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(9) K. L. Smith, Rev. Sci. Instrum., 44, 1108 (1973). (10) R. F. Browner, R. M. Dagnall, and T. S. West, Anal. Chim. Acta, 45, 163 (1969). (11) . , D. 0. KnaDD. K. E. Zacha. and J. D. Winefordner. Soectrochim. Acta. Pari B , 23,' 389 (1968). (12) R. M. Dagnall and T. S. West, Appl. Opt., 7, 1287 (1968). (13) R. F. Browner and J. D. Winefordner, Spectrochim Acta, Part B , 28, 263 (19731 (14) RIM.'Dagnall, M. D. Sibester, andT. S. West, Taianta, 18, 1103(1971). (15) W. E. Bell, A. L. Bloom, and J. Lynch, Rev. Sci. Instrum., 32, 688 (1961). (16) R. G. Brewer, Rev. Sci. Instrum., 32, 1356 (1961). (17) N. P. Ivanov, L. V. Minervina, S. V. Baranov, L. G. Pofraldi, and I. I. Olikov, Zh.Anal. Kbim.. 21. 1129 (1966). (18) J. Reader, J . Oit.-Soc Am., 65,' 988 (1975).
J. Reader, J . Opt SOC.Am., 65. 286 (1975). L. Minnhagen and L. Stigmark, Ark. Fys., 13 (Z), 27 (1957). L. Minnhagen, B. Peterson, and L. Stigmrk, Ark f y s . , 16 (45), 571 (1960). H. LJ. Eckert, Report No. ATR-77(9472W (1977), Aerospace Corporation, El Seaundo, Calif. (23) J. P. Haarsrna, G. J. DeJong, and J. Agterdenbos, Spectrochim. Acta, Part B , 29, 1 (1974). (24) W. W. Macalpine and R. 0. Schildknecht, Electronics, 33, 140 (1960). (25) F. C. Gabriel, Rev. SO. Instrum.. 47, 484 (1976).
s.
RECEIVED for review October
17, 1977. Accepted December
12,1977
Stationary Cold-Vapor Atomic Absorption Spectrometric Method for Mercury Determination Soo-Loong Tong Department of Chemistry, University of Malaya, Kuala
Lumpur, Malaysia
A new stationary cold-vapor atomic absorption method using an ordinary 4-cm UV-cell for mercury determination is proposed. Mercury(11) is reduced and then partitioned between an aqueous and a gas phase in a stoppered UV-cell. Direct atomic absorption measurement is taken by allowing the mercury resonant light beam to pass through the vapor phase of the system while non-atomic absorption is corrected using an automatic background corrector. The calibration graph obtained for Hg(I1) in 4 M H2S04is linear from 0 to 30 ppb and absorbance at concentrations up to 50 ppb shows only slight deviation from linearity. The slope of the linear region is 0.0253 ppb-' and the detection limit is 0.02 ppb or 0.1 ng. The absorbance was found to be dependent on the concentration of the common acids used. The partition constant of elemental mercury between the two phases was also determined employing a radiotracer technique. The value obtained was 0.66 f 0.04 for Hg(I1) in 2 M H2S04.
Most determinations of mercury by atomic absorption spectrophotometry a t ppb and sub-ppb levels are based on the cold-vapor method reported by Poluektov et al. ( I ) and Hatch and Ott (2). There are many modifications and improvements on this principle which have become standard in many laboratories ( 3 ) . Practically all of these involved measurements of transient atomic absorption of reduced mercury. In one approach, mercury reduced by stannous ion is bubbled and swept with a carrier gas through the absorption cell or, alternatively, the carrier gas is continuously recirculated so that more steady absorbance readings can be obtained. In another method, the reduced mercury is partitioned between the liquid and a fixed volume of air by agitation, after which the mercury-laden air is blown directly through the absorption cell. In principle, the reduction and partition of mercury may be carried out in a closed vessel with UV-transparent windows followed by direct stationary atomic absorption measurement if accurate non-atomic absorption correction can be made easily. Such a system with a minimum dead volume for the air phase would then provide better detection sensitivity, further simplification in operation and be subject to less 0003-2700/78/0350-0412$01 .OO/O
analytical variables. In this report, a detailed study on the use of an ordinary rectangular 4-cm UV-cell for this purpose is described. In addition, the partition constant of reduced mercury between the solution and air phase determined by a radiotracer technique is also reported.
EXPERIMENTAL Apparatus. All atomic absorption measurements were made on an Instrumentation Laboratory IL-251 double beam spectrophotometer equipped with an automatic background corrector. The burner in the atomization compartment was replaced by a specially designed holder (Figure 1)for a 4-cm UV-cell (Spectrosil, dimensions: 10 X 32 X 40 mm, Thermal Syndicate Limited, England) which allows for proper alignment with the atomic light beam. A Varian mercury hollow-cathode lamp and a hydrogen continuum lamp for background correction were used. Gamma activity for *03Hgwas measured using a 50 X 50 mm well-type NaI(T1) detector in conjunction with an ORTEC single channel analyzer. Reagents. Reagent grade chemicals and deionized-distilled water were used for all the preparation of solutions. Stock mercury solution (1000 pg/mL) was prepared by dissolving 1.354 g mercury(I1) chloride in 50 mL of concentrated hydrochloric acid and then dilute to 1 L. Working standards (0.2-1.0 wg/mL) were prepared weekly by appropriate dilution from this solution with 570 "0,-0.01% K2Cr20isolution. The reductant consisted of 1070 ( w / v ) SnCl,, 5% (w/v) NaC1, and 10 mL H2S04in 100 mL solution. Radioactive roeHgwas purchased as mercuric chloride in 0.1 N HC1 solution (Radiochemical Centre Ltd., Amersham) with specific activity of 0.68 mCi/mg. Procedure. A pair of 4-cm CV-cells, of volumes 12.9 mL and 13.0 mL each have been used alternatively for the reductionpartition and subsequent cold-vapor atomic absorption measurement. To obtain the calibration graphs, 5.0 mL of acid solutions were pipetted into the cell followed by the addition of appropriate volumes (0.020-0.50 mL) of the working standards of Hg(I1) and 0.20 mL of the reducing agent and the cell was tightly stoppered. After shaking for 2 min, the cell was placed in the holder fixed to the atomic absorption spectrophotometer. The holder has been previously aligned to allow the atomic light beam to pass through the upper gas phase of the cell with maximum intensity. Lamp currents applied t o the mercury hollow-cathode lamp and hydrogen continuum lamp were 4 mA and 15 mA, respectively. The mercury 253.7-nm resonant line was used with slit width of 320 nm and photomultiplier high 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978
a3
SIDE VIEW
FRONT VIEW
413
L-cm
0.2 W
i a 0 In 4
I
i TO BURNER SLPFQRT HOLE
SCALE
0.1
20 nrr
Figure 1. Cell holder attachment for stationary cold-vapor atomic absorption measurements
voltage of 460 volts. Atomic absorption of the mercury vapor was taken using double-beam and simultaneous background correction mode with I-s integration time. After the measurement, the cell was washed with dilute nitric acid-dichromate solution in a fume cupboard and thoroughly rinsed with deionized water and shaken dry. Experimental conditions for the determination in hydrochloric, nitric, and sulfuric acids of various concentrations have been studied. Linear and working ranges of the mercury solution of the proposed procedure, and the possible effect due to room temperature fluctuation during an analysis, were investigated. Direct determination of the partition constant of elemental mercury between the solution and gaseous phases has been carried out using varying concentrations of '03Hg-labeled standards ranging from 1.2-59.6 ppb in 2 M HzS04.After carrying out the reduction and partition equilibration in the cell as described above, exactly 4 n L of the solution were pipetted cautiously into a 4.5-mL vial. The vial was stoppered tightly and counted immediately with the single-channel y-ray counter. The concentrations of mercury remaining in the solution phase at equilibrium were determined by comparing the counting data with the count-rate of a '03Hg-labeled standard.
RESULTS AND DISCUSSION T h e optimum volume of the sample solution to be used in the system described here was found to be not more than 6 mL, in order to give a minimum dead volume possible for the gas phase upon which the atomic light beam is being passed through. Larger solution volume would cause partial absorption interference which cannot be corrected by simultaneous background correction. Sample solution of 5.0 mL with 0.20-0.40 mL of the reducing agent was found to be satisfactory for Hg(I1) concentration ranging from 0-50 ppb. Partition equilibrium of reduced mercury between the two phases was established in less than 1 min by applying moderate shaking; normally each solution was shaken for 2 min before absorbance was taken. Cell positioning with respect to the beam path is quite critical and cell-in cell-out reproducibility has to be observed carefully to achieve optimum precision. T h e absorbance reading usually became stabilized within 30 s after insertion of the cell into the holder. Initial fluctuations observed are suspected to be due to the thin film of liquid adhered on the cell windows. From repetitive measurements of a 5.0 ppb Hg(I1) solution, relative precision of 2% was obtained. Temperature effect on the partition equilibrium has been investigated although no strict temperature regulation of the present system is possible. No noticeable changes were found in the absorbance readings, however, when room temperature fluctuated between 22-26
"C. T h e calibration graph for Hg(I1) in 4 M H2S04solution is linear from 0 to 30 ppb and absorbance a t concentrations up
0
1
2
3
4
5
6
ACID C O N C E N l H A T I O N , M
Figure 2. Cold-vapor absorbance of 10 ppb Hg(I1) in H,S04 (X), in HC1 ( O ) ,and in HNO, (0)
to 50 ppb (1.230 absorbance) shows only slight deviation from linearity. The sensitivity in terms of the slope of the calibration graph in the linear region is 0.0253 ppb-'. The detection limit, defined as the concentration which yields an absorbance twice that of the standard deviation of the absorbance of a blank, is 0.02 ppb or 0.1 ng under the present instrumental conditions. These results compare favorably even with the best detection limits for the transient cold-vapor absorption method as reported by Hawley and Ingle, Jr. ( 4 ) . T h e slopes of the atomic absorption calibration and the detection limits obtained by them were 0.0219 ppb-' and 3 ppt Hg(II),respectively, for a 20-cm cell, and 0.0636 ppb-' and 1 ppt Hg(II), respectively, using a 60-cm cell. These results were achieved through the reduction of dead volume of the reducing apparatus, increasing the efficiency of diffusion of elemental mercury into the carrier gas, and by modifying the instrument light source and detector. Our method, being simpler in operation, can be further improved in sensitivity with the use of a similar UV-cell of longer pathlength (such as 10 or 20 cm). As shown in Figure 2, the absorbance measured is strongly dependent on the concentration of the acid medium. T h e gradual increase observed in the absorbance of 10 ppb Hg(I1) solutions in H2S04of increasing concentration is qualitatively consistent with the observation of Koirtyohann and Khalil ( 5 )who have found a more rapid change. The absorbance was found to decrease rapidly in hydrochloric acid of more than 2 M and nitric acid of more than 3.5 M concentration. Contrary to these trends, the latter authors found practically no variations for both acids three to four times more concentrated. In the system we employed, the volatility of these acids possibly causes not only reduction in the amount of elemental mercury distributed in the gas phase but also errors in the automatic background correction. As a test of feasibility for real sample analysis, the method described has been used for the analysis of fish samples digested according to the procedure of Ramirez-Munoz (6). T h e digested solution was filtered after the oxidation but before the addition of hydroxylamine sulfate for the reduction of excess permanganate. Results based on the standard addition method are shown in Figure 3. Linear regressions carried out for each set of the known addition data gives correlation coefficients of 0.993-0.998. I t is therefore con-
414
ANALYTICAL CHEMISTRY, VOL. 50, NO. 3, MARCH 1978 ABSORBANCE
Table I. Partition Constant of Reduced Mercury between
( 2 X expansion)
the Liquid and Gas Phase
Hg(I1) concn, ppb 1.2 2.4 7.2 14.4
28.8 59.6 Mean f std. dev a
0
5
10
15
20
SPIKED Hg CONCENTRATION, ppb
Flgure 3. Known addition determination of mercury in digested fish sample solutions. Concentrations of Hg(I1) found in solution 1: 3.9 ppb; solution 2: 1.4 ppb; and solution 3: 0.3 ppb. Blank solution: -x-x-
cluded that ions commonly present in digested fish solution do not interfere with the determination. However, incomplete digestion often encountered for fish tissues of high fat content have been found to render the method inapplicable because of foaming problems. T h e system is expected to be generally useful for monitoring mercury in natural and polluted water as well as mercury in other digested samples. Common oxidants normally required for sample treatments such as potassium dichromate, potassium permanganate, hydrogen peroxide, and bromine, do not interfere with the determination. The partition constant of the reduced mercury between the liquid and vapor phases is of fundamental interest for the cold-vapor methods. However, few studies concerning this have appeared in the literature. A rough estimate based on t h e results of Ure and Shand (7) yields a partition constant value of 0.25, which is defined as
K=
Concentration of Hg in air Concentration of Hg in liquid
This value is significantly lower than the values reported by
Partition constant, Ka 0.61 0.70
0.63 0.68 0.66 0.69 0.66
i
0.04
Average of triplicate determinations.
Koirtyohann and Khalil ( 5 ) . The latters found K values of 0.40 for mercury in hydrochloric and nitric acids, and from 0.4M.70 for mercury in 0-6 M H2S04.Our results from using radioactive '03Hg tracer and the system described above are presented in Table I. T h e mean partition constant is 0.66 f 0.04 for Hg(I1) in 2 M H2S04ranging from 1.2-59.6 ppb, as compared with the value of approximately 0.50 obtained by Koirtyohann and Khalil ( 5 ) under the same acid condition. As shown previously in Figure 2, variations of the partition constant as a function of the concentration of some common acids are prominent for our system. In conclusion, transient peak atomic absorption and peak area integration measurements for the cold-vapor method can be replaced by a steady-state atomic absorption method. Although simultaneous background correction is essential while employing the technique, it is basically simpler in operation than many other techniques for rapid mercury determinations. With the use of a set of 4 reduction-absorption cells, an average of 20 determinations can be carried out per hour.
ACKNOWLEDGMENT T h e author thanks M. C. Lim and C. K. Chu for many helpful suggestions related to this work.
LITERATURE CITED (1) N. S. Poluektov, R . A . Vitkun, and T. V. Zelyukova, Zh. Anal. Khim., 19, 937 (1964). (2) W. R. Hatch and W. L. Ott. Anal. Chem., 40, 2085 (1968). (3) A . M. Ure, Anal. Chim. Acta, 76, 1 (1975). (4) J. E. Hawley and J. D. Ingle, Jr., Anal. Chem., 47, 719 (1975). (5) S. R. Koirtyohann and M. Khalil, Anal. Chem., 48, 136 (1976). (6) J. Ramirez-Munoz, Beckman Instruments Inc., Appl. Res. Tech. Rep.. No. 556, (1971). ( 7 ) A . M. Ure and C. A. Shand, Anal. Chim. Acta, 72, 63 (1974).
RECEIVED for review August 30, 1977. Accepted October 17, 1977.
