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l. Levels of mercury in water by electrothermal atomization atomic-absorption spectrometry after solvent extraction
Tdanta, Vol. 37,No. 12,pp. 11194122,1990 Printedin Great Britain
0039-9140/90 $3.00+ 0.00 Pergamon Press plc
DETERMINATION OF 1 rig/l.. LEVELS OF MERCURY WATER BY ELECTROTHERMAL ATOMIZATION ATOMIC-ABSORPTION SPECTROMETRY AFTER SOLVENT EXTRACTION
IN
A. LE BEIAN* and J. Y. CABON URA CNRS 322, Universitk de BretagneCkcidentale,6, Avenue Le Gorgeu, 29287 Brest Cedex, France (Received 14 December 1989. Revised 26 June 1990. Accepted 10 July 1990) Summary-4ptimization of the furnace parameters for electrothermal atomization of mercury leads to a characteristic mass of 20 pg in aqueous solution and 30pg in chloroform extracts (with Zeeman correction). With a single-step solvent extraction from 100 ml of sample, performed in the sampling vessel, and direct injection of 400 ~1 of the extract into the furnace, a characteristic concentration of -0.8 rig/l.. is reached.
Trace determinations of mercury are currently generally based on the work of Hatch and Ott.’ The sensitivity of these methods is high but not sufficient for the direct determination of mercury in unpolluted natural waters (at concentrations of a few ng/l.). To reach such low levels it is necessary to preconcentrate the mercury atoms in the cold vapour; the means generally used are amalgamation in a gold trap before determination by atomic-absorption2-5 or atomicfluorescence spectrometry6 with long-path cells. An alternative is amalgamation in gold-’ or platinum-coated’ furnaces before electrothermal atomic-absorption (AA) spectrometry. In such conditions the characteristic concentration can be decreased by using a larger sample volume. Trace mercury determination has been reviewed by Drabaek and Iverfeldt,’ who emphasize that sampling and storage, and the addition of reagents are critical steps in the determination of ultratrace levels. To overcome these problems we propose a procedure for determination of mercury in the rig/l.. range with sample volumes around 100 ml, by use of only conventional electrothermal AA spectrometers. To reduce the risk of mercury losses (or gains by contamination) the metal is determined in the solvent used for the extraction, which is performed directly in the sampling vessel.
*Author for correspondence.
EXPERIMENTAL
Apparatus
Sampling and extraction were performed in lOO-ml glass flasks fitted with Teflon stoppers. The AA spectrometer was a Perkin-Elmer 23030 equipped with an AS-60 automatic sampler in which the plastic cups had been replaced by glass tubes (2.5 cm high). Fumacewall atomization from pyrolytically graphitecoated graphite tubes was used. The light-source was a Perkin-Elmer EDL working at 5 W; the 253.7 nm line was used. The solvents and reagents were pipetted with a Gilson Pipetman pneumatic syringe modified to accommodate glass Pasteur pipette tips. Glassware was used because it allows the elimination of adsorbed mercury by heating. Reagents
Analytical reagent grade chloroform was washed before use, by shaking it with an equal volume of Millipore MQ ultrapure water, and its mercury level was checked by AAS after evaporation of 250ml of the washed solvent to low volume in a glass beaker and final evaporation in the furnace. The concentration found was 0.08 pg/ml. Analytical reagent grade sodium diethyldithiocarbamate (DDTC) was used as a 10 mg/ml aqueous solution, purified by extraction with an equal volume of purified chloroform. The pH of this solution was around 9.3 and the complexing capacity did not decrease significantly during three months storage at 20”.
1119
1120
A. LE BUN and J. Y. CABON
For calibration a 6 pg/l. mercury diethyldithiocarbamate solution in chloroform was prepared monthly from analytical grade mercuric nitrate and DDTC. Procedure One ml of DDTC solution, 1.5 ml of chloroform and a sample volume close to 100 ml are introduced into a loo-ml flask; the exact amount of sample is measured by weighing. At room temperature the solubility of chloroform in water is 5.0ml/l., the solubility of water in the solvent being negligible (0.8 ml/l.). To prevent contamination from the calomel electrode, the pH of the water sample, which should be between 3 and 9, is not adjusted beforehand but is measured after the extraction. The flask is then shaken for 2 min, and the two phases are separated by centrifugation. The organic phase is transferred to the automatic-sampler cup by pneumatic syringe, and covered with 0.5 ml of ultrapurified water to prevent evaporation of the solvent. A sample (80 ~1) of the organic phase is introduced into the graphite furnace and dried by heating at 80” for 25 sec. When necessitated by a low mercury concentration, multiple injection and drying steps are performed. Atomization is performed by heating at 700” for 4 set; the furnace temperature is then lowered to about 30”. Three see after the beginning of atomization, integration of the signal for 10 set is started. The nitrogen flow is stopped during the whole of the atomization and measurement steps. A high-temperature cleanout step follows the integration. Calibration over the 0.7-20 rig/l.. mercury concentration range is performed by injecting 5-80 ~1 of the 6 pg/l. mercury diethyldithiocarbamate chloroform solution into the graphite furnace. Under these conditions, a single injection of 80 ~1 of the chloroform extract leads to a characteristic concentration of mercury in the water sample of about 3.8 rig/l.. To ensure access to the 1 rig/l.. level it is necessary to inject 400 ~1 of solvent (5 successive injections of 80~1), the characteristic concentration then being about 0.8 rig/l.. when measured in the peak area mode.
when the peak height is measured instead of the peak area. When the atomization temperature is lowered, m, decreases when measured in the peak-area mode, but increases if the peak height is used. Thus, the sensitivity is always better when peak areas are measured. Moreover, with the mercury EDL, the integrated noise signal is in the 0.001 set range for a lo-set integration time, whereas the maximum absorbance of the noise signal in the peak-height mode is about 0.003. For a CHClJDDTC extract of mercury the background absorbance is always below 0.03 and consequently is not increased significantly by the noise signal in the peak-height mode. However, the signal to noise ratio is more than five times better in the peak-area mode than in the peak-height mode, so peak-area measurements are used throughout. From work by L’vov et al.” it appears that under fixed experimental conditions m, is proportional to D, the diffusion coefficient of the neutral atoms in the gas phase, when the peak area is measured. The diffusion coefficient varies with the temperature: D, = D2,J T/273) where T is the absolute temperature and n varies from 1.5 to 1.8.11-‘3 Thus it appears that the sensitivity is maximum when the temperature is high enough for efficient generation of neutral atoms. In the L’vov experiments, under isothermal conditions with the STPF (stabilized temperature platform furnace) technique, at 1200”, the characteristic mass was m, = 52 pg of mercury. Also with the STPF technique at 10&Y, but in the presence of palladium as matrix modifier,‘4*‘s Grobenski et al. obtained an m. value of 85 pg with a Perkin-Elmer furnace.16 Under our experimental conditions, with mercuric nitrate solution, the signal is visible when an atomization temperature as low as 200” is reached. Thus operating at low temperatures for the minimum time necessary for efficient transformation of ionic mercury into neutral atoms increases the sensitivity. Although our Table 1. Furnace parameters for determination of mercury in chloroform extracts (80 ~1) Drying
RESULTS AND DISCUSSION
Optimized conditions for the AAS measurements At high atomization temperatures the characteristic mass of mercury, m,, is twice as high
Temperature, “C Ramp time, set Hold time, set Integration, set N, flow, ml/min
Atomization
80
700
30
1
1
1
24 300
3
8 9 0
1 0
Clean-out 2500 1 4 300
~ete~nation
of 1 r&l. leveIs of mercury in water
experiments are not performed by the STPF technique, the delay in the appearance of neutral atoms, at between 500 and llOO”, is so large (more than 3 set) that the gas and wall thermal equilib~a are reached when the neutral atoms are in the furnace atmosphere.” The variation of mo, for a IO-set integration time, with temperature during the atomization is represented in Fig. 1 for mercury introduced as mercuric nitrate solution. The values of I) are also reported in Fig. 1. The quasi-constant value of about 30 pg for m0 from 200 to 500” indicates that the increase in diffusion with temperature is compensated by more efficient atomization, At above 500” and up to 1200”, the variation of m, is parallel to the change in diffusion coefficient; diffusion is the governing factor at these temperatures. At above 1200” the increase in characteristic mass is steeper but in this case the gas thermal equilibrium is not reached. When the temperature of the furnace is lowered from 500 to 30” as soon as the absorption maximum is reached, the diffusion coefficient decreases and the mercury atoms are kept in the optical path by contraction of the gas. Under such conditions the characteristic mass is lowered from 30 to 20 pg. When the mercury is introduced as its diethyldithiocarbamate complex the minimum temperature for efficient atomization is about 650”. This increase in the atomization temperature is probably due to the stability of the rne~u~-sulphide compounds resulting from
Tf*C)
Fig. I. Characteristic mass and diffusion coefficient of mercury as a function of atomization temperature: (a) characteristic mass (pg); (0) diffusion coefficient (cm2/sec).
1121
decomposition of the DDTC. Owing to the lack of precision for the temperature measurements, the atomization temperature is set to a higher value, XKl”, a precaution which ensures the efficiency of neutral atom production. The characteristic mass m. is 30 pg under these conditions. It should be mentioned that m, varies slightly not only from one furnace to another, but also with the number of atomizations performed.
Filipelli has shown that mercury and its organo-derivatives are fully extracted into chloroform from sea-water at pH > 3, with ammonium p y~olidine~t~ocarbamate (APDC) as the complexing agent.” From his results the distribution ratio, P, was estimated to be larger than 3000.18To prevent losses of metal, samples are generally stored under acidic conditions; in such a medium, APDC is more stable than DDTC and is generally used as the complexing agent. In the experimental procedure proposed here, mercury losses are prevented by performing the complexation at the time of sample collection; the extraction is then also done in the same flask. Under such conditions acidification of the sample is not necessary and consequently diethyldithiocarbamate (DDTC) can be used; this is an advantage as DDTC is more stable than APDC in weakly acidic media. By means of two consecutive extractions with chlorofo~ from the same water sample, the distribution ratio P between water and chloroform has been measured for mercuric ions, and methylmercury and phenylmercury chlorides, with DDTC as’the complexing extractant.‘* For aqueous phase pH values between 3 and 9, P is greater than 2500, for all three mercury species. Consequently, the yield is better than 96% for a single-step extraction, if the water to solvent volume ratio is 100. Use of more than one extraction is time-consuming and increases the risk of dilution and contamination errors. Under our experimental conditions, the reproducibility for the introduction and atomization steps is better than 1.5% (RSD for 10 measurements) for chloroform solutions at the 500 pg mercury level. The variation in ~on~ntration that could result from evaporation from glass tubes open to the air at room temperature is less than 5% during half an hour, in the case of chloroform, and in this case the evaporation can be
A. Lx BMANand J. Y. CABON
1122
suppressed by adding a few drops of water on top of the organic phase. To avoid any hazards associated with chloroform it is possible to extract the mercury complex with freon but in this case the reproducibility of the introduction and atomization steps increases to about 3% (RSD for 10 measurements). The experimental method described here is the first step towards our goal, the determination of mercury in rain water. In this case, the sample volumes collected are variable and the total mass of mercury collected can be as low as 1OOpg or even lower. Under the experimental conditions described the relative standard deviation is 30% for 10 measurements at the 1 rig/l.. level of mercury in water; the relative standard deviation decreases to 15% for concentrations around 5 rig/l.. The measurement takes only 5 min, and the complete procedure about 10. CONCLUSION The determination of mercury in water, at the 1 rig/l.. level, requires optimization of the experimental conditions for both the atomicabsorption spectrophotometry and the extraction. A low characteristic mass is achieved by integration of the signal and by choosing an atomization temperature close to 700”; in such conditions the characteristic mass is 30 pg. When the extraction with DDTC/CHCl, is performed directly in the sampling vessel at a sample-to-solvent volume ratio of 100, the characteristic concentration is better than 1 ng/l., with a 30% relative standard deviation. Besides high sensitivity, the advantages of the
procedure are the saving in time and the reduced risks of contamination with or loss of mercury. REFERENCES 1. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 2. S. J. Long, D. R. Scott and J. R. Thompson, ibid., 1973, 45, 2227. 3. N. S. Bloom and E. A. Crecelius, Mar. Chem., 1983,14, 49. 4. R. Ahmed and M. Stoeppler, in M. Stoeppler and H. W. Diirbek (eds.), Contributions to Environmental Specimen Banking, Jiil-Spez-349, KFA Jiilich, 1986. 5. D. Coma and J. Noel, Mar. Chem., 1987, 28, 389. 6. N. Bloom and W. Fitzgerald, Anul. Chim. Acta, 1988, 208, 151. 7. S. H. Lee, K. H. Jung and D. S. Lee, Tafanta, 1989,36, 999. 8. D. C. Baxter and W. Freoh, Anal. Chim. Acta, 1989, 225, 175. 9. I. Drabaek and A. Iverfeldt, in Hazardous Metals in the Environment, M. Stoeppler (ed.), Chapter 7, Elsevier, Amsterdam, in the press. 10. B. V. L’vov, V. G. Nikolaev, E. A. Norman, L. K. Polxik and M. Mojioa, Spectrochim. Acta, 1986, 41B, 1043. 11. F. Fagioli, C. Looatelli, R. Vecohietti and G. Torxi, J. Anal. Atom. Spectrom., 1988, 3, 159. 12. B. V. L’vov, L. K. Polxik and L. F. Yatsenko, Talanta, 1987, 34, 141. 13. W. M. G. T. van den Broek and L. de Galan, Anal. Chem., 1987, 49, 2176. 14. X.-Q. Shan and Z.-M. Ni, Acta Chim. Sin., 1979, 37, 261. 15. G. F. Kirkbright, H.-C. Shan and R. D. Snook, At. Spectrosc., 1980, 1, 85. 16. Z. Grobenski, W. Erler and U. Voellkopf, ibid., 1985, 6, 91. 17. C. J. Radmeyer and H. G. C. Human, J. Anal. Atom. Spectrom., 1989, 4, 393. 18. M. Filipelli, Analyst, 1984, 109, 515. 19. A. Le Bihan and J. Y. Cabon. Analwis. , 1990.,, 18. 126.
0039-9140/90 $3.00+ 0.00 Pergamon Press plc
DETERMINATION OF 1 rig/l.. LEVELS OF MERCURY WATER BY ELECTROTHERMAL ATOMIZATION ATOMIC-ABSORPTION SPECTROMETRY AFTER SOLVENT EXTRACTION
IN
A. LE BEIAN* and J. Y. CABON URA CNRS 322, Universitk de BretagneCkcidentale,6, Avenue Le Gorgeu, 29287 Brest Cedex, France (Received 14 December 1989. Revised 26 June 1990. Accepted 10 July 1990) Summary-4ptimization of the furnace parameters for electrothermal atomization of mercury leads to a characteristic mass of 20 pg in aqueous solution and 30pg in chloroform extracts (with Zeeman correction). With a single-step solvent extraction from 100 ml of sample, performed in the sampling vessel, and direct injection of 400 ~1 of the extract into the furnace, a characteristic concentration of -0.8 rig/l.. is reached.
Trace determinations of mercury are currently generally based on the work of Hatch and Ott.’ The sensitivity of these methods is high but not sufficient for the direct determination of mercury in unpolluted natural waters (at concentrations of a few ng/l.). To reach such low levels it is necessary to preconcentrate the mercury atoms in the cold vapour; the means generally used are amalgamation in a gold trap before determination by atomic-absorption2-5 or atomicfluorescence spectrometry6 with long-path cells. An alternative is amalgamation in gold-’ or platinum-coated’ furnaces before electrothermal atomic-absorption (AA) spectrometry. In such conditions the characteristic concentration can be decreased by using a larger sample volume. Trace mercury determination has been reviewed by Drabaek and Iverfeldt,’ who emphasize that sampling and storage, and the addition of reagents are critical steps in the determination of ultratrace levels. To overcome these problems we propose a procedure for determination of mercury in the rig/l.. range with sample volumes around 100 ml, by use of only conventional electrothermal AA spectrometers. To reduce the risk of mercury losses (or gains by contamination) the metal is determined in the solvent used for the extraction, which is performed directly in the sampling vessel.
*Author for correspondence.
EXPERIMENTAL
Apparatus
Sampling and extraction were performed in lOO-ml glass flasks fitted with Teflon stoppers. The AA spectrometer was a Perkin-Elmer 23030 equipped with an AS-60 automatic sampler in which the plastic cups had been replaced by glass tubes (2.5 cm high). Fumacewall atomization from pyrolytically graphitecoated graphite tubes was used. The light-source was a Perkin-Elmer EDL working at 5 W; the 253.7 nm line was used. The solvents and reagents were pipetted with a Gilson Pipetman pneumatic syringe modified to accommodate glass Pasteur pipette tips. Glassware was used because it allows the elimination of adsorbed mercury by heating. Reagents
Analytical reagent grade chloroform was washed before use, by shaking it with an equal volume of Millipore MQ ultrapure water, and its mercury level was checked by AAS after evaporation of 250ml of the washed solvent to low volume in a glass beaker and final evaporation in the furnace. The concentration found was 0.08 pg/ml. Analytical reagent grade sodium diethyldithiocarbamate (DDTC) was used as a 10 mg/ml aqueous solution, purified by extraction with an equal volume of purified chloroform. The pH of this solution was around 9.3 and the complexing capacity did not decrease significantly during three months storage at 20”.
1119
1120
A. LE BUN and J. Y. CABON
For calibration a 6 pg/l. mercury diethyldithiocarbamate solution in chloroform was prepared monthly from analytical grade mercuric nitrate and DDTC. Procedure One ml of DDTC solution, 1.5 ml of chloroform and a sample volume close to 100 ml are introduced into a loo-ml flask; the exact amount of sample is measured by weighing. At room temperature the solubility of chloroform in water is 5.0ml/l., the solubility of water in the solvent being negligible (0.8 ml/l.). To prevent contamination from the calomel electrode, the pH of the water sample, which should be between 3 and 9, is not adjusted beforehand but is measured after the extraction. The flask is then shaken for 2 min, and the two phases are separated by centrifugation. The organic phase is transferred to the automatic-sampler cup by pneumatic syringe, and covered with 0.5 ml of ultrapurified water to prevent evaporation of the solvent. A sample (80 ~1) of the organic phase is introduced into the graphite furnace and dried by heating at 80” for 25 sec. When necessitated by a low mercury concentration, multiple injection and drying steps are performed. Atomization is performed by heating at 700” for 4 set; the furnace temperature is then lowered to about 30”. Three see after the beginning of atomization, integration of the signal for 10 set is started. The nitrogen flow is stopped during the whole of the atomization and measurement steps. A high-temperature cleanout step follows the integration. Calibration over the 0.7-20 rig/l.. mercury concentration range is performed by injecting 5-80 ~1 of the 6 pg/l. mercury diethyldithiocarbamate chloroform solution into the graphite furnace. Under these conditions, a single injection of 80 ~1 of the chloroform extract leads to a characteristic concentration of mercury in the water sample of about 3.8 rig/l.. To ensure access to the 1 rig/l.. level it is necessary to inject 400 ~1 of solvent (5 successive injections of 80~1), the characteristic concentration then being about 0.8 rig/l.. when measured in the peak area mode.
when the peak height is measured instead of the peak area. When the atomization temperature is lowered, m, decreases when measured in the peak-area mode, but increases if the peak height is used. Thus, the sensitivity is always better when peak areas are measured. Moreover, with the mercury EDL, the integrated noise signal is in the 0.001 set range for a lo-set integration time, whereas the maximum absorbance of the noise signal in the peak-height mode is about 0.003. For a CHClJDDTC extract of mercury the background absorbance is always below 0.03 and consequently is not increased significantly by the noise signal in the peak-height mode. However, the signal to noise ratio is more than five times better in the peak-area mode than in the peak-height mode, so peak-area measurements are used throughout. From work by L’vov et al.” it appears that under fixed experimental conditions m, is proportional to D, the diffusion coefficient of the neutral atoms in the gas phase, when the peak area is measured. The diffusion coefficient varies with the temperature: D, = D2,J T/273) where T is the absolute temperature and n varies from 1.5 to 1.8.11-‘3 Thus it appears that the sensitivity is maximum when the temperature is high enough for efficient generation of neutral atoms. In the L’vov experiments, under isothermal conditions with the STPF (stabilized temperature platform furnace) technique, at 1200”, the characteristic mass was m, = 52 pg of mercury. Also with the STPF technique at 10&Y, but in the presence of palladium as matrix modifier,‘4*‘s Grobenski et al. obtained an m. value of 85 pg with a Perkin-Elmer furnace.16 Under our experimental conditions, with mercuric nitrate solution, the signal is visible when an atomization temperature as low as 200” is reached. Thus operating at low temperatures for the minimum time necessary for efficient transformation of ionic mercury into neutral atoms increases the sensitivity. Although our Table 1. Furnace parameters for determination of mercury in chloroform extracts (80 ~1) Drying
RESULTS AND DISCUSSION
Optimized conditions for the AAS measurements At high atomization temperatures the characteristic mass of mercury, m,, is twice as high
Temperature, “C Ramp time, set Hold time, set Integration, set N, flow, ml/min
Atomization
80
700
30
1
1
1
24 300
3
8 9 0
1 0
Clean-out 2500 1 4 300
~ete~nation
of 1 r&l. leveIs of mercury in water
experiments are not performed by the STPF technique, the delay in the appearance of neutral atoms, at between 500 and llOO”, is so large (more than 3 set) that the gas and wall thermal equilib~a are reached when the neutral atoms are in the furnace atmosphere.” The variation of mo, for a IO-set integration time, with temperature during the atomization is represented in Fig. 1 for mercury introduced as mercuric nitrate solution. The values of I) are also reported in Fig. 1. The quasi-constant value of about 30 pg for m0 from 200 to 500” indicates that the increase in diffusion with temperature is compensated by more efficient atomization, At above 500” and up to 1200”, the variation of m, is parallel to the change in diffusion coefficient; diffusion is the governing factor at these temperatures. At above 1200” the increase in characteristic mass is steeper but in this case the gas thermal equilibrium is not reached. When the temperature of the furnace is lowered from 500 to 30” as soon as the absorption maximum is reached, the diffusion coefficient decreases and the mercury atoms are kept in the optical path by contraction of the gas. Under such conditions the characteristic mass is lowered from 30 to 20 pg. When the mercury is introduced as its diethyldithiocarbamate complex the minimum temperature for efficient atomization is about 650”. This increase in the atomization temperature is probably due to the stability of the rne~u~-sulphide compounds resulting from
Tf*C)
Fig. I. Characteristic mass and diffusion coefficient of mercury as a function of atomization temperature: (a) characteristic mass (pg); (0) diffusion coefficient (cm2/sec).
1121
decomposition of the DDTC. Owing to the lack of precision for the temperature measurements, the atomization temperature is set to a higher value, XKl”, a precaution which ensures the efficiency of neutral atom production. The characteristic mass m. is 30 pg under these conditions. It should be mentioned that m, varies slightly not only from one furnace to another, but also with the number of atomizations performed.
Filipelli has shown that mercury and its organo-derivatives are fully extracted into chloroform from sea-water at pH > 3, with ammonium p y~olidine~t~ocarbamate (APDC) as the complexing agent.” From his results the distribution ratio, P, was estimated to be larger than 3000.18To prevent losses of metal, samples are generally stored under acidic conditions; in such a medium, APDC is more stable than DDTC and is generally used as the complexing agent. In the experimental procedure proposed here, mercury losses are prevented by performing the complexation at the time of sample collection; the extraction is then also done in the same flask. Under such conditions acidification of the sample is not necessary and consequently diethyldithiocarbamate (DDTC) can be used; this is an advantage as DDTC is more stable than APDC in weakly acidic media. By means of two consecutive extractions with chlorofo~ from the same water sample, the distribution ratio P between water and chloroform has been measured for mercuric ions, and methylmercury and phenylmercury chlorides, with DDTC as’the complexing extractant.‘* For aqueous phase pH values between 3 and 9, P is greater than 2500, for all three mercury species. Consequently, the yield is better than 96% for a single-step extraction, if the water to solvent volume ratio is 100. Use of more than one extraction is time-consuming and increases the risk of dilution and contamination errors. Under our experimental conditions, the reproducibility for the introduction and atomization steps is better than 1.5% (RSD for 10 measurements) for chloroform solutions at the 500 pg mercury level. The variation in ~on~ntration that could result from evaporation from glass tubes open to the air at room temperature is less than 5% during half an hour, in the case of chloroform, and in this case the evaporation can be
A. Lx BMANand J. Y. CABON
1122
suppressed by adding a few drops of water on top of the organic phase. To avoid any hazards associated with chloroform it is possible to extract the mercury complex with freon but in this case the reproducibility of the introduction and atomization steps increases to about 3% (RSD for 10 measurements). The experimental method described here is the first step towards our goal, the determination of mercury in rain water. In this case, the sample volumes collected are variable and the total mass of mercury collected can be as low as 1OOpg or even lower. Under the experimental conditions described the relative standard deviation is 30% for 10 measurements at the 1 rig/l.. level of mercury in water; the relative standard deviation decreases to 15% for concentrations around 5 rig/l.. The measurement takes only 5 min, and the complete procedure about 10. CONCLUSION The determination of mercury in water, at the 1 rig/l.. level, requires optimization of the experimental conditions for both the atomicabsorption spectrophotometry and the extraction. A low characteristic mass is achieved by integration of the signal and by choosing an atomization temperature close to 700”; in such conditions the characteristic mass is 30 pg. When the extraction with DDTC/CHCl, is performed directly in the sampling vessel at a sample-to-solvent volume ratio of 100, the characteristic concentration is better than 1 ng/l., with a 30% relative standard deviation. Besides high sensitivity, the advantages of the
procedure are the saving in time and the reduced risks of contamination with or loss of mercury. REFERENCES 1. W. R. Hatch and W. L. Ott, Anal. Chem., 1968, 40, 2085. 2. S. J. Long, D. R. Scott and J. R. Thompson, ibid., 1973, 45, 2227. 3. N. S. Bloom and E. A. Crecelius, Mar. Chem., 1983,14, 49. 4. R. Ahmed and M. Stoeppler, in M. Stoeppler and H. W. Diirbek (eds.), Contributions to Environmental Specimen Banking, Jiil-Spez-349, KFA Jiilich, 1986. 5. D. Coma and J. Noel, Mar. Chem., 1987, 28, 389. 6. N. Bloom and W. Fitzgerald, Anul. Chim. Acta, 1988, 208, 151. 7. S. H. Lee, K. H. Jung and D. S. Lee, Tafanta, 1989,36, 999. 8. D. C. Baxter and W. Freoh, Anal. Chim. Acta, 1989, 225, 175. 9. I. Drabaek and A. Iverfeldt, in Hazardous Metals in the Environment, M. Stoeppler (ed.), Chapter 7, Elsevier, Amsterdam, in the press. 10. B. V. L’vov, V. G. Nikolaev, E. A. Norman, L. K. Polxik and M. Mojioa, Spectrochim. Acta, 1986, 41B, 1043. 11. F. Fagioli, C. Looatelli, R. Vecohietti and G. Torxi, J. Anal. Atom. Spectrom., 1988, 3, 159. 12. B. V. L’vov, L. K. Polxik and L. F. Yatsenko, Talanta, 1987, 34, 141. 13. W. M. G. T. van den Broek and L. de Galan, Anal. Chem., 1987, 49, 2176. 14. X.-Q. Shan and Z.-M. Ni, Acta Chim. Sin., 1979, 37, 261. 15. G. F. Kirkbright, H.-C. Shan and R. D. Snook, At. Spectrosc., 1980, 1, 85. 16. Z. Grobenski, W. Erler and U. Voellkopf, ibid., 1985, 6, 91. 17. C. J. Radmeyer and H. G. C. Human, J. Anal. Atom. Spectrom., 1989, 4, 393. 18. M. Filipelli, Analyst, 1984, 109, 515. 19. A. Le Bihan and J. Y. Cabon. Analwis. , 1990.,, 18. 126.