AnalyticalMethods

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Spectroscopy Unit, Chemistry Division,

Investigaciones Energeticas, Medioambie

Complutense 40, E-28040 Madrid, Spain

isabel.rucandio@ciemat.es

Cite this: Anal. Methods, 2013, 5, 4131

Received 5th April 2013Accepted 10th May 2013

DOI: 10.1039/c3ay40566d

www.rsc.org/methods

This journal is ª The Royal Society of

A simplified method for determination of organicmercury in soils

Rodolfo Fernandez-Martınez and Isabel Rucandio*

Monitoring levels of organic mercury species at very low concentrations in the environment is of concern

due to their high toxicity. However, conventional methods for organic mercury determination are usually

expensive and time consuming because they involve many preparation steps and require instrumentation

which is not available in most laboratories. In order to make it easier the organic mercury determination

this paper presents a simple, fast and reliable extraction method for isolating and quantifying the

organic mercury fraction in soil samples. The proposed method is based on one single digestion stage

using a CuBr2 solution in HCl to release the organic mercury compounds from the solid matrix and their

simultaneous and selective extraction into dichloromethane. After the separation of the organic phase,

reextraction into aqueous media using N-acetyl-L-cysteine solution allows the determination of the

extracted organic mercury by electrothermal atomic absorption spectrometry with the direct mercury

analyzer DMA-80. Experimental and instrumental variables were optimized by the analysis of synthetic

samples of methylmercury dispersed in pulverized silica. The method was validated by the analysis of

the certified CRM 580 reference material. The detection limit of the procedure is 9.6 ng of organic

mercury per gram of dry soil. The applicability of the proposed method to real samples was

demonstrated through recovery studies of methylmercury in spiked soils. In addition, the influence of

the TOC (Total Organic Carbon) content in soils was studied. The recoveries obtained under optimal

experimental conditions ranged from 90% to 105% for all tested samples, indicating the suitability of

the proposed method for determination of the organic mercury fraction in soils.

Introduction

The overall picture of the regional ecological situation showsthat mercury is one of the most hazardous toxic elements inrural and urban soils. Organic mercury compounds representan extremely toxic mercury species group that poses a concernfrom a human health point of view, due to its marked tendencyto be bioaccumulated throughout the food chain. Organicmercury species have been typically investigated in sediments,waters and biological matrices. However signicant organicmercury contents have also been observed in oodplain soilsand soils from mining areas.1–3 The occurrence of organicmercury compounds may be attributed to biomethylationprocesses by sulphate-reducing bacteria.4 Other factorsaffecting methylation are dissolved organic carbon (DOC) andpH.3,5 Abiotic methylation can occur through transalkylationreactions in highly contaminated sites.6 Even fulvic acids havemethylating properties at high temperatures.7 Although otherorganic mercury species can be present in natural samples,

Technology Department, Centro de

ntales y Tecnologicas (CIEMAT), Av.

. E-mail: rodolfo.fernandez@ciemat.es;

Chemistry 2013

monomethylmercury (MeHg) is clearly predominant in naturalwaters, soils and sediments, representing almost the wholeorganic mercury fraction in these environments. Consequently,the evaluation of the organic mercury fraction in environmentalsamples can provide very useful and practical informationabout its potential toxicity to human health.

Owing to the toxic character of MeHg and other organicmercury species, notable efforts have been dedicated to developanalytical tools to determine these compounds in environ-mental samples. The general trend in the development of thosemethods has been the use of a number of established stages:preconcentration, extraction, derivatization to form adequatespecies able to be separated by a chromatographic techniqueand quantication by a suitable detection technique such asElectron Capture Detection (ECD), Cold Vapour AtomicAbsorption Spectrometry (CVAAS), Cold Vapour Atomic Fluo-rescence Spectrometry (CVAFS) or Inductively Coupled PlasmaMass Spectrometry (ICPMS).8–15 Furthermore, a previous diges-tion stage is necessary, in the case of solid samples such as soilsor sediments, to release the organic mercury compounds intosolution without C–Hg bond alterations. These methods arereally suitable for determination of mercury speciation innatural samples, but they can be considered as time consumingbecause of the need for a large number of stages. Moreover, they

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are not available in most conventional laboratories because ofthe highly sophisticated required equipment. On the otherhand, the possibility of articial formation of MeHg duringsample preparation steps is well known. The extension of thisprocess varies with the kind of matrix and the techniqueapplied.16 In this sense, extraction procedures based onextraction with moderately acidic bromide solutions combinedwith the use of a Cu(II) solution (e.g. 1 M CuSO4) present thelowest potential for artefact formation.17

In contrast, specic mercury analysers which can directlydetermine mercury contents in solid and liquid samples offernowadays a simple and relatively cost-effective alternative todetermine mercury contents in environmental samples. One ofthem is the DMA-80 from Milestone (Sorisole, Italy). Thedevelopment of new methods allowing the determination ofseveral remarkable mercury fractions directly in solid andliquid samples has increased in the last few years. The DMA-80has been used to determine total mercury contents in a growingnumber of biological matrices18,19 such as soils and sedi-ments,20,21 food,22,23 biological products and air particles.24 Thedetermination of the organic mercury fraction in solid envi-ronmental samples has also been reported.25–27 These methodsare based on conventional sequential stages: (i) release oforganic mercury species by an appropriate solution; (ii) selectiveextraction with toluene; (iii) back-extraction to an aqueousphase and (iv) determination with a direct mercury analyzer(DMA).

The present work describes the development of a newmethod for the determination of the organic mercury fractionin soil samples, which successfully emphasizes the advantagesof the DMA-80 mercury specic analyzer. The novelty of thismethod lies on the use of simultaneous acid extraction withCuBr2 solution and selective extraction with dichloromethane(extractive digestion) followed by subsequent extraction with N-acetyl-L-cysteine and determination with a DMA-80 instrument.In this sense the lower number of stages reduces the analysistime and the potential sources of errors. In addition, a CuBr2solution was selected in the rst stage as a new releasing agentof organic mercury species. Naturally and synthetically spikedsamples were thoroughly used to optimize experimental andinstrumental conditions. The effects of different parameterssuch as agitation type, extraction time, CuBr2 concentration,evaporation time, sample stability and a number of matrixeffects were evaluated.

ExperimentalReagents

All reagents were of analytical-reagent grade with the exceptionof dichloromethane (CH2Cl2) (HPLC grade). Ultrapure waterfrom a Milli-Q system (Millipore Bedford, MA, USA) was usedthroughout.

CuBr2 solutions were prepared weekly at different concen-trations (0.3 or 0.5 M) by dissolving the appropriate amount inhydrochloric acid (5, 10, 20 or 25% v/v acid concentrationaccording to the test). N-Acetyl-L-Cysteine (NAC) solutions wereprepared daily by dissolving this reagent in ultrapure water.

4132 | Anal. Methods, 2013, 5, 4131–4137

A 1000 mg L�1 stock solution of methylmercury chloride wasobtained fromMerck (Darmstadt, Germany) andMeHg workingstandard solutions were prepared freshly by serial dilution with0.5 N HNO3.

CRM 580 reference sediment is certied by the CommunityBureau of Reference (BCR) of the European Community.

O2 gas (99.999%) was supplied from AlphaGaz and usedwithout further purications.

Instruments, apparatus and materials

Mercury concentrations were quantied using a MilestoneDMA-80 (Sorisole, Italy) instrument. The solid or liquid sampleis weighed, introduced into the sample boat and then put intothe autosampler. The sample is rst dried and then thermallydecomposed in a continuous ow of oxygen. Combustionproducts are removed and further decomposed in a hot catalystbed. Mercury vapours are trapped on a gold amalgamator andsubsequently desorbed for quantication. The mercury contentis determined using atomic absorption spectrophotometry at254 nm. All processing steps are self-contained within the DMAunit and the results are displayed in a control terminal.

Solid samples and reagents were weighed using an analyticalbalance (Sartorius BP210D). A drying thermostatized oven(Proeti S.A.) with a maximum adjustable temperature of 200 �C,automatic agate mortars (Fischer Scientic) and an automatichomogenizer (Spex Mixer/Mill Cat. 8000) were used for thesynthetic sample preparation. 40 mL glass centrifuge tubes(Alamo) were used for extracting solid samples. Shakingsystems include a vortex mixer (Heidolph Reax 2000) withvariable speed, an end-over-end shaker (Bunsen AAR-8) and anultrasonic bath (Selecta Ultrasons 9L) with a switch-timerclockwise connected with a water bath equipped with animmersion thermostat (Selecta Tectron-200) with adjustabletemperature from +5 �C to 100 �C. A bench-top centrifuge(Eppendorf 5804) was employed to separate phases from theextraction process. Organic phase aliquots were taken usinggraduate (500 mL maximum volume) gas tight HPLC syringes(Hamilton).

Cleaning procedure

Owing to the low concentration of MeHg in natural samples, itis very important to carry out a good cleaning procedure inorder to avoid any possible contamination prior to the sampletreatment. One of the most common sources of contaminationis laboratory ware, particularly glassware and Teon ware.Consequently, an adequate cleaning procedure is essential toensure good control of any contamination source and to keepthe integrity of obtained data. The large number of stagescommonly involved in the cleaning procedures designed forMeHg determination are oen called ultraclean procedures.28,29

The procedure proposed is based on the use of nitric acid as themain cleaning reagent and it was performed as follows: all glassand Teon materials, particularly the centrifuge tubes thatcontain residues from the previous extractions, were exhaus-tively washed with a common detergent, thoroughly rinsed withtap water and dried in an oven at 105 �C. The Teon and

This journal is ª The Royal Society of Chemistry 2013

Table 1 Operational conditions of the DMA-80 instrument

Parameter Value

Oxygen ow pressure 4 barDrying temperature 200 �CDrying time 120 sDecomposition temperature 650 �CDecomposition time 180 sPurge time 60 sHg releasing time fromamalgam

12 s

Record time 30 s

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glassware were soaked in a 25% clean nitric acid bath overnight,with the exception of centrifuged tubes that were lled withapproximately 20 mL of that solution and shaken overnight inan end-over-end shaker. Aerwards, all the materials wererinsed three times with ultrapure water, dried in an oven at 105�C and stored until analysis.

Spiked sample preparation

Spiked samples were obtained by a procedure based on thatdescribed by Vazquez et al.30 with slight modications. Solidsamples were dried for 4 days in an oven at 50 �C, pulverized inan agate mortar and homogenized. Then 20 g of each matrixsample was weighed, mixed with 2 mL of 250 ng L�1 MeHgstandard solution prepared in methanol, and dried in an oven at45 �C for 2 days. This sample was transferred into a glass vial, and5 plexiglass balls were added and then homogenized. Finally thespikes were stored at�18 �C until analysis. Different kinds of soilmatrices were spiked for the optimization condition and recoverystudies. Concentrations of MeHg were determined for eachspiked sample by analysis with the DMA-80 instrument.

Extraction procedures and measurement

The following sequential scheme corresponds to the optimizedprocedure: about 0.5 g of each sample was accurately weighedinto a 40 mL centrifuge glass tube. 5 mL of 0.3 M CuBr2 solutionin 5% v/v HCl and 10 mL of CH2Cl2 were successively added.The centrifuge tubes were closed and then agitated by vortexingfor 1 minute. The tubes were placed in an end-over-end rotaryshaker for 20 minutes at 35 rpm. Samples were centrifuged at4000 rpm for 10 minutes. 0.2 mL of the organic phase wascollected by using a HPLC syringe, and placed in a quartz boatwith 0.1 mL of 1% w/v N-Acetyl-L-Cysteine (NAC) solution. Thenorganic aliquots were evaporated at room temperature (20–25�C) into a fume hood for 25 minutes. 0.2 mL of 5% v/v HNO3

were added to each boat and the mercury concentration wasdetermined by a DMA-80 using the operational conditions listedin Table 1.

Fig. 1 Conditions and efficiencies of the different extraction proceduresexpressed as % of MeHg extracted with regard to the total MeHg spiked.

Results and discussionOrganic mercury extraction procedure

Among the different extractant solutions reported in the liter-ature acidic leaching using the H2SO4/KBr/CuSO4 mixture is the

This journal is ª The Royal Society of Chemistry 2013

most appropriate one for isolating MeHg from solid samplesavoiding undesirable methylation of inorganic mercury.17,31 Inthis work, the use of a CuBr2 solution in HCl is proposed as thereleasing agent for organic mercury species. Briey, organicmercury species bound to the soil or sediment matrix can bereleased by the combined action of the bromide and Cu(II) ionswhile HCl supplies adequate acidic media to the leachingprocess.32 Cu(II) ions displace alkylmercury cations frombinding sites in soil samples due to their high charge densityand smaller size.8,33,34 On the other hand, bromide ions wereselected as the halide source, since it has been established thatorganomercury bromide derivatives (RHgBr) have a morefavourable distribution between organic and aqueous phasescompared with other halides such as chloride or iodide.33 Thesecompounds can be subsequently extracted into the organicsolvent.

The optimal conditions for the quantitative extraction of theorganic mercury fraction were studied. For this purpose, a seriesof experiments were performed to evaluate the effect of extrac-tion conditions, that is, CuBr2 and HCl concentrations, time,temperature and agitation type on the organic mercury extrac-tion yield from spikes. Extractions were conducted with silicaspiked with a known amount of MeHg (81.3� 3.0 ng g�1). Fig. 1shows the MeHg recovery percentage obtained for each experi-ment. First, only ultrasonic agitation assays were attemptedunder different conditions. Non-acceptable recoveries wereobtained when ultrasonic agitation was used at moderate HClconcentrations and temperatures. When these parameters wereincreased up to 20% v/v HCl and 60 �C, MeHg recoveries higherthan 90% were achieved. Theoretically, there are not problemsassociated with the use of high HCl concentrations andtemperatures. Some authors have reported that HCl concen-trations below 60% v/v do not decompose MeHg,8 and isotope-specic extraction experiments have demonstrated that MeHgcan be extracted at temperatures up to 90 �C.35,36 However, theuse of such temperatures avoids the possibility of simultaneousextraction with dichloromethane, because of its high volatility.Dichloromethane losses result in organic mercury quantica-tion errors. Hence it would be necessary to keep the solution atroom temperature. The increase of the extraction time by usingonly ultrasonic agitation did not provide a satisfactory MeHgrecovery at room temperature, probably due to the non-feasiblecontact between solid and liquid phases. Consequently the useof only ultrasonic agitation had to be discarded.

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In order to overcome ultrasonic limitations and to achieveacceptable recoveries at room temperature rotary agitation wastested. This ensures a close contact between the sample andsolvent which can help to achieve a more extensive extraction ofthe organic mercury fraction. Although times below 10 minuteswere shown to be insufficient to extract quantitatively thespiked MeHg, increases of the extraction time allowed recov-eries higher than 95% to be obtained. Finally an extraction timeof 20 minutes was selected for this stage using room tempera-ture and concentrations of 0.3 M for CuBr2 and 5% for HCl. Thetest series also demonstrated the suitability of CuBr2 solution torelease organic mercury from the spiked samples. While onlyHCl is not able to extract MeHg appropriately, quantitativerecoveries were achieved even at low HCl concentrations (5%) inthe presence of 0.3 M CuBr2.

Once released, organic mercury compounds must be sepa-rated from the inorganic ones that can be coextracted with theCuBr2/HCl solution. For this purpose an organic solvent isusually used to selectively extract organomercury species.Conventionally, this extraction is carried out aer the releasingstep. However, Carro et al.37 developed a method based on thesimultaneous release and extraction of MeHg in marine sedi-ments using toluene as the organic solvent. In this work, the useof dichloromethane is proposed due to its high selectivity fororganic mercury compounds.38 In addition, its high volatilityfacilitates the subsequent solvent evaporation stage. In thisstage optimization studies were focused on the possibility ofsimultaneously carrying out the release and the selectiveextraction of organic mercury species. According to the litera-ture, the organic solvent is added aer the acid digestion stepfor most methods.17,31,39 Fig. 2 compares the MeHg extractionefficiencies of two procedures: a conventional procedure(Procedure A) with the extraction step performed aer thedigestion of samples under the optimized conditions, and thesimultaneous digestion and extraction of organic mercuryunder the same conditions (Procedure B). High MeHg recov-eries were obtained for the simultaneous procedure (B) similarto those obtained for the sequential method (A). Therefore, thesimultaneous procedure is advantageous since it helps to

Fig. 2 Comparison of sequential and simultaneous procedures for the releasing a

4134 | Anal. Methods, 2013, 5, 4131–4137

reduce time and steps of sample processing, and it was nallyadopted. The time required for this stage is notably shorter thanthose reported for other similar methods based on a selectiveextraction and determination with a DMA.26,27 Although ashorter time is described in one of these methods,25 theproposed method is a more simplied one since (i) it does notrequire microwave assisted digestion and (ii) the transfer of thesample to a centrifuge tube for subsequent steps is not neces-sary. As a consequence the risk of contamination and MeHglosses by the containers in the proposed method is much lowerdue to the use of single-use tubes.

Following the previous stage, the organic phase must beseparated and evaporated. The separation of the organic phaseand preservation of the samples until analysis require theapplication of specic protocols to avoid organic mercury los-ses. In the present work a more practical procedure is per-formed by taking aliquots of the organic phase. They were takenusing HPLC syringes. These can take accurately the desiredvolume out without stirring the sample. A standard volume of0.20 mL was selected, but higher volumes could be taken whenthe sensitivity of the method needs to be improved. In order tosimplify sample processing, the solvent evaporation and back-extraction of organic mercury species were simultaneouslycarried out in quartz boats with 0.10 mL N-Acetyl-L-Cysteine(NAC) solution added. It has been proved that this reagenteffectively prevents Hg losses from solutions containing alkylHg,40 avoiding MeHg degradation processes that occur in thepresence of other cysteine derivatives.41 Besides, this solutionprepared in water provides an aqueous solvent for back-extraction. A comparison of MeHg recoveries using differentNAC concentrations was carried out. Very poor recoveries wereobtained without NAC addition (<4% MeHg), which means thatimportant MeHg losses occur during the evaporation process.By increasing the NAC concentration to 0.5% w/v higher MeHgrecoveries were obtained (60.9 � 13.4%). However, almostquantitative recoveries (95.8 � 3.3%) were obtained with addi-tion of NAC solution (1%). Therefore, NAC solution seems tostabilize organic mercury compounds by retaining them in theaqueous phase. Since NAC solution does not interfere with Hg

nd selective extraction of the organic mercury fraction.

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quantication at the tested levels, 1% w/v NAC concentrationwas selected in order to assess a quantitative recovery duringthe back extraction process. The time taken for organic solventevaporation was further optimized (Fig. 3). MeHg recoveriesfrom spiked samples increased with time from less than 85%for 10 min up to about 94.5% for 20 minutes, and remainedconstant thereaer. From these results a time of 25 minutes(96% Hg recovery) was xed for this stage. This time is notice-ably shorter than those reported by other authors.17,42,43 Incontrast with other similar methods additional heating at hightemperature or purging with nitrogen is not necessary.38

Analytical determination of the organic mercury fraction

The last stage of the method consists of the determination ofthe extracted organic mercury with the DMA-80 instrument.According to the EPA method 7473,44 calibration curves wereobtained using Hg(II) stock standard solutions. The accuracy ofthe determination was evaluated by the analysis of controlstandards of MeHg in each batch of samples. In all cases theobtained Hg concentration was within �5% deviation of thestandard Hg concentration. The main aspect to take intoconsideration was the stability of the samples during analysis.Fig. 4 compares the obtained absorbances when replicates ofthe same sample were successively analysed. Absorbances weredecreasing during analysis without addition of any stabilizingreagent and the obtained results were not comparable. It seems

Fig. 3 Effect of the evaporation time of CH2Cl2.

Fig. 4 Evolution of the absorbance signal during analysis for replicates withoutaddition of any stabilising reagent and with addition of 5% v/v HNO3.

This journal is ª The Royal Society of Chemistry 2013

that extracted organic mercury losses occur in the sample boatprobably due to the high generated temperatures in theinstrument. This problem could be solved by addition of 0.20mL of a 5% nitric acid solution to each boat aer the evapora-tion process. The use of this solution prevents Hg losses fromthe samples and provides stable absorbances for the replicates.Matrix effects were evaluated by adding spikes of 1 mg L�1

prepared in 5% nitric acid to the sample boats. Those spikerecoveries ranged from 90 to 101% indicating that no matrixeffects occur.

Method assessment: application to soils spiked with MeHgand validation with CRM 580

It is well documented that MeHg and other organic mercurycompounds have a high affinity to organic matter.45 A rela-tionship between MeHg concentrations and the TOC content insoils can be established; generally MeHg levels increase as TOCincreases.46,47 To our knowledge, there are no available soilreference materials with MeHg certied content. Hence vali-dation assays for new analytical methods must be carried out byother procedures such as analysis of spiked samples as well ascertiedmaterials of othermatrices such as sediments. Analysisof matrix-spiked samples is a common procedure in analyticalchemistry to ensure that the data collected meet the objectivesoutlined in the method development plans. In order to make anappropriate assessment of the organic mercury fraction recov-eries and to study the inuence of TOC on the extraction effi-ciency, a set of three different solid matrices were spiked withknown amounts of a MeHg stock standard solution in the sameway as described above. Pulverized silica was used as the inertmatrix and two soil samples collected from the surroundingarea of CIEMAT were selected as solid matrices in the presentstudies. The rst soil sample was labelled as SBMO, it presents alow content of TOC (0.87%) and corresponds to a poor-vegeta-tion area. The second soil sample, labelled as SAMO, corre-sponds to a moisture and rich-vegetation area with a signicantTOC concentration (4.20%). Table 2 presents the results fromthe application of the proposed method to the spiked matrices.Average recoveries were higher than 90%, which indicate thesuitability of the method for quantitative extraction of theorganic mercury fraction present in soils. On the other hand,quantitative recoveries for the two tested soil samples indicatethat organic mercury extraction is not affected by the TOCcontent. Results obtained for organic Hg in the CRM 580revealed a good agreement at a 95% condence level (t-test) with

Table 2 Recovery of MeHg spiked in several matrices and CRM 580

MatrixMeHg added(ng g�1)

MeHg found(ng g�1)

MeHgrecovery (%)

Silica 81.3 � 3.0 86.2 � 3.4 106.0 � 4.2SBMO 113.8 � 3.9 104.9 � 9.1 91.9 � 8.0SAMO 130.5 � 8.4 129.1 � 1.8 98.9 � 1.3CRM 580 75.5 � 3.7a 72.8 � 2.8b 96.4 � 0.7

a Certied value. b Mean value � standard deviation of 4 replicates.

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the certied value (Table 2). In addition, the selectivity of themethod is also demonstrated since MeHg represents only0.06% of total Hg in this CRM (132 � 4 mg kg�1 total Hg).

The repeatability of the analytical signal of absorbance with95% condence level was determined by the product t

ffiffiffiffiffi

2sp

using8 injections of a 100 ng L�1 MeHg standard solution. The meanpeak was 0.0046 and the repeatability was equal to 0.096,meaning that the coefficient of variation (s/�x) was lower than3%. The LOD was calculated from the calibration curve andbased on the amount necessary to yield a net signal equal tothree times the standard deviation (SD) of the blank, corre-sponding to 0.48 mg L�1. A total of 24 experiments were carriedout with spike samples in order to determine the LOD and therepeatability for the proposed method. The LOD for the soilsamples was 9.6 mg kg�1 and the repeatability (coefficient ofvariation) was less than 10%.

Conclusions

A simple and relatively cost-effective methodology has beendeveloped for the determination of the organic mercury presentin soil samples. CuBr2 allows organic mercury compounds to bequantitatively released under mild conditions preventingorganic mercury losses. The use of this reagent in 5% HCltogether with CH2Cl2 in a single extraction step provides arelatively fast determination (<1 h) of organic mercury contents.The proposed method can be simultaneously applied to severalsamples, up to 16 samples at one time, and organic mercury canbe sequentially measured in those samples. This means that areasonable number of samples can be processed per day.Recovery studies of spiked samples demonstrated that theoptimized experimental conditions combined with determina-tion by a DMA work satisfactorily for organic mercury, obtaininghigh efficiencies and accuracies. Differences in the TOC contenthave no effect on organic mercury extraction and determina-tion. The method was applied to two real soil samples and asediment reference material obtaining recoveries higher than92%, which indicates the suitability of the proposed method. Insummary, the developed method is a good and cheap alterna-tive to determine the organic mercury concentration in soilsamples. The main advantages of this method compared withother real speciation methods are: (i) it is notably faster, with atime of processing and analysis of about 1 h, (ii) it requires littlespecialized instrumentation as compared to other methods,avoiding the necessity of a large number of previous steps, and(iii) it minimizes the MeHg artifact formation broadly describedin the literature and the risk of MeHg losses by transferringsamples to different containers. In addition, it offers a highersimplicity and speed than other similar methods based on theselective extraction of organic Hg fraction and determinationwith a DMA.

Acknowledgements

The authors are very grateful to Rosa Marıa Fernandez, VicentaFelisa Sanz, Candelas Alonso and Luis Vicente Gomez for theireffort and assistance in the experimental work, as well as to

4136 | Anal. Methods, 2013, 5, 4131–4137

Dolores Marıa Sanchez for the TOC determinations. Theauthors thank the Spanish Ministry of Education and Science(previously Ministry of Science and Technology) for the nan-cial support (project BQU2003-07509-C02-02).

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