Analytica Chimica Acta 551 (2005) 192–198

Determination of methylmercury and mercury(II) in a marine ecosystemusing solid-phase microextraction gas chromatography-mass spectrometry

S. Mishra, R.M. Tripathi∗, S. Bhalke, V.K. Shukla, V.D. PuranikEnvironmental Assessment Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Maharashtra, India

Received 15 May 2005; received in revised form 30 June 2005; accepted 14 July 2005Available online 29 August 2005

Abstract

A solvent free solid-phase microextraction (SPME) method has been developed to determine methyl mercury and Hg(II) in sediment,seawater and biota samples from TTC area (Mumbai, India) using gas chromatograph-mass spectrometer (GC-MS). The analytical methodconsists of phenylation with Na[B(C6H5)4], simultaneous solid-phase microextraction of the derivatives, followed by a final GC-MS analysis.Experimental design methodology was used for optimization of important process parameter, like SPME fiber coating (nature and thickness),extraction time, extraction temperature, and pH. After extraction, the fiber is directly injected to the injector port of GC for desorption,s 5 ng as Hg,r t matricesw©

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eparation and quantification. The absolute detection limits obtained for methylmercury and inorganic mercury were 0.02 and 0.0espectively. Standard reference materials were analyzed for validation of the methodology. The total mercury content in differenas determined using hydride generation atomic adsorption spectrometry (HG-AAS).2005 Elsevier B.V. All rights reserved.

eywords: SPME; GC-MS; Mercury speciation; Methyl mercury; HG-AAS

. Introduction

The mercury threat is rising day-by-day. As developedations get tough with mercury generating industries, theeveloping world is becoming a hotspot for this deadly metal

hat plays havoc with human health. Mercury is consideredhighly toxic element because of its accumulative and per-

istent character in the environment. Inorganic and organicpecies of mercury are generated in different industrial activ-ties, mainly pharmaceutical, paper, electrochemical andlaguicide industries[1]. Geochemical and environmentaltudies have shown that elemental and ionic mercury cane converted into highly toxic organo-mercury compoundsy biological processes such as methyl mercury which is theost common form of mercury found in the environment

2–4]. Various mercury species differ greatly in their bio-hysico-chemical properties; among them the methyl form is

he most hazardous[5,6]. Organometallic form shows accu-

∗ Corresponding author. Tel.: +91 22 25598272; fax: +91 22 25505151.E-mail address: rmt@apsara.barc.ernet.in (R.M. Tripathi).

mulation in superior organism due to its high affinity toSHgroup of proteins and lipid tissues in living organisms[7]. Thetarget organ for methyl mercury toxicity is the central nvous system, especially the brain, and may occur at doslow as 3�g kg−1 in humans (WHO, 1976). The high toxicof mercury species at low concentration levels has stimuthe development of species selective analytical methogies for accurate and sensitive qualitative and quantitdetermination of these species in environmental matrice

Liquid chromatography-piezoelectric detection[8], GC-MS [9], CV-AFS [10,11], CV-AAS [12,13], ICP-MS[14–16], MIP-AES [14,17], FAPES[18], GC-FAPES[19]and HPLC-AFS[20] are the general methods for specselective analysis of organomercury and Hg(II) in the eronment. However, these techniques demand hydrideeration or alkylation followed by purge and trap. For ecient derivatization of inorganic mercury and its orgacompounds, reagents like, sodium tetraethylborate, sotetraphenylborate[21,22], grignard reagents and sodiutetrahydroborate[17,23] have been in use. Procedemployed, for extraction of mercury species from solid s

003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.aca.2005.07.026

S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198 193

ples are often based on liquid–liquid extraction, acidic andalkaline leaching, aqueous distillation, supercritical fluidextraction (SFE) and microwave-assisted derivatization sol-vent extraction (MADSE)[24–28]. These tedious extractionsample-processing protocols have stressed the need for thedevelopment of new methods in this field.

Solid phase microextraction (SPME) method was envis-aged to reduce the disadvantages associated with above saidextraction procedures. This technique offers attractive alter-natives to the current methods[29]. In this technique, analytesestablish an equilibrium between the matrix and the station-ary phase of fiber. Commercially different polarity fibers areavailable and analyte adsorption depends essentially on itsaffinity with the stationary phase and on the thickness of thecoating material. After adsorption equilibrium, the fiber isthermally desorbed in the injection port of the GC system[30].

In this study, the suitability of SPME-GCMS for mer-cury speciation in environmental matrices was examined. Themethod is based on in situ aqueous phase phenylation withNa[B(C6H5)4] followed by SPME and GC-MS determina-tion. Factors that affected the performance of SPME havebeen studied and optimized. The optimized method was thensuccessfully applied to the analysis of several environmentalmatrices, like seawater, sediment and biota sample.

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with1 lle-f tiona asd sc pec-t

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is toa were

washed with detergent, thoroughly rinsed with double dis-tilled water and soaked in dilute electronic grade HNO3(10%) bath. Afterwards, the materials were rinsed with dou-ble distilled water before further use. Pyrex, quartz glass andteflon (PTFE) were used for sample storage and processes.

2.3. Reagents

Stock Solutions (1000 mg L−1) of Hg(II) and methylmer-cury (CH3Hg+) were prepared by dissolution of appropri-ate amount of mercury(II) chloride (HgCl2) (99.6%, FlukaChemie GmbH) and methylmercury chloride (CH3HgCl)(99.4%, Fluka Chemie GmbH) salts, respectively, and storedin the dark at 4◦C. The methylmercury chloride was dis-solved in methanol. Working standards of CH3HgCl andHgCl2 were prepared by serial dilutions of the 1000 mg L−1

stock solutions using HPLC grade methanol and double dis-tilled water acidified to 1% (v/v) with electronic grade HNO3,respectively.

A fresh solution of sodium tetraphenylborate(Na[B(C6H5)4]) (1%, m/v, Merck Limited) was pre-pared daily. Buffer solutions (1 mol L−1) having pH valuesbetween 2 and 9 were prepared by mixing appropriateamount of acetic acid and sodium acetate (both from,Thomas Baker, Mumbai, India).

All chemicals used were of analytical reagent grade. Allt 245( 0,1 an)w

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bayi ando west

. Experimental

.1. Instrumentation

A Shimadzu model QP5050A (Kyoto, Japan)hromatograph-mass spectrometer was used. GC equith a 15 mm× 0.25 mm i.d. fused capillary column coaith a 0.25�m film thickness of DB-5 (J&W ScientificA, USA) was used. The column temperature was ramt 5◦C min−1 from an initial temperature of 80◦C to thenal temperature of 300◦C. A thermogreen LB-2 SeptuSupelco) was used in the splitless mode, maintaine70◦C, at the temperature recommended by the SPMEfacturer. Helium (99.99%) was used as a carrier gaow rate of 1.8 ml min−1. Mass spectrometer equipped wuadrupole mass analyzer and electron impact ionizource was used. Interface temperature was set at 2◦C,hile mass scan range used between 45 and 400 amu.SPME was manually performed using fiber coated

00�m polydimethylsiloxane (PDMS) (Supelco Inc., Beonet, PA, USA). Extraction was done in immerse condit 40◦C for 15 min under stirring condition. Desorption wone at 270◦C for 3 min. Analysis of total mercury waarried out using hydride generation-atomic absorption srophotometer (GBC-HG 3000).

.2. Cleaning procedure

Extreme care was taken at all steps of the analysvoid contamination. All glasswares and plastic wares

he solutions were prepared using a Mettler-Toledo AGSwitzerland) analytical balance (±0.02 mg) and 20, 10000 and 5000 Nichipet EX micropipettes (Tokyo, Japith a precision of±1–0.7%.

.4. Safety considerations

Organo-mercury compounds, especially methyl mers extremely toxic. It may cause neurological damagell as kidney malfunction. Direct contact with the say lead to death. Mercury and methyl mercury standere handled in fume hood, using appropriate protelothing. Special care was taken to avoid breathing meercury vapors. Disposal of all mercury containing w

rom the experiment was done in accordance with facuidelines.

.5. Certified reference materials

Two certified reference materials were used to checccuracy of the developed method for methylmercury dination, e.g., IAEA-405 (sediment) and IAEA-142/T

muscle homogenate). All were obtained from IAEA, Aria.

.6. Study area

Thane Creek, which is adjacent to Mumbai harbours a triangular mass of brackish water which widens outpens to the Arabian Sea in the south. Along the east and

194 S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198

sides of the creek, many industrial units have come up. Thanecreek is the ultimate recipient of all the liquid dischargesfrom these industries. The Trans-Thane creek industrial areahouses a number of major, medium and small scale indus-trial units largely involved in the manufacture, storage anduse of chemicals, petrochemicals, pharmaceuticals and finechemical products, pesticide formulation etc. Of the 1800 oddindustries registered in the area, nearly 50 could be termed asmajor and the rest classified as small and medium scale. Theeffluent discharges both treated and untreated are let into thecreek.

2.7. Sample collection, storage and preservation

Seawater samples were collected from TTC coastal area incleaned PTFE bottles. Samples were acidified in 1% HNO3to preserve mercury speciation[31]. Sediment samples weremanually collected from the top sediments (5–10 cm depth),stored in sealed plastic bags, transported to the laboratory incold boxes, sieved through 63�m nylon sieve and stored inclosed pyrex vial at 4◦C until analysis. Biota samples werecollected in different points of the creek and transported tothe laboratory in cold boxes inside sealed plastic bags. Edi-ble parts were removed; freeze dried and then stored tillanalysis.

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25 ml

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following the same experimental conditions described in sea-water.

2.8.3. Biota samplesBiota like fish, crab, prawn and bivalves were selected

for the present study. For digestion of these samples, theprocedure given by Fischer et al.[22] was followed. Thefrozen, edible portions (0.3–0.8 g) of these samples were firsthydrolyzed using 25% KOH (10–20 ml) for 2 h under ultra-sonic treatment. In most of the cases, the solid phase wascompletely dissolved but sometimes it was necessary to cen-trifuge for 10 min at 3500 rpm to separate the solid residuefrom the liquid phase. In a 25 ml glass vial about 13.8 mldeionised water, 10 ml acetate buffer of pH 4.5, 0.2 ml ofdigested biota sample and 0.4 ml of 1% (Na[B(C6H5)4]) wasadded. Pre-concentration and analysis of different mercuryspecies was done following the same conditions describedfor seawater and sediment.

The instrument was calibrated using known methyl mer-cury and Hg(II) standards. The identification of mercury com-pounds was carried out by retention time and mass fragmen-tation pattern. The quantitative estimation was done usingcalibration curve as well as standard addition method to avoidmatrix effect. Total mercury concentrations were found outusing hydride generation atomic absorption spectrophotome-ter (HG-AAS).

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.8. Analytical procedure

.8.1. SeawaterSamples were filtered through 0.45�m Membrane filter

Cellulose nitrate) (Whatman, Maidstone, England). Ab5–30 ml of seawater sample was taken in 50 ml glass viaH was adjusted to 4.5 by addition of acetate buffer (pHhe final volume was always made to 48 ml in order to h

he sample volume constant and pH to 4.5 for every SPxperiment for seawater sample. Then 1 ml of the derivatgent (Na[B(C6H5)4], 1%) was added and allowed to react0 min. Finally, the SPME fiber was exposed to the samp

mmersed condition for 15 min at 40◦C to extract the phenerivative of Hg(II) and methyl mercury. After this sampl

ime, the fiber was retraced into the needle and immedinserted into the GC injection port for thermal desorptionC-MS analysis.

.8.2. Sediment samplesApproximately, 0.5 g of sample was extracted using 1

HCl:CH3OH, 1:1 (v/v)) and by sonicating for 2 h. After txtraction step, the mixture was centrifuged for 10 mi500 rpm and supernatant was separated from the sedy decantation. The sediment was rinsed with double

illed water and all the fractions were mixed and take25 ml glass vial. Volume and pH was adjusted to 2

nd 4.5, respectively, with addition of acetate buffer.5), followed by addition of 0.4 ml of 1% (Na[B(C6H5)4]).re-concentration of analytes on SPME fiber and analysifferent mercury species using GC-MS technique was

t

. Result and discussion

Non-polar phenyl methyl mercury and diphenyl mercormed by the reaction of methyl mercury species and Hith Na[B(C6H5)4] has been analyzed by GC-MS in tresent study. Immersed SPME is ideally suited for ex

ion, pre-concentration and convenient introduction tohe optimization of different analytical parameters for pheethyl mercury is described below. In a 25 ml glass vial a4.6 ml deionised water, 10 ml acetate buffer of pH 4.5,�lliquot of (0.8�g/ml as Hg) of methyl mercury and 0.4f 1% (Na[B(C6H5)4]) was added and allowed to react0 min. Finally, the SPME fiber was exposed to the samp

mmersed condition for 15 min at 40◦C to extract the phenerivative of methyl mercury. After this sampling time,ber was retraced into the needle and immediately insnto the GC injection port for thermal desorption and GC-nalysis.

.1. Effect of the fiber coating material and thickness

PDMS fiber (100�m) was found more effective amoDMS/DVB, CW-DVB and polyacrylate, because it is nolar in nature shown inFig. 1A. Fig. 1B shows that thethyl mercury extracted by 100�m PDMS is more effec

ive than those of 30 and 7�m coating. This is because tmount of analyte extracted by the fiber coating is direroportional to the volume of the coating[32].

S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198 195

Fig. 1. (A) Effect of fiber type on extraction of phenyl derivative of MeHg. (B) Effect of fiber coating thickness on extraction of phenyl derivative of MeHg.(C) Effect of pH on extraction of phenyl derivative of MeHg. (D) Effect of extraction time on extraction of phenyl derivative of MeHg. (E) Effect of extractiontemperature on extraction of CH3Hg.

3.2. Effect of pH

pH is an important factor influencing not only the deriva-tization, but also the sorption behavior. The adjustment of pHof the sample can improve the sensitivity of the method forbasic and acidic analytes. This is related to the fact that unless

ion exchange coatings are used, SPME can extract only neu-tral (non-ionic) species from water. By properly adjusting thepH, week acids and bases can be converted to their neutralforms for extraction by the SPME fibre[33]. However, inthis case, pH of the solution affect the derivatization (pheny-lation) process only, that is why it is important to optimize pH

196 S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198

of the solution to get maximum conversion of polar analytesto non-polar species. We studied the pH effect between 2 and9 for methyl mercury chloride species. As shown inFig. 1C,at pH 4.5, maximum amount of phenyl methyl mercury wasextracted by the fiber.

3.3. Effect of exposure time and extraction temperature

During the SPME sampling the analyte should achievean equilibrium distribution between the sample matrix andthe fibre coating. The equilibration time is defined as thetime required for the amount of analyte extracted to becomeconstant, within the limits of experimental error, indepen-dent of further increases in extraction time. The equilibrationtime should be determined for each analyte, since it dependson its partition coefficient[33]. In the present study theeffect of extraction time on extraction efficiency is shownin Fig. 1D. From figure, it is clear that, after 50–60 min onlythe equilibrium is attained but 15 min was considered a suit-able compromise between maximizing the extraction yieldand minimizing the extraction time for routine analysis. It isalready reported that, when equilibrium times are excessivelylong, shorter extraction times can be used[34]. However, thisrequires that the mass transfer conditions and the system tem-perature remain constant for all experiments.

eciest car-rm dd

urya ut bys andH iotaf ith as esf mla 996.Ta s Hg,r imest r 10sT plesd g/g)i ng/l)i for

0.8 ng/ml as Hg of CH3Hg+ and Hg2+ standards were 6 and13% for (n = 4).

Carryover or memory effect is a common problem encoun-tered in the analysis of mercury compounds using SPMEmethod. For that two carryover experiments given by[25]were carried out. The first consisted of running a second des-orption of the same fibre after the initial desorption, followingexposure to a standard solution. The second involved run-ning a blank using a same procedure, except that no standardwas added, after the initial desorption of the same fibre. Asobserved by[25], in the first case there was no carryover. Butin the second one a very small peak due to diphenyl mercurywas observed. But it mainly depends upon type and thicknessof fibre and the concentration of analyte. For 100�m PDMSfibre and 0.8 ng/ml CH3Hg+ standard solution the carryoverwas less than 0.5% for a subsequent blank. The memory effectcan be subsequently reduced by using a longer desorptiontime, or by running blanks.

4. Quality assurance

The reliability of the procedure for estimation of methylmercury in environmental samples using SPME-GC-MS hasbeen assessed by analyzing two standard reference materialso cleH ellw rp anced tifiedfw

n ofm iotas ples,p -MSs frag-m li fterp ents curya tifi-c massfa d bya otalm fromT .18,

TC

S Meth

Unit

1 �g kg−1 )2 �g kg−1

Temperature also affects the extraction of mercury spo SPME fibre. The extraction of mercury species wasied out at different temperature mainly ambient to 60◦C andaximum response was observed at 40◦C (Fig. 1E). Standareviation for four measurements is given by bar inFig. 1.

Method performance of the analysis of methyl mercnd Hg(II) in sediment and biota samples was carried opike recovery analysis. Recovery for methyl mercuryg(II) in sediments varied from 84 and 93% and in b

ound to be 75–85%. The instrument was calibrated weries of CH3Hg+ and Hg2+ standards. The linearity rangor both CH3Hg+ and Hg2+ are at least from 0.05 and 6 ng/s Hg and correlation coefficients vary from 0.9925 to 0.9he absolute detection limits for CH3Hg+ and Hg2+ using thebove methodology were found to be 0.02 and 0.05 ng aespectively. Detection limits were determined as three the standard deviation of the background measured fouccessive SPME extractions of buffer and Na[B(C6H5)4].he concentration detection limits (considering the samize taken for measurement) for both CH3Hg+ and Hg2+ forifferent matrices were calculated to be (0.04 and 0.1 n

n sediment (0.5 and 12.5 ng/g) in biota and (0.8 and 2n water, respectively. The relative standard deviation

able 1oncentrations of methyl mercury in standard reference materials

.N. Reference material

Sediment IAEA-405Muscle homogenate IAEA-142/TM

btained from IAEA, i.e. (Sediment IAEA-405 and Musomogenate IAEA-142/TM). The results agreed very with certified values (Table 1). F-test was carried out forecision comparison, which shows there is no significifference between the precisions of observed and cer

or 10% probability. Confidence interval given in (Table 1)as calculated for 95% confidence level.The optimized method was used for the determinatio

ethyl mercury and Hg(II) in seawater, sediment and bamples collected from TTC area. The derivatised samre-concentrated on SPME fibres were injected to GCystem and their total ion chromatograms and massentation patterns were obtained.Fig. 2A presents the tota

on chromatogram for the methyl mercury and Hg(II) ahenylation with sodium tetraphenyl borate in a sedimample. The retention times observed, for methyl mernd Hg(II) were 4.9 and 14.4 min, respectively. The idenation of the compounds was done by retention time andragmentation pattern, which is given inFig. 2B and C. Thebsence of diphenyl mercury in the sample was confirmenalyzing the sample as a blank without derivatization. Tercury concentration in sediment samples collectedrombay, Airoli and Koperkhairane were found to be 0

yl mercury (five replicates)

Observed value Certified value

5.38 (4.82–5.94) 5.49 (4.96–6.0244.9 (41.7–48.1) 47 (43–51)

S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198 197

Fig. 2. (A) Total ion chromatogram of phenyl derivative of CH3HgCl (retention time, 4.94 min) and Hg(II) (retention time, 14.45 min) in a typical sedimentsample. (B) Mass spectrum of phenyl derivative of CH3HgCl (CH3HgPh). (C) Mass spectrum of phenyl derivative of Hg(II) (PhHgPh).

1.54 and 0.17�g g−1, respectively (Table 2). In which inor-ganic mercury i.e. Hg(II) constitutes a large fraction about90–96% whereas methyl mercury contribution is only 3–10%(10.5–63.4 ng g−1). High mercury concentration found inAiroli than Trombay and Koperkharane, may be because

of high industrial discharges in that area. In bivalves andcrabs mercury concentration was found to be high where asin fish and prawns concentration is in a lower side. In biotamethylmercury concentration was found to be about 50–90%of total mercury whereas inorganic mercury only contributes

Table 2Concentration of total mercury, Hg(II) and methyl mercury in sediment samples collected at Trans-Thane creek area

Sample Location Concentration of mercury

Total mercury(�g g−1) dry weight

Inorganic mercury(�g g−1) dry weight

Methyl mercury(ng g−1) dry weight

Mean Min Max Mean Min Max Mean Min Max

1 Trombay (n = 12) 0.18 0.08 0.23 0.17 0.07 0.21 10.5 7.6 20.32 Airoli (n = 7) 1.54 0.98 1.62 1.48 0.93 1.60 63.4 50.3 713 Koparkhairane (n = 11) 0.17 0.08 0.21 0.15 0.06 0.19 15.7 10 35

Table 3Concentration of total mercury, inorganic mercury Hg(II) and methyl mercury in biota samples collected at Trans-Thane creek area

S.N. Sample Unit Total mercury Methyl mercury Inorganic mercury

1 Fish (n = 5) ng g−1 (dry weight) 45± 5 40 ± 6 < 12.02 Bivalves (n = 3) ng g−1 (dry weight) 407± 22 224± 15 125± 113 Prawns (n = 10) ng g−1 (dry weight) 82± 7 98 ± 9 14 ± 3.44 Crabs (n = 6) ng g−1 (dry weight) 295± 17 208± 13 78± 9

198 S. Mishra et al. / Analytica Chimica Acta 551 (2005) 192–198

1–30% (Table 3). High concentration of methyl mercuryin biota samples can be attributed to bio-accumulation. Inmost of the seawater samples total mercury concentrationwas found to vary from 5 and 10 ng l−1, while methyl mer-cury in these samples were found at below detection limit(<0.8 ng l−1).

5. Conclusion

Gas chromatography-mass spectrometer combined withsolid phase microextraction using 100�m PDMS-coatedfiber offers an alternative method of choice for extraction andquantitative analysis of ultratrace levels of methyl mercuryand inorganic Hg simultaneously in sediments and biota sam-ples. Sample extraction using SPME method demonstratesthat it is a fast, simple, solvent-free alternative technique tothe traditional extraction techniques. The absolute detectionlimit obtained with SPME-GC-MS for methyl mercury andHg2+ was 0.02 and 0.05 ng as Hg, respectively. The studydemonstrates that sodium tetra phenyl borate can be success-fully used as a derivatizing agent for methyl mercury andHg(II) at parts per billion (ppb) levels.

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