to cover them all. We will rather

afterglow regime: (a) positive ion spectrum 1–300 m/z, (b) negative ion spectrumctra. The spectra were collected under identical discharge conditions.

a Chimica Acta 679 (2010) 7–16 13

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B. Fernández et al. / Analytic

elect a few representative examples that, we believe, will allowhe reader to get more familiar with the potential of the GD-basedechniques for the analysis of nanostructures, the goal of this shorteview.

.1. Quantitative in-depth profile analysis of filledanostructured templates

The development of highly ordered and self-assembled mag-etic nanowires is receiving great attention these days due to theirpecial magnetic properties at the nanoscale, allowing extraordi-ary information storage capability. As it will be shown in the

ollowing, GD based techniques can contribute to optimize the syn-hesis of metallic nanowires obtained from the filling of nanoporousnodic alumina templates (NAATs) through electrodeposition tech-iques. GD-based direct solid analysis techniques can provide

nformation about the depth of the wires, as well as to alert of theiranufacturing defects (e.g. non-homogeneous filling throughout

he length of the nanopore, presence of contaminants), in a quickanner.The main reason for using NAATs as precursor patterns is

ased on the possibility of obtaining thermally stable periodicelf-organised and highly ordered hexagonally dense packedanoporous arrays (see Fig. 5a). Fig. 5b and c collects rf-GD-OESuantitative depth profiles (concentration versus depth) for twolled NAATs on a pure aluminium substrate. Compositional depthrofile analysis was achieved by a relative simple multimatrix cal-

bration procedure using homogeneous materials [12].Fig. 5b shows the obtained depth profile for Ni-filled nanopores

f 1.2 �m length with Ni. As can be observed, the profile is in goodgreement with the estimated pore length. The Ni signal inside theores is constant, demonstrating a homogeneous electrodeposi-ion. Moreover, it can be noted that the Ni signal at the beginningf the profile is very high because the channels were overflowedith this metal. Fig. 5c shows the depth profile corresponding

o 5 �m long NAATs filled with a layer of 1.5 �m of gold (inner)nd another of 1.5 �m nickel. It can be observed that the Au layereaches the bottom of the nanopore and that both Ni and Au layersre well discriminated. Results, compared with other techniquesuch as scanning electron microscopy and energy-dispersive X-raypectroscopy, show that the rf-GD-OES proves to be adequate andromising for this challenging application.

.2. Analysis of thin layers and multilayers

Many examples can be found in the literature for the analysis ofayers in the interval 10–100 nm. Here, as a representative example,he analysis of hard disks for magnetic data storage will be brieflyresented. The fast sputtering capabilities of the GD techniques,llowing for the in-depth profile analysis of thin films and relativelyhick coatings, makes this analytical tool specially appropriate forhe quality control (e.g. thickness of layers and presence of contam-nants) of hard disks, usually consisting of layers (both conductingnd non-conducting) of widely differing thicknesses (from a fewens of nanometers to several tens of microns). Shimizu [65] and

arcus groups [75] have published good examples of the analysisy rf-GD-OES of this type of widely used materials.

Fig. 6 shows the near surface region of two commercial hardisks of different technological generations [75]. The profile inig. 6a reveals the existence of at least three distinct layers on topf the Ni–P film of the older disk. At the onset of the discharge, only

arbon (of the list of analytes) is present to an appreciable extent.his is likely a primary lubricant layer. The second layer (whichoes not appear to be entirely discrete) is seen to consist of car-on and calcium and extends into the cobalt–chromium magnetic

ayer. This is an amorphous carbon layer that protects the mag-

is 200 nm). Insert side view of the template (scale is 1 �m). (b) Rf-GD-OES depthprofiles for filled NAATs of 1.2 �m nanopore length with Ni. (c) Rf-GD-OES depthprofiles for filled NAATs of 5 �m nanopore length with Au/Ni layers.

netic medium. The chromium layer is not well defined, showing itspresence in the cobalt layer (the trailing edge of Cr shows muchmore definition at the interface with the Ni–P layer). For compar-ison, the profile of a latter produced disk is presented in Fig. 6b.Though the basic profile structure is similar, the thicknesses of thecobalt–chromium and chromium layers are approximately 50% of

the older disk. Another difference is seen in the relative concentra-tion of the chromium layer. Simple comparison between the Co, Crand Ni intensities suggests that the purity of the chromium layer ismuch greater in the newer disk. In general, the profiles presented

14 B. Fernández et al. / Analytica Chimica Acta 679 (2010) 7–16

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ig. 6. Rf-GD-OES depth profiles of the top of commercial hard disks for magnetictorage. From Ref. [75], with permission of the Royal Society of Chemistry. (a) Hardisk produced in 1992. (b) Hard disk produced in 1998.

ere represent well the technical evolution in hard disk storageevices.

.3. In-depth profile analysis of ultra-thin multilayers andonolayers

An example of the ultimate depth profile capabilities of the GD-ES has been recently shown by Escobar Galindo et al. [6] for thenalysis of a multilayered system containing six 2.5-nm Cr layersmarkers) buried in a titanium matrix plus an extra marker at theutermost surface. The comparative depth profiles obtained by GD-ES and SIMS of such sample are shown in Fig. 7. Both, GD-OES andIMS were able to detect the ultra-thin Cr layers but with differ-nt sensitivities. GD-OES resolved the markers, including the oneeposited on the outermost surface, but showed some degrada-ion of the profiles (Fig. 7a). By contrast, the SIMS depth profilehowed no degradation for the Cr peaks located at depths below0 nm from the surface (Fig. 7b). However, the two markers locatedear the surface (at 0 and 50 nm) were modified by the so-calledtransient effect” (due to non-stabilized oxidation during the firsttages of the SIMS erosion process), which led to higher sputter-ng and ionization yields [6]; the first marker was practically lost,

hile the second was found at a sputtering time shorter than thatorresponding to the average sputtering rate. Finally, it has to beighlighted that the analysis are about two orders of magnitude

aster using the GD compared to SIMS.Moreover, it has been demonstrated that GD-OES allows to

etect a single monolayer of an organic molecule (e.g. thiourea,enzothiazole and benzotriazole) adsorbed in a copper surface26,76] by monitoring the C, H, O and S optical emissions.

.4. Analysis of polymers

The use of rf-GD-OES for the analysis of organic layers such asrominated flame retardant coatings [77] and paints [78] have beenemonstrated. However, we believe that for this particular field ofpplication, mass spectrometry has much more to offer than OESetection. While the use of MS for elemental and isotopic analysis

Fig. 7. Depth profile analysis of a multilayer coating containing seven 2.5 nm thickCr layers (markers) embedded in a Ti matrix. Only the first six Cr layers are shown.From Ref. [6] with permission of Elsevier. (a) GD-OES. (b) SIMS.

was its first reported applications, clearly the strongest feature ofthe technique in general is its ability to obtain molecular mass andstructural information of organic compounds. It is this area wherethe rf-GD-MS might be truly complementary of its OES counter-part.

Interesting examples of polymer identification have beendescribed, using rf-GD-MS for samples such as poly(vinylidene flu-oride) [78], PTFE [63,79] and flame retardants [80]. In this context,recent studies have shown that the pulsed rf-GD-ToFMS techniqueis an excellent tool allowing to discriminate similar elementalcomposition but different polymer structures by measuring in theafterglow region as well as to obtain in depth profile molecularinformation [81,82] as can be observed in Fig. 8 for the analysisof a multilayer consisting of a layer (approx. 100 nm) of poly-4-bromo/styrene (PBrS) followed by a layer (approx. 500 nm) ofpoly(methylmethacrylate) marked with Cu (Cu-PMMA) on top ofpolyethylene terephthalate (PET) substrate. The inorganic mark-ers were included to assure that molecular ions and marker ionsshow similar depth profile and that molecular ions would allowto discriminating the layers under analysis. As can be seen, the

polystyrene (PS) is clearly defined following the fragment at mass37 (C3H+), as it is shown by the simultaneous bromine signal. Then,the observed Cu signal from PMMA behaved in the same way asthe C5H7

+ signal. Finally, the sample substrate is reached when theC2O2

+ increases (at 40 s).

B. Fernández et al. / Analytica Chim

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ig. 8. Pulsed rf-GD-ToFMS molecular depth profile of a PBrS/Cu-PMMA/PET mul-ilayer.ource: From Ref. [82] with permission of Springer.

. Conclusions

It is really amazing to certify how fast the application fieldf glow discharge techniques has broadened throughout the lastecade, moving from just the in-depth profile analysis of coatings

n the micrometer range (mostly in conducting samples, e.g. gal-anized steels) to the characterization of thin and ultra-thin layersn materials of varied nature. Nowadays, GD-OES and GD-MS haveemonstrated to be fast and reliable tools for the analysis of surfacesnd nanolayers. It appears that such remarkable developments cane rationalised thanks to the merging of research efforts in twoain areas: (i) advances in the field of instrumentation, such as

evelopment of pulsed power GDs, design of GD sources allow-ng for good crater shapes (and, so, high depth resolution) for MSetection, use of improved vacuum technology reducing sourceontaminants, and incorporation of CCDs (for OES) and ToF (forS) detection allowing really fast spectra acquisition, (ii) develop-ent of improved operation methodologies, particularly addressed

o minimization of source contamination at the beginning of theischarge and analytical exploitation of the features brought abouty using pulsed GD sources (such as the possibility to analyse ther-ally unstable samples like certain polymers, or the high sensitivity

nd molecular information capabilities of the afterglow region).Although a field still in its infancy, the increasing number of suc-

essful applications of GDs for the characterization at the nanoscale,r in the “nanoworld”, is continuously increasing, confirming theigh analytical value and still uncovered potentialities of GD-OESnd GD-MS in nanotechnology.

cknowledgements

Financial support from “Plan Nacional de I+D+I” (Spanish Min-stry of Science and Innovation and EU FEDER Programme) throughhe projects MAT2007-65097-C02 and CTQ2006-02309/BQU isratefully acknowledged. In addition, Beatriz Fernandez acknowl-dges financial support from “Juan de la Cierva” Research Programf the Ministry of Science and Innovation of Spain co-financed byhe European Social Fund.

eferences

[1] M.P. Soriaga, J. Stickney, L.A. Bottomley, Y. Kim (Eds.), Thin Films: Preparation,

Characterization, Applications, Springer, 2002.

[2] S.A. Dayeh, C. Soci, X.Y. Bao, D. Wang, Nano Today 4 (2009) 347–358.[3] P.R. Bandaru, P. Pichanusakorn, Semicond. Sci. Technol. 25 (2010), 024003

(16pp).[4] B. Fernández, J.M. Costa, R. Pereiro, A. Sanz-Medel, Anal. Bioanal. Chem. 396

(2010) 15–29.

[

[

ica Acta 679 (2010) 7–16 15

[5] R. Escobar-Galindo, R. Gago, D. Duday, C. Palacio, Anal. Bioanal. Chem. 396(2010) 2725–2740.

[6] R. Escobar-Galindo, R. Gago, A. Lousa, J.M. Albella, Trends Anal. Chem. 28 (2009)494–505.

[7] B. Fernández, F. Claverie, C. Pecheyran, O.F.X. Donard, Trends Anal. Chem. 26(2007) 951–966.

[8] J. Pisonero, B. Fernández, R. Pereiro, N. Bordel, A. Sanz-Medel, Trends Anal.Chem. 25 (2006) 11–18.

[9] N. Jakubowski, R. Dorka, E. Steers, A. Tempez, J. Anal. Atom Spectrom. 22 (2007)722–735.

10] N. Trigoulet, H. Hashimoto, I.S. Molchan, P. Skeldon, G.E. Thomson, A. Tempez,P. Chapon, Surf. Interface Anal. 42 (2010) 328–333.

11] R. Payling, M. Aeberhard, D. Delfose, J. Anal. Atom Spectrom. 16 (2001) 50–55.12] D. Alberts, V. Vega, R. Pereiro, N. Bordel, V.M. Prida, A. Bengtson, A. Sanz-Medel,

Anal. Bioanal. Chem. 396 (2010) 2833–2840.13] J. Angeli, A. Bengtson, A. Bogaerts, V. Hoffmann, V.D. Hodoroaba, E. Steers, J.

Anal. Atom Spectrom. 18 (2003) 670–679.14] W.W. Harrison, C.M. Barshick, J.A. Klingler, P.H. Ratliff, Y. Mei, Anal. Chem. 62

(1990) 934A–949A.15] R.E. Steiner, C.M. Barshick, A. Bogaerts, in: R.A. Meyers (Ed.), Encyclopedia of

Analytical Chemistry, John Wiley & Sons Ltd., 2009, pp. 1–28.16] M.R. Winchester, R. Payling, Spectrochim. Acta Part B 59 (2004) 607–666.17] A.I. Drobyshev, Y.I. Turkin, Spectrochim. Acta Part B 36 (1981) 1153–1161.18] J.A. Kingler, P.J. Savickas, W.W. Harrison, J. Am. Soc. Mass Spectrom. 1 (1990)

138–143.19] W.O. Walden, W. Hang, B.W. Smith, J.D. Wineforder, W.W. Harrison, Fresenius’

J. Anal. Chem. 355 (1996) 442–446.20] C.L. Lewis, L. Li, T. Millay, S. Downey, J. Warrick, F.L. King, J. Anal. Atom Spectrom.

18 (2003) 527–532.21] V. Majidi, M. Moser, C. Lewis, W. Hang, F.L. King, J. Anal. Atom Spectrom. 15

(2000) 19–25.22] Ph. Belenguer, M. Ganciu, Ph. Guillot, Th. Nelis, Spectrochim. Acta Part B 64

(2009) 623–641.23] W.W. Harrison, W. Hang, Fresenius’ J. Anal. Chem. 355 (1996) 803–807.24] V. Hoffmann, H.J. Uhlemann, F. Präbler, K. Wetzig, D. Birus, Fresenius’ J. Anal.

Chem. 355 (1996) 826–830.25] A. Bengtson, Spectrochim. Acta Part B 63 (2008) 917–928.26] D. Klemm, V. Hoffmann, K. Wetzig, J. Eckert, Anal. Bioanal. Chem. 395 (2009)

1893–1900.27] S.S. Inayoshi, S. Tsukahara, A. Kinbara, Vacuum 53 (1999) 281–284.28] I.S. Molchan, G.E. Thompson, P. Skeldon, N. Trigoulet, P. Chapon, A. Tempez, J.

Malherbe, L. Lobo-Revilla, N. Bordel, Ph. Belenguer, Th. Nelis, A. Zahri, L. Therese,Ph. Guillot, M. Ganciu, J. Michler, M. Hohl, J. Anal. Atom Spectrom. 24 (2009)734–741.

29] D. Klemm, V. Hoffmann, C. Edelmann, Vacuum 84 (2010) 299–303.30] M.R. Winchester, R.K. Marcus, Anal. Chem. 64 (1992) 2067–2074.31] X. Yan, Y. Lin, R. Huang, W. Hang, W.W. Harrison, J. Anal. Atom Spectrom. 25

(2010) 534–543.32] Th. Nelis, M. Aeberhard, M. Hohl, L. Rohr, J. Michler, J. Anal. Atom Spectrom. 21

(2006) 112–125.33] D. Alberts, P. Horvath, Th. Nelis, R. Pereiro, N. Bordel, J. Michler, A. Sanz-Medel,

Spectrochim. Acta Part B 65 (2010) 533–541.34] V. Efimova, A. Derzsi, A. Zlotorowicz, V. Hoffmann, Z. Donko, J. Eckert, Spec-

trochim. Acta Part B 65 (2010) 311–315.35] B. Fernández, R. Pereiro, N. Bordel, A. Sanz-Medel, J. Anal. Atom Spectrom. 20

(2005) 462–466.36] L. Therese, Z. Ghalem, Ph. Guillot, Ph. Belenguer, Anal. Bioanal. Chem. 386 (2006)

163–168.37] D. Alberts, L. Therese, Ph. Guillot, R. Pereiro, A. Sanz-Medel, Ph. Belenguer, M.

Ganciu, J. Anal. Atom Spectrom 25 (2010) 1247–1252.38] J. Pisonero, J.M. Costa, R. Pereiro, N. Bordel, A. Sanz-Medel, Anal. Bioanal. Chem.

397 (2004) 17–29.39] C.B. Douthitt, J. Anal. Atom Spectrom. 23 (2008) 685–689.40] T. Gusarova, T. Hofmann, H. Kipphardt, C. Venzago, R. Matschat, U. Panne, J.

Anal. Atom Spectrom. 25 (2010) 314–321.41] D.C. Duckworth, D.L. Donohue, D.H. Smith, T.A. Lewis, R.K. Marcus, Anal. Chem.

65 (1993) 2478–2484.42] M. Voronov, T. Hofmann, P. Smid, C. Venzago, J. Anal. Atom Spectrom. 24 (2009)

676–679.43] M. Voronov, P. Smid, V. Hoffmann, Th. Hofmann, C. Venzago, J. Anal. Atom

Spectrom. 25 (2010) 511–518.44] W. Hang, C. Baker, B.W. Smith, J.D. Winefordner, W.W. Harrison, J. Anal. Atom

Spectrom. 12 (1997) 143–149.45] R.E. Steiner, C.L. Lewis, V. Majidi, J. Anal. Atom Spectrom. 14 (1999) 1537–1541.46] J. Pisonero, J.M. Costa, R. Pereiro, N. Bordel, A. Sanz-Medel, Anal. Bioanal. Chem.

379 (2004) 658–667.47] M. Vazquez-Pelaez, J.M. Costa-Fernandez, R. Pereiro, N. Bordel, A. Sanz-Medel,

J. Anal. Atom Spectrom. 18 (2003) 864–871.48] J.P. Guzowski Jr., G.M. Hieftje, Anal. Chem. 72 (2000) 3812–3820.49] N.H. Bings, J.M. Costa-Fernández, J.P. Guzowski Jr., A.M. Leach, G.M. Hieftje,

Spectrochim. Acta Part B 55 (2000) 767–778.50] M. Hohl, A. Kanzari, J. Michler, T. Nelis, K. Fuhrer, M. Gonin, Surf. Interface Anal.

38 (2006) 292–295.51] A.C. Muniz, J. Pisonero, L. Lobo, C. Gonzalez, N. Bordel, R. Pereiro, A. Tempez, P.

Chapon, N. Tuccitto, A. Licciardello, A. Sanz-Medel, J. Anal. Atom Spectrom. 23(2008) 1239–1246.

1 ca Chim

[

[[

[

[

[[

[

[

[[

[

[

[

[

[

[

[

[

[[

[

[

[[

[

[[

6 B. Fernández et al. / Analyti

52] R. Valledor, J. Pisonero, N. Bordel, J.I. Martín, C. Quirós, A. Tempez, A. Sanz-Medel, Anal. Bioanal. Chem. 396 (2010) 2881–2887.

53] T. Tanaka, T. Kubota, H. Kawaguchi, Anal. Sci. 10 (1994) 895–899.54] N. Jakubowski, I. Feldmann, D. Stuewer, J. Anal. Atom Spectrom. 12 (1997)

151–157.55] C. Beyer, I. Feldmann, D. Gilmour, V. Hoffmann, N. Jakubowski, Spectrochim.

Acta Part B 57 (2002) 1521–1533.56] J. Pisonero, I. Feldmann, N. Bordel, A. Sanz-Medel, N. Jakubowski, Anal. Bioanal.

Chem. 382 (2005) 1965–1974.57] M. Voronov, V. Hoffmann, J. Anal. Atom Spectrom. 22 (2007) 1184–1188.58] A. Martin, R. Pereiro, N. Bordel, A. Sanz-Medel, J. Anal. Atom Spectrom. 22 (2007)

1179–1183.59] J. Pisonero, K. Turney, N. Bordel, A. Sanz-Medel, W.W. Harrison, J. Anal. Atom

Spectrom. 18 (2003) 624–628.60] L. Lobo, J. Pisonero, N. Bordel, R. Pereiro, A. Tempez, P. Chapon, J. Michler, M.

Hohl, A. Sanz-Medel, J. Anal. Atom Spectrom. 24 (2009) 1373–1381.61] J.A. Klinger, C.M. Barshick, W.W. Harrison, Anal. Chem. 63 (1991) 2571–2576.62] P. Vega, J. Pisonero, N. Bordel, A. Tempez, M. Ganciu, A. Sanz-Medel, Anal.

Bioanal. Chem. 394 (2009) 373–382.63] S. Canulescu, J. Whitby, K. Fuhrer, M. Hohl, M. Gonin, Th. Horvath, J. Michler, J.

Anal. Atom Spectrom. 24 (2009) 178–180.64] S. Canulescu, I.S. Molchan, C. Tauziede, A. Tempez, J.A. Whitby, G.E. Thom-

son, P. Skeldon, P. Chapon, J. Michler, Anal. Bioanal. Chem. 396 (2010) 2871–2879.

65] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompson, Spectrochim. Acta Part B58 (2003) 1573–1583.

66] I.S. Molchan, G.E. Thompson, P. Skeldon, N. Trigoulet, A. Tempez, P. Chapon, N.Tuccito, A. Licciardello, Surf. Interface Anal. 42 (2010) 211–214.

[

[

[

ica Acta 679 (2010) 7–16

67] M. Uemura, T. Yamamoto, K. Fushimi, Y. Auki, K. Shimizu, H. Habazaki, Corro-sion Sci. 51 (2009) 1554–1559.

68] J.A. García, R.J. Rodríguez, R. Martínez, C. Fernández, A. Fernández, R. Payling,Appl. Surf. Sci. 235 (2004) 97–102.

69] R. Bayón, A. Igartua, X. Fernández, R. Martinez, R.J. Rodríguez, J.A. García, A. deFrutos, M.A. Arenas, J. de Damborenea, Tribol. Int. 42 (2009) 591–599.

70] B. Fernández, A. Martín, N. Bordel, R. Pereiro, A. Sanz-Medel, Anal. Bioanal.Chem. 384 (2006) 876–886.

71] P. Kern, P. Schwaller, J. Michler, Thin Solid Films 494 (2006) 279–286.72] B. Yuksel, E.D. Sam, O.C. Aktas, M. Urgen, A.F. Cakir, Appl. Surf. Sci. 255 (2009)

4001–4004.73] P. Sánchez, B. Fernández, A. Menéndez, R. Pereiro, A. Sanz-Medel, J. Anal. Atom

Spectrom. 25 (2010) 370–377.74] P. Schwaller, M. Aeberhard, T. Nelis, A. Sifher, R. Thapliyal, J. Michler, Surf.

Interface Anal. 38 (2006) 757–760.75] W. Luesaiwong, R.K. Marcus, J. Anal. Atom Spectrom. 19 (2004) 345–353.76] K. Shimizu, R. Payling, H. Habazaki, P. Skeldon, G.E. Thompson, J. Anal. Atom

Spectrom. 19 (2004) 692–695.77] A.S. Vázquez, A. Martín, J.M. Costa-Fernández, J.R. Encinar, N. Bordel, R. Pereiro,

A. Sanz-Medel, Anal. Bioanal. Chem. 389 (2007) 683–690.78] R.K. Marcus, J. Anal. Atom. Spectrom. 15 (2000) 1271–1277.79] C.R. Shick Jr., P.A. DePalma Jr., R.K. Marcus, Anal. Chem. 68 (1996) 2113–2118.

80] L. Li, C.M. Barshick, J.T. Millay, A.V. Welty, F.L. King, Anal. Chem. 75 (2003)

3953–3961.81] N. Tuccitto, L. Lobo, A. Tempez, I. Delfanti, P. Chapon, S. Canulescu, N. Bordel, J.

Michler, A. Licciardello, Rapid Commun. Mass Spectrom. 23 (2009) 549–556.82] L. Lobo, N. Tuccitto, N. Bordel, R. Pereiro, J. Pisonero, A. Licciardello, A. Tempez,

P. Chapon, A. Sanz-Medel, Anal. Bioanal. Chem. 396 (2010) 2863–2869.