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Radiotracer investigation of the cold-vapour atomic absorption method of analysis for trace mercury
Chimica Acta, 101 (1978) 429-432 o XlsevierScientific Publishing Company, Amsterdam - Printed in The Netherlands
Anclytica
Short Communication
RADIOTRACER INVESTIGATION OF THE COLD-VAPOUR ATOMIC ABSORPTION METHOD OF ANALYSIS FOR TRACE MERCURY
D. C. STUART*
Nuclear Research
Building,
AfcMaster
University,
Hamilton,
Ontario
(Canada)
(Received 27th February 1978)
Trace analysis for mercury in environmental samples, and especially in foodstuffs, has become of great interest because of the highly toxic nature of this element and its compounds. The analytical technique most commonly employed is atomic absorption [l] , because it is widely available and relatively straightforward. Normal atomic absorption spectrometers can be converted for cold-vapour mercury analysis by commercially available kits [Z] . Many devices have been reported in the literature, but all such systems are basically the same, following the methods of Hatch and Ott [3] and Poluetkov et al. [4,51Because of certain problems found in application of this method, a careful check of the procedures used was undertaken, with radiotracer mercury to facilitate the investigation. The results obtained in conjunction with those of sample ashing procedures 161, indicate that the method is less straightforward than its simplicity suggests. Eqerimen tai Appamtrts and reagents. A schematic diagram of the setcup used, based on the Jarrell-Ash kit, is shown in Fig. 1. Radioactivities were measured with a 7.6 X 7.6-cm sodium iodide well-type scintillation detector connected to a Canberra model 8100 multichannel pulse-height analyzer. Ail chemicals used were of reagent-grade purity. 19’Hg was produced in the McMaster Nuclear Reactor by irradiating known amounts of msrcury(II)
chloride sealed in quartz capsules; the capsules were then frozen in liquid nitrogen and broken open, and solutions were prepared for use as standards. Procedures. Each step in the recommended Jarrell-Ash procedure [2] was followed carefully by means of the radiotracer mercury standards. The test sample must contain the mercury to be analyzed as Hg(II) in solution, neither organically bound nor strongly complexed. An aliquot of tin@) chloride solution is added to the test sample to reduce the mercury present, and the reaction vessel is attached to the aeration apparatus at point A *Present Address: SLOWPOKE Nuclear Reactor, Trace Analysis Research Centre, Dalhousie University, Halifax, N-S., Canada B3H 451.
430
A,:'
.*’
I
u _,’
;’
,=’
Fig. 1. Mercury accessory kit. (A) Reaction vessel (13 ml); (B) tubing and stopcock (6.5 ml); (D) drying tube and plastic tubing (10 ml); (E) absorption cell (29 ml); (F) flowmeter.
(Fig. 1). Stopcocks I3 and C are turned to bubble air through the test solution. This sweeps the reduced mercury into the absorption cell, E, where the atomic absorption at 253.7 nm is monitored. Stability of mercury solutions Standards containing 1.8 X lo-? g of radiotracer mercury were placed in the reaction vessels. While standing, no loss of mercury was observed, but after addition of the tin(n) chloride, immediate loss occurred followed by a continuous slow loss. If the reaction vessel was shaken to promote mixing, immediate losses of about 10% were measured. Figure 2 shows typical data. This demonstrates the necessity of ensuring that the sample be connected to the apparatus the instant that reductant is added; better still, reductant should be added to the already connected apparatus.
0
5
IO
20
15 Tune
25
30
(mid
Fig. 2. Stability of mercury soiution. (A) Addition of tin@) briefly.
chloride; (B) solution shaken
431
Trcpping
in the system standards were aerated through the flowthrough system, it was found that significant quantities of mercury were trapped. The drying agent was especially bad. In particular, calcium chloride, when it became slightly moist, trapped all the radiotracer mercury. Magnesium perchlorate was considerably better, typically trapping only several per cent of 2 1.8 X lo-’ g standard. During these experiments, water vapour was tested and not observed to cause significant absorption at the mercury wavelength. A drying agent is, therefore, not required. It is necessary, however, to prevent the aerosol which is formed by the action of the aeration bubbler horn being carried into the system, because this results in a transfer of the test solution which may cause a spurious absorption signal and slowly coats the walls with solute. Careful insertion of filter paper discs in place of the drying agent (point D in Fig. 1) was effective in removing the aerosol. Glass wool plugs were found to trap about 10% of the activity_ The latter mercmy is “exchangeable”, as shown by the fact that running a non-radioactive mercu_rystandard through the system displaced the originally trapped radioactive mercury. Metal fittings (such as the brass parts of the flowmeter) also act as effective traps for the mercury vapour, as evidenced by the buildup of radioactivity during the experiments_ After the method had been modified as suggested by the above results, the trapping of mercury in the flow system was further investigated. Starting with a freshly cleaned and assembled system, six radiotracer mercury standards of 2.9 X lo-’ g were run consecutively through the system, and the amount recovered in a trap at the exhaust end was measured. The system was then flushed with two non-radioactive standards of 1.9 X lo-’ g each, and the recovery of activity in a fresh trap was measured. The data (Table 1) show that 1.5 X 10es g of radiotracer mercury was lost to the system during the first six runs. This amount was subsequently recovered in the seventh and eighth runs, demonstrating the exchangeable nature of the trapping_ The mercury was not localized in any one spot, but was rather spread over the whole system. Long periods of aeration did not remove the trapped mercury. These results show that everything the mercury vepour contacts in the system presents a potential problem of trapping which should be checked. It is very desirable to have the minimum possible surface area exposed. Even with When
TABLE Trapping
of mercury
radiokacer
mercury
of mercury
in the system
1
Samplenumbera
1
2
Radiotracer recovered (lo)
90 + 4
8Ok
=Samples contained
4
3
4
88~
4
94*
4
1-6 each contained 2.9 X IO-’ g of radiotracer 1.0 x lo-’ g of non-active mercury.
5
6
7
8
98*4
9924
4823
2&l
mercury.
Samples
7 and 3
432
all precautions, some mercury is retained in the system, and this is exchanged during successive runs. These observations cast doubt on the reliability of the absorption signals obtained during the analytical procedure, particularly if the mercury content of successive samples varies widely, or if samples are run which contain less than the amount of mercury which is exchangeably trapped in the system. The first few samples run through a newly cleaned system always yield low values. The fiiancial support of the National Research Council of Canada is gratefully acknowledged. This research was carried out in the laboratories of Professor K. Fritze, McMaster University, whose advice and encouragement are deeply appreciated. REFERENCES
1 See e.g., A_ IM. Ure, Anal. Chim. Acta, 76 (1975) 1. 2 See e. g., Jar&l-Ash Atomic -4bsorption Application Laboratory, No. Hg-1, Aug. 1970; H. L. Kahn, At. Absorpt. Newsl., 10 (1971) .58; H. M. Mittelhauser, At. Absorpt. NewsL, 9 (1970) 34. 3 W. R. Hatch and W. L. Ott, Anal. Chem., 40 (1968) 2085. 4 N. S. Poluektov and R. A. Vitkun, Zb. Anal. Kbim., 18 (1963) 33. 5 N. S. Priluekiov, R. k Vitkun and Y. V. Zelytiova, Zh. Anal. Khim., 19 (1964) 937. 6 C. C. Stuart, Anal. Cbim. Acta., 96 (1978) 83.
Anclytica
Short Communication
RADIOTRACER INVESTIGATION OF THE COLD-VAPOUR ATOMIC ABSORPTION METHOD OF ANALYSIS FOR TRACE MERCURY
D. C. STUART*
Nuclear Research
Building,
AfcMaster
University,
Hamilton,
Ontario
(Canada)
(Received 27th February 1978)
Trace analysis for mercury in environmental samples, and especially in foodstuffs, has become of great interest because of the highly toxic nature of this element and its compounds. The analytical technique most commonly employed is atomic absorption [l] , because it is widely available and relatively straightforward. Normal atomic absorption spectrometers can be converted for cold-vapour mercury analysis by commercially available kits [Z] . Many devices have been reported in the literature, but all such systems are basically the same, following the methods of Hatch and Ott [3] and Poluetkov et al. [4,51Because of certain problems found in application of this method, a careful check of the procedures used was undertaken, with radiotracer mercury to facilitate the investigation. The results obtained in conjunction with those of sample ashing procedures 161, indicate that the method is less straightforward than its simplicity suggests. Eqerimen tai Appamtrts and reagents. A schematic diagram of the setcup used, based on the Jarrell-Ash kit, is shown in Fig. 1. Radioactivities were measured with a 7.6 X 7.6-cm sodium iodide well-type scintillation detector connected to a Canberra model 8100 multichannel pulse-height analyzer. Ail chemicals used were of reagent-grade purity. 19’Hg was produced in the McMaster Nuclear Reactor by irradiating known amounts of msrcury(II)
chloride sealed in quartz capsules; the capsules were then frozen in liquid nitrogen and broken open, and solutions were prepared for use as standards. Procedures. Each step in the recommended Jarrell-Ash procedure [2] was followed carefully by means of the radiotracer mercury standards. The test sample must contain the mercury to be analyzed as Hg(II) in solution, neither organically bound nor strongly complexed. An aliquot of tin@) chloride solution is added to the test sample to reduce the mercury present, and the reaction vessel is attached to the aeration apparatus at point A *Present Address: SLOWPOKE Nuclear Reactor, Trace Analysis Research Centre, Dalhousie University, Halifax, N-S., Canada B3H 451.
430
A,:'
.*’
I
u _,’
;’
,=’
Fig. 1. Mercury accessory kit. (A) Reaction vessel (13 ml); (B) tubing and stopcock (6.5 ml); (D) drying tube and plastic tubing (10 ml); (E) absorption cell (29 ml); (F) flowmeter.
(Fig. 1). Stopcocks I3 and C are turned to bubble air through the test solution. This sweeps the reduced mercury into the absorption cell, E, where the atomic absorption at 253.7 nm is monitored. Stability of mercury solutions Standards containing 1.8 X lo-? g of radiotracer mercury were placed in the reaction vessels. While standing, no loss of mercury was observed, but after addition of the tin(n) chloride, immediate loss occurred followed by a continuous slow loss. If the reaction vessel was shaken to promote mixing, immediate losses of about 10% were measured. Figure 2 shows typical data. This demonstrates the necessity of ensuring that the sample be connected to the apparatus the instant that reductant is added; better still, reductant should be added to the already connected apparatus.
0
5
IO
20
15 Tune
25
30
(mid
Fig. 2. Stability of mercury soiution. (A) Addition of tin@) briefly.
chloride; (B) solution shaken
431
Trcpping
in the system standards were aerated through the flowthrough system, it was found that significant quantities of mercury were trapped. The drying agent was especially bad. In particular, calcium chloride, when it became slightly moist, trapped all the radiotracer mercury. Magnesium perchlorate was considerably better, typically trapping only several per cent of 2 1.8 X lo-’ g standard. During these experiments, water vapour was tested and not observed to cause significant absorption at the mercury wavelength. A drying agent is, therefore, not required. It is necessary, however, to prevent the aerosol which is formed by the action of the aeration bubbler horn being carried into the system, because this results in a transfer of the test solution which may cause a spurious absorption signal and slowly coats the walls with solute. Careful insertion of filter paper discs in place of the drying agent (point D in Fig. 1) was effective in removing the aerosol. Glass wool plugs were found to trap about 10% of the activity_ The latter mercmy is “exchangeable”, as shown by the fact that running a non-radioactive mercu_rystandard through the system displaced the originally trapped radioactive mercury. Metal fittings (such as the brass parts of the flowmeter) also act as effective traps for the mercury vapour, as evidenced by the buildup of radioactivity during the experiments_ After the method had been modified as suggested by the above results, the trapping of mercury in the flow system was further investigated. Starting with a freshly cleaned and assembled system, six radiotracer mercury standards of 2.9 X lo-’ g were run consecutively through the system, and the amount recovered in a trap at the exhaust end was measured. The system was then flushed with two non-radioactive standards of 1.9 X lo-’ g each, and the recovery of activity in a fresh trap was measured. The data (Table 1) show that 1.5 X 10es g of radiotracer mercury was lost to the system during the first six runs. This amount was subsequently recovered in the seventh and eighth runs, demonstrating the exchangeable nature of the trapping_ The mercury was not localized in any one spot, but was rather spread over the whole system. Long periods of aeration did not remove the trapped mercury. These results show that everything the mercury vepour contacts in the system presents a potential problem of trapping which should be checked. It is very desirable to have the minimum possible surface area exposed. Even with When
TABLE Trapping
of mercury
radiokacer
mercury
of mercury
in the system
1
Samplenumbera
1
2
Radiotracer recovered (lo)
90 + 4
8Ok
=Samples contained
4
3
4
88~
4
94*
4
1-6 each contained 2.9 X IO-’ g of radiotracer 1.0 x lo-’ g of non-active mercury.
5
6
7
8
98*4
9924
4823
2&l
mercury.
Samples
7 and 3
432
all precautions, some mercury is retained in the system, and this is exchanged during successive runs. These observations cast doubt on the reliability of the absorption signals obtained during the analytical procedure, particularly if the mercury content of successive samples varies widely, or if samples are run which contain less than the amount of mercury which is exchangeably trapped in the system. The first few samples run through a newly cleaned system always yield low values. The fiiancial support of the National Research Council of Canada is gratefully acknowledged. This research was carried out in the laboratories of Professor K. Fritze, McMaster University, whose advice and encouragement are deeply appreciated. REFERENCES
1 See e.g., A_ IM. Ure, Anal. Chim. Acta, 76 (1975) 1. 2 See e. g., Jar&l-Ash Atomic -4bsorption Application Laboratory, No. Hg-1, Aug. 1970; H. L. Kahn, At. Absorpt. Newsl., 10 (1971) .58; H. M. Mittelhauser, At. Absorpt. NewsL, 9 (1970) 34. 3 W. R. Hatch and W. L. Ott, Anal. Chem., 40 (1968) 2085. 4 N. S. Poluektov and R. A. Vitkun, Zb. Anal. Kbim., 18 (1963) 33. 5 N. S. Priluekiov, R. k Vitkun and Y. V. Zelytiova, Zh. Anal. Khim., 19 (1964) 937. 6 C. C. Stuart, Anal. Cbim. Acta., 96 (1978) 83.