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Determination of mercury in tuna fish tissue using isotope dilution-inductively coupled plasma mass spectrometry
Microchemical Journal 80 (2005) 233 – 236 www.elsevier.com/locate/microc
Determination of mercury in tuna fish tissue using isotope dilution-inductively coupled plasma mass spectrometry Sang Hak Leea,*, Jung Ki Suhb b
a Department of Chemistry, Kyungpook National University, Taegu, 702-701, Korea Division of Chemical Metrology and Materials Evaluation, Korea Research Institute of Standards and Science, P.O. Box 102, Yusung, Taejon, 305-600, Korea
Received 22 July 2004; accepted 25 July 2004 Available online 12 October 2004
Abstract Isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP/MS) was applied to determine mercury in living tissue. Microwave digestion method using HNO3/H2O2 media for the dissolution of solid sample was studied. The procedure for accurate determination of total mercury in tuna fish tissue sample by ID-ICP/MS is described. For the method validation, total Hg concentration in tuna fish CRM (BCR CRM 463) was determined by ID-ICP/MS after addition of 202Hg to CRM followed by acid decomposition of the spiked sample. This method was applied to the determination of Hg in tuna fish CCQM-P39 sample provided by IRMM (Institute for Reference Materials and Measurement, GEEL, Belgium) for the international comparison study. D 2004 Elsevier B.V. All rights reserved. Keywords: Hg; Isotope dilution-inductively coupled plasma mass spectrometry; CCQM-P39; Tuna fish tissue
1. Introduction Mercury is a serious environmental toxicant and there are several reviews on different aspects of mercury toxicology [1–3]. The main source of human intake of mercury contaminants originates from methyl-mercury in fish and fishery products. Fish and marine mammals and birds preying on fish may contain considerable amounts of mercury [4]. The mercury in fish originates from mercury in water, which is methylated, and accumulates in the fish as such. Mercury is commonly determined in environmental and biological samples by atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectrometry (ICP-MS). These analytical techniques are suitable for routine analysis because of their high sensitivity and reasonable accuracy and precision. In general, it is well known that isotope dilution-inductively
* Corresponding author. Tel.: +82 53 950 5338; fax: +82 53 950 6330. E-mail address: [email protected] (S. Hak Lee). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.07.007
coupled plasma/mass spectrometry (ID-ICP/MS) has a high potential for routine analysis of trace elements if the accuracy of results is of predominant analytical importance. Therefore, ID-ICP/MS has frequently been employed in the certification of element contents in certified reference materials (CRMs) [5–8]. Although IDICP/MS method has been widely used for the determination of trace element in various matrices, only a few applications have been reported for the determination of Hg. The problem in Hg analysis is known that the memory effect increases the blank counts and worsens the analytical performance of ICP-MS [9]. Another problem is the possibility of Hg loss during sample decomposition procedure due to its volatility. This paper describes a method to determine Hg by IDMS using quadrupole-inductively coupled plasma mass spectrometry. Microwave digestion with HNO3/H2O2 media was applied to the dissolution of solid sample. For the method validation, total Hg concentration in tuna fish CRM (BCR CRM 463) was determined by ID-ICP/MS after addition of 202 Hg to samples followed by the acid decomposition of the
234
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
Table 1 Operating conditions for ICP-MS ICP-MS instrument
Perkin-Elmer SCIEX Elan 6100DRC
Nebulizer gas flow Auxiliary gas flow Plasma gas flow Lens voltage ICP RF power Analog stage voltage Pulse stage voltage Discriminator threshold Dwell time Sweeps Replicates
0.90 l/min 1.20 l/min 16.00 l/min 6.75 V 1350 W 2550 V 1400 V 80.00 100 ms 30 5
spiked samples. Finally, the present method was applied to the determination of Hg in tuna fish CCQM-P39 sample provided by IRMM for the international comparison study.
2. Experimental 2.1. Instrument The ICP-MS used in this study is a ELAN 6100 DRCICP-MS (Perkin-Elmer SCIEX, Concord, ON, Canada) equipped with a Fassel torch, a Gilson Minipuls peristaltic pump, a Meinhard-type concentric glass nebulizer and a cyclonic-type spray chamber. All data was acquired in normal mode conditions without reaction cell gas. The settings of ion lens system, gas flow rates and other parameters were tuned daily to obtain maximum stability and sensitivity by using tuning solution containing 1 ppb each of Mg, In and U. Under the optimum conditions, the general intensities of Mg, In and U were about 6000, 50,000 and 30,000 counts/s, respectively. The material of sampling cone and skimmer is platinum. Typical operating conditions are summarized in Table 1. 2.2. Reagents A stock standard solution of Hg was prepared by dissolution of pure metals (99.999995%, Alfa, Catalogue
#10634). Working primary standard solutions were prepared by serial dilutions of the stock standard solution. Electronic grade HNO3 (Dong Woo Pure Chemicals, IkSan, Korea) was used with further purification by sub-boiling distillation. The 202Hg-enriched spike isotope (IRMM 640) and isotopic standards for mass bias correction (IRMM 639) were obtained from IRMM (Retieseweg, B-2440 GEEL, Belgium). These isotopes were dissolved in dilute HNO3 and stored in clean Teflon FEP bottles. Low-density polyethylene (LDPE) containers were utilized. These bottles were cleaned by immersing the vessels in 20% HNO3 for 2 days and washed successively with de-ionized water. The composition of enriched isotope (IRMM 640) and isotopic standard (IRMM 639), and potential interferences of Hg isotopes in ICP-MS are shown in Table 2. 2.3. Sample decomposition procedure by microwave digestion method The tuna fish sample, typically 400 mg, was decomposed by microwave digestion method, after adding an appropriate amount of the 202Hg spike solution. On the basis of preliminary investigations and the certified Hg contents of the reference material BCR-463 and CCQM-P39 sample, the optimum blend ratio was calculated as a compromise between lowest error magnification factor and sufficient counting rate [10]. In this case, the calculated optimum 200 Hg/202Hg ratio was in the range 0.06–0.3, with a corresponding error magnification factor of approximately 1.1–1.6. The sample was weighed accurately into the Teflon PTFE microwave vessel together with the 202Hg spike solution. The spike addition was varied with regard to an optimized value of 0.06–0.3 for the amount of mercury from sample to spike in the isotope diluted sample. Concentrated HNO3 (10 ml) and H2O2 (1 ml) were added as a sample decomposition media. It should be noted that overnight digestion at room temperature in the closed vessel was performed before pressurized microwave digestion to ensure enough isotope equilibrium. Microwave heating was then performed on samples in three steps: 250 W for 5 min, 450 W for 5 min and 600 W for 20 min. After dilution with water, the solution was ready for the measurement of 200 Hg/201Hg isotope ratio by ICP-MS.
Table 2 Potential interferences of Hg isotopes and isotopic compositions of natural mercury, isotopic standard and 202
202
Hg-enriched isotope used for IDMS method
Isotope
Natural abundances (%)
Mass fraction of Hg-enriched isotope (IRMM 640)a
Mass fraction of IRMM 640 isotopic standarda
Potential interferences
196
0.15 9.97 16.87 23.10 13.18 29.86 6.87
0.001715(36) 0.0596(11) 0.1543(16) 0.5319(33) 1.2979(50) 97.6985(68) 0.2560(21)
0.1454(12) 9.769(52) 16.689(64) 23.000(58) 13.237(25) 30.148(53) 7.011(30)
Pt, TaO, HfO, WO Pt, TaO, HfO, WO TaO, WO WO WO WO Pb, WO
Hg Hg 199 Hg 200 Hg 201 Hg 202 Hg 204 Hg 198
a
The values are taken from the certificates of supplier (IRMM, GEEL, Belgium). Numbers in parentheses indicate the uncertainty of the reported value.
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
2.4. Dry mass correction The tuna fish sample absorbs ambient moisture at typical laboratory temperature and humidity conditions. Therefore, the sample bottle was opened immediately before weighing aliquots for the IDMS blend preparation. For correction of measured values to dry mass, water content measurement was made on a separate portion of the same material with a mass of 0.5 g sample. The sample was dried before weighing for a 24 h in a ventilated oven at 85F2 8C. In this experiment, the content of moisture of tuna fish sample was 2.07%.
235
Table 4 Summary of analytical results for Hg in BCR CRM 463 according to the variation of reference isotope Reference isotope/ enriched isotope
Measured concentration (A/g)
Certified value and its uncertaintya (A/g)
199
2.875 2.908 2.877
2.85F0.16
Hg/202Hg Hg/202Hg 201 Hg/202Hg 200
a This uncertainty value was expressed as a half-width of the 95% confidence interval of the mean.
when the measured isotope ratio were 199Hg/202Hg, Hg/202Hg and 201Hg/202Hg, respectively, for applying IDMS. The certified value of Hg in this CRM was 2.85F0.16 A/g. There is no meaningful difference between the measured and certified values. The results also indicate that the variation of reference isotope used for the determination of Hg does not influence the analytical result. However, it is desirable to choose 200Hg as a reference isotope considering the highest natural abundance. If the sample contains tungsten (W) component, there is possibility of formation of WO that is interfering molecular ion on the Hg isotopes as shown in Table 2. However, in the preliminary test, tungsten was not detected in the present sample.
200
3. Results and discussion 3.1. Analysis of tuna fish CRM (BCR 463) For the application of IDMS, it is necessary to prepare several kinds of solutions. These are a primary assay standard solution, an enriched spike isotopic solution, a spike calibration solutions (the mixed solution of primary assay standard solution and an enriched spike isotopic solution) and sample blend (a mixed solution of sample and enriched spike isotope). The level of concentration of Hg in sample blends, spike calibration solutions and isotopic standard solution for IDMS was about 15–20 ng/g. The typical value of intensities and isotopic ratios for each solution in ICP-MS was shown in Table 3. The choice of reference isotope and enriched isotope considering possible interferences and natural abundances is important for the accurate determination by IDMS. In general, the isotope with the highest abundance in the sample was chosen as a reference isotope if there is no interference from another element. In the present studies, IDMS was applied to obtain the content of Hg in tuna fish CRM (BCR 463) according to the variation of reference isotope shown in Table 4. As shown in this table, the content of Hg in tuna fish CRM (BCR 463) was found to be 2.875, 2.908 and 2.887 A/g
3.2. Analysis of CCQM-P39 sample and uncertainty evaluations Inorganic Analysis Working Group of CCQM meetings (Paris, Ottawa 2002) agreed that IRMM will act as the pilot laboratory of the CCQM-P39 pilot study, bHg, Pb, Se, As and methyl-mercury in tuna fishQ. Therefore, CCQM-P39 sample was provided from IRMM for international comparison study on April 2003. This sample is a freeze-dried and ground tuna fish powder in amber glass vials. The present method was applied to the determination of Hg in CCQM-P39 sample. The measured Hg concentration in this sample was 21.0710 6 mol/kg and this value agreed well with the mean value of the results obtained from all the participants in the interna-
Table 3 Typical value of intensities and ratios for isotopic standard, spike calibration solutions and sample blends in ICP-MS Solutions
isotope
Intensity (cps)
Ratios
%RSD for ratio
Isotopic standard (IRMM-639)
199
14836 20484 11767 27218 9233 12794 7455 30409 8515 11824 6929 31658
0.545 0.753 0.432 1 0.304 0.421 0.245 1 0.269 0.374 0.219 1
0.60 0.48 0.70
Spike calibration solutions
Sample blend (sample+202Hg)
Hg 200 Hg 201 Hg 202 Hg 199 Hg 200 Hg 201 Hg 202 Hg 199 Hg 200 Hg 201 Hg 202 Hg
0.50 0.46 0.80 0.38 0.39 0.41
236
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
tional comparison study. The uncertainty in our value was evaluated according to the ISO/EURACHEM guides [11,12]. The combined relative standard uncertainty (u c) was found to be 0.5%. When multiplied by the coverage factor k=2, this gives an expanded uncertainty (U=k u c) of 1.04%.
4. Conclusions Despite its possibility of a loss by volatilization and its high memory effect, the content of Hg in a tuna fish reference material (BCR CRM 427) and CCQM-P39 sample could be accurately and precisely determined by ID-ICP/MS using quadrupole-type ICP-MS. The HNO3/H2O2 mixture was found to be a suitable medium for sample decomposition using a microwave digestion system. As is apparent from the results, mercury concentration in CCQM-P39 sample could be analyzed with a total expanded uncertainty of about 1.0%.
Acknowledgements This work was supported by grant No. R05-2002-00000741-0 from the Basic Research Program of the Korea Science & Engineering Foundation.
References [1] M. Berlin, L. Friberg, G.F. Nordberg, V.G. Vouk, Handbook of Toxicology Metals, second edition, Specific Metals, vol. 2, Elsevior Biomedical Press, Amsterdam, 1986, p. 387.
[2] L. Friberg, J. Vostal, Mercury in the Environmental, CRC Press, Cleveland, OH, 1972, p. 214. [3] A. Schqtz, G. Skarping, S. Skerfving, Trace Element Analysis in Biological Specimens, Chapter 20, Mercury, Elsevior Biomedical Press, Amsterdam, 1994, pp. 404 – 467. [4] WHO, Environmental Health Criteria, Mercury-Environmental Aspects, vol. 86, World Health Organization, Geneva, 1989, p. 115. [5] J.W. McLaren, D. Beauchemin, S.S. Berman, Application of isotope dilution inductively coupled plasma mass spectrometry to the analysis of marine sediments, Anal. Chem. 59 (1987) 610. [6] K. Okamoto, Isotope dilution/inductively coupled plasma mass spectrometric determination of total tin in NIES fish tissue reference material, Spectrochim. Acta, Part B: Atom. Spectrosc. 46 (1991) 1615. [7] D. Beauchemin, J.W. McLaren, S.S. Berman, Determination of trace metals in marine biological reference materials by inductively coupled plasma mass spectrometry, Anal. Chem. 60 (1988) 687. [8] M.J. Campbell, G. Vermeir, R. Dams, Influence of chemical species on the determination of mercury in a biological matrix (cold muscle) using inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 7 (1992) 617. [9] J. Youshinaga, M. Morita, Determination of mercury in biological and environmental samples by inductively coupled plasma mass spectrometry with the isotope dilution technique, J. Anal. At. Spectrom. 12 (1997) 417. [10] K.G. Heumann, Inorganic Mass Spectrometry, Willy, New-York, 1988, p. 301; Acta 441 (2001) 135. [11] ISO, Guide to the Expression of Uncertainty in Measurement, International Organization for Standardization, Geneva, ISBN: 9267-10188-9, 1993. [12] EURACHEM, Quantifying Uncertainty in Analytical Measurement (Crown Copyright), ISBN: 0-948926-08-2, 1995; D. Chakrabarti, W.D. Jonghe, F. Adams, Anal. Chim. Acta 120 (1980) 121.
Determination of mercury in tuna fish tissue using isotope dilution-inductively coupled plasma mass spectrometry Sang Hak Leea,*, Jung Ki Suhb b
a Department of Chemistry, Kyungpook National University, Taegu, 702-701, Korea Division of Chemical Metrology and Materials Evaluation, Korea Research Institute of Standards and Science, P.O. Box 102, Yusung, Taejon, 305-600, Korea
Received 22 July 2004; accepted 25 July 2004 Available online 12 October 2004
Abstract Isotope dilution-inductively coupled plasma mass spectrometry (ID-ICP/MS) was applied to determine mercury in living tissue. Microwave digestion method using HNO3/H2O2 media for the dissolution of solid sample was studied. The procedure for accurate determination of total mercury in tuna fish tissue sample by ID-ICP/MS is described. For the method validation, total Hg concentration in tuna fish CRM (BCR CRM 463) was determined by ID-ICP/MS after addition of 202Hg to CRM followed by acid decomposition of the spiked sample. This method was applied to the determination of Hg in tuna fish CCQM-P39 sample provided by IRMM (Institute for Reference Materials and Measurement, GEEL, Belgium) for the international comparison study. D 2004 Elsevier B.V. All rights reserved. Keywords: Hg; Isotope dilution-inductively coupled plasma mass spectrometry; CCQM-P39; Tuna fish tissue
1. Introduction Mercury is a serious environmental toxicant and there are several reviews on different aspects of mercury toxicology [1–3]. The main source of human intake of mercury contaminants originates from methyl-mercury in fish and fishery products. Fish and marine mammals and birds preying on fish may contain considerable amounts of mercury [4]. The mercury in fish originates from mercury in water, which is methylated, and accumulates in the fish as such. Mercury is commonly determined in environmental and biological samples by atomic absorption spectrometry (AAS) or inductively coupled plasma mass spectrometry (ICP-MS). These analytical techniques are suitable for routine analysis because of their high sensitivity and reasonable accuracy and precision. In general, it is well known that isotope dilution-inductively
* Corresponding author. Tel.: +82 53 950 5338; fax: +82 53 950 6330. E-mail address: [email protected] (S. Hak Lee). 0026-265X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2004.07.007
coupled plasma/mass spectrometry (ID-ICP/MS) has a high potential for routine analysis of trace elements if the accuracy of results is of predominant analytical importance. Therefore, ID-ICP/MS has frequently been employed in the certification of element contents in certified reference materials (CRMs) [5–8]. Although IDICP/MS method has been widely used for the determination of trace element in various matrices, only a few applications have been reported for the determination of Hg. The problem in Hg analysis is known that the memory effect increases the blank counts and worsens the analytical performance of ICP-MS [9]. Another problem is the possibility of Hg loss during sample decomposition procedure due to its volatility. This paper describes a method to determine Hg by IDMS using quadrupole-inductively coupled plasma mass spectrometry. Microwave digestion with HNO3/H2O2 media was applied to the dissolution of solid sample. For the method validation, total Hg concentration in tuna fish CRM (BCR CRM 463) was determined by ID-ICP/MS after addition of 202 Hg to samples followed by the acid decomposition of the
234
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
Table 1 Operating conditions for ICP-MS ICP-MS instrument
Perkin-Elmer SCIEX Elan 6100DRC
Nebulizer gas flow Auxiliary gas flow Plasma gas flow Lens voltage ICP RF power Analog stage voltage Pulse stage voltage Discriminator threshold Dwell time Sweeps Replicates
0.90 l/min 1.20 l/min 16.00 l/min 6.75 V 1350 W 2550 V 1400 V 80.00 100 ms 30 5
spiked samples. Finally, the present method was applied to the determination of Hg in tuna fish CCQM-P39 sample provided by IRMM for the international comparison study.
2. Experimental 2.1. Instrument The ICP-MS used in this study is a ELAN 6100 DRCICP-MS (Perkin-Elmer SCIEX, Concord, ON, Canada) equipped with a Fassel torch, a Gilson Minipuls peristaltic pump, a Meinhard-type concentric glass nebulizer and a cyclonic-type spray chamber. All data was acquired in normal mode conditions without reaction cell gas. The settings of ion lens system, gas flow rates and other parameters were tuned daily to obtain maximum stability and sensitivity by using tuning solution containing 1 ppb each of Mg, In and U. Under the optimum conditions, the general intensities of Mg, In and U were about 6000, 50,000 and 30,000 counts/s, respectively. The material of sampling cone and skimmer is platinum. Typical operating conditions are summarized in Table 1. 2.2. Reagents A stock standard solution of Hg was prepared by dissolution of pure metals (99.999995%, Alfa, Catalogue
#10634). Working primary standard solutions were prepared by serial dilutions of the stock standard solution. Electronic grade HNO3 (Dong Woo Pure Chemicals, IkSan, Korea) was used with further purification by sub-boiling distillation. The 202Hg-enriched spike isotope (IRMM 640) and isotopic standards for mass bias correction (IRMM 639) were obtained from IRMM (Retieseweg, B-2440 GEEL, Belgium). These isotopes were dissolved in dilute HNO3 and stored in clean Teflon FEP bottles. Low-density polyethylene (LDPE) containers were utilized. These bottles were cleaned by immersing the vessels in 20% HNO3 for 2 days and washed successively with de-ionized water. The composition of enriched isotope (IRMM 640) and isotopic standard (IRMM 639), and potential interferences of Hg isotopes in ICP-MS are shown in Table 2. 2.3. Sample decomposition procedure by microwave digestion method The tuna fish sample, typically 400 mg, was decomposed by microwave digestion method, after adding an appropriate amount of the 202Hg spike solution. On the basis of preliminary investigations and the certified Hg contents of the reference material BCR-463 and CCQM-P39 sample, the optimum blend ratio was calculated as a compromise between lowest error magnification factor and sufficient counting rate [10]. In this case, the calculated optimum 200 Hg/202Hg ratio was in the range 0.06–0.3, with a corresponding error magnification factor of approximately 1.1–1.6. The sample was weighed accurately into the Teflon PTFE microwave vessel together with the 202Hg spike solution. The spike addition was varied with regard to an optimized value of 0.06–0.3 for the amount of mercury from sample to spike in the isotope diluted sample. Concentrated HNO3 (10 ml) and H2O2 (1 ml) were added as a sample decomposition media. It should be noted that overnight digestion at room temperature in the closed vessel was performed before pressurized microwave digestion to ensure enough isotope equilibrium. Microwave heating was then performed on samples in three steps: 250 W for 5 min, 450 W for 5 min and 600 W for 20 min. After dilution with water, the solution was ready for the measurement of 200 Hg/201Hg isotope ratio by ICP-MS.
Table 2 Potential interferences of Hg isotopes and isotopic compositions of natural mercury, isotopic standard and 202
202
Hg-enriched isotope used for IDMS method
Isotope
Natural abundances (%)
Mass fraction of Hg-enriched isotope (IRMM 640)a
Mass fraction of IRMM 640 isotopic standarda
Potential interferences
196
0.15 9.97 16.87 23.10 13.18 29.86 6.87
0.001715(36) 0.0596(11) 0.1543(16) 0.5319(33) 1.2979(50) 97.6985(68) 0.2560(21)
0.1454(12) 9.769(52) 16.689(64) 23.000(58) 13.237(25) 30.148(53) 7.011(30)
Pt, TaO, HfO, WO Pt, TaO, HfO, WO TaO, WO WO WO WO Pb, WO
Hg Hg 199 Hg 200 Hg 201 Hg 202 Hg 204 Hg 198
a
The values are taken from the certificates of supplier (IRMM, GEEL, Belgium). Numbers in parentheses indicate the uncertainty of the reported value.
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
2.4. Dry mass correction The tuna fish sample absorbs ambient moisture at typical laboratory temperature and humidity conditions. Therefore, the sample bottle was opened immediately before weighing aliquots for the IDMS blend preparation. For correction of measured values to dry mass, water content measurement was made on a separate portion of the same material with a mass of 0.5 g sample. The sample was dried before weighing for a 24 h in a ventilated oven at 85F2 8C. In this experiment, the content of moisture of tuna fish sample was 2.07%.
235
Table 4 Summary of analytical results for Hg in BCR CRM 463 according to the variation of reference isotope Reference isotope/ enriched isotope
Measured concentration (A/g)
Certified value and its uncertaintya (A/g)
199
2.875 2.908 2.877
2.85F0.16
Hg/202Hg Hg/202Hg 201 Hg/202Hg 200
a This uncertainty value was expressed as a half-width of the 95% confidence interval of the mean.
when the measured isotope ratio were 199Hg/202Hg, Hg/202Hg and 201Hg/202Hg, respectively, for applying IDMS. The certified value of Hg in this CRM was 2.85F0.16 A/g. There is no meaningful difference between the measured and certified values. The results also indicate that the variation of reference isotope used for the determination of Hg does not influence the analytical result. However, it is desirable to choose 200Hg as a reference isotope considering the highest natural abundance. If the sample contains tungsten (W) component, there is possibility of formation of WO that is interfering molecular ion on the Hg isotopes as shown in Table 2. However, in the preliminary test, tungsten was not detected in the present sample.
200
3. Results and discussion 3.1. Analysis of tuna fish CRM (BCR 463) For the application of IDMS, it is necessary to prepare several kinds of solutions. These are a primary assay standard solution, an enriched spike isotopic solution, a spike calibration solutions (the mixed solution of primary assay standard solution and an enriched spike isotopic solution) and sample blend (a mixed solution of sample and enriched spike isotope). The level of concentration of Hg in sample blends, spike calibration solutions and isotopic standard solution for IDMS was about 15–20 ng/g. The typical value of intensities and isotopic ratios for each solution in ICP-MS was shown in Table 3. The choice of reference isotope and enriched isotope considering possible interferences and natural abundances is important for the accurate determination by IDMS. In general, the isotope with the highest abundance in the sample was chosen as a reference isotope if there is no interference from another element. In the present studies, IDMS was applied to obtain the content of Hg in tuna fish CRM (BCR 463) according to the variation of reference isotope shown in Table 4. As shown in this table, the content of Hg in tuna fish CRM (BCR 463) was found to be 2.875, 2.908 and 2.887 A/g
3.2. Analysis of CCQM-P39 sample and uncertainty evaluations Inorganic Analysis Working Group of CCQM meetings (Paris, Ottawa 2002) agreed that IRMM will act as the pilot laboratory of the CCQM-P39 pilot study, bHg, Pb, Se, As and methyl-mercury in tuna fishQ. Therefore, CCQM-P39 sample was provided from IRMM for international comparison study on April 2003. This sample is a freeze-dried and ground tuna fish powder in amber glass vials. The present method was applied to the determination of Hg in CCQM-P39 sample. The measured Hg concentration in this sample was 21.0710 6 mol/kg and this value agreed well with the mean value of the results obtained from all the participants in the interna-
Table 3 Typical value of intensities and ratios for isotopic standard, spike calibration solutions and sample blends in ICP-MS Solutions
isotope
Intensity (cps)
Ratios
%RSD for ratio
Isotopic standard (IRMM-639)
199
14836 20484 11767 27218 9233 12794 7455 30409 8515 11824 6929 31658
0.545 0.753 0.432 1 0.304 0.421 0.245 1 0.269 0.374 0.219 1
0.60 0.48 0.70
Spike calibration solutions
Sample blend (sample+202Hg)
Hg 200 Hg 201 Hg 202 Hg 199 Hg 200 Hg 201 Hg 202 Hg 199 Hg 200 Hg 201 Hg 202 Hg
0.50 0.46 0.80 0.38 0.39 0.41
236
S. Hak Lee, J. Ki Suh / Microchemical Journal 80 (2005) 233–236
tional comparison study. The uncertainty in our value was evaluated according to the ISO/EURACHEM guides [11,12]. The combined relative standard uncertainty (u c) was found to be 0.5%. When multiplied by the coverage factor k=2, this gives an expanded uncertainty (U=k u c) of 1.04%.
4. Conclusions Despite its possibility of a loss by volatilization and its high memory effect, the content of Hg in a tuna fish reference material (BCR CRM 427) and CCQM-P39 sample could be accurately and precisely determined by ID-ICP/MS using quadrupole-type ICP-MS. The HNO3/H2O2 mixture was found to be a suitable medium for sample decomposition using a microwave digestion system. As is apparent from the results, mercury concentration in CCQM-P39 sample could be analyzed with a total expanded uncertainty of about 1.0%.
Acknowledgements This work was supported by grant No. R05-2002-00000741-0 from the Basic Research Program of the Korea Science & Engineering Foundation.
References [1] M. Berlin, L. Friberg, G.F. Nordberg, V.G. Vouk, Handbook of Toxicology Metals, second edition, Specific Metals, vol. 2, Elsevior Biomedical Press, Amsterdam, 1986, p. 387.
[2] L. Friberg, J. Vostal, Mercury in the Environmental, CRC Press, Cleveland, OH, 1972, p. 214. [3] A. Schqtz, G. Skarping, S. Skerfving, Trace Element Analysis in Biological Specimens, Chapter 20, Mercury, Elsevior Biomedical Press, Amsterdam, 1994, pp. 404 – 467. [4] WHO, Environmental Health Criteria, Mercury-Environmental Aspects, vol. 86, World Health Organization, Geneva, 1989, p. 115. [5] J.W. McLaren, D. Beauchemin, S.S. Berman, Application of isotope dilution inductively coupled plasma mass spectrometry to the analysis of marine sediments, Anal. Chem. 59 (1987) 610. [6] K. Okamoto, Isotope dilution/inductively coupled plasma mass spectrometric determination of total tin in NIES fish tissue reference material, Spectrochim. Acta, Part B: Atom. Spectrosc. 46 (1991) 1615. [7] D. Beauchemin, J.W. McLaren, S.S. Berman, Determination of trace metals in marine biological reference materials by inductively coupled plasma mass spectrometry, Anal. Chem. 60 (1988) 687. [8] M.J. Campbell, G. Vermeir, R. Dams, Influence of chemical species on the determination of mercury in a biological matrix (cold muscle) using inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 7 (1992) 617. [9] J. Youshinaga, M. Morita, Determination of mercury in biological and environmental samples by inductively coupled plasma mass spectrometry with the isotope dilution technique, J. Anal. At. Spectrom. 12 (1997) 417. [10] K.G. Heumann, Inorganic Mass Spectrometry, Willy, New-York, 1988, p. 301; Acta 441 (2001) 135. [11] ISO, Guide to the Expression of Uncertainty in Measurement, International Organization for Standardization, Geneva, ISBN: 9267-10188-9, 1993. [12] EURACHEM, Quantifying Uncertainty in Analytical Measurement (Crown Copyright), ISBN: 0-948926-08-2, 1995; D. Chakrabarti, W.D. Jonghe, F. Adams, Anal. Chim. Acta 120 (1980) 121.