Anal. Chem. 1980, 52, 449-453
449
Sub-Part-per-Billion Determination of Total Dissolved Selenium and Selenite in Environmental Waters by X-ray Fluorescence Spectrometry Harry J. Robberecht and Rent5
E. Van
Grieken
Department of Chemistry, University of Antwerp (U.I.A.),Universiteitsplein
The tetravalent and hexavalent selenium content of different environmental water types can be determined after preconcentration on activated carbon by simple energy-dispersive X-ray fluorescence. The elemental selenlum Is adsorbed on the activated carbon after specific reduction of the selenite by /-ascorbic acid. The total selenium content is determined after refluxing the water samples wtth thiourea in sulfuric acid medium and subsequent adsorption of the elemental form. The contribution of hexavalent selenium is obtained by subtraction. All of the experimental reduction reaction parameters have been optimized and the influence of humic material, salts, and oxidizing substances is investigated. The limit of detection is 50 ng L-’ for the tetravalent selenium and 60 ng L-’ for the total selenium content. The coefflcient of variation amounts to approximately 10% for both species at the 0.5-1 pg Se L-’ level. The suspended material can easily be analyzed separately.
Selenium has recently been the focus of several epidemiological studies. I t is probably one of the few indicted toxic elements, that is also essential for vegetation and man ( 1 ) and serves as a dietary nutrient for animals ( 2 , 3 ) . T h e range between the essential and toxic concentration levels is rather narrow. The oxidation states of selenium are -11, 0, +IV, and +VI. I n natural waters, the predominant oxidation state is not well established. Sillen (4) has suggested that most of the selenium in seawater should be present as the thermodynamically stable hexavalent state, but in view of the oxidation-reduction potential, Chau et al. ( 5 ) concluded that the tetravalent state was the most probable. Most selenium in organic form, including compounds, in biological systems, seems to be in the -11 oxidation state. In nature the abundance of selenium is very low as compared to that of sulfur. Because of the very low concentration in natural waters, simple procedures for high sensitivity determinations are very scarce. The procedure, which has been applied most commonly for the determination of selenium, involves coprecipitation followed by spectrophotometry (5-10). Coprecipitation by iron(II1) hydroxide is most frequently cited for the separation of selenium (5,6). By adsorption colloid flotation (IO), it is possible to shorten the procedure, but difficulties are encountered in transferring the foam completely from the water surface. A direct determination of selenium in seawater without preconcentration has been reported ( 11-13) but its applicability is uncertain. By hydride generation and quartz tube atomic absorption spectrophotometry, it is also possible to determine selenium without preconcentration (14),but the method is prone to several interferences (15-18) and therefore cannot be applied to chlorinated water. Neutron activation analysis after preconcentration by adsorption on activated carbon (19) is highly sensitive but not widely available. An extensive review on the different determination methods has been presented by Shendrikar (20). In the present work, the preconcentration is based on specific selenite and selenate reduction and adsorption of the
7, 6-26 70 Wilrdk, Belgium
resulting elemental selenium onto activated carbon. The preconcentration has been optimized and evaluated for simple economic measurements by energy-dispersive X-ray fluorescence. By using different reduction conditions, it is possible to measure tetravalent as well as hexavalent selenium separately. If any of the selenium was originally present in the elemental state, it is collected on the activated carbon or together with the suspended material. Organic species are unlikely to be important in natural waters, since they are easily hydrolyzed into selenide, which is oxidized bacterially (21). Anyway, activated carbon is known to be a good adsorbant for organics, hence also for organic selenium compounds (22-25).
EXPERIMENTAL Reagents. All reagents were of analytical grade. The water used for dilution and for the standards was deionized and double distilled in quartz material. Standards solutions were prepared from Titrisol standards or K2Se04. The activated carbon, a “Baker analyzed’’ reagent, was treated with concentrated HF and HC1, washed with water, and dried at 110 “C. This treatment reduces the trace metal contamination by a mean factor of five (26). Different commercial types of thiourea were analyzed for their selenium content; the Fluka product 88810 was selected because its Se content was around 0.01 pg g-’ while other commercially available brands contained up to 0.4 pg g-l. Apparatus. The energy-dispersive X-ray fluorescence (EDXRF) instrumentation included a Siemens Kristalloflex-2 high voltage power supply and an X-ray tube with tungsten target, a Mo secondary fluorescer and filter, a 16-position sample holder, and a Kevex Si(Li) semiconductor detector with related electronics. A 1024-channelanalyzer coupled to a magnetic tape unit recorded the spectra, which were analyzed by a PDP 11/45 computer. Radiotracer experiments for the study of adsorption loss were performed by means of a Ge(Li) detector and a 4096-channel analyzer. In all experiments 75Se(T,,2= 120 days) was used. Sodium selenite (specific activity 6.4 pCi (pg Se)-’) and sodium selenate (specific activity 12.8 m Ci (pg Se)-’) were obtained from the Radiochemical Centre (Amersham). The filtration unit was a polycarbonate magnetic filter-funnel Gelman No. 4209 with 9.6 cm2 active area. Optimized Preconcentration Procedures. Numerous tests, discussed below, have been carried out to optimize the preconcentration for total selenium and selenite, for subsequent combination with XRF-analysis. The recommended procedures are as follows. Preconcentration of Total Dissolved Selenium. To a 1-L water sample in a 3-L flask provided with a reflux-cooler, are added sequentially: 50 mL of concentrated sulfuric acid, 1 g of thiourea, and 100 mg of activated carbon (AC). The resulting suspension is heated to boiling and refluxed for at least 15 min. After cooling, the AC with the adsorbed elemental selenium is filtered off on a 47-mm diameter, 0.4-pm pore-size Nuclepore membrane. Preconcentration ofselenite. To a I-L sample in a 2-L beaker, 3 g of l-ascorbic acid and 100 mg AC are added. After stirring for at least 15 min, the AC-suspension is filtered off on a Nuclepore membrane. The loaded filter is washed with 100 mL of double distilled water. The difference in the results for both precon-
0003-2700/80/0352-0449$01.00/0P 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
centration procedures is assumed to be the selenate-content. Overall Analysis Procedure. Immediately after the sampling in plastic containers, pretreated with HCl, "OB, and double distilled water, the natural water samples are filtered through a Nuclepore membrane to collect the suspended matter and acidified with 1mL L-'of 32% HC1. The samples can then be stored for several weeks without significant loss of selenate or selenite. Significant equilibrium shifts were not observed. The preconcentration procedure for the total dissolved selenium and the specific preconcentration procedure for selenite are applied to different aliquots of the sample. The filters loaded with the AC, to which the resulting elemental selenium is adsorbed, are measured in the ED-XRF-unit for 3000 s. The spectrum analysis makes use of a nonlinear least-squares fitting program (27). The area under the Se K, peak (11.21 keV) is computed. If the thickness of AC loading on the filter is not homogeneous, a recently developed method (28) for the evaluation of the effective sample thickness in the X-ray beam can be applied and the measured selenium content of the AC can be corrected by the ratio of what was actually added to the apparently analyzed AC quantity. The filter containing the particulate matter from the natural water sample can be analyzed separately for selenium, if desired.
RESULTS AND DISCUSSION Determination of Total Dissolved Selenium. Activated carbon is a powerful adsorbant for colloidal elemental selenium. However, a t p H 2, less than 1%of the selenite and the selenate are adsorbed. Several reducing conditions were tested out for simultaneous selenite and selenate reduction to the elemental state. Addition of 3 g of 1-ascorbic acid to aqueous samples at p H 2 reduces selenite quantitatively (see below) but the yield for selenate was found to be only 0.8%. Massee e t al. (19) proposed reducing the hexavalent selenium to the tetravalent state by acidifying the sample with 4 N HC1 and refluxing the solutions for a t least 15 min. The Se(1V) thus obtained was further reduced to elemental selenium by 1-ascorbic acid, adsorbed on the AC, and determined by instrumental neutron activation analysis. When this procedure was combined with ED-XRF analysis, however, the determination of the adsorbed selenium soon appeared to be less accurate and sensitive, owing to the interference of the bromine K, peak (11.8keV) in the energy range of the Se K, peak. Bromine, contaminant of hydrochloric acid and present in environmental waters, is also adsorbed on the activated carbon, either as bromide or in the elemental form after oxidation by chlorine. For the commercial preparation of selenium, a reduction of the hexavalent form in strong sulfuric medium can be used (29). A quantitative recovery of as little as 0.1 kg of selenium by thiourea reduction was claimed by Treadwell and Hall (30). Thiourea has also been used for a quantitative gravimetric determination of selenium by Deshmukh and Sankaranarayanan ( 3 1 ) . T h e reduction of hexavalent and tetravalent selenium by thiourea in sulfuric acid medium was therefore studied for combination with ED-XRF analysis. Influence of Reduction Reaction Parameters. Tracer experiments showed that losses of both selenate and selenite from simulated natural waters onto Pyrex and polyethylene containers were negligible, even after prolonged storage. Elemental selenium, however, appeared to be rapidly lost on both materials. The activated carbon does prevent these losses, therefore the AC has to be added before the reduction step. Radiotracer experiments of different selenate solutions did not show any significant selenium loss as possible volatile compounds, produced by the reflux. T h e sensitivity of the method is enhanced with target thickness, but simultaneously also the absorption of the Se K, rays increases, implying larger corrections and deteriorated accuracy. As an optimal compromise 10 mg (cm)-2AC targets were preferred ( 3 2 ) .
Table I. Recovery of 5 H g of Selenium (VI)a added % recovery and stand. dev. per blank values, thiourea, p g Se L-' g measurement 0.1 42 i 10 0.058 0.5 821 3 0.18 1 861 7 0.37 3 922 4 0.88 1 L,50 mL sulfuric acid, 100 mg AC, and 30-min reflux.
Table 11. Recovery of 5 F g of Selenium(VI)a added recovery ( 7 0 ) and stand. dev. per blank values, sulfuric p g Se L-' acid, mL measurement 1 11 z 5 0.19 10 58 i 9 0.16 50 8 5 I5 0.18 100 a
87
i
0.17
2
1 L, 0.5 g thiourea, 100 mg AC, and 30-min reflux.
Table 111. Recovery of 5
p g of
Selenium(VI)= %
recovery
and stand. dev. per
reflux time, min n o reflux (30-min stirring)
0.4
until bubbling
85
5 15 30 a
measurement 0.5 2 891 3 93 z 3 86 I 5 i
i
blank
values,
s,"
p gL-
0.089 0.31 0.31 0.34
0.37
1 L, 1 g thiourea, 5 0 mL sulfuric acid, and 100 mg
.4C.
The reaction parameters (thiourea and sulfuric acid concentrations, reflux time, and sample volume) were all investigated and optimized by carrying out the following experiments in threefold. The influence of the thiourea concentration on the selenate recovery was studied under the following conditions: 1 L of a selenate solution containing 5 kg Se(V1) was refluxed for 30 min after addition of 50 mL of concentrated sulfuric acid, 100 mg of AC, and different amounts of thiourea. The selenium content of the obtained AC filters is shown in Table I, together with corresponding blank values. The experiments leading to Tables I, 11, and I11 were carried out using thiourea available from UCB. T h e purest thiourea out of 15 brands did afterwards allow us to reduce the blank levels below those listed (see below). The influence of different amounts of concentrated sulfuric acid on the recovery of selenate from 1-L solutions is presented in Table 11. The addition of 50 mL of sulfuric acid seemed to be necessary for efficient reduction to elemental selenium. The analytical grade sulfuric acid does not contain significant selenium impurity as is proved by the constant blank value. For the reduction of the selenate, it appears necessary to warm the solution to its boiling point (Table 111). Heating also increases the amount of selenium collected from the blank solutions. This indicates that the selenium present as a contaminant in the thiourea is also reduced to the elemental form. In the range from 200 mL to 1L, no influence of the sample volume on the recovery percentage was observed. All the following work made use of the optimal conditions derived from these experiments. Influence of Sample Salinity. To test the applicability of the procedure for seawater and brines, the recovery of selenate
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
451
______
Table IV. Recovery Yield of Selenium from Selenate Solutions of 1 L in the Presence of 1 7 0 ppm of Iron(III)a 7c recovered ferric ion selenium and added content, stand. dev. per selenate, p g Se L-I mg L-' measurement 8 2 * 26 0.1 0.1 170 96 z 23 0.75 _89 i 1 0 0.75 170 76 = 4 5 __ 93 + 3 5 170 83 I 7 a 1 g thiourea, 50 mL sulfuric acid, 1 0 0 mg AC, and 15-min reflux.
Table VI.
____-___ Influence of Concentration on the Recovery
of Selenate and Selenite'" %
as selenate
as selenite
0.050 0.100 0.5
0.75 1.0 1.0 2.5
2.9 4.25 5.0
Table V. Recovery of Selenium from 1 - L Selenate Solutions Containing Humic Acid and Ferric Ionsa added selenate, pg
Se L-'
0.095 0.095 0.76 0.76 5 5 5
humic material content, mg L-'
__
34 34 _6
34
20 20 a
34
ferric ion content, p g Fe L-' .-
__ __
170 170
% recovered selenium anc: dev. per measurement
8 2 * 26 91 I 2 0 89 2 20 7 0 5 22
93 91 80 84
2
3
4 i 5 702 5 +
1 g thiourea, 50 mL sulfuric acid, 100 mg AC, a n d
30-min reflux. from saturated solutions of sodium chloride was studied under optimized reaction conditions. The presence of large amounts of sodium chloride does not influence the reduction of selenate t o elemental selenium by thiourea. Indeed, the average recovery of 0.25 and 2.5 pg L-' of Se(VI), found in threefold experiments, amounted to 83 and 87 %, respectively, in distilled water and to 79 and 92%, respectively, for a 350-g NaCl L-' brine, with standard deviations per measurement ranging between 3 and 13%. Influence of Ferric Ions. It is possible that by the addition of thiourea to natural water or wastewater, other oxidizing substances interfere with selenate reduction. The influence of 170 ppm of ferric ions on the recovery of different amounts of selenate was studied in triplicate experiments. The results, presented in Table IV, show that the reduction of selenate by thiourea and the adsorption of the elemental selenium on activated carbon are not influenced to a significant degree by the presence of excessively high ferric ion concentrations. Influence of H u m i c Material. Most of the environmental waters contain a few tenths up to several ppm of humic substances. The disturbance caused by these substances on the reduction and recovery procedure of selenate was tested. One-liter solutions of different selenate concentrations, some containing high ferric ion levels, were equilibrated for 24 h in the presence of 34 mg of humic acid. Because the p H of the solutions was about 7, some of the humic material settled out. After 24 h the suspensions were stirred and 1 mL of concentrated hydrochloric acid was added to reduce adsorption losses. After 12 h of desorption, 1 g of thiourea, 50 mL of concentrated sulfuric acid, and 100 mg of AC were added. The analysis procedure was applied. The data from threefold runs (Table V) indicate t h a t for these unnaturally high concentrations of humic acid a loss of 10 to 15% of the selenium
5.3 10
25 100 100
measurement 82 82
0.2
recovered
i
15 (3)b
5
26 ( 3 )
101 i 11 ( 3 ) 8 3 t 11 ( 3 )
89 Il O ( 8 ) 96 i 11 ( 3 ) 92 t 4 (3)' 87 2 3 ( 3 ) 93 (1) 81 * l ( 3 ) 93 + 3 ( 3 ) 99 i 6 ( 3 ) 85 i 5 (3) 84 + 5 ( 3 ) 71
-
2(3)
93 = 3 ( 3 )
a 1 L, 1 g thiourea, 50 mL sulfuric acid, 1 0 0 mg AC, and 15-min reflux. Number of determinations in parentheses. In addition to 1 g thiourea, in this experiment 3 g of I-ascorbic acid were added to the reduction reaction mixture.
occurs. It is unclear whether this is due to adsorption losses in the polyethylene bottles of Se-humic acid complexes or to a reduced accessibility of the complexed selenium. In the presence of 6 ppm of humic material, radioactive tracer experiments showed that the recovery loss at the 5-ppb level of selenate was small. In practice, humic material will thus not hamper the applicability of the procedure. Reduction o f Selenite t o Elemental Selenium by Thiourea i n Sulfuric M e d i u m . Solutions containing 0.2 to 100 pg Se L-' as selenite were preconcentrated and analyzed by EDXRF. The results are included in Table VI where the results for Se(1V) are the averages of threefold to sixfold measurements. Up to 10 ppb it is possible to effectively reduce the added tetravalent selenium. It appears unnecessary to add I-ascorbic acid, specific reducing agent for selenite (see below) t o reduce the tetravalent selenium to the elemental form. Indeed the reduction of 1 pg of selenium(1V) with or without the supplementary addition of 3 g of 1-ascorbic acid after the refluxing with thiourea, gives the same recovery yield and the same amount of selenium as the selenite reduction by 1ascorbic acid alone (see below). Standard additions of 0.4 pg of selenium(1V) to a 1-L river water sample (1.25 pg Se(1V)) also give a recovery of 89 i lo%, consistent with the data for synthetic solutions in Table VI. I t is thus possible to determine by this reduction method the total dissolved selenium in water samples, because tetravalent as well as hexavalent selenium is reduced to the elemental state. Recovery us. Selenate Concentration. In numerous experiments the recoveries of different selenate concentrations were determined either by tracer experiments or by ED-XRF measurements. When standard solutions were prepared by weighing H,SeO, crystals, erratic results were found because of hygroscopicity effects. Therefore K2Se0, powder, stored under argon, was ultimately used. The final average results are included in Table VI. Each point represents the average of three- to eightfold measurements. For all practical purposes, the collection yield can be considered constant around 88% for Se(V1) levels between 0.05 and 10 pg L Addition of 0.76 yg of selenium as selenate to a 1 L of river water sample (containing 1.58pg of total dissolved selenium) showed a recovery of 86%, consistent with the data in Table VI.
'.
452
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
Table VII. Influence of NaCl on the Recovery of Selenitea selenite concn, pg
Se L" 0.5 0.8 5 13
% recovery
without NaCl 97
i
9
and stand. dev. per measurement 3.5% 15% 35% NaCl
NaCl
94
89
t
8
5
Table VIII. Effect of the Concentration o n the Recovery of Selenitea % recovered
added selenite
NaCl
quantity,
9
88ir 7
0.1 0.2 0.25
9 3 6~ 94 I 1 0
0.5
a 1 L, 3 g l-ascorbic acid, pH 2 , 1 0 0 mg AC, 15-min reaction time.
Reproducibility. For all the experiments leading to the data in Table VI, it appeared that the precision for selenate and selenite determinations was not significantly different. The average relative standard deviations were as follows: 25% for up to 0.1 pg Se L-', 11% for the 0.1-1 pg Se L-' range (21 measurements), 4.8% for the 1-25 pg Se L-'range (22 measurements), and 3% for the 100 pg Se L-' level (6 measurements). This reproducibility seems satisfactory for applications to environmental waters. Detection L i m i t . Apart from the scattered X-rays in the ED-XRF spectrum, the minimum detectable amount is mainly defined by the impurities in the used thiourea. Therefore 15 commercial brands of thiourea were analyzed for their selenium contents. The amount varied from 0.01 up to 0.4 pg g-'. With the purest, Fluka product no. 88810, one reaches a limit of detection of 0.06 pg Se L-' for this selenium preconcentration procedure and ED-XRF analysis. Specific D e t e r m i n a t i o n of T e t r a v a l e n t Selenium. In aqueous solutions, selenite can be reduced quantitatively to t h e elemental state by l-ascorbic acid a t p H 2 (33, 34). Selenate is reduced very slowly or not a t all by this reagent. The resulting colloidal selenium can be efficiently collected by adsorption on activated carbon. In this way a selective preconcentration of selenite can be carried out. Massee et al. (19) successfully combined reduction with 1-ascorbic acid and filtration through an AC layer bed with neutron activation through the reaction 74Se(n,y)75Se to perform high sensitivity determination of selenite in tap water and seawater. In the present work, the characteristics of this preconcentration procedure combined with ED-XRF were evaluated. Influence of Reduction Reaction Parameters (19). T o obtain an efficient reduction of selenite and a high and constant recovery yield, the acidity of the sample should be between and 4 N HC1, a t least 2 g L-' ascorbic acid should be used, and a reaction time of a t least 10 min should be allowed. A 100-mg quantity of AC is sufficient to capture the selenium from a 1-L natural water sample nearly quantitatively. Similar conditions appeared to hold when the adsorption step was carried out in batch mode on an AC suspension, rather than by filtering through an AC layer (32). T h e order of addition of the reagents was not found to be critical (32). T h e optimal analysis conditions for selenite determinations, given in the Experimental section, were applied in all further experiments. I n f l u e n c e of Various Salts. Under optimized conditions, the recovery of 0.5 and 5 pg selenite from 1 L of 3.5% and 1 L of 15% NaCl solution, respectively, does not change drastically with respect to recoveries from distilled water as can be seen in Table VI1 where the averages of triplicate experiments are listed. Also in the presence of 10,100, or lo00 ppm of Fe(II1) as FeCl, there was no systematic recovery loss of 0.5 and 5 pg of selenite in 1-L solution samples. The possible reduction of the trivalent state of iron to the divalent state by ascorbic acid does not interfere with the reduction of selenite t o elemental selenium. T h e reducing agent is present in sufficient amount. Also the presence of
Se L - '
0.05
92t 2 80- 8
pg
0.8 1 5
measurement 70 85 80 87 85
93 89 89
10
79
25
82 65
100
selenium
and stand. dev. per 6 (3)b 7 (3) 19 ( 3 ) lO(3) I5 ( 3 ) ? 6(13) i 4 (3) t 7 (7) I5 ( 5 ) i 4 (2) + 5 (2) i i i i
1 L 3 g /-ascorbic, p H 2, 1 0 0 mg AC, 15-min reaction
time.
Number of determinations in parentheses.
50 to 5000 ppm of MgS04.7H20,Na2CO3.10H20,NH4C1,and CaC12.2H20appeared not to influence to a significant extent the recovery of 0.25 pg L-' selenite. Influence of Humic Material. Tracer experiments involving solutions with 0.3,1.2, and 10.2 pg L-' of selenite to which 0.4 to 6 ppm of fulvic acid had been added showed that, for short contact times, the selenium recovery is not significantly influenced by the presence of fulvic acids. At an elevated level of 15 ppm of fulvic acid, a 6% reduction in recovery was observed. For all practical purposes, the humic matter influence can be neglected. Recovery us. Selenite Concentration. In three series of experiments, the collection efficiencies for concentrations of selenite were measured either by tracer experiments or by ED-XRF measurements. The average results are represented in Table VIII. The recovery yield appears high and practically constant from below 0.1 pg L-' up to a few pg L-', hence for the usual environmental range. At the high concentration end, the recovery can be improved either by increasing the contact time with the activated carbon (the recovery of 100 pg L-' selenite increases to 86% after 24-h contact) or by increasing the amount of activated carbon (using 200 mg activated carbon per liter instead of 100 mg increases the recovery of 100 pg L-' selenite to 82% after a 15-min contant time). Standard addition of 0.4 pg Se(1V) to a 1-L river water sample (containing 1.3 pg Se(1V)) showed that 95% of the selenium is recovered in agreement with the synthetic solution data in Table VIII. Reproducibility. The data in Table VI11 give an indication of the reproducibility of the proposed procedure. The percent standard deviation appears to be around 6% up from 0.5 pg Se L-',and somewhat less favorable a t lower concentration levels. Accuracy. Three samples taken from a polluted river estuary were analyzed both by the ED-XRF method and by neutron activation analysis. T h e results from the former analysis method were 0.13, 0.67, and 1.45 pg Se L-' as selenite, while the latter gave (35): 0.15, 0.60, and 1.55 pg Se L-', respectively. Both from these data and from the consistency of the recovery yields on synthetic solutions, it seems that the accuracy of the proposed procedure is around 10%. Detection L i m i t . From the selenium blank and the scattered X-ray contribution in the Se K, energy region, the detection limit for the ED-XRF analysis combined with the selenite preconcentration procedure appeared to be at the 0.05 pg Se L-'level. Applicability. Various samples from rivers, estuaries and the sea, rain and snow, drinking water supply, swimming pools, and cold and geothermal springs have been analyzed by EDX R F for selenium in the particulate matter and for total
Anal. Chem. 1980. 52, 453-457
dissolved selenium and for selenite by the methods discussed above. I n general selenite, and selenate (taken as the difference between the result for total dissolved selenium and for selenite) were above the detection limits of 0.06 and 0.05 wg Se L-', respectively. For the quite polluted Scheldt river, its estuary, and the North Sea, the concentrations for selenate were generally lower than those for selenite. In drinking water, swimming water, and spring water, the reverse seemed to be true. The methods are at present being applied in a large scale screening program for selenium in the Belgian environment.
ACKNOWLEDGMENT We acknowledge the help and suggestions of H. A. Van der Sloot, Energy Research Foundation ECN, Petten, The Netherlands, in the initial phase of this work.
LITERATURE CITED I. Rosenfield and 0. A. Beath, "Selenium", Academic Press, New York, 1964. K. Schwarz, Nutr. Rev., 18, 193 (1960). E. Wolf, V. Kolionitsch, and C. H. Kline, J . Agric. Food Chem., 1I.355 (1963). L. G. Sillen, Svensk. Kem. Tidskr., 75, 161 (1963). Y. K. Chau and J. P. Riley. Anal. Chim. Acta, 33,36 (1965). 0. Yoshii, K . Hiraki, Y. Nishikawa, and T. Shigematsu, Bunseki Kagaku, 26, 91 (1977). Y. Sugimura, Y. Suzuki. and Y . Miyake. J . Oceanogr. SOC. Jpn., 32, 235 (1976). Y. Sugimura and Y. Suzuki, J . Oceanogr. SOC.Jpn., 33, 23 (1977). T. Yamatshige, Y. Ohmoto, and Y. Shigetomi, BunsekiKagaku, 27,607 (1978). J. H. Tzeng and H. Zeitlin, Anal. Chim. Acta, 101, 71 (1978). Y. Shimoishi, Anal. Chim. Acta, 64, 465 (1973). Y. Shimoishi and K . Toei, Anal. Chim. Acta. 100, 65 (1978). C. I. Measures and J. D. Burton, Nature (London), 273, 293 (1978). G. A. Cutter, Anal. Chim. Acta, 98, 59 (1978). F. D. Pierce and H. R. Brown, Anal. Chem.. 48, 693 (1976). F. D. Pierce and H. R. Brown, Anal. Chem., 49, 1417 (1977).
453
(17) M. Verlinden and H. Deelstra, Fresenius' Z . Anal. Chem., 296, 253 (1979). (18) A. Meyer, C. Hofer, G. Tolg, S. Raptis, and G. Knapp. Fresenius' Z . Anal. Chem., 296, 337 (1979). (19) R. Masshe, H. A. van der Sloot, and H. A. Das, J . Radioanal. Chem., 35, 157 (1977). (20) A. D. Shendrikar, Sci. Total Environ., 3, 155 (1974). (21) G. Gissel-Nielsen, Risb Report 370, Risb National Laboratory, Roskilde, Denmark, November 1977. (22) B. G. Lewis, C. M. Johnson, and T. C. Broyer. Pbnf Soil, 40, 107 (1974). (23) B. G. Lewis and C. M. Johnson, J . .4gric. Food Chem., 14, 638 (1966). (24) C. S. Evans and C. M. Johnson, J . Chromatogr., 21, 202 (1966). (25) C. S. Evans, C. J. Asher, and C. M. Johnson, Aust. J . Bioi. Sci., 21, 13 (1968). (26) B. M. Vanderborght and R. E. Van Grieken, Anal. Chim. Acta, 89. 399 (1977). (27) P. Van Espen, H. Nullens, and F. Adams. Nucl. Instrum. Methods, 142, 243 (1977). (28) P. J. Van Espen. F. C. Adams, L. M Van't dack, and R. E. Van Grieken, Anal. Chem., 51, 961 (1979). (29) M. L. Hollander and Y. E. Lebedeff, U.S. Patent 2 834652 (13-5-1958) as described in "Selenium", R. A. Zingaro and W. C. Cooper, Eds., Van Nostrand Reinhold Company, New York, 1974. (30) F. P. Treadweil and W. T. Hall, "Analytical Chemistry", Vol. I, 9th ed.. John Wiley, New York. 1948, p 110. (31) G. S. Deshmukh and K. M. Sankaranarayanan, J . Sci. Res. Banaras Hindu Univ., 3, 5 (1952-53). (32) H. Robberecht, R. Van Grieken and H. A. van der Sloot, Proceedings of the International Congress on Analytical Techniques in Environmental Chemistry, Barcelona, November 1978, Pergamon, Elmsford, N.Y., 1979, in press. (33) D. M. Fogg and N. T. Wilkinson, Analyst (London), 81, 525 (1956). (34) J. G. Sherrat and E . C. Conchie, J . Assoc. Off. Anal. Chem., 7 , 109 (1969). (35) H. A. van der Sloot, Netherlands, Energy Research Foundation ECN, Petten, The Netherlands, private communication, 1979
RECEIVED for review August 14, 1979. Accepted November 14, 1979. This work is supported in part by the Belgian Ministry of Health through the "Selenium Impact" research project (Promotor: D. Vanden Berghe, Department of Medicine, University of Antwerp).
Vacuum-Ultraviolet Atomic Absorption Spectrometry of Mercury with Cold Vapor Generation Kiyoshi Tanabe, Jun'ichi Takahashi, Hiroki Haraguchi,
and Keiichiro Fuwa
Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Vacuum-uitraviolet atomic absorption spectrometry of mercury has been investigated at 185.0 nm, using an argon gas-purged monochromator (50-cm focal length). The cold vapor generation technique was employed for atomization of mercury from the solution phase. An absorption cell equipped with T-shaped tubes has been devised to connect the optical path between the light source and entrance slit of the monochromator, and purging of the optical path with nitrogen gas is performed. The sensitivity and detection limit for mercury, 0.06 ng and 0.01 ng, respectively, were dependent on the type of the electrodeless discharge lamp.
Current environmental concern over the danger of mercury pollution has accelerated progress of analytical methods for mercury. Especially in the field of atomic absorption spectrometry, the cold vapor generation technique has made it possible to determine mercury at the sub-ppb (ng/mL) level (1-31, and such methods have been extensively applied to mercury analysis in various samples. However, some natural samples (e.g., seawater) contain mercury only a t a few ppt 0003-2700/80/0352-0453$01.00/0
(pgjmL) level (4-6). Therefore, improved methods are required to directly determine such low-level mercury concentration without resorting to tedious preconcentration procedures. Almost all the studies so far performed have utilized the spin-forbidden resonance line a t 253.7 nm, caused by the transition 6s1So-6p3P,. On the other hand, it has been known that the main resonance line of mercury is a t 185.0 nm in the vacuum-ultraviolet (VUV) region. The VUV atomic line, caused by the transition 6s1So-6p1P,, has a higher oscillator strength cf = 1.18) than the 253.7 nm line (f = 0.026). Consequently, improved atomic absorption sensitivity may be expected a t 185.0 nm compared to 253.7 nm, since the absorption coefficient is proportional to the oscillator strength of each line ( 7 ) . Despite the advantage in the use of the 185.0-nm line, based on the consideration of oscillator strength, only a few investigations have been reported in terms of VUV atomic absorption spectrometry o f mercury (8-11). This may be due to the experimental difficulty in the VUV region because of the oxygen molecular absorption interference and background molecular absorption by molecules produced in the atomization process. Dagnall et al. investigated the senC 1980 American Chemical Society
