Anal. Chem. 1982, 5 4 , 579-581
579
Detection of Preatomization Losses of Mercury in the Graphite Tube with the Tracer Technique Lothar Lender0 and Viliam Krlvan* Sektion Analytik uncl Hochstreinigung, Universitat Ulm, Oberer Eselsberg N-26, 0-7900 Ulm/Donau, Federal Republlc of Germany
The posslblllty of determlnlng mercury by conventlbnal flameless atomlc absorptlon spectrometry was Investigated by means of the tracer technlque uslng 203Hgas radloactlve Indlcator. Uslng f,hls technique, It Is posslble to detect the preatomlratlon mercury losses dlrectly. From solutions of HCI, “OS, H2S04,H,02, and the mlxtures HN03/H,0, and 40-90 % of trace mercury Is lost during drylng H,SO,/H,O,, at 70 OC. On the other hand, nondetectable losses of mercury occur from the mixtures HCI/H,O, and HCI/HN03/H202 up to 220-250 OC. A slmllar stablllzatlon effect is achieved also by formation of HgS wlth H,S In a graphite tube before drying. On thls basls, mercury can be determined In many instances uslng tire graphlte tube atomlzation without the vapor technlque. The sensltlvlty was found to be 0.4 ng/l % absorptlon. The tochnlque was applied to the determlnation of mercury in human hair,,
Atomic absorption spectrometry (AAS) has become one of the most widely used routiine methods for the determination of traces of mercury. Among the several variations, the cold vapor technique introduced by Brandenberger and Bader ( I , 2) and by Hatch anid Ott (3),meanwhile extensively developed and reviewed ( 4 4 , is, because of its high sensitivity and reliability, by far of greatest practical significance. This technique involves the generation of monoatomic mercury chemically in the cold prior to its determination by AAS. Alternatively, mercury can be reduced by electrolytical amalgamation on copper or gold and then released as vapor by heating. However, independent of the way of mercury vapor formation, the cold vapor technique requires additional equipment to the conventional instrumentation. In addition, the large number of reagents used can result in high blank values. The flame technique provides a rather poor sensitivity (9, 10) and for this rearion it is mot applicable to the determination of mercury in the most biological and environmental samples. For a large number of mercury determinations, the conventional flameless AAS would be the technique of choice. However, its appliication has been very limited because of significant losses of mercury occurring in the preatomization pretreatment in the graphite tube, i.e., in the drying and pyrolysis stage. This serious problem has stimulated a number of investigations ainned at the chemical stabilization of mercury in the graphite furnace. An increase of the thermal stability of mercury has been achieved in different ways including addition of tellurium (11) or potassium dichromate (12) to a nitric acid solution, formation of mercury sulfide after addition of ammonium sulfide (13),and formation of amalgam with gold in a gold-plated graphite furnace (14,15). Nakano et al. described a technique in which mercury is fixed on an ion exchanger and processed in the graphite furnace (16).The most attractive procedure for avoiding losses of mercury seems to be that using a mixture of HC1 and H202or of HC1, and H202(17-19). This procedure is simple and, in the most cases, it requires only a slight variation of the sample solution 0003-2700/82/0354-0579$01.25/0
obtained after the decomposition. However, to a considerable part, the conclusions of the above cited papers disagree with one another. All of them are based on absorption measurements in which the information on losses during drying andl charring cannot be obtained directly but from absorption signals in the atomization step. In this paper, the use of 203Hgas a radiotracer for the investigation of losses of mercury during the preatomization steps in the heated graphite furnace and for working out a1 flameless AAS procedure for the determination of mercury is reported.
EXPERIMENTAL SECTION Chemicals and Apparatus. Reagents used in the tracer experiments were of “pro analysi” grade and those used in the analysis of hair samples for mercury were of “suprapur” grade. In both the tracer and the absorption experiments, HgC12 was the salt form of nonradioactive mercury. All reagents were obtained from Merck, Darmstadt. The acids were additionally purified by subboiling distillation. The radiotracer 2osHgfor spiking the test solutions was supplied by NEN Chemicals GmbH, Dreieich, as a nitric acid solution. The radioactive purity was checked by y-ray spectrometry and the carrier concentration was determined by reverse substoichiometric isotopic dilution analysis wereby mercury was extracted with dithizone into CCl,. The specific activity of the labeled solutions varied between 8 X lo3 and 5 X lo4 dps/p.g. In the tracer experiments, a single-channel analyzer with a well-type NaI(T1) detector was used for counting the 279-keV y-ray of 20sHg. The graphite tube furnace with power supply and programmer Perkin-Elmer HGA-500 Model without optical system with both pyrolytically coated and uncoated tubes was used for the tracer studies. In order to prevent contamination of the environment, we placed the equipment in a closed chamber made of Plexiglass which was connected to an exhaust hood of the radiochemical laboratory. The analysis of the hair samples was carried out using a Perkin-Elmer Model 400 atomic absorption spectrometer equipped with an HGA 76 graphite furnace and programmer, and pyrolytically coated tubes. The spectrometer was operated at the absorption line of 253.7 nm with a current from the electrodeless discharge lamp of 5 mA. Procedures for Tracer Experiments. A 10-20 pL portion of the mercury solutions labeled with 203Hgcontaining about 0.3 pg of Hg/mL was injected into the graphite tube and counted with the well-type detector placed in a horizontal position in order to hold the drop position constant. The tube was placed into the furnace and the corresponding program was started. The drying step was investigated in the temperature Tange 70-140 “C and the charring step in the range 180-350 OC. At appropriate intervals, the tubes were removed and counted. The counting times were so chosen that the error caused by counting statistics was lower than 1%. Generally, this was achieved within some minutes of counting. In each measurement, the complete atomization program was executed in order to remove the mercury from the tube which was used for the next experiment. In the investigation of the stabilization effects of sulfide formation, 10 p L of a mercury solution containing 0.3 pg of Hg in 1mL of 2% nitric acid was injected into the graphite tube and the mercury converted into sulfide either by placing the tube for 2 min in a H2Satmosphere or by addition of 20 pL of 5% solution of (NH&S. The tube was then processed in the same way as described above. 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982 4
7oof
5oj
a) 2 % H C I , 2 % H 2 0 2
b) 2 % H C I , 2 % H N 0 3 , 2 % H 2 0 2
501
L 70
100
130
"
4
t 280 (OC)
200
100
--------
\
e) 10% HCI, 6,5% HN03, 2% H 2 0 2
50
loo]
50
f) 5% HCI, 2% HCOOH, 2% H 2 0 2
.
130
200
300
PC)
Flgure 1. Effect of furnace drying and charring temperature on mercury losses for various media.
Table I. Operating Parameters for the Determination of Hg in Human Hair step temp, "C ramp time, s hold time, s gas flow, mL/min
drying
charring
atomization
cleaning
120 30 30 300
200 1 20 300
1000 1 5 0
2500 1 5 300
In some exemplary cases, the drying time and the concentration of mercury were varied.
Procedure for the Determination of Mercury in Hair. Hair amounts of 60-100 mg were decomposed in 0.5 mL of concentrated nitric acid in a pressure bomb (20) for 1h at 170 O C . The resulting sample solution was diluted with a mixture containing 5 % HC1 and 2 % HzOzto 10 mL. By use of the atomization program summarized in Table I, mercury was determined by the standard addition method.
RESULTS AND DISCUSSION Preatomization Losses of Mercury in the Graphite Furnace. The experiment series was started by investigating
Table 11. Percentage of Mercury Lost during Drying in a Pyrocoated Tube at 70 "C by Using Different Solutions Containing 0.3 r g of Hg/mL for a Ramp Time of 20 s and a Hold Time of 50 s solution 2% HNO, 2% H,SO, 2% H,O, 2%HCl 2% HNO, + 2% H,O, 2% H,SO, + 2%H,O, 2% HC1+ 2% H,O,
% mercury lossa
77.7 f 86.7 f 42.7 f 49.3 f 48.7 ?r 44.3 f
4.7 4.3 3.1 5.1 4.0 5.7
not detectable a Average values and standard deviations were obtained from six separate experiments. the losses of mercury from solutions of HC1, "03, HZS04, HzOz,and the mixture of the individual acids with H,Oz a t a drying temperature of 70 "C. As can be seen from Table I1 where these data are shown, drying of all investigated mercury solutions excluding the mixture of HC1 and HzOz produce even at 70 O C losses lying between 40% and 90%. On the other hand, no detectable losses occurred from the
ANALYTICAL CHEMISTRY, VOL. 54, NO. 3, MARCH 1982
mixture of 2% HCl and 2% H202 Therefore, this system was investigated also at higher temperatures and the results are illustrated in Figure la. It is evident that temperatures up to 200 "C can be used in the pretreatment steps without causing significant losses of mercury. As nitric acidl is one of the most common reagents for the decomposition of biological and environmental samples, the stabilization effect of the HC1/H202mixture in the presence of nitric acid was also investigated in more detail. The results obtained are given in Figure lb-e. They show that the presence of nitric acid noticeably decreases the stabilization effect of the HC1/H2OZsystem, but this influence can be maintained if the concentration of hydrochloric acid is increased. For example, with the system containing 5% HC1, 2% "Os, and 2% H202(Figure IC) the losses of mercury can be neglected up to 250 "C. Further increase of the HC1 concentration, e.g., to 10% (Figure ld) does not improve the stabilization effect. However, higher HN03 concentrations require an increase of the HC1 concentration. For instance, whereas losses of 20.5 & 3.2 % were measured already at 140 "C for the system consisting of 5 % HC1+ 6.5% "OB + 2% Hz02,acceptablie stabilization is achieved if the concentration of HCl is increased to KO% (Figure le). The nitric, acid resulting from the decomposition can be destroyed by addition of formic acid, an exces8 of which remains in the solution and does not influence the ijtabilization effect of the HCl/H202 system as can be seen jfrom Figure If. The effect of other experimental parameters on the thermal stabilization of mercury in the graphite tube was investigated on exemplary cases. No noticeable difference was found between the behavior of an uncoated and a pyrolytically coated tube. Also the variation of the hold time (50-100 s) and of the concentratbon of mercury, for example, within 0.03-0.3 pg/mL in the solution of 10% HC1+ 2% HN03 2% H202, did not affect significantly the percentage losses. The results show that the losses of mercury from a hydrogen peroxide solution and from the mixture of hydrogen peroxide and nitric acid are significantly lower than those from a nitric acid solution alone. The observation made by Issaq and Zielinski (17) concerning increased stabilization of mercury and enhancement of the signal by additioin of hydrogen peroxide was verified, but the losses of mercury were still found to be unacceptably high if hydrogen peroxide alone is added (see Table 11). Our investigations confirm that a decisive improvement of the stabilization of mercury is achieved by the presence of the mixture of hydrochloric acid and hydrogen peroxide as suggested by Owens and Gladney (18)and Alder and Hickman (19). The resulhi obtained by the radiotracer technique, which enables direct mercury transport measurements, prove that in this way losses of mercury during the preatomization steps are not only further minimized but completely eliminated. The stabilization of mercury in the graphite furnace by sulfide formation was investigated for two different experimental conditions. Good stabilization could be achieved by formation of HgS in the HzS atmosphere (Figure lg). However, the necessiity of working with a H2Satmosphere introduces some experimental disadvantages as compared with the simple stabilization procedure with HC1/HzO2 On the other hand, we could not achieve any satisfactory stabilization of mercury by addition of ammonium sulfide to the sample
+
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solution in the furnace as proposed earlier (13). Our expierimental conditions were only slightly modified: we added 20 pL of 5% (NH4)2Ssolution to 10 pL of mercury solution (0.3 pg/mL in 2% "OB) whereas 10 p L of 5% (NH4)2Ssolution was added to 20 p L of mercury solution (1 pg/mL in 11% "Os) in the original work. For example, the losses at 200 OC were found to be 30.5 f 3.2%. From the comparison of the different stabilization procedures it can be concluded that the stabilization with HC1/H202 is more convenient than that with gaseous H2S and more effective and reliable than that with (NH4),S solution. By use of the described stabilization with HC1/H202,an absorbance of unity is obtained with 0.4 ng of mercury. Determination of Mercury i n Human Hair. The procedure described for the decomposition of hair was also checked by means of the tracer technique and there were no detectable losses of mercury. On the basis of the resudts obtained with the tracer technique for stabilization of mercury, the atomization program summarized in Table I was developed and used for the analysis of human hair samples. In the analysis of human hair obtained as reference material from the Joint Research Centre, Commission of the European Communities, Ispra, Italy, we found by the above described technique, from six replicate determinations, a mercury content of 11.6 f 0.5 pg/g, and the value certified by the supplier was 11.3 pg/g. From the results of the tracer experiments and from the good agreement of the analytical data on human hair as reference material, it can be seen that reliable direct determination of mercury is possible if appropriate measures are taken to avoid losses during the drying and charring steps.
LITERATURE CITED (1) Brandenberger, ti.; Bader, H. Helv. Chlm. Acfa 1967, 5 0 , 1409-1415. (2) Brandenberger, H.; Bader, H. At. Absorpt. Newsl. 1968, 7, 53-57. (3) Hatch, W. R.; Ott, W. L. Anal. Chem. 1988, 40, 2085-2087. (4) Ankersmit, R.; Barjhoux, J.; Cappeilina, F.; Carter, W. T.; Deetman A. A.; Diirr, W.; Killens, Ch.; Lutz, J.; Melard, P., Norberg, Sven, A.; Nouyrigat, F.; Dlivier, M.; Pouillot, M.; Reiners, P.; Romeis, H. Anal. Chlm. Acta 1974, 72, 37-48. (5) Gladney, E. S.;Owens, J. W. Anal. Chim. Acta 1977, 90, 271-274. (6) Stuart, D. C. Anal. Chlm. Acta 1978, 101, 429-432. (7) Meicher, M. Perkin-Elmer, Atomic Absorption Application Laboratory No. 19, Bodenseawerk Perkln-Elmer, D-7770 Uberiingen, F.R.G., April 1976. (8) Kaiser, G.; Wtz, D.; Tolg, G.; Knapp, G.; Maichln, B.; Spitzy, H. Z. Anal. Chem. 1978, 291, 278-291. (9) Rubeska, I.; Bolkan, B. " Atomlc Absorption Spectrometry", 2nd ed.; CRC Press: Cleveland, OH, 1971; p 135. (IO) Fuller, C. W. "Electrothermal Atomization for Atomic Absorption Spectrometry"; The Chemical Society Buriington House: London, 1977; p 75. (1 1) Perkin-Elmer, Analytical Methods for ,Furnace Atornlc Absorption Spectroscopy, Report 332-A1, D-7770 Uberilngen, F.R.G., 1979. (12) Rattonettl, A. Instrumentation Laboratory Inc.; Report No. 12, WIC mington, MA, Feb 1980. (13) Edlger, R. D. At. Absorpt. Newsl. 1975, 14. 127-130. (14) Lech, J. F.; Slemer. D.; Woodriff, R. Appl. Spectrosc. 1974, ;?8, 88-71. (15) Lech, J. F.; Siemer, D.; Woodriff, R. Appl. Spectrosc. 1974, ;?8, 76-79, (16) Nakano, K.; Takada, T.; Fujita, K. Chem. Lett. 1979, 869-872. (17) Issaq, H. J.; Zlelinski, W. L. Anal. Chem. 1974, 46, 1436-1439. (18) Owens, J. W.; Gladney, E. S.Anal. Chem. 1976, 48, 787-788. (19) Alder, J. F.; Hlckman, D. A. Anal. Chem. 1977, 49. 336-339. (20) Kotz. L.; Kaiser, G.; Tschopel, P.; Tolg. G. 2.Anal. Chem. 1972, 260, 207-209.
RECEIVED for review September 30,1981. Accepted Decembler 14, 1981.
