Mercury speciation in seafood samples by LC–ICP-MS with a rapid ultrasound-assisted extraction procedure: Application to the determination of mercury in Brazilian seafood samples

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  Mercury speciation in seafood samples by LC–ICP-MS with a rapid ultrasound-assisted extraction procedure: Application to the determination of mercury in Brazilian seafood samples
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  Analytical Methods Mercury speciation in seafood samples by LC–ICP-MS with a rapidultrasound-assisted extraction procedure: Application to the determinationof mercury in Brazilian seafood samples Bruno Lemos Batista, Jairo L. Rodrigues, Samuel S. de Souza, Vanessa C. Oliveira Souza,Fernando Barbosa Jr. ⇑ Laboratório de Toxicologia e Essencialidade de Metais, Faculdade de Ciências Farmacêuticas de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brazil a r t i c l e i n f o  Article history: Received 23 November 2009Received in revised form 7 December 2010Accepted 14 December 2010Available online 21 December 2010 Keywords: ICP-MSSpeciationFood samplesMercurySample preparationMethylmercuryLiquid chromatography a b s t r a c t This paper describes a simple method for mercury speciation in seafood samples by LC–ICP-MS with afast sample preparation procedure. Prior to analysis, mercury species were extracted from food sampleswith a solution containing mercaptoethanol,  L  -cysteine and HCl and sonication for 15 min. Separation of mercury species was accomplished in less than 5 min on a C8 reverse phase column with a mobile phasecontaining 0.05%-v/v mercaptoethanol, 0.4% m/v  L  -cysteine and 0.06 mol L   1 ammonium acetate. Themethod detection limits were found to be 0.25, 0.20 and 0.1 ng g  1 for inorganic mercury, ethylmercuryand methylmercury, respectively. Method accuracy is traceable to Certified Reference Materials (DOLT-3and DORM-3) from the National Research Council Canada (NRCC). With the proposed method there is aconsiderable reduction of the time of sample preparation. Finally, the method was applied for the speci-ation of mercury in seafood samples purchased from the Brazilian market.   2010 Elsevier Ltd. All rights reserved. 1. Introduction Our understanding for the mechanisms of biological activitiesand biogeochemical cycling of mineral and trace element specieshas been substantially advanced during recent years with the helpof chemical speciation studies. The safety and nutritional quality of food are determined by both the total level and the speciation, i.e.chemical form(s), of trace elements in foods. Then, speciation anal-ysis of food samples is gradually becoming more widely acceptedand recommended by food authorities to ensure food safety.According to IUPAC, speciation analysis is defined as the analyticalprocess of identifying and/or measuring quantities of one or moreindividual chemical forms in a sample, and speciation of an ele-ment is defined as the distribution of an element amongst definedchemical species in a system (Templeton et al., 2000).Mercury (Hg) is one of the most hazardous pollutants in theenvironment. It exists in three basic forms: elemental mercury(Hg 0 ) known as metallic mercury, inorganic mercury compounds(Ino-Hg), primarily mercuric chloride, and organic mercury, pri-marily methylmercury (Met-Hg) (ATSDR, 1999). Organic formsare more toxic than inorganic (ATSDR, 1999).Mercuryis present in fish and seafood products largely as meth-ylmercury. Food sources other than fish and seafood products maycontain mercury, but mostly in the form of inorganic mercury.However, the proportion between chemical forms of mercury infood samples may vary significantly from sample to sample. Thismakes it essential to have analytical methods, based on speciationanalysis, which can differentiate between chemical forms in foodproducts to better characterise the risks of toxicity (EFSA, 2009).Guideline for the presence of Hg derived from MeHg in seafoodhave been established; the US Food and Drug Administration set aguideline for MeHg in seafood at 1 l g g  1 (on edible portion or wetmass) (Food, 2009). However, in other countries the same guide-line is 0.5 l g g  1 . Consequently, suitable analytical methodologyfor routine Hg and MeHg analysis by control laboratories mustbe developed.The most effective instrumental based techniques for chemicalspeciation analysis rely on the use of chromatography (mainly gaschromatography (GC) (Baxter, Rodushkin, Engstrom, & Waara,2007; Gibicar et al., 2007; Rahman, Fahrenholz, & Kingston, 2009;Yan,Yang,&Wang,2008)orliquidchromatography(LC)(Carbonell,Bravo, Fernandez, & Tarazona, 2009; Chiou, Jiang, & Danadurai,2001; Meng et al., 2007; Morton, Carolan, & Gardiner, 2002;Qvarnstrom & Frech, 2002; Santoyo, Figueroa, Wrobel, & Wrobel,2009; Storelli, Storelli, Giacominelli-Stuffler, & Marcotrigiano,2000; Vallant, kadnar, & Goessler, 2007) coupled to a specific and 0308-8146/$ - see front matter    2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.foodchem.2010.12.068 ⇑ Corresponding author. Tel.: +55 16 36024701. E-mail address:  fbarbosa@fcfrp.usp.br (F. Barbosa).Food Chemistry 126 (2011) 2000–2004 Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem  sensitivedetector,suchasICP-MS.ComparedwithGC,LCisthepre-ferred separation technique used for mercury speciation, becausethe mercury species do not need to be derived to volatile com-pounds before HPLC separation.Although several methods have been developed for measuringmercury in food samples (Augelli, Munoz, Richter, Cantagallo, &Angnes, 2007; Nardi et al., 2009; Voegborlo & Akagi, 2007), thereis merely a few proposing speciation analysis (Kuballa, Leonhardt,Schoeberl, Lachenmeier, & Dirk, 2009; Liu, 2010; Santoyo et al.,2009; Vallant et al., 2007).One of the most important steps during speciation analysis isthe sample preparation protocol. Different procedures have beenproposed for the extraction of mercury species in biological sam-ples for speciation purposes based on HPLC–ICP-MS (Meng et al.,2007; Rodrigues, Souza, Souza, & Barbosa, 2010) or GC–ICP-MS (Gibicar et al., 2007). In general, protocols are based on acid(Rahman et al., 2009) or basic extractions (Gibicar et al., 2007; Qvarnstrom & Frech, 2002) mediums. However, most of thesemethodologies require very tedious and time-consuming proce-dures. Moreover, as far as a compatible pH value for the reverse-phase column is concerned, a laborious procedure usually has tobe adopted to adjust appropriate pH of the extracted solution priorto injection into the HPLC. Secondly, Hg species transformationmight occur during sample preparation (Liang & Lazoff, 1999;Qvarnstrom & Frech, 2002). In order to avoid some of the afore-mentioned limitations, alternative extraction procedures havebeen suggested with reagents containing thiol ligands, such asmercaptoethanol (Meng et al., 2007), or  L  -cysteine (Chiou et al.,2001). These procedures are associated with the use of microwaveenergy (Rahman et al., 2009). On the other hand, laboratories mustcope with an increasing demand of food samples for inorganic andmethylmercury determination in response to the concerns of mer-cury intake from food consumption. Then, fast sample preparationprocedures with minimal handling are extremely desirable in rou-tine analysis to respond to this increasing demand.The aim of this paper was therefore to evaluate a simple meth-od for methylmercury and inorganic mercury determination infood samples by high-performance liquid chromatography coupledto inductively coupled plasma mass spectrometry (ICP-MS) with afast sample preparation procedure prior to analysis. The methodwas then applied for speciation of mercury in seafood samplescommercialised in the Brazilian markets. 2. Material and methods  2.1. Instruments and apparatus All measurements were made with an ICP-MS (Elan DRC II Perk-inElmer, Norwalk, CT) for total mercury determination and for spe-ciation. A microwave oven equipped with PTFE vessels, modelEthos 1600 (Milestone, Monroe, CT) was used for sample digestion.  2.2. Measuring of mercury species A Perkin Elmer model L-200 LC pump, six-port injector(Rheodyne 9725) with a reverse-phase column (C8, 3 l m,33  4.6 mm, Brownlee Columns PerkinElmer (USA)) comprisedthe LC system. Samples were loaded with a syringe into a 100 l L sample loop. All separations were performed at room temperatureunder isocratic conditions. The isocratic mobile phase was 0.05%v/v mercaptoethanol, 0.4% m/v  L  -cysteine, 0.06 mol L   1 ammoniumacetate. The flow rate was 1.0 mL min  1 . The effluent from the LCcolumn was directly connected to the nebuliser with PEEK tubing(1.59 mm o.d.) and a low dead volume PEEK connector. Data eval-uation was performedusing Chromera  softwaresupplied withtheinstrument, and quantification was based on peak high by externalcalibration.The optimum experimental conditions for both ICP-MS and LCare given in Table 1.  2.3. Reagents All reagents used were of analytical grade and the solutionswere prepared using high-purity water with a resistivity of 18.2 M X  cm, obtained from a Milli-Q Plus water purification sys-tem (Millipore, Bedford, MA, USA). Hydrochloric acid (Merck,Darmstadt, Germany), was doubly distilled in a quartz sub-boilingapparatus (Kürner Analysentechnik, Rosenheim, Germany).A clean laboratory and laminar-flow hood capable of producingclass 100 were used for preparing solutions and samples. All solu-tions were stored in high-density polyethylene bottles. Plastic bot-tles and glassware materials were cleaned by soaking in 10% (v/v)HNO 3  for 24 h, rinsed five times with Milli-Q water and dried in aclass 100 laminar-flow hood before use. All operations were per-formed on a clean bench.A 10 mg L   1 standard solution of inorganic mercury wasobtained from Perkin-Elmer (PerkinElmer, Norwalk, CT). A 1000mg L   1 standard solution of methylmercury chloride (CH 3 HgCl)and 1000 mg L   1 standard solution of ethylmercury chloride(CH 3 CH 2 HgCl) in water were obtained from Alfa Aesar. Analyticalcalibration standards of mercury species were prepared daily overthe range of 0.0–20.0 l g L   1 for the LC–ICP-MS method by suitableserial dilutions of the stock solution in the mobile phase.Additional chemicals for the speciation studies were HPLCgrade methanol (99.9% v/v) and mercaptoethanol (Sigma–Aldrich,USA),  L  -cysteine (Fluka, Japan). Ammonium acetate (99.99%) wasobtained from Aldrich Chemical Company (Milwaukee, USA).  2.4. Sample preparation for speciation analysis Edible parts of each seafood sample was homogenised using amixer. Then, samples were lyophilised at   50  C (ThermoVLP200, Thermo, CA, USA). After that, samples were grinded byusing a cryogenic mill with a self-container liquid nitrogen bath(SPEX model 6800 Freezer Mill). Then, 0.2 g of the resulting homo-genated samples were weight and transferred to a polypropylenetest tubes (15 mL) following addition to 10 mL of a solution con-taining 0.10% v/v HCl + 0.05% m/v  L  -cysteine + 0.10% v/v 2-mercap-toethanol. The mixture was sonicated for 15 min in an ultrasonicbath 1400 A (UNIQUE, Brazil). The resulting solution was centri-fuge and then filtered through 0.20 l m Celulose filters (Millipore,USA). Sample extraction was performed in triplicate and extraction  Table 1 Liquid chromatography and ICP-MS operating conditions for Hg speciation in seafoodsamples. LC conditions Column C8 (3 l m, 33  4.6 mm)Mobile phase 0.05% v/v mercaptoethanol0.4% m/v  L  -cysteine0.06 mol L   1 ammonium acetateMobile phase flow rate 1 mL min  1 Sample loop 100 l L Measurement Peak height ICP-MS experimental conditions Radio frequency power/W 1200Scan mode Peak hoppingNebuliser gas flow/L min  1 0.58Resolution/amu 0.7Replicates 3Isotopes  202 Hg B.L. Batista et al./Food Chemistry 126 (2011) 2000–2004  2001  blanks were prepared in the same manner. A complete descriptionof the sample preparation procedure is shown in Fig. 1. Since the sample was homogenised, our values represent a mean of thewhole edible sample.  2.5. Sample preparation for the determination of total mercury For comparative purposes the total amount of mercury wasdetermined in ordinary seafood samples by ICP-MS. For this anal-ysis, samples were digested and analysed according to the methodproposed by Nardi et al. (2009). Briefly, samples (0.10–0.25 g) wereaccurately weight in a PFA digestion vessel, and then 4 mL of nitricacid 14 mol/L + 2 mL of 30% (v/v) H 2 O 2  were added. After that, thedigestate were left to cool and then the volume made up to 50 mL with Milli-Q water. Then, rhodium was added as internal standardto a final concentration of 10 l g L   1 .  2.6. Standard reference materials and ordinary food samples In order to verify the accuracy and precision of the proposedmethod, Certified Reference Materials (CRMs) DOLT-3 and DORM-3 from National Research Council Canada (NRCC) were analysedby the proposed method.Additional samples (fish, mussels, shrimps, octopus, tunafishand squids) were purchased from the Brazilian markets and ana-lysed by the proposed method. 3. Results and discussion Our preliminary experiments were carried out to explore theefficiency of the combination of thiol-containing compounds( L  -cysteine, 2 mercaptoethanol) and a dilute solution of HCl(0.10% v/v) for a fast extraction of Hg in seafood samples. For thepreliminary experiments, the CRM DOLT-3 from National ResearchCouncil Canada (NRCC) was selected. Several combinations of thereagents concentration were evaluated and mercury was deter-mined directly in the liquid phase by ICP-MS. Better recoveriesof total mercury in 241 the CRM (30–40%) was obtained withthe use of a solution containing 0.10% v/v HCl + 0.05% m/v L  -cysteine + 0.10% v/v 2-mercaptoethanol. Then, it was furtherevaluated the combination of this extractor solution with ultra-sound energy.  3.1. Evaluation of the use of ultrasound energy Quantitative mercury extractions from biological samples havebeen demonstrated even in low acid conditions when associatedwith ultrasound energy (Rio-Segade & Bendicho, 1999). Then, asan alternative to the use of microwave-assisted extraction and todecrease the time for sample preparation, our experiments werecarried out to explore the efficiency of ultrasound energy to im-prove the recoveries of mercury in food samples. Different timesof extraction (from 0 to 50 min) were evaluated with the extractorsolution containing 0.10% v/v HCl + 0.05% m/v  L  -cysteine + 0.10%v/v 2-mercaptoethanol. The CRM DOLT-3 from National ResearchCouncil Canada (NRCC) was used for this experiment. Quantitativeextraction (>90%) of both inorganic and methylmercury was ob-served for the CRM by using 15 min of ultrasound energy as shownin Fig. 2. Then, for the subsequent experiments, mercury specieswere extracted from the seafood samples with this optimisedcondition.  3.2. Optimisation of LC operating conditions After the optimisation of mercury extraction from seafood sam-ples, we optimised the mobile phase composition. Different combi-nations of reagents in the mobile phase are usually recommendedfor the speciation of Hg in biological samples by HPLC–ICP-MS.Some authors recommend the use of   L  -cysteine and mercap-toethanol (Chiou et al., 2001) while others recommend methanol,mercaptoethanol and ammonium acetate (Morton et al., 2002) ora mixture of   L  -cysteine, pyridine and methanol (Vallant et al.,2007). Our preliminary experiments demonstrated more promis-ing results (time of separation, resolution, selectivity and sensitiv-ity) for the mixture of mercaptoethanol,  L  -cysteine and ammoniumacetate. According to Chiou et al. (2001) the retention time of mercury species increases with the increase in mercaptoethanolconcentration in the mobile phase. We have observed the same re- Fig. 1.  Schematic representation of the proposed procedure for Hg speciation inseafood samples with the use of ultrasound extraction/LC–ICP-MS. Fig. 2.  Recovery of Hg species in the DOLT-3 Certified Reference Material byapplying LC–ICP-MS and different times of ultrasonic energy application. For theexperimental conditions see text and Table 1.2002  B.L. Batista et al./Food Chemistry 126 (2011) 2000–2004  sults (data not shown). Thus, we fixed mercaptoethanol concentra-tion at 0.05% v/v as a compromise between selectivity and time of analysis. Ammonium acetate was fixed at 0.06 mol L   1 . Then, opti-misation of   L  -cysteine concentration in the mobile phase wascarried out. Separation of mercury species can take place basedon the cysteine-mercury complexes on the polymeric-based C8reverse-phase column. Concentrations of   L  -cysteine between0.05% and 0.4% m/v were evaluated. It has been observed that thehigher the concentration of   L  -cysteine, the lower the retention timeof the three mercury species and the higher the sensitivity for allmercury species. For an  L  -cysteine concentration of 0.4% m/v, theseparation of the three mercury species is achieved in less than5 min compared to 20 min when 0.05% m/v  L  -cysteine is used inthe mobile phase. As a result, a solution containing 0.4% m/v L  -cysteine, 0.05% v/v mercaptoethanol, 0.06 mol L   1 ammoniumacetate was used as the mobile phase. Calibration curves withthe optimised conditions for all mercury species present coeffi-cients of correlation always higher than 0.999.  3.3. Validation studies, detection limits and repeatability Validation of the proposed method was accomplished usingCRMs DOLT-3 and DORM-3from NationalResearch Council Canada(NRCC). For additional validation, it was also analysed severalseafood samples commercialised in the Brazilian markets. Dataobtained with the proposed method were compared to the resultsobtained using direct sample introduction for total mercury deter-mination by ICP-MS. Results for DOLT-3 and DORM-3 are shown inTable 2. Values found using the proposed method are in goodagreement with established target values.The LC–ICP-MS proposed method detection limit (3 SD) was 0.1,0.2 and 0.25 ng g  1 for methylmercury, ethylmercury and inor-ganic mercury, respectively. Typical within-day precision was al-ways lower than 9.0% (DOLT-3), while between-day precisionwas <14.0% RSD (DOLT-3) for both methylmercury and inorganicmercury determinations.  3.4. Speciation of mercury in seafood samples commercialised in theBrazilian markets For the application of the proposed method 19 different types of seafood samples (fish, tunafish, mussels, octopus, shrimps andsquids) were purchased at a local supermarket and analysed bythe proposed method. Results are shown in Table 3. Methylmer-cury was the predominant mercury form in all samples. In someof them inorganic mercury was also identified. Moreover, totalmercury levels found with the proposed method as a sum of inor-ganic and methylmercury are in good agreement with total Hg val-ues found by applying the methodology of  Nardi et al. (2009)(Table 3). All analysed samples have concentrations below the0.5 l g g  1 limit recommended by the FAO (2009) and adopted bymany countries. Tuna fish presented the higher levels of mercuryin the samples purchased from the Brazilian market. The concen-tration of mercury varied from 94.4 to 160 ng g  1 . These valuesare in good agreement with those found by Emami-Khansaria,Ghazi-Khansaria, and Abdollahic (2005) in tuna fish from the Per-sian gulf area of Iran (43–253 ng g  1 ) and lower than those foundby Voegborlo, El-Methnani, and Abedin (1999) and Carbonell et al. (2009) in tuna fish from the Mediterranean coast of Libya(200–660 ng g  1 ) and from a municipal fish market from Spain(110–678 ng g  1 ), respectively. A chromatogram with mercury  Table 2 Concentrations ( l g g  1 ) of total mercury and methylmercury (Met-Hg) in the Certified Reference Materials (DOLT-3 and DORM-3). Found values are denoted as mean ± standarddeviation,  n  = 3. SampleCRMTarget values LC–ICP-MS methodMet-Hg concentration Total concentration Ino-Hg concentration Met-Hg concentration Total concentrationDOLT-3 1.59 ± 0.12 3.37 ± 0.14 1.8 ± 0.1 1.61 ± 0.08 3.4DORM-3 0.355 ± 0.056 0.382 ± 0.060 0.012 ± 0.001 0.40 ± 0.05 0.41Ethylmercury was not detected in these reference materials.  Table 3 Mercury speciation in seafood samples obtained in the Brazilian market (values are denoted as mean (SD),  n  = 3). ND = not detected. Sample Scientific name Met-Hg (ng g  1 ) Ino-Hg (ng g  1 ) Et-Hg (ng g  1 ) Total Hg proposedmethod (ng g  1 )Total Hg Nardiet al. (2009) (ng g  1 )Shrimp 1 26.7 (1.1) <0.25 <0.2 26.7 27.1 (1.4)Shrimp 2 4.6 (0.2) <0.25 <0.2 4.6 5.0 (0.2)Shrimp 3 19.3 (0.3) 2.7 (0.6) <0.2 22.0 21.7 (0.3)Shrimp 4  Triops cancriformis  21.3 (1.9) <0.25 <0.2 21.3 22.2 (1.1)Shrimp 5 9.2 (0.7) <0.25 <0.2 9.2 8.9 (0.4)Shrimp 6 13.6 (1.2) <0.25 <0.2 13.6 13.2 (0.5)Octopus 1  Octopus vulgaris  4.2 (1.1) <0.25 <0.2 4.2 3.9 (0.2)Octopus 2 3.8 (0.6) <0.25 <0.2 3.8 4.2 (0.3)Fish 1  Rhomboplites aurorubens  80.5 (4.1) 5.3 (0.7) <0.2 85.8 91.7 (4.2)Fish 2 35.9 (2.4) 1.9 (0.1) <0.2 37.8 37.2 (1.0)Canned Tuna fish 1 94.4 (2.8) <0.25 <0.2 94.4 93.2 (1.3)Canned Tuna fish 2  Thunnus atlanticus  82.9 (3.3) 2.1 (0.2) <0.2 85.0 86.4 (2.1)Canned Tuna fish 3 160.1 (3.8) 13.8 (0.6) <0.2 173.9 179.3 (4.9)Mussel 1  Mytella guyanensis  23.2 (1.1) 2.7 (0.2) <0.2 25.9 24.6 (1.1)Mussel 2 36.8 (0.2) 3.0 (0.2) <0.2 39.8 41.0 (1.0)Squid 1 11.9 (0.5) <0.25 <0.2 11.9 12.1 (0.5)Squid 2  Illex illecebrosus  9.2 (0.9) <0.25 <0.2 9.2 9.5 (0.3)Squid 3 13.6 (1.1) <0.25 <0.2 13.6 13.2 (0.4)Squid 4 18.4 (0.2) <0.25 <0.2 18.4 17.8 (1.0) B.L. Batista et al./Food Chemistry 126 (2011) 2000–2004  2003  speciation in a sample of mussel with the proposed method isshown in Fig. 3. 4. Conclusion A simple method for mercury speciation in seafood samplesbased on LC–ICP-MS is described. Sample preparation procedureis very fast and simple with a quantitative extraction of mercuryin 15 min. In addition, the number of handling steps, sample prep-aration and analysis time, as well as potential sources of analyticalerrors, is reduced. The method was successfully applied for thespeciation of mercury in seafood samples commercialised in theBrazilian markets.  Acknowledgments The authors are grateful to Fundação de Amparo à Pesquisa doEstado de São Paulo (FAPESP) and Conselho Nacional de Desen-volvimento Científico e Tecnológico (CNPq) for financial supportand fellowships. References ATSDR. (1999).  Toxicological profile for mercury (update) . 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