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Determination of Total Mercury in Biological Materials by Cold Vapor Atomic Absorption Spectrometry after Microwave Digestion
MICROCHEMICAL JOURNAL ARTICLE NO.
54, 168–173 (1996)
0090
Determination of Total Mercury in Biological Materials by Cold Vapor Atomic Absorption Spectrometry after Microwave Digestion UJANG TINGGI1
AND
GRAHAM CRAVEN
Queensland Department of Health, Government Chemical Laboratory, 39 Kessels Road, Coopers Plains, Queensland, 4108 Australia Received July 28, 1995; accepted January 22, 1996 The use of several acid mixtures for digestion of marine biological materials with microwaveassisted heating was investigated. The mixtures of HNO3 –H2SO4 –H2O2 and of HNO3 –H2SO4 – HCl were most effective after microwave digestion, resulting in complete decomposition of samples. Recoveries of added mercury to marine biological tissues were satisfactory, ranging from 91 to 108%. This indicated that no matrix interference or loss of volatile mercury occurred during the analysis. The mercury content of standard reference materials after microwave digestion corresponded closely to the certified values. q 1996 Academic Press, Inc.
INTRODUCTION
Increased industrial activity has resulted in elevated levels of toxic metals such as mercury (Hg) in the environment. These metals accumulate in animal tissues and eventually are taken up by humans through the food chain. The adverse toxic effects of Hg on humans are well documented (1). Fish and other seafood products usually contain significantly higher concentrations of Hg than other foodstuffs (2). Concern over environmental pollution for Hg has intensified the search for analytical methods that require minimal sample preparation, and give contaminant-free analysis and good analytical sensitivity. The problem of losing Hg through volatilization during sample digestion at high temperature precludes the use of fusion and dry ashing procedures. To avoid Hg losses, low temperature open wet digestion procedures using several combinations for mixtures of nitric, sulfuric, and perchloric acids have been used widely (3). However, some acid digestion procedures give low recoveries, being partly the result of incomplete dissolution of samples (4). The use of a sealed Teflon vessel enclosed in a stainless steel container and heated at high temperature can prevent losses of volatile Hg (5). Since the introduction of microwave digestion in 1975 by Abu-Samra and colleagues (6), there has been significant interest in using microwave heating to replace conventional heating during acid digestion. The major advantages of microwave digestion using a closed-vessel container are that the digestion time is significantly reduced and problems caused by contamination and losses through volatilization are also minimized 1
To whom correspondence should be addressed. 168
0026-265X/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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(7). Owing to these advantages, the microwave digestion technique is becoming more widely used in the modern analytical laboratories. Several analytical techniques such as electrothermal atomic absorption spectrometry, inductively coupled plasma mass spectrometry, nuclear magnetic resonance, and capillary column gas chromatography have been used for Hg analysis (8). The most commonly used method is cold vapor atomic absorption spectrometry (CV-AAS) due to its ease of operation, high sensitivity, and relatively inexpensive operational cost (9). In the present study, the efficacy of microwave digestion using different combination of acid mixtures for determination of Hg in marine biological tissues prior to CV-AAS analysis was investigated. EXPERIMENTAL
Apparatus All glassware and labware were acid-washed by soaking in 2% (v/v) HCl overnight, rinsing twice with deionized water, and leaving to dry. A Varian Techtron SpectrAA 300 atomic absorption spectrophotometer, equipped with a mercury hollow cathode lamp operated at 4 mA with the spectral band pass and resonance wavelength set at 0.5 and 253.7 nm, respectively, was used. A modified mercury cold vapor apparatus was attached to the spectrophotometer and resulting signals were displayed on a computer (Epson, PC AX2) and then printed (Epson 400X). A commercially available microwave oven Model MDS-81D (CEM Corp, Matthews, NC) with programmable time and power settings and a rotating sample carousel was used for digestion. PFA-Teflon pressure-relief type digestion vessels (120 ml capacity) and capping station were obtained from CEM. The microwave apparatus has a maximum power of 630 W (100%) that is adjustable in 1% increments. Reagents Analytical grade reagents of nitric acid (HNO3) (70%, BDH), hydrochloric acid (HCl) (37%, BDH), sulfuric acid (H2SO4) (98%, BDH), and hydrogen peroxide (H2O2) (30%, BDH) were used. A stock mercury standard (1000 mg/liter) was prepared from mercury(II) chloride in nitric acid and made to 1 liter. A working mercury standard (100 mg/liter) was prepared by appropriate dilution. A stannous chloride solution (20% w/v) was prepared by dissolving in 10% (v/v) HCl. All dilutions were prepared using deionized water obtained by reverse osmosis, and purified by ion-exchange cartridge. Antifoam agent (Dow Corning) was used to prevent vigorous foaming. Sample Preparation Samples of seafood mix were prepared from a mixture of tissues of fish, mussels, prawns, and octopus. These samples were macerated and homogenized together in a commercial blender. Samples were freeze dried and ground to pass through a 200 mm sieve. Fresh seafood samples were macerated in a commercial blender, and an equal weight of water was added to facilitate sample homogenization.
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TINGGI AND CRAVEN TABLE 1 Comparison of the Efficiency of Acid Mixtures for Decomposition of Sample Hg released (mean { s.d.)a
Acid mixture HNO3 (5 ml) HNO3/H2O2 (4:2 v/v; 6 ml) HNO3/H2SO4 (4:0.25 v/v; 4.25 ml) HNO3/H2SO4/H2O2 (4:0.25:2 v/v; 6.25 ml) HNO3/H2SO4/HCl (4:0.25:0.5 v/v; 4.75 ml) a b
0.46 0.61 0.57 0.96 0.92
{ { { { {
0.10 0.04 0.15 0.03 0.05
(n (n (n (n (n
Å Å Å Å Å
4)b 5) 5) 5) 5)
Comments turbid digest turbid digest turbid digest clear digest clear digest
Hg content is expressed as mg/g (dry weight). n is the number of replicate determinations.
Sample Digestion The dry sample was accurately weighed (250 mg) directly into a digestion vessel, and the appropriate amount of acid mixture was added (Table 1). When HCl or H2O2 was required, samples were predigested in HNO3 , cooled, and the appropriate volume of HCl or H2O2 was then added (this would prevent vigorous reactions and losses of volatile Hg). The vessels were tightly capped using the capping station and vented into a collecting container holding water to trap excess acid fumes that might escape. The vessels were then placed in the microwave oven and heated at three stages: at 25% (90 W) for 10 min, at 35% (126 W) for 10 min, and at 50% (180 W) for 8 min. The vessels were cooled in ice water for 20 min. CV-AAS Analysis After the digested samples had cooled, they were transferred into test tubes and made up to 20 ml with deionized water. Two drops of antifoam solution and 2 ml of stannous chloride were added and vortex mixed for 60 s. The reduced mercury vapor was then swept into the optical cell for absorbance reading with a nitrogen gas flow of 3 liter/min. The absorbance signal was registered and the concentration was calculated from a calibration graph (linear range: 0–300 ng) by the computer and results were printed. RESULTS AND DISCUSSION
Effect of Acid Mixture The efficacy of acid mixtures for the microwave digestion of marine biological materials was investigated by comparing recoveries of Hg from a prepared seafood mix. Results in Table 1 show that mixtures of HNO3/H2SO4/H2O2 and HNO3/H2SO4/ HCl gave the highest levels of Hg. Poor recoveries of Hg were obtained when HNO3 , HNO3/H2O2 , and HNO3/H2SO4 were used. These poor recoveries may be due to incomplete decomposition of samples as shown by the turbidity and yellowish appearance of the solution. Turbidity due to poor digestion efficacy has also been reported by Kojima and colleagues (10) using a mixture of HNO3/HCl. They found that botanical samples were more difficult to digest than zoological samples even when HClO4 was added to the HNO3/HCl mixture. Microwave digestion using nitric acid alone,
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DETERMINATION OF TOTAL MERCURY TABLE 2 Comparison of Microwave Digestion with the Low Temperature Water Bath Digestion
Sample
Microwave digestion (HNO3/H2SO4/H2O2)
Water bath digestion (HNO3/H2SO4/HCl)
Seafood mix No. 89 Seafood mix No. 12 Seafood mix No. 94
0.43 { 0.02 (n Å 5)a 0.96 { 0.03 (n Å 5) 0.13 { 0.01 (n Å 5)
0.39 { 0.01 (n Å 9) 0.96 { 0.04 (n Å 6) 0.12 { 0.01 (n Å 5)
Note. All values are expressed in mg/g (dry weight). a n is the number of replicate determinations.
especially for samples containing more fat, has also been reported to give turbid solution as a result of incomplete digestion (11). To obtain a clear digestion solution, they recommended adding 1 ml of 30% H2O2 . Mohd and colleagues (12) have stated that the efficiency of microwave digestion is affected by several parameters which need to be optimized. These include the amount of sample to be used, the type and amount of acid used, the microwave power applied, and the time of dissolution. Poor digestion efficacy could result in incomplete breakdown of resistant organomecurials such as mono- or dimethyl, ethyl, or phenyl Hg, which are found in most biological materials (13). These organomecurials are difficult to reduce to elemental Hg with stannous chloride, and therefore could interfere with the final measurement of total Hg during the CV-AAS analysis (13, 3). However, others have shown that the use of HNO3 alone is very effective for determination of Hg in a variety of biological materials (8, 9). These investigators used Parr-type high pressure vessels irradiated at maximum microwave power for digestion. In this present study, digestion was carried out at 50% microwave power using low pressure vessels. Setting microwave power at 65 and 75% resulted in lower Hg recoveries. At increased power and temperature, vigorous fuming occurred in the tubing connected to the water container. Volatile Hg may have been lost with the fumes and trapped in the water container. It was also observed that high temperature and the presence of sulfuric acid caused damage to the surface of the Teflon digestion vessels. Losses of Hg were also encountered if the digestion vessels were allowed to cool overnight at room temperature. These losses were probably due to adsorption and diffusion of Hg through the Teflon vessels. TABLE 3 Recovery of Mercury from Standard Reference Materials and Microwave Digestion Material
This study (mean { s.d.)
Certified values (mean { s.d.)
Oyster tissue 1566a (n Å 3)a Dorm-II (n Å 4) Dorm-I (n Å 4)
0.062 { 0.004 4.27 { 0.26 0.76 { 0.05
0.0642 { 0.0067 4.64 { 0.26 0.798 { 0.074
Note. All values are expressed in mg/g (dry weight). a n is the number of replicate determinations.
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TINGGI AND CRAVEN TABLE 4 Mean Recovery of Mercury (ng) from Spiked Selected Fresh Marine Samples
Sample
Hg present
Hg added
Hg found
Recovery (%)
Prawns Squids Crab #1 Crab #2 Fish #1 Fish #2
20.0 35.0 52.1 67.1 14.7 19.0
100 100 150 50 100 50
123 140 191 113 124 63
103 105 93 92 108 91
Note. Each value is the mean of triplicate determinations.
Of the acid mixtures tested, HNO3/H2SO4/H2O2 was the most effective. The addition of H2O2 to a HNO3/H2SO4 mixture was preferred over HCl because of its greater oxidizing power and its ability to give a less-turbid digestion solution. Results in Table 2 for mercury in a prepared seafood mix were comparable to those obtained by the method proposed by Louie and colleagues (14) using a low temperature (607C) water bath with a mixture of HNO3/H2SO4/HCl. However, when this method was used, especially for samples that contained more fat, an incomplete decomposition of tissues was encountered as indicated by the appearance of yellowish residues floating in the solution. Accuracy The accuracy of the method was validated by determining Hg in standard reference materials of oyster tissue, SRM 1566a (National Institute of Standards & Technology), and dogfish muscle, DORM-1 and DORM-2 (National Research Council Canada). Hg levels from these samples (Table 3) were in close agreement with the certified values. Recoveries of spiked Hg (Table 4) in fresh seafood tissues were satisfactory and ranged from 91 to 108%. This indicates that no matrix interference and no losses of volatile Hg were encountered during the analysis. CONCLUSIONS
A procedure using an acid mixture of HNO3/H2SO4/H2O2 was found to be very effective with good accuracy for the analysis of Hg in marine biological materials after microwave digestion. Digestion time was also greatly reduced and a complete decomposition of samples was obtained. This would make the microwave digestion an ideal method for routine analysis which, in the long run, would reduce costs. Problems with contamination, high blank, and losses of volatile Hg were minimized because digestion was carried out in a sealed Teflon vessel. REFERENCES 1. World Health Organization. Mercury—Environmental Aspects. World Health Organization, Geneva, 1989. 2. Reilly, C. Metal Contamination of Food, 2nd Ed. Elsevier Applied Science, London, 1991. 3. de Vargas, M. C.; Romero, R. A. Atom. Spectro., 1989, 10, 160–164. 4. Burguera, J. L.; Burguera, M. J. Food Comp. Anal., 1988, 1, 159–165.
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5. Vermier, G.; Vandecasteele, C.; Temmerman, E.; Dams, R.; Versieck, J. Mikrochim. Acta, 1988, 111, 305–313. 6. Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Anal. Chem., 1975, 47, 1475–1477. 7. Mincey, D. W.; Williams, R. C.; Giglio, J. J.; Graves, G. A.; Pacella, A. J. Anal. Chim. Acta, 1992, 264, 97–100. 8. Tahan, J. E.; Granadillo, V. A.; Sanchez, J. M.; Cubillan, H. S.; Romero, R. A. J. Anal. Atom. Spectro., 1993, 8, 1005–1010. 9. Navarro-Alarcon, M.; Lopez-Martinez, M. C.; Sanchez-Vinas, M.; de la Serrana, H. L. J. Agric. Food Chem., 1991, 39, 2223–2225. 10. Kojima, I.; Kato, A.; Iida, C. Anal. Chim. Acta, 1992, 264, 101–106. 11. Xu, L.; Shen, W. Fresenius Z Anal. Chem., 1988, 332, 45–47. 12. Mohd, A. A.; Dean, J. R.; Tomlinson, W. R. Analyst, 1992, 117, 1743–1748. 13. Landi, S.; Fagioli, F.; Locatelli, C. J. AOAC Inter., 1992, 75, 1023–1028. 14. Louie, H. W.; Go, D.; Fedczina, M.; Judd, K.; Dalins, J. J. Assoc. Off. Anal. Chem., 1985, 68, 891– 892.
ah02$$1318
08-21-96 22:21:41
mica
AP: MICROCHEM
54, 168–173 (1996)
0090
Determination of Total Mercury in Biological Materials by Cold Vapor Atomic Absorption Spectrometry after Microwave Digestion UJANG TINGGI1
AND
GRAHAM CRAVEN
Queensland Department of Health, Government Chemical Laboratory, 39 Kessels Road, Coopers Plains, Queensland, 4108 Australia Received July 28, 1995; accepted January 22, 1996 The use of several acid mixtures for digestion of marine biological materials with microwaveassisted heating was investigated. The mixtures of HNO3 –H2SO4 –H2O2 and of HNO3 –H2SO4 – HCl were most effective after microwave digestion, resulting in complete decomposition of samples. Recoveries of added mercury to marine biological tissues were satisfactory, ranging from 91 to 108%. This indicated that no matrix interference or loss of volatile mercury occurred during the analysis. The mercury content of standard reference materials after microwave digestion corresponded closely to the certified values. q 1996 Academic Press, Inc.
INTRODUCTION
Increased industrial activity has resulted in elevated levels of toxic metals such as mercury (Hg) in the environment. These metals accumulate in animal tissues and eventually are taken up by humans through the food chain. The adverse toxic effects of Hg on humans are well documented (1). Fish and other seafood products usually contain significantly higher concentrations of Hg than other foodstuffs (2). Concern over environmental pollution for Hg has intensified the search for analytical methods that require minimal sample preparation, and give contaminant-free analysis and good analytical sensitivity. The problem of losing Hg through volatilization during sample digestion at high temperature precludes the use of fusion and dry ashing procedures. To avoid Hg losses, low temperature open wet digestion procedures using several combinations for mixtures of nitric, sulfuric, and perchloric acids have been used widely (3). However, some acid digestion procedures give low recoveries, being partly the result of incomplete dissolution of samples (4). The use of a sealed Teflon vessel enclosed in a stainless steel container and heated at high temperature can prevent losses of volatile Hg (5). Since the introduction of microwave digestion in 1975 by Abu-Samra and colleagues (6), there has been significant interest in using microwave heating to replace conventional heating during acid digestion. The major advantages of microwave digestion using a closed-vessel container are that the digestion time is significantly reduced and problems caused by contamination and losses through volatilization are also minimized 1
To whom correspondence should be addressed. 168
0026-265X/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
ah02$$1318
08-21-96 22:21:41
mica
AP: MICROCHEM
DETERMINATION OF TOTAL MERCURY
169
(7). Owing to these advantages, the microwave digestion technique is becoming more widely used in the modern analytical laboratories. Several analytical techniques such as electrothermal atomic absorption spectrometry, inductively coupled plasma mass spectrometry, nuclear magnetic resonance, and capillary column gas chromatography have been used for Hg analysis (8). The most commonly used method is cold vapor atomic absorption spectrometry (CV-AAS) due to its ease of operation, high sensitivity, and relatively inexpensive operational cost (9). In the present study, the efficacy of microwave digestion using different combination of acid mixtures for determination of Hg in marine biological tissues prior to CV-AAS analysis was investigated. EXPERIMENTAL
Apparatus All glassware and labware were acid-washed by soaking in 2% (v/v) HCl overnight, rinsing twice with deionized water, and leaving to dry. A Varian Techtron SpectrAA 300 atomic absorption spectrophotometer, equipped with a mercury hollow cathode lamp operated at 4 mA with the spectral band pass and resonance wavelength set at 0.5 and 253.7 nm, respectively, was used. A modified mercury cold vapor apparatus was attached to the spectrophotometer and resulting signals were displayed on a computer (Epson, PC AX2) and then printed (Epson 400X). A commercially available microwave oven Model MDS-81D (CEM Corp, Matthews, NC) with programmable time and power settings and a rotating sample carousel was used for digestion. PFA-Teflon pressure-relief type digestion vessels (120 ml capacity) and capping station were obtained from CEM. The microwave apparatus has a maximum power of 630 W (100%) that is adjustable in 1% increments. Reagents Analytical grade reagents of nitric acid (HNO3) (70%, BDH), hydrochloric acid (HCl) (37%, BDH), sulfuric acid (H2SO4) (98%, BDH), and hydrogen peroxide (H2O2) (30%, BDH) were used. A stock mercury standard (1000 mg/liter) was prepared from mercury(II) chloride in nitric acid and made to 1 liter. A working mercury standard (100 mg/liter) was prepared by appropriate dilution. A stannous chloride solution (20% w/v) was prepared by dissolving in 10% (v/v) HCl. All dilutions were prepared using deionized water obtained by reverse osmosis, and purified by ion-exchange cartridge. Antifoam agent (Dow Corning) was used to prevent vigorous foaming. Sample Preparation Samples of seafood mix were prepared from a mixture of tissues of fish, mussels, prawns, and octopus. These samples were macerated and homogenized together in a commercial blender. Samples were freeze dried and ground to pass through a 200 mm sieve. Fresh seafood samples were macerated in a commercial blender, and an equal weight of water was added to facilitate sample homogenization.
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TINGGI AND CRAVEN TABLE 1 Comparison of the Efficiency of Acid Mixtures for Decomposition of Sample Hg released (mean { s.d.)a
Acid mixture HNO3 (5 ml) HNO3/H2O2 (4:2 v/v; 6 ml) HNO3/H2SO4 (4:0.25 v/v; 4.25 ml) HNO3/H2SO4/H2O2 (4:0.25:2 v/v; 6.25 ml) HNO3/H2SO4/HCl (4:0.25:0.5 v/v; 4.75 ml) a b
0.46 0.61 0.57 0.96 0.92
{ { { { {
0.10 0.04 0.15 0.03 0.05
(n (n (n (n (n
Å Å Å Å Å
4)b 5) 5) 5) 5)
Comments turbid digest turbid digest turbid digest clear digest clear digest
Hg content is expressed as mg/g (dry weight). n is the number of replicate determinations.
Sample Digestion The dry sample was accurately weighed (250 mg) directly into a digestion vessel, and the appropriate amount of acid mixture was added (Table 1). When HCl or H2O2 was required, samples were predigested in HNO3 , cooled, and the appropriate volume of HCl or H2O2 was then added (this would prevent vigorous reactions and losses of volatile Hg). The vessels were tightly capped using the capping station and vented into a collecting container holding water to trap excess acid fumes that might escape. The vessels were then placed in the microwave oven and heated at three stages: at 25% (90 W) for 10 min, at 35% (126 W) for 10 min, and at 50% (180 W) for 8 min. The vessels were cooled in ice water for 20 min. CV-AAS Analysis After the digested samples had cooled, they were transferred into test tubes and made up to 20 ml with deionized water. Two drops of antifoam solution and 2 ml of stannous chloride were added and vortex mixed for 60 s. The reduced mercury vapor was then swept into the optical cell for absorbance reading with a nitrogen gas flow of 3 liter/min. The absorbance signal was registered and the concentration was calculated from a calibration graph (linear range: 0–300 ng) by the computer and results were printed. RESULTS AND DISCUSSION
Effect of Acid Mixture The efficacy of acid mixtures for the microwave digestion of marine biological materials was investigated by comparing recoveries of Hg from a prepared seafood mix. Results in Table 1 show that mixtures of HNO3/H2SO4/H2O2 and HNO3/H2SO4/ HCl gave the highest levels of Hg. Poor recoveries of Hg were obtained when HNO3 , HNO3/H2O2 , and HNO3/H2SO4 were used. These poor recoveries may be due to incomplete decomposition of samples as shown by the turbidity and yellowish appearance of the solution. Turbidity due to poor digestion efficacy has also been reported by Kojima and colleagues (10) using a mixture of HNO3/HCl. They found that botanical samples were more difficult to digest than zoological samples even when HClO4 was added to the HNO3/HCl mixture. Microwave digestion using nitric acid alone,
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DETERMINATION OF TOTAL MERCURY TABLE 2 Comparison of Microwave Digestion with the Low Temperature Water Bath Digestion
Sample
Microwave digestion (HNO3/H2SO4/H2O2)
Water bath digestion (HNO3/H2SO4/HCl)
Seafood mix No. 89 Seafood mix No. 12 Seafood mix No. 94
0.43 { 0.02 (n Å 5)a 0.96 { 0.03 (n Å 5) 0.13 { 0.01 (n Å 5)
0.39 { 0.01 (n Å 9) 0.96 { 0.04 (n Å 6) 0.12 { 0.01 (n Å 5)
Note. All values are expressed in mg/g (dry weight). a n is the number of replicate determinations.
especially for samples containing more fat, has also been reported to give turbid solution as a result of incomplete digestion (11). To obtain a clear digestion solution, they recommended adding 1 ml of 30% H2O2 . Mohd and colleagues (12) have stated that the efficiency of microwave digestion is affected by several parameters which need to be optimized. These include the amount of sample to be used, the type and amount of acid used, the microwave power applied, and the time of dissolution. Poor digestion efficacy could result in incomplete breakdown of resistant organomecurials such as mono- or dimethyl, ethyl, or phenyl Hg, which are found in most biological materials (13). These organomecurials are difficult to reduce to elemental Hg with stannous chloride, and therefore could interfere with the final measurement of total Hg during the CV-AAS analysis (13, 3). However, others have shown that the use of HNO3 alone is very effective for determination of Hg in a variety of biological materials (8, 9). These investigators used Parr-type high pressure vessels irradiated at maximum microwave power for digestion. In this present study, digestion was carried out at 50% microwave power using low pressure vessels. Setting microwave power at 65 and 75% resulted in lower Hg recoveries. At increased power and temperature, vigorous fuming occurred in the tubing connected to the water container. Volatile Hg may have been lost with the fumes and trapped in the water container. It was also observed that high temperature and the presence of sulfuric acid caused damage to the surface of the Teflon digestion vessels. Losses of Hg were also encountered if the digestion vessels were allowed to cool overnight at room temperature. These losses were probably due to adsorption and diffusion of Hg through the Teflon vessels. TABLE 3 Recovery of Mercury from Standard Reference Materials and Microwave Digestion Material
This study (mean { s.d.)
Certified values (mean { s.d.)
Oyster tissue 1566a (n Å 3)a Dorm-II (n Å 4) Dorm-I (n Å 4)
0.062 { 0.004 4.27 { 0.26 0.76 { 0.05
0.0642 { 0.0067 4.64 { 0.26 0.798 { 0.074
Note. All values are expressed in mg/g (dry weight). a n is the number of replicate determinations.
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TINGGI AND CRAVEN TABLE 4 Mean Recovery of Mercury (ng) from Spiked Selected Fresh Marine Samples
Sample
Hg present
Hg added
Hg found
Recovery (%)
Prawns Squids Crab #1 Crab #2 Fish #1 Fish #2
20.0 35.0 52.1 67.1 14.7 19.0
100 100 150 50 100 50
123 140 191 113 124 63
103 105 93 92 108 91
Note. Each value is the mean of triplicate determinations.
Of the acid mixtures tested, HNO3/H2SO4/H2O2 was the most effective. The addition of H2O2 to a HNO3/H2SO4 mixture was preferred over HCl because of its greater oxidizing power and its ability to give a less-turbid digestion solution. Results in Table 2 for mercury in a prepared seafood mix were comparable to those obtained by the method proposed by Louie and colleagues (14) using a low temperature (607C) water bath with a mixture of HNO3/H2SO4/HCl. However, when this method was used, especially for samples that contained more fat, an incomplete decomposition of tissues was encountered as indicated by the appearance of yellowish residues floating in the solution. Accuracy The accuracy of the method was validated by determining Hg in standard reference materials of oyster tissue, SRM 1566a (National Institute of Standards & Technology), and dogfish muscle, DORM-1 and DORM-2 (National Research Council Canada). Hg levels from these samples (Table 3) were in close agreement with the certified values. Recoveries of spiked Hg (Table 4) in fresh seafood tissues were satisfactory and ranged from 91 to 108%. This indicates that no matrix interference and no losses of volatile Hg were encountered during the analysis. CONCLUSIONS
A procedure using an acid mixture of HNO3/H2SO4/H2O2 was found to be very effective with good accuracy for the analysis of Hg in marine biological materials after microwave digestion. Digestion time was also greatly reduced and a complete decomposition of samples was obtained. This would make the microwave digestion an ideal method for routine analysis which, in the long run, would reduce costs. Problems with contamination, high blank, and losses of volatile Hg were minimized because digestion was carried out in a sealed Teflon vessel. REFERENCES 1. World Health Organization. Mercury—Environmental Aspects. World Health Organization, Geneva, 1989. 2. Reilly, C. Metal Contamination of Food, 2nd Ed. Elsevier Applied Science, London, 1991. 3. de Vargas, M. C.; Romero, R. A. Atom. Spectro., 1989, 10, 160–164. 4. Burguera, J. L.; Burguera, M. J. Food Comp. Anal., 1988, 1, 159–165.
ah02$$1318
08-21-96 22:21:41
mica
AP: MICROCHEM
DETERMINATION OF TOTAL MERCURY
173
5. Vermier, G.; Vandecasteele, C.; Temmerman, E.; Dams, R.; Versieck, J. Mikrochim. Acta, 1988, 111, 305–313. 6. Abu-Samra, A.; Morris, J. S.; Koirtyohann, S. R. Anal. Chem., 1975, 47, 1475–1477. 7. Mincey, D. W.; Williams, R. C.; Giglio, J. J.; Graves, G. A.; Pacella, A. J. Anal. Chim. Acta, 1992, 264, 97–100. 8. Tahan, J. E.; Granadillo, V. A.; Sanchez, J. M.; Cubillan, H. S.; Romero, R. A. J. Anal. Atom. Spectro., 1993, 8, 1005–1010. 9. Navarro-Alarcon, M.; Lopez-Martinez, M. C.; Sanchez-Vinas, M.; de la Serrana, H. L. J. Agric. Food Chem., 1991, 39, 2223–2225. 10. Kojima, I.; Kato, A.; Iida, C. Anal. Chim. Acta, 1992, 264, 101–106. 11. Xu, L.; Shen, W. Fresenius Z Anal. Chem., 1988, 332, 45–47. 12. Mohd, A. A.; Dean, J. R.; Tomlinson, W. R. Analyst, 1992, 117, 1743–1748. 13. Landi, S.; Fagioli, F.; Locatelli, C. J. AOAC Inter., 1992, 75, 1023–1028. 14. Louie, H. W.; Go, D.; Fedczina, M.; Judd, K.; Dalins, J. J. Assoc. Off. Anal. Chem., 1985, 68, 891– 892.
ah02$$1318
08-21-96 22:21:41
mica
AP: MICROCHEM