Anal. Chem. 1987, 59, 1466-1470
1466
T a b l e IV. B a c k g r o u n d C o r r e c t e d Integrated Absorbance f o r Cadmium in U r i n e Samples U s i n g 200 pg o f (NH,),HPO, 10 pg of Mg(NO& and 50 pg o f Pd 500 pg o f NH4N03, Respectively, as the Modifier
+
+
urine
dilution
control
1x5 1:2 1:2
1
6b"
integrated absorbance, A s PO4 + Mg(NO,), Pd + NH4N03 0.082 0.041 0.004
0.084 0.094 0.016
Sample taken during the day.
ammonium phosphate or ammonium phosphate-magnesium nitrate modifier for the determination of cadmium in biological materials. For the determination of cadmium in urine, the proposed modifier is clearly superior because an interference that is found with the phosphate modifier even if Zeemaneffect background correction is used and is not observed with the palladium nitrate-ammonium nitrate modifier. Registry
No. Cd, 7440-43-9; Pd(N02)2,18608-81-6;NH4+,
14798-03-9. LITERATURE CITED
which is not observed for the palladium nitrate-ammonium nitrate modifier. Even more revealing is the comparison of the integrated absorbance readings for these urine samples obtained by using the ammonium phosphatemagnesium nitrate and palladium nitrate-ammonium nitrate modifier, respectively, and Zeeman-effect background correction (Table IV). While essentially the same integrated absorbance is obtained for the 1:5 diluted control urine, the values are significantly different for the 1:2 diluted fresh urine samples. This indicates an interference in the presence of the ammonium phosphatemagnesium nitrate modifier, which is not observed with the palladium nitrate-ammonium nitrate modifier. While the data collected in this study were not conclusive, it is likely that this interference is of spectral origin. Spectral interferences are quite common at short wavelengths when continuum-source background correction such as a deuterium arc lamp is used (20). Spectral interferences, while much less likely, may also occur with Zeeman-effect background correction. It is known that molecular lines can also split into several components in a strong magnetic field (21) and can cause spectral interferences if they overlap with the line profile emitted by the spectral source lamp. Such a gaseous molecular species may be formed from the phosphate modifier in the presence of the urine matrix. CONCLUSION The proposed palladium nitrate-ammonium nitrate modifier is at least equivalent to the previously recommended
Drasch. G. A. Scl. Total Envlron. 1983, 2 6 , 111. Siavin, W.; Manning, D. C.; Carnrick, 0.R. At. Spectrosc. 1981, 2 , 137. Fernandez, F. J.; Bohler, W.; Beaty, M. M.; Barnett W. B. At. Spectrmc. 1981. 2 , 73. Stoeppier, M. Spectrochlm. Acta, Part 6 1983, 388. 1559. Stoeppler, M. Presented at the 5th Cadmium Conference, San Francisco, CA, Feb. 4-6. 1986. Edlger, R. D. At. Absorpt. News/. 1975, 7 4 , 127. Wetzeil, L. T.; Beii, J. U. Clin. Chem. (Winston-Salem, N.C.) 1980, 26, 1796. Bruhn, F.; Navarrete, A. Anal. Chim. Acta 1981, 730, 209. Hindetberger, E. J.; Kaiser, M. L.; Koirtyohann, S. R. At. Spectrosc. l e a l , , ? , 1. Subramanian, K. S.; Meranger, J. C.; Mac Keen, J. E. Anal. Chem. 1983, 55, 1064. Pruszkowska, E.; Carnrick, G. R.; Siavin, W. Clln. Chem. (WlnstonSalem, N . C . ) 1983, 29, 477. Slavln, W.; Carnrick, G. R.; Manning, D. C.; Pruszkowska, E. At. Spectrosc. 1983, 4 , 69. Weir, B.; Schlemmer, 0.J . Anal. At. Spectrom. 1988, 7 , 119. Schiemmer G.; Welz, B. Spectrochim. Acta, Part 6 1988. 4 78. 1157. Schlemmer, G.; Welz, B. Int. Symp. 8bl. Ref. Mater.. 2nd 1988, 57. Beaty, M.; Barnett, W.; Grobenski, 2. At. Spectrosc. 1980, 7 , 72. Versieck, J. J . Res. Nat. 6w. Stand. ( U S . ) 1986, 9 1 , 87. Versieck, J. personal communication, 1986, Ghent, Belgium. Techniques in Oraphite Furnace Atomic Absorptbn Spectrophotome try; Perkin-Elmer Corp.: Rldgefield, CT, 1985; Part No. 0993-8150. Slavin, W.; Carnrick, G. R. At. Spectrosc. 1988, 7 , 9. Herzberg, G. Molecular Spectra and Molecular Structure, I . Spectra of Dlatomic Molecules, 2nd ed.;Van Nostrand-Reinhold: New York, 1950.
-
RECEIVED for review November 3,1986. Accepted February 4,1987. X.Y. gratefully acknowledges a fellowship from the government of the Federal Republic of Germany.
CORRESPONDENCE Extension of Elution Range in Micellar Electrokinetic Capillary Chromatography Sir: Micellar electrokinetic capillary chromatography (MECC) is a highly efficient separation technique in which solutes are separated based on their differential partitioning between an electroosmoticallypumped aqueous mobile phase and a slower moving, electrophoretically retarded, micellar phase. The technique was first reported by Terabe (1). Factors that influence retention behavior have been investigated (2,3)and, recently, we have reported the results of studies concerning the influence of experimental parameters on column efficiency in MECC ( 4 ) , including the effects of injection procedures (5). In addition to these fundamental studies, MECC has provided an efficient means of separating a fairly wide variety of compounds of both biological and environmental significance (3,6, 7). All of these applications 0003-2700/87/0359-1468$01.50/0
have demonstrated efficiencies exceeding 100 OOO theoretical plates/m of column length. High efficiency and the aqueous mobile phases that have been used in MECC facilitate the analysis of biological samples. One obstacle in the application of MECC to complex sample analysis is its limited elution range. The elution range can be described as follows: A solute that is totally soluble in the aqueous mobile phase will elute at a time, to,determined solely by the magnitude of the electroosmotic flow velocity. Alternately, a solute that is completely micelle solubilized will elute at a maximum elution time, t,, which is equal to the time required for the electrophoretically retarded micelles to traverse the column, Sample components of intermediate micelle solubility will then elute within the range defined by 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987 t o and .,t Thus, the available peak capacity of MECC is clearly dependent on these parameters. In fact, it has been shown theoretically that peak capacity in MECC is directly proportional to In @,/to) (2). Any measures taken to increase the ratio t,/to should therefore increase the number of components capable of being resolved by the technique. In MECC, the net flow velocity of the micelles, urn,is determined by the summation of their electrophoretic velocity, and the larger and opposing electroosmotic flow of the solvent, u,. In our work with sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) surfactants, u,, has typically been about 3640% larger than umC.Thus u, and u, have the same sign, the former being about a fador of 3 smaller than the latter. It should be possible to extend elution range by increasing t, through a reduction in u,. Our first attempt at reducing u, involved the use of a relatively short chain-length surfactant, sodium decyl sulfate (STS),with the aim of forming smaller micelles with higher electrophoretic velocities (8). Since the electroosmoticflow of solvent did not increase appreciably, the result was a slower net micelle velocity. Although elution ranges obtained when using STS were larger than those obtained for SDS or CTAC, solute retention time reproducibility was poor with the STS system. Another limitation to this approach is that as surfactant chain-length decreases, critical micelle concentration increases dramatically. The high surfactant concentrations needed for micelle formation result in high column currents, which can cause heat dissipation problems and a degradation of separation efficiency (4). A second approach involves adjustment of electroosmotic flow rather than the electrophoretic flow velocities of the micelles. In addition to the properties of the solvent, it has been shown that electroosmotic flow depends on the magnitude of the { potential at the capillary wall-liquid interface (9),which is dependent on the nature of the solid surface and the ionic nature of the liquid. Electroosmotic flow velocity is generally directly proportional to the ( potential. It has been shown in capillary zone electrophoresis (CZE), using fused silica capillaries, that reductions in solvent pH reduce u, ( 1 0 , l l ) . According to these reports, the larger hydrogen ion content of the mobile phase serves to deactivatethe surface of the column, causing a decrease in { potential. Unfortunately, pH adjustment has the added effect of changing the nature of the separation in MECC, since solutes with acid and/or base functionalities are often separated with the technique. It has also been demonstrated that capillaries fabricated from Teflon exhibit lower (potentials in CZE (12). However, two problems are associated with the use of Teflon capillaries. First, the use of Teflon capillaries would necessitate the coupling of the capillary to detection flow cells, thus providing an additional source of band dispersion. Second, the heat dissipating capability of Teflon is inferior to fused silica and this could increase the aforementioned thermal problems. In view of these considerations, attempts were made to modify the inside surfaces of the fused silica capillaries so as to reduce the {potential, and hence reduce ueo,as shown in CZE (I1,13),without modifying the mobile phases typically employed in MECC. This paper reports the successful extension of elution range in MECC through the use of surface-silanated, fused-silica capillaries. EXPERIMENTAL SECTION Column Preparation. Fused-silica capillaries were obtained from Scientific Glass Engineering (Austin, TX). Untreated (i.e., unsilanated) columns used in this work were first acid washed with 0.10 N HCI for 1 h and then filled with the mobile phase. Columns that were to be modified were first acid washed with 0.10 N HCI for 1 h, rinsed with methanol to remove excess acid, and then dried overnight under flowing nitrogen at 160 OC. A
0
5 10 TIME (minutes)
1467
15
Figure 1. Chromatogram of the separation of six test solutes on an untreated column. Column dimensions and separatlon conditions are given In the text. Components in order of elution are NBD-ethylamine, NBD-n -propylamine, NBD-n -butylamine, NBD-cyclohexylamine, NBD-n -hexylamine, and tetrahydro(trifluoromethy1)benzopyranoquinoiizin-1lane (coumarin 540A).
10% trimethylchlorosilane (TMCS) solution in toluene was then pumped through the column for 3 h at 95 O C . After silanation, the column was rinsed with toluene, methylene chloride, and methanol and finally filled with the mobile phase. Chromatography. The chromatographic system and the electroinjection procedure used in this work were previously described (5). On-column laser-based fluorescence detection was carried out by using a Cyonics Model 2001-20BL argon ion laser for excitation (488 nm and 20 mW). Fluorescence emission signals were isolated at 540 nm with a 6-nm band-pass by using an Instruments SA Model H-10 monochromator and detected with an RCA Model 1P28 photomultiplier tube. Photocurrents were processed with a Keithley Model 485 picoammeter and chromatograms were recorded on a Kipp and Zonen Model BD40 strip chart recorder. Chemicals. Sodium dodecyl sulfate (SDS),99% purity, was supplied by Sigma Chemical Co. Trimethylchlorosilane (TMCS) was purchased from Petrarch, Inc. All mobile phases were prepared in triply distilled, deionized water. All other chemicals were reagent grade (Fisher Scientific). Amines were derivatized with the fluorescent label 4-chloro-7-nitrobenzofurazan (NBDchloride), obtained from Sigma. RESULTS AND DISCUSSION
Figure 1shows the separation of the six test solutes used in this work, on an untreated, 50-pm-i.d. X 65-cm-long, fused-silica column. The aqueous mobile phase was 0.025 M in SDS and 5 X M in Na2HP04. This and all subsequent separations in this study were performed at 20 kV. In order to estimate to for this column, a mixture of 4-ethylamino-7nitrobenzofurazan (NBD-ethylamine) and 4-cyclohexylamino-7-nitrobenzofurazan(NBD-cyclohexylamine) was injected into this column and separated by using a mobile phase that did not contain surfactant. As shown in Figure ZA, a single peak was observed indicating the absence of solute retention. As such, the elution time for these components was determined solely by the electroosmotic flow velocity, and to of 4.6 min was obtained. With this value used for to,and with t, designated as the elution time for the last component in Figure 1, the lipophilic fluorescent dye coumarin 540A, the elution ratio, to/t,, for this separation is 0.4. This is consistent with our previous experiences with this technique ( 4 ) .
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
1488
I
1
4 a TIME (minuter)
o
0
8 12. TIME (minutes)
4
A
16
B
‘,L
I
0
imation of to. In all probability, the real to value is slightly less than this, but since we are interested in determiniig to/t,, an overestmation of towould actually provide a conservative estimate of the extent to which the elution range had been extended. With a t o of 9.6 min and a t , of 86.5 min (last component in Figure 3), a t o / t , of 0.1 was obtained for the silanated column. This ratio will vary if careful attention is not given to column treatment procedures. We have produced other columns by using this procedure with to/tm values ranging from 0.08 to 0.2. It is particularly important to ensure that all traces of water have been removed from the apparatus prior to silanation. Residual water will react with TMCS and result in variable surface coverages. The resolution Rs,between any two components in MECC is given by eq 1 (2),where N is the average efficiency of the
io
5
peaks, a is the column selectivity, given by (k2//kl’),and k’ is the capacity factor as described in eq 2 (2). Resolution can
is
TIME (minutes)
C
F@ro 2. (A) Chromatogram of separation of NBD-ethylamine and
NBD-cyclohexylamine on an untreated column, using a mobile phase without surfactant. (B) Chromatogram of the separation of same two solutes on a column treated with TMCS, using a mobiJe phase wlthout surfactant. (C) Same as (B), but with a mobile phase containing 10% by volume 2-propanol.
0
20
40
60
80
100
TIME Irnlnutee)
Flgure 8. Chromatogram of the separation of the solutes (see Figure 1) on a column treated with TMCS. Column dimensions and mobile phase identical with those of Figure 1.
Figure 3 shows the separation of the same six solutes on a 50-rm-i.d. X 65-cm long column that was treated with TMCS. The mobile phase employed for the separation was identical with that used to obtain the chromatogram shown in Figure 1. The same procedure was used to estimate tofor this silanated column, and this result is shown in Figure 2B. It should be noted that although some separation, hence retention, of these two components is indicated, no micelles were used in t h i s mobile phase. The significance of this retention mechanism will be discussed later. Previous experiencein the separation of these solutes on reversed-phase columns indicates that since k’ for the second solute to elute, NBDcyclohexylamine, is rather low, the retention time for the first solute to elute, NBD-ethylamine (9.6 rnin), is a fair approx-
also be determined strictly from an experimentally obtained chromatogram by inserting into eq 3 the observed retention
times, t R 1 and tb, and base-line peak widths, W1 and W,, for the peaks in question. Based on the to/t, and k values obtained for the two columns used in this work, eq 1 would predict that the experimentally observed resolution of 3.3, obtained for NBDcyclohexylamine and 4-n-hexylamino-7-nitrobenzofurazan (NBD-n-hexylamine) on the untreated column (fourth and fifth peaks in Figure l ) ,would increase by a factor of 6 when the silanated column is employed. However, the actual improvement in resolution between these two components was only half the expected value. In fact, for any two components in these separations, the same result was obtained (Le., the observed improvement in resolution was approximately half of what would have been expected based solely on changes in to/tm).Of course, one can readily see that the efficiencies obtained on the silanated column were inferior to those of the untreated column. By multiplication of the last factor in eq 1by the experimentally observed efficiencies, the predicted improvements in resolution were in agreement with those shown. For example, for NBD-cyclohexylamine and NBDn-hexylamine shown in Figure 1, N = 80000, a = 3.8, k2/ = 75.1, k,’ = 19.7, and to/t, = 0.4. Insertion of these values in eq 1 results in a calculated resolution of 3.3, which is in agreement with the value obtained by using eq 3. Similarly, for these same components in Figure 3,N = 12 000,a = 3.6, k’, = 45.6, k’, = 12.6, and to/tm= 0.1. Insertion in eq 1yields a resolution of 7.6, which again is in agreement with the value obtained by using eq 3. Thus, the equation derived by Terabe is valid for predicting changes in resolution brought about by changes in to/t,, provided any changes in efficiency are taken into account. Furthermore, changes in to/t, obtained by lowering electroosmotic flow do yield improvements in resolution. Unfortunately, these improvements are less than expected due to a concomitant degradation in efficiency. Our initial supposition as to the cause of band broadening for the silanted column, a weak wall interaction between solute and the surface of the column, was reinforced by the chromatograms shown in Figure 2A,B. Since no micelles were present, one would not expect any retention, and this is the
io,
ANALYTICAL CHEMISTRY, VOL. 59. NO.
-0
30
SO
TIME (minutes)
90
-
120
Chromatogram of the separatlon of the test mixture on the siianated column. conditions the same as in Flgure 3 except the mobile phase contained 10% by volume 2-propanol.
I
Flgure 4.
result illustrated in Figure 2A for the untreated column. In Figure 2B, however, a separation of low capacity, with efficiency on the order that one would expect for a 50-rm-i.d., low-capacity bonded-phase, open capillary column, was observed. This clearly indicates that, in addition to lowering the { potential, the silanation of the column created an additional mechanism for solute retention in MECC. This at least partly accounts for the band broadening observed in Figure 3. Small percentages of organic modifiers, particularly 1propanol, have been shown to improve separation efficiency in micellar liquid chromatography (14). Two explanations were set forth to account for the improvements. The first involved an improvement in the wetting of the lipophilic stationary phase with the aqueous mobile phase. Second, it was proposed that the added alcohol tended to “loosen” the micellar structure, so that the local microviscosity experienced by a solubilized solute was diminished, thereby increasing mass-transfer kinetics. On the basis of these observations, we decided to examine the effect of adding small percentages of 2-propanol on efficiency in MECC. Figure 2, part C, is a chromatogram of the same two components used in Figure 2, parts A and B, on the silanated column. The mobile phase used here was 5 X M Na2HP04 that was also 10% by volume 2-propanol. As shown, only one peak was observed for the two solutes, thus indicating that wall interaction (retention) had been diminished in favor of improved partitioning in the mobile phase. It should be noted that 2-propanol also appears to further decrease u,, when compared to the results shown in Figure 2B. This should not be surprising, however, since it already has been stated that an effect of adding alcohol is to improve wetting of the bonded phase. As such, the surface is further modified, resulting in additional changes in ,(potential and, consequently, electroosmotic flow. Figure 4 shows the separation of the six test solutes on the silanated column, using a micellar mobile phase that was 0.025 M in SDS, 5 X M in Na2HP04,and 10% by volume in 2-propanol. When compared to Figure 3, the effect of the added 2-propanol is obvious. The efficiency for the last component in Figure 3 is only 12 000 theoretical plates, while in Figure 4 it is 65000. An enhancement in efficiency of approximately 5 is obtained by adding 2-propanol to the mobile phase on the silanated column. The alcohol also produced some change in the selectivity for the separation. It is expected
0
MAY 15,1987
5
10
1469
I
15
TIME (minutes)
Chromatogram of the separatlon of the test mixture on the untreated column. Conditions the same as in Flgure 1 except the mobile phase contained 10% by volume 2-propanol. Flgure 5.
that changes in selectivity could be minimized by using lower 2-propanol concentrations, while still observing significant improvements in efficiency. To test whether or not the observed efficiency enhancement was due solely to the minimization of wall interactions, the 2-propanol/micelle mobile phase was used in the untreated column. This chromatogram is shown in Figure 5. Once again, the effect of 2-propanol was to increase the efficiency of the separation. The last component in Figure 5 is approximately 180000 theoretical plates, while the same component without 2-propanol (Figure 1) is 80 000. Since this column did not undergo silanization with TMCS, one would not expect that the efficiency enhancement was due to minimization of wall interactions. In fact, as shown in Figure 2A, two components injected onto this column gave one sharp peak, indicating no wall interactions at all. When this result is considered along with the observations made by Dorsey (14), more support is given to the argument that one effect of the alcohol is to improve mass-transfer kinetics in and out of the micelles. Another interesting point made by comparison of Figures 1 and 5, and Figures 3 and 4,is that the addition of 2-propanol improved efficiency on the untreated column by a fsctor slightly greater than 2, while the observed improvement on the silanated column was greater than a factor of 5. It appears that the effects of 2-propanol are to enhance mass-transfer rates across the micelles and, more importantly, to minimize wall interactions created by the presence of trimethylsilane on the fused silica surface. We have successfully demonstrated one approach to extend elution range in MECC. By silanating the surfaces of fused-silica capillaries with TMCS, we lowered electroosmotic flow velocity and, more importantly, the net flow velocity of the micelle phase, thereby increasing elution range. This is accomplished at the expense of a loss in column efficiency which primarily results from solute-wall interactions. Addition of 2-propanol to the mobile phase serves to minimize this effect, thereby restoring much of the efficiency lost in the process. Registry No. TMCS, 75-77-4;SDS,151-21-3.
LITERATURE CITED (1) Tarabe, S.; Otsuka, K.; Ichikawa, K.; Tsuchiya, A,; Ando, T. Anal. Chem. 1984, 56, 111-113.
Anal. Chem. 1007, 59, 1470-1471
1470
(2) Terabe, S.; Otsuka, K.; Ando, T. Anal. Chem. 1985, 5 7 , 834-841. (3) Otsuka, K.; Terabe, S.; Ando. T. J. Chromatcgr. 1985. 348, 39-47. (4) Sepanlak, M. J.; Cole, R . 0. Anal. Chem. 1987, 59, 472. (5) Burton, D. E.;Sepanlak, M. J.; Maskarinec, M. P. Chromatographla, in press. (6) Otsuka, K.;Terabe, S.; Ando, T. J . Chtumatogr. 1985, 332, 219-226. (7) Burton, D. E.; Sepanlak, M. J.; Maskarinec, M. P. J . Chromatogr. Sci. 1988, 24, 347-351. (8) Burton, D. E.;SeDaniak, M. J.; Maskarinec, M. P. J . Chromatogr. Sci., submitted for publication. (9) Pretorius, V.; Hopkins, B. J.; Schieke. J. D. J . Chromatogr. 1974, 99, 23-30. (10) Tsuda, T.: Nomura, K.; Nakagawa, G. J. Chromatogr. 1983, 264, 385-392. (11) Jorgenson, J. W.; Lukacs. K. D. Anal. Chem. 1981, 53, 1298-1302. (12) Lukacs, K. D.;Jorgenson, J. W. HRC CC,J . Hlgh Resolut. Chromatogr. Chromatcgr. Common. 1985, 8 . 407-411. (13) HJertin, S. J. Chromatogr. 1985, 347, 191-198.
(14) Dorsey, J. G.; DeEchegaray, M. J.; Landy. J. S. Anal. Chem. 1983, 55, 924-928.
A. T. Balchunas M. J. Sepaniak* Department of Chemistry University of Tennessee Knoxville, Tennessee 37996
RECEIVED for review November 12,1986. Accepted January 12, 1987. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US. Department of Energy, under Contract DE-FG05-86ER13613 with the University of Tennessee (Knoxville).
Quantitative Reduction of Selenate Ion to Selenite in Aqueous Samples Sir: Trace analysis for selenium in biological materials and water has chiefly been concerned with determination of the total amount of the element (I). However, speciation of selenium with respect to oxidation state is important in assessing the bioavailability of the element and ita toxicity (2). Electroanalysis is especially well suited to this type of speciation and has the additional advantage that other trace elements can be determined simultaneously. Stripping analytical methods for determining the selenite anion a t the part-per-billion level are well documented in the literature (3). The other principal source of water-soluble inorganic selenium is the aelenate anion @eo4%). Electroanalysis of this species is not possible, since the corresponding reduction reaction to selenite, namely SeOd2-
+ 3H+ + 2e-
-
HSeOy
+ H20
(1)
which has a very positive standard potential (1.075 V), is so slow that no faradaic current is observed in the usual voltammetric experiments (4). One procedure for determining selenate is to reduce it first to selenite in hot concentrated HC1 solution and then to determine it by difference with respect to selenite. Cutter (5, 6) has recommended that the reduction be carried out in boiling 4 M HC1 for 15 min and warned that prolonged boiling could lead to reduction of selenite to lower oxidation states (5). The latter conclusion was challenged on thermodynamic grounds by Bye (7)who suggested that loss of selenium from samples which were boiled too long was due to the formation of volatile Se(1V) compounds. Bye also concluded that the important component of the reducing solution was the C1- ion and that the reduction could be carried out quantitatively in concentrated HC1 solutions (7). The purpose of the present investigation was to assess more carefully the conditions under which selenate is quantitatively converted to selenite. In this regard, it seemed that measuring the rate constant for the reaction
Se042-+ 4H+ + 2C1-
-
HZSeO3+ HzO
+ Clz
(2)
would help the analyst determine the best conditions for the reduction reaction. The results of this study are presented in this paper. EXPERIMENTAL SECTION Electroanalysis. Selenite concentration at the parts-permillion level was determined by differential pulse polarography (DPP) using a PARC polarographic analyzer (Model 174A)with 0003-2700/87/0359-1470$01.50/0
a PARC static mercury drop electrode (Model 303A). Current against voltage curves were recorded with a 50-mV modulation voltage at a scan rate of 1 mV s-l from an initial potential of -400 mV against a AgjAgC1 reference electrode. Analysis was based on the two-electron reduction of HgSe which occurs at -0.57 V (8). The sample was deoxygenated with argon which also provided a protective sheath over the sample during analysis. Digestion Procedure. Selenate was converted to selenite by acidifying 8 mL of aqueous solution containing the sample with 8 mL of concentrated (12 M)HC1. The resulting solution was then placed in a capped test tube and heated in a water bath at 91.0 "C for 30 min. The test tube made of borosilicate glass was placed in the bath so that the portion containing the sample (approximately 10 cm of a total length of 12 cm) was totally immersed. The screw cap wm made of Bakelite and had a rubber liner coated with Teflon such that Teflon was the only material that could come into contact with the sample. The bath temperature (91.0 "C) was chosen to be sufficientlybelow boiling that the level of water in the bath did not change significantly during digestion, but otherwise was arbitrary. After the sample was allowed to cool to room temperature, the solution was diluted with distilled water to 32 mL so that the analysis was carried out in a matrix of 3 M HC1. It should be noted that results for samples heated in uncapped test tubes were significantly lower, demonstrating that loss of selenium through formation of volatile compounds is a problem for samples heated in open systems. Kinetic Analysis. In order to determine the kinetic parameters for reaction (2),a solution containing 20 ppm SeO?- was acidified with concentrated HC1 to either 4.0 or 6.0 M. The acidified solution was placed in capped test tubes and heated for known lengths of time in the range 0-90 min in a water bath maintained at 91.0 0.1 "C. Test tubes were removed at recorded intervals, and the reaction was stopped by freezing the sample in a dry iceacetone bath. The sample was brought to room temperature prior to analysis and the Se03" concentration determined by differential pulse polarography. Reagents. All chemicals used were reagent grade. Sodium selenate was obtained from Thiokol/Ventron and sodium selenite from Aldrich Chemical. Aqueous, solutionswere prepared by using water with a resistivity of 17 Ma cm or greater. Standard solutions were obtained by dilution of stock solutionscontaining either IO00 ppm SeO?- or 1000 ppm Se0,2-.
*
RESULTS AND DISCUSSION Analytical Determination. Selenite was determined by DPP with either 0.12 or 3.0 M HCl as background electrolyte. Calibration curves of peak current against selenite concentration were prepared in the concentration range 2-15 ppm. An excellent linear relationship between these variables was obtained for a given background electrolyte, the precision of 0 1987 American Chemical Society
