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Total Mercury Determination in Biological and Environmental Standard Samples by Gold Amalgamation Followed by Cold Vapor Atomic Absorption Spectrometry
JOBNAME: 53#2 96 PAGE: 1 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
MICROCHEMICAL JOURNAL ARTICLE NO. 0028
53, 195–200 (1996)
Total Mercury Determination in Biological and Environmental Standard Samples by Gold Amalgamation Followed by Cold Vapor Atomic Absorption Spectrometry JACKSON M. OMBABA1 Department of Chemistry, Jackson State University, Jackson, Mississippi 39217 Received September 22, 1995; accepted October 13, 1995 As a result of industrialization and changes in the environment during the twentieth century, human and animals are exposed to numerous chemical forms of mercury, including elemental mercury vapor (Hg0), inorganic mercurous (Hg+) and mercuric (Hg2+) compounds, and organic mercuric (R–Hg+ or R–Hg–R, where R represents any organic ligand) compounds. All forms of mercury have toxic effects, therefore, it is important to determine total mercury in biological and environmental samples to establish baseline levels. This paper highlights the methodology developed for total mercury determination in biological and environmental samples using preconcentration by mercury amalgamation accessory followed by cold vapor atomic absorption spectrometry. The high-pressure microwave digested mercury samples were chemically reduced by stannous chloride and the vapor generated was collected on a mercury amalgamation tube (MAT). The concentrated mercury was revolatilized by rapid heating of the MAT to 500–800°C and was transferred to the absorption cell for determination by cold vapor atomic absorption spectrometry. The preconcentration on the MAT and subsequent revolatilization methodology was compared with the cold vapor atomic absorption determination without the preconcentration step. The preconcentration proved to be sensitive and as the result of the high sensitivity, small sample volumes were analyzed and only short sampling times were required. The values for NIST certified human urine, apple leaves, rice flour, bovine liver, buffalo sediments, estuarine sediments, and coal fly ash standard samples were found to be 0.103, 0.067, 0.013, 0.006, 1.267, 0.013, and 0.233 ppm, respectively, versus the certified values of 0.105, 0.044, 0.0058, 0.004, 1.440, 0.063, and 0.16 ppm, respectively. The method is thus suitable for continuous monitoring of mercury and for the fast and reliable determination of mercury in biological and environmental samples, even at background levels. © 1996 Academic Press, Inc.
INTRODUCTION Increasing industrialization and large scale use of agriculture chemicals are potential sources of environmental pollutants which may ultimately find their way to human body. Accumulation of toxic heavy metals such as mercury (Hg) in the body may adversely affect some physiological functions. Because of the adverse toxic effects of mercury compounds, their accurate determination in biological and environmental samples has received great attention (1–24). Numerous techniques have been developed for the determination of low mercury levels in both biological and environmental samples. These techniques include colorimetry (1), radiochemical neutron activation analysis (RNAA) (2), atomic fluorescence spectrometry (3), helium-microwave induced plasma atomic emission spectrometry (4), and graphite furnace atomic absorption spectrometry (5). Some of these techniques have limitations. For example, the application of colorimetry or microwave induced plasma results in poor 1
To whom correspondence should be addressed. 195 0026-265X/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
JOBNAME: 53#2 96 PAGE: 2 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
196
JACKSON M. OMBABA
detection limits. Neutron activation analysis requires expensive equipment, access to an irradiation source, and the method itself is time consuming. Graphite furnace atomic absorption spectrometry (GFAAS) is a widely used technique for the determination of heavy and toxic metals in biological and environmental samples. However, GFAAS suffers from chemical and matrix interferences. The utilization of atomic fluorescence spectrometry for the direct determination of mercury is difficult because of the quenching of the fluorescence signal by nitrogen and oxygen (6–8). One of the most widely used techniques for the determination of mercury is the cold vapor generation method followed by atomic absorption spectrometry (9–11). Either stannous or borohydride compounds are used to reduce the mercury compounds to elemental mercury (Hg0), followed by sweeping the free mercury atoms into an absorption cell and subsequent detection by appropriate mercury line. To improve on the sensitivity and to be able to determine mercury at very low concentrations, several enrichment techniques have been proposed. Some of these proposals include preconcentration of mercury on copper wire (12) or platinum (9, 13–15) and preconcentration of the volatilized mercury on gold (16–19) or silver (20). Although cold vapor atomic absorption spectrometry is one of the most sensitive techniques for the determination of trace and ultra-trace concentrations of mercury in biological and environmental samples, direct determination of the element is not feasible because of the potential interferences from the large organic and inorganic matrix usually present in these type of samples. To overcome this problem, mercury in these samples often requires decomposition of the organic and inorganic matrix and the conversion of the mercury to the inorganic divalent form which is easily reduced by the appropriate reducing agent. Decomposition is usually achieved by wet digestion. Several digestion methods have been proposed for the pretreatment of environmental and biological samples for total mercury determinations. Typically, these wet digestions involve combinations of strong acids and/or oxidants at elevated temperatures in open or closed systems (21–24). There is no standard digestion method for any one particular sample but it is generally agreed that the application of high-temperature and high-pressure microwave systems drastically reduces the digestion time and cuts down on the use of high amount of acids and samples. In this paper, the application of mercury amalgamation accessory (MAC) for the determination of total mercury in environmental and biological standard samples is reported. The high-temperature and high-pressure microwave system was used for the digestion of the samples. The MAC data are compared to those obtained without the preconcentration step. EXPERIMENTAL Instrumentation A Varian atomic absorption spectrophotometer (Palo Alto, CA, Model SpectrAA-400P) equipped with a 17 cm T-shaped absorption cell (i.d. 4 1.5 cm, with windows) was used. Absorbance signals were recorded on a citizen printer. A mercury hollow-cathode lamp was used as the line source at 253.7 nm. Mercury vapor was generated in the laboratory by employing a Varian vapor generator accessory (Model VGA-76) under a continuous flow of sample and reductant. The generated vapor was carried out using a flow of
JOBNAME: 53#2 96 PAGE: 3 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
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nitrogen gas on a continuous flow mode. The generated mercury vapor was sent from the VGA to the absorption cell placed in the optical path of the AAS. Elemental mercury was measured by employing integration of the peak area absorbance and with a calibration curve based on known mercury standards. A Varian mercury concentration accessory (Model MCA-90) was used to concentrate mercury vapor. The MCA-90 is equipped with a mercury amalgamation tube (MAT). The tube has gold sites which traps the mercury vapor during the concentration step. The MAT was placed between the vapor generator accessory and the T-shaped absorption cell placed on the optical path of the mercury line. During the concentration step, the temperature of the MAT was kept at or near room temperature and the MAT was rapidly heated to a temperature of 500–800°C during the measurement period. Peak height was used for absorbance evaluation. Peak area could not be used because the signal was not transient. The digestion procedures were conducted by utilizing a high-pressure, high-temperature microwave unit (Model MSD-81D, CEM Corporation, Indian Trail, NC), equipped with a closed vessel accessory specially designed to control the pressure and temperature during a given digestion procedure. Teflon PFA (polyfluoroalkol) vessels were used with the digestion procedure. This microwave unit is capable of holding a maximum of 11 samples per digestion time period. Reagents and Standard Solutions All solutions were prepared from analytical reagent-grade chemicals with Millipore distilled and deionized water and stored in polyethylene bottles. The reduction solution was prepared by dissolution of 50 g of SnCl2 · 2H2O (Merck) in 100 ml of ultrex HCl with constant stirring. The solution was warmed on a hot plate to complete dissolution and then was allowed to cool. The solution was added to about 300 ml of distilled water with constant stirring. The volume was adjusted to 500 ml with distilled water. It was freshly prepared weekly. Mercury stock solutions were prepared from 1000 ppm standard solutions. Working standards were prepared daily from the stock solution by serial dilution using 1% ultrex nitric acid (J. T. Baker). Certified reference materials (CRM) from the National Institute of Standards and Technology (NIST) (U.S. Department of Commerce, Gaithersburg, MD), as well as laboratory testing quality control urine materials from Biorad (Germany), were used to validate the procedures and method developed here and establish the percentage of analyte recovery. Digestion Procedures NIST certified human urine (SRM #2670). Standard reference freeze dried human urine for mercury determination was reconstituted by adding 20.0 ml of distilled and deionized water. The reconstituted urine (2.0 ml) was mixed with potassium persulfate (0.2 g), ultrex nitric acid (1.0 ml), and distilled and deionized water (1.0 ml). The mixture was vortexed for 30 s to make sure that the urine was mixed with the acid very well before closing the vessels and starting the digestion procedure. A spiked urine sample was prepared by spiking 2.0 ml of the reconstituted sample with a known amount of mercury standard and the spiked urine sample was treated the same way as described above. Both the urine sample and the spiked urine sample were close digested at 100% microwave power for 2 min, followed by 50% power for 4 min. The vessels were allowed to cool, 0.5 ml of
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H2SO4 acid was added, and the samples were subjected to open digestion at 10% microwave power for 10 min. Apple leaves (SRM #1515), Citrus leaves, Rice flour (SRM #1568a), and Bovine liver (SRM #1577b). Approximately 0.2 g each of NIST certified apple leaves, citrus leaves, and rice flour were weighed and dissolved in a solution of 3.0 ml of ultrex nitric acid and 2.0 ml distilled and deionized water. The mixtures were vortexed for 30 s and microwave close digested at 100% power for 3 min, at 50% power for 10 min, and lastly at 20% power for 20 min. The vessels were allowed to cool and the samples were diluted with distilled and deionized water and then analyzed both with and without a preconcentration step. Buffalo sediment (SRM #2704), estuarine sediment (SRM #1646), urban particulates (SRM #1648), and coal fly ash (SRM #1633a). Approximately 0.2 g each of NIST certified buffalo sediment, estuarine sediment, coal fly ash, and urban particulates were weighed and dissolved in a mixture solution of 3.0 ml of ultrex nitric acid and 1.0 ml of distilled and deionized water. Each mixture solution was vortexed for 30 s and microwave close digested at 100% power for 3 min, at 50% power for 5 min, and at 30% power for 20 min. After the vessels had cooled to room temperature, they were opened and 2.0 ml of ultrex HCl and 3.0 ml of ultrex HF were added to each. The digestion recipe outlined above was followed. RESULT AND DISCUSSION Five (5) biological samples (human urine, apple leaves, citrus leaves, rice flour, and bovine liver) and four (4) environmental samples (buffalo sediment, estuarine sediment, urban particulate, and coal fly ash) were digested in triplicate and analyzed with and without a preconcentration step. The results of these experiments are listed in Tables 1, 2, and 3. Table 1 shows the result of total mercury determination for standard reference freeze dried human urine. The efficiency of this digestion methodology was evaluated by measuring the recoveries of known standard solution and spiked human urine with known certified mercury values. It can be seen from the table that the recovery of mercury was satisfactory and ranged from 92 to 100%. Table 1 also shows that there is a good agreement between the preconcentration and the nonpreconcentration step. As shown in Table 1, the values of Hg found in this study are in close agreement with the values of the certified references materials. The precision of the methodology was determined from the TABLE 1 Analytical Values for Total Mercury Concentrations for Standard Solution and NIST Freeze Dried Urine Sample Results (ppm) Sample
VGA76-CVAASa
MAC90-CVAASb
Certified value (ppm)
Standard solution Human urine Spiked human urine
0.059 0.103 0.110
0.058 0.104 0.109
0.061 0.105 0.109 (expected)
a b
Vapor generator accessory-cold vapor atomic absorption. Mercury concentration accessory-cold vapor atomic absorption spectrometry.
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TABLE 2 Analytical Values for Total Mercury Concentrations for NIST Certified Biological Samples Results (ppm) Sample
VGA76-CVAAS
MAC90-CVAAS
Certified value (ppm)
Apple leaves Rice flour Citrus leaves Bovine liver
0.067 0.013 0.0023 0.006
0.059 0.006 0.0001 0.024
0.044 0.0058 — 0.004
relative standard deviation (RSD) of over 10 replicate digestions of human urine. RSD values fall below 10%. Table 2 summarizes the mercury values obtained from the biological samples digested as previously discussed in the Procedure section. The table also compares the two methods of analysis. As clearly shown in Table 2, there is good agreement between the results obtained here and the certified values reported by NIST. In this particular analysis, the results obtained by the mercury amalgamation accessory are more in agreement with the certified values than the results obtained by the mercury vapor generator accessory without the preconcentration step. There is no significant statistical difference between the two methods of analysis. Table 3 summarizes the results obtained from the environmental samples, viz., buffalo sediment, estuarine sediment, urban particulates, and coal fly ash. These particular samples are known to be difficult to digest. For example, it is extremely difficult to completely digest coal fly ash due to the presence of insoluble silicates. Some workers have found that even when mercury is almost quantitatively released from environmental samples into the acid solution, there is potential for interferences due to the presence of other metal ions. With the MAC technique approach, this potential for interference from metal ions is drastically reduced. The results obtained from these environmental samples were in good agreement with the certified values as shown in Table 3. Of the four environmental samples analyzed, the results for buffalo sediments were found to be the best. The higher values reported for the others could be due to possible chemical interferences from metals ions. CONCLUSION Application of mercury amalgamation accessory has been shown to be effective equipment for total mercury determination of microwave digested environmental and biological TABLE 3 Analytical Values for Total Mercury Concentrations for NIST Certified Environmental Samples Results (ppm) Sample
VGA76-CVAAS
MAC90-CVAAS
Certified value (ppm)
Buffalo sediments Estuarine sediments Urban particulates Coal fly ash
1.267 0.013 0.125 0.233
1.745 0.015 0.025 0.302
1.440 0.063 — 0.16
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samples. With the limited experiments reported here it appears that the results obtained confirm that the digestion methodologies outlined here are acceptable techniques for the determination of mercury in biological and environmental samples. It should be noted that pressurized microwave digestion is appropriate equipment for complete destruction of the organic and inorganic materials which will otherwise bring about some interference during the analysis of mercury in biological and environmental samples. There is no universal recipe for different types of matrices, but the ones outlined above may be regarded as the starting point for microwave digestion methodological development. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Horvat, M.; Lupsina, V.; Pihlar, B. Anal. Chem. Acta., 1991, 273, 71. Zmijeska, J.; Radioanal. Chem., 1977, 35, 389. Airey, D.; Sci. Total Environ., 1982, 25, 19. Fukushi, K.; Willie, S. N.; Sturgeon, R. E. Anal. Lett., 1993, 26(2), 325. Tinggi, U.; Reilly, C.; Hahn, S.; Capra, M. J. Sci. Total Environ., 1992, 125, 15. Thomson, K. C.; Reynold, G. D. Analyst, 1971, 96, 771. Robinson, J. W.; Araktingi, Y. E. Anal. Chim. Acta., 1973, 63, 29. Cavalli, P.; Rossi, G. Analyst, 1976, 101, 272. Topping, G.; Pirie, J. M. Anal. Chim. Acta., 1972, 62, 200. Lee, S. H.; Jung, K. H.; Lee, D. S. Talanta, 1989, 36, 99. Hladky, Z.; Risova, J.; Fisera, M. J. Anal. At. Spectrom., 1990, 5, 691. Brandenberger, H.; Bader, H. Helva. Chim. Acta., 1967, 50, 1409. Welz, B.; Melcher, M.; Sinemus, H. W.; Maier, D. At. Spectrom., 1984, 5, 37. Schroeder, W. H.; Environ. Sci. Technol., 1982, 16, 394A. Fitzergerald, W. F.; Gill, G. A. Anal. Chem., 1979, 51, 1714. Olafsso, J.; Anal. Chim. Acta., 1974, 68, 208. Schroeder, W. H.; Hamilton, M. C.; Stobart, S. R. Rev. Anal. Chem., 1985, 8, 179. Scott, J. E.; Ottaway, J. M. Analyst, 1981, 106, 1076. Temmerman, E.; Vandecasteele, C.; Vermeir, G.; Layman, R.; Dams, R. Anal. Chim. Acta., 1990, 236, 371. Kalb, G. W.; Absorpt. Newsl., 1970, 9, 84. Uchino, E.; Kosuga, T.; Konishi, S.; Nishimura, M. Environ. Sci. Technol., 1987, 21, 920. Ahmed, R.; May, K.; Stoeppler, M. Fresenius’ Z. Anal. Chem., 1987, 326, 510. Ping, L.; Dasgupta, P. K. Anal. Chem., 1989, 61, 1230. Hatch, W. R.; Ott, W. L. Anal. Chem., 1968, 40, 2085.
MICROCHEMICAL JOURNAL ARTICLE NO. 0028
53, 195–200 (1996)
Total Mercury Determination in Biological and Environmental Standard Samples by Gold Amalgamation Followed by Cold Vapor Atomic Absorption Spectrometry JACKSON M. OMBABA1 Department of Chemistry, Jackson State University, Jackson, Mississippi 39217 Received September 22, 1995; accepted October 13, 1995 As a result of industrialization and changes in the environment during the twentieth century, human and animals are exposed to numerous chemical forms of mercury, including elemental mercury vapor (Hg0), inorganic mercurous (Hg+) and mercuric (Hg2+) compounds, and organic mercuric (R–Hg+ or R–Hg–R, where R represents any organic ligand) compounds. All forms of mercury have toxic effects, therefore, it is important to determine total mercury in biological and environmental samples to establish baseline levels. This paper highlights the methodology developed for total mercury determination in biological and environmental samples using preconcentration by mercury amalgamation accessory followed by cold vapor atomic absorption spectrometry. The high-pressure microwave digested mercury samples were chemically reduced by stannous chloride and the vapor generated was collected on a mercury amalgamation tube (MAT). The concentrated mercury was revolatilized by rapid heating of the MAT to 500–800°C and was transferred to the absorption cell for determination by cold vapor atomic absorption spectrometry. The preconcentration on the MAT and subsequent revolatilization methodology was compared with the cold vapor atomic absorption determination without the preconcentration step. The preconcentration proved to be sensitive and as the result of the high sensitivity, small sample volumes were analyzed and only short sampling times were required. The values for NIST certified human urine, apple leaves, rice flour, bovine liver, buffalo sediments, estuarine sediments, and coal fly ash standard samples were found to be 0.103, 0.067, 0.013, 0.006, 1.267, 0.013, and 0.233 ppm, respectively, versus the certified values of 0.105, 0.044, 0.0058, 0.004, 1.440, 0.063, and 0.16 ppm, respectively. The method is thus suitable for continuous monitoring of mercury and for the fast and reliable determination of mercury in biological and environmental samples, even at background levels. © 1996 Academic Press, Inc.
INTRODUCTION Increasing industrialization and large scale use of agriculture chemicals are potential sources of environmental pollutants which may ultimately find their way to human body. Accumulation of toxic heavy metals such as mercury (Hg) in the body may adversely affect some physiological functions. Because of the adverse toxic effects of mercury compounds, their accurate determination in biological and environmental samples has received great attention (1–24). Numerous techniques have been developed for the determination of low mercury levels in both biological and environmental samples. These techniques include colorimetry (1), radiochemical neutron activation analysis (RNAA) (2), atomic fluorescence spectrometry (3), helium-microwave induced plasma atomic emission spectrometry (4), and graphite furnace atomic absorption spectrometry (5). Some of these techniques have limitations. For example, the application of colorimetry or microwave induced plasma results in poor 1
To whom correspondence should be addressed. 195 0026-265X/96 $18.00 Copyright © 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
JOBNAME: 53#2 96 PAGE: 2 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
196
JACKSON M. OMBABA
detection limits. Neutron activation analysis requires expensive equipment, access to an irradiation source, and the method itself is time consuming. Graphite furnace atomic absorption spectrometry (GFAAS) is a widely used technique for the determination of heavy and toxic metals in biological and environmental samples. However, GFAAS suffers from chemical and matrix interferences. The utilization of atomic fluorescence spectrometry for the direct determination of mercury is difficult because of the quenching of the fluorescence signal by nitrogen and oxygen (6–8). One of the most widely used techniques for the determination of mercury is the cold vapor generation method followed by atomic absorption spectrometry (9–11). Either stannous or borohydride compounds are used to reduce the mercury compounds to elemental mercury (Hg0), followed by sweeping the free mercury atoms into an absorption cell and subsequent detection by appropriate mercury line. To improve on the sensitivity and to be able to determine mercury at very low concentrations, several enrichment techniques have been proposed. Some of these proposals include preconcentration of mercury on copper wire (12) or platinum (9, 13–15) and preconcentration of the volatilized mercury on gold (16–19) or silver (20). Although cold vapor atomic absorption spectrometry is one of the most sensitive techniques for the determination of trace and ultra-trace concentrations of mercury in biological and environmental samples, direct determination of the element is not feasible because of the potential interferences from the large organic and inorganic matrix usually present in these type of samples. To overcome this problem, mercury in these samples often requires decomposition of the organic and inorganic matrix and the conversion of the mercury to the inorganic divalent form which is easily reduced by the appropriate reducing agent. Decomposition is usually achieved by wet digestion. Several digestion methods have been proposed for the pretreatment of environmental and biological samples for total mercury determinations. Typically, these wet digestions involve combinations of strong acids and/or oxidants at elevated temperatures in open or closed systems (21–24). There is no standard digestion method for any one particular sample but it is generally agreed that the application of high-temperature and high-pressure microwave systems drastically reduces the digestion time and cuts down on the use of high amount of acids and samples. In this paper, the application of mercury amalgamation accessory (MAC) for the determination of total mercury in environmental and biological standard samples is reported. The high-temperature and high-pressure microwave system was used for the digestion of the samples. The MAC data are compared to those obtained without the preconcentration step. EXPERIMENTAL Instrumentation A Varian atomic absorption spectrophotometer (Palo Alto, CA, Model SpectrAA-400P) equipped with a 17 cm T-shaped absorption cell (i.d. 4 1.5 cm, with windows) was used. Absorbance signals were recorded on a citizen printer. A mercury hollow-cathode lamp was used as the line source at 253.7 nm. Mercury vapor was generated in the laboratory by employing a Varian vapor generator accessory (Model VGA-76) under a continuous flow of sample and reductant. The generated vapor was carried out using a flow of
JOBNAME: 53#2 96 PAGE: 3 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
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nitrogen gas on a continuous flow mode. The generated mercury vapor was sent from the VGA to the absorption cell placed in the optical path of the AAS. Elemental mercury was measured by employing integration of the peak area absorbance and with a calibration curve based on known mercury standards. A Varian mercury concentration accessory (Model MCA-90) was used to concentrate mercury vapor. The MCA-90 is equipped with a mercury amalgamation tube (MAT). The tube has gold sites which traps the mercury vapor during the concentration step. The MAT was placed between the vapor generator accessory and the T-shaped absorption cell placed on the optical path of the mercury line. During the concentration step, the temperature of the MAT was kept at or near room temperature and the MAT was rapidly heated to a temperature of 500–800°C during the measurement period. Peak height was used for absorbance evaluation. Peak area could not be used because the signal was not transient. The digestion procedures were conducted by utilizing a high-pressure, high-temperature microwave unit (Model MSD-81D, CEM Corporation, Indian Trail, NC), equipped with a closed vessel accessory specially designed to control the pressure and temperature during a given digestion procedure. Teflon PFA (polyfluoroalkol) vessels were used with the digestion procedure. This microwave unit is capable of holding a maximum of 11 samples per digestion time period. Reagents and Standard Solutions All solutions were prepared from analytical reagent-grade chemicals with Millipore distilled and deionized water and stored in polyethylene bottles. The reduction solution was prepared by dissolution of 50 g of SnCl2 · 2H2O (Merck) in 100 ml of ultrex HCl with constant stirring. The solution was warmed on a hot plate to complete dissolution and then was allowed to cool. The solution was added to about 300 ml of distilled water with constant stirring. The volume was adjusted to 500 ml with distilled water. It was freshly prepared weekly. Mercury stock solutions were prepared from 1000 ppm standard solutions. Working standards were prepared daily from the stock solution by serial dilution using 1% ultrex nitric acid (J. T. Baker). Certified reference materials (CRM) from the National Institute of Standards and Technology (NIST) (U.S. Department of Commerce, Gaithersburg, MD), as well as laboratory testing quality control urine materials from Biorad (Germany), were used to validate the procedures and method developed here and establish the percentage of analyte recovery. Digestion Procedures NIST certified human urine (SRM #2670). Standard reference freeze dried human urine for mercury determination was reconstituted by adding 20.0 ml of distilled and deionized water. The reconstituted urine (2.0 ml) was mixed with potassium persulfate (0.2 g), ultrex nitric acid (1.0 ml), and distilled and deionized water (1.0 ml). The mixture was vortexed for 30 s to make sure that the urine was mixed with the acid very well before closing the vessels and starting the digestion procedure. A spiked urine sample was prepared by spiking 2.0 ml of the reconstituted sample with a known amount of mercury standard and the spiked urine sample was treated the same way as described above. Both the urine sample and the spiked urine sample were close digested at 100% microwave power for 2 min, followed by 50% power for 4 min. The vessels were allowed to cool, 0.5 ml of
JOBNAME: 53#2 96 PAGE: 4 SESS: 16 OUTPUT: Sat Jun 1 06:16:43 1996 /xypage/worksmart/tsp000/04ç5/4
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H2SO4 acid was added, and the samples were subjected to open digestion at 10% microwave power for 10 min. Apple leaves (SRM #1515), Citrus leaves, Rice flour (SRM #1568a), and Bovine liver (SRM #1577b). Approximately 0.2 g each of NIST certified apple leaves, citrus leaves, and rice flour were weighed and dissolved in a solution of 3.0 ml of ultrex nitric acid and 2.0 ml distilled and deionized water. The mixtures were vortexed for 30 s and microwave close digested at 100% power for 3 min, at 50% power for 10 min, and lastly at 20% power for 20 min. The vessels were allowed to cool and the samples were diluted with distilled and deionized water and then analyzed both with and without a preconcentration step. Buffalo sediment (SRM #2704), estuarine sediment (SRM #1646), urban particulates (SRM #1648), and coal fly ash (SRM #1633a). Approximately 0.2 g each of NIST certified buffalo sediment, estuarine sediment, coal fly ash, and urban particulates were weighed and dissolved in a mixture solution of 3.0 ml of ultrex nitric acid and 1.0 ml of distilled and deionized water. Each mixture solution was vortexed for 30 s and microwave close digested at 100% power for 3 min, at 50% power for 5 min, and at 30% power for 20 min. After the vessels had cooled to room temperature, they were opened and 2.0 ml of ultrex HCl and 3.0 ml of ultrex HF were added to each. The digestion recipe outlined above was followed. RESULT AND DISCUSSION Five (5) biological samples (human urine, apple leaves, citrus leaves, rice flour, and bovine liver) and four (4) environmental samples (buffalo sediment, estuarine sediment, urban particulate, and coal fly ash) were digested in triplicate and analyzed with and without a preconcentration step. The results of these experiments are listed in Tables 1, 2, and 3. Table 1 shows the result of total mercury determination for standard reference freeze dried human urine. The efficiency of this digestion methodology was evaluated by measuring the recoveries of known standard solution and spiked human urine with known certified mercury values. It can be seen from the table that the recovery of mercury was satisfactory and ranged from 92 to 100%. Table 1 also shows that there is a good agreement between the preconcentration and the nonpreconcentration step. As shown in Table 1, the values of Hg found in this study are in close agreement with the values of the certified references materials. The precision of the methodology was determined from the TABLE 1 Analytical Values for Total Mercury Concentrations for Standard Solution and NIST Freeze Dried Urine Sample Results (ppm) Sample
VGA76-CVAASa
MAC90-CVAASb
Certified value (ppm)
Standard solution Human urine Spiked human urine
0.059 0.103 0.110
0.058 0.104 0.109
0.061 0.105 0.109 (expected)
a b
Vapor generator accessory-cold vapor atomic absorption. Mercury concentration accessory-cold vapor atomic absorption spectrometry.
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TABLE 2 Analytical Values for Total Mercury Concentrations for NIST Certified Biological Samples Results (ppm) Sample
VGA76-CVAAS
MAC90-CVAAS
Certified value (ppm)
Apple leaves Rice flour Citrus leaves Bovine liver
0.067 0.013 0.0023 0.006
0.059 0.006 0.0001 0.024
0.044 0.0058 — 0.004
relative standard deviation (RSD) of over 10 replicate digestions of human urine. RSD values fall below 10%. Table 2 summarizes the mercury values obtained from the biological samples digested as previously discussed in the Procedure section. The table also compares the two methods of analysis. As clearly shown in Table 2, there is good agreement between the results obtained here and the certified values reported by NIST. In this particular analysis, the results obtained by the mercury amalgamation accessory are more in agreement with the certified values than the results obtained by the mercury vapor generator accessory without the preconcentration step. There is no significant statistical difference between the two methods of analysis. Table 3 summarizes the results obtained from the environmental samples, viz., buffalo sediment, estuarine sediment, urban particulates, and coal fly ash. These particular samples are known to be difficult to digest. For example, it is extremely difficult to completely digest coal fly ash due to the presence of insoluble silicates. Some workers have found that even when mercury is almost quantitatively released from environmental samples into the acid solution, there is potential for interferences due to the presence of other metal ions. With the MAC technique approach, this potential for interference from metal ions is drastically reduced. The results obtained from these environmental samples were in good agreement with the certified values as shown in Table 3. Of the four environmental samples analyzed, the results for buffalo sediments were found to be the best. The higher values reported for the others could be due to possible chemical interferences from metals ions. CONCLUSION Application of mercury amalgamation accessory has been shown to be effective equipment for total mercury determination of microwave digested environmental and biological TABLE 3 Analytical Values for Total Mercury Concentrations for NIST Certified Environmental Samples Results (ppm) Sample
VGA76-CVAAS
MAC90-CVAAS
Certified value (ppm)
Buffalo sediments Estuarine sediments Urban particulates Coal fly ash
1.267 0.013 0.125 0.233
1.745 0.015 0.025 0.302
1.440 0.063 — 0.16
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samples. With the limited experiments reported here it appears that the results obtained confirm that the digestion methodologies outlined here are acceptable techniques for the determination of mercury in biological and environmental samples. It should be noted that pressurized microwave digestion is appropriate equipment for complete destruction of the organic and inorganic materials which will otherwise bring about some interference during the analysis of mercury in biological and environmental samples. There is no universal recipe for different types of matrices, but the ones outlined above may be regarded as the starting point for microwave digestion methodological development. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
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