Anal. Chem. 1991, 63,699-703 (25) Bayfield, R. F.; Romaiis, L. F. Anal. Blochem. 1985, 144, 569. (26) Yatsimirskii, K. E. Kinetic Methods of Analysis; Pergamon Press: Oxford, 1966. (27) West, P. W.; Ramakrishna, T. V. Anal. Chem. 1968, 40, 966. (28) Kawashima, T.; Tanaka, M. Anal. Chim. Acta 1988, 4 0 , 137. (29) Hwang, J. M.; Wei, T. S.; Chen, Y. M. J . Chin. Chem. Soc. 1986,33, 109. (30) Linares, P.; Luque de Castro, M. D.; Valcarcel, M. Analyst 1986, 1 7 1 , 1405. (31) Betteridge, D.; Oates, P. 6.;Wade, A. P. Anal. Chem. 1987, 59, 1236. (32) Rios, A.; Luque de Castro, M. D.; Valcarcei, M. Anal. Chem. 1988, 6 0 , 1540. (33) Canete, F.; Rios, A.; Luque de Castro, M. D.; Valcarcel, M. Anal. Chem. 1988, 6 0 , 2354. (34) Ruzicka, J.; Marshall, G. D.; Christian, G. D. Anal. Chem. 1990, 62,
1861.
690
(35) Wentzeil, P. D.; Hatton. M. J.; Shiundu, P. M.; Ree. R. M.; Wade, A. P.; Betteridge, D.; Sly, T. J. J. Automatic Chem. 1989, 7 1 (54, 227. (36) Shiundu, P. M.; Wentzeil, P. D.; Wade, A. P. Taknta 1990, 37,329. 237,361. A. P.;Shiundu, P. M.; Wentzell, P. D. Anal. Chim. Acta 1990, (37) Wade, (38) Shlundu, P. M.; Wade, A. P.; Jonnalagadda, S. 6. Can. J . Chem. 1990, 68, 1750.
RECEIVED for review September 4,1990. Accepted December 20,1990.This work was supported by NSERC operating grant No. 5-80246and by UBC-NSERC equipment grant No. 580085. This work was presented by P.M.S. a t The 1990 FACSS Conference for the 1990 Tomas Hirshfeld Student Award.
Indirect Photometric Chromatography of Anions Using Sodium Naphthalenetrisulfonate Shahbaz A. Maki and Neil D. Dadelson* Department of Chemistry, Miami University, Oxford, Ohio 45056
A triply charged eluent has been characterlzed for anion-exchange chromatography with lndlrect photometric detectlon. Sodium naphthalenetrlsulfonatehas shown particular promise for the separation and detectlon of NO2-, Br-, NO9-, SO:-, I-, and SCN-. The analysis of a mixture contalning these anions can be performed In less than 18 min with detection limits of 0.4-1 ng for ail anions. The determination of the strongly retained sulfur oxides such as dithlonite, tetrathlonate, and other poiythionates can be easily accomplished with this highly charged eluent in a short time with high sensitivity. The chromatographlc performance of thls eluent requires no pH adjustment of the mobile phase. I n addition, chromatograms obtalned with thls eluent were free from any system peaks.
INTRODUCTION The analysis and detection of inorganic and organic anions using ion-exchange chromatography with indirect detection continues to be of interest as evidenced by two recent review articles ( I , 2). Indirect photometric chromatography (IPC) involves the use of a light-absorbing eluent that provides the means of both chromatographic separation and detection of non-light-absorbing analytes (3). This IPC technique offers several advantages over conventional ion-exchange chromatography, the primary one being universal detection (2). Several eluents have been characterized as mobile phases for anion IPC, such as benzoate, phthalate, salicylate, and other carboxylate salts (3-8).These mobile phases possess relatively large molar absorptivities in the UV region as well as selective ion-exchange capabilities for separating the most common anions. The pH of the mobile phase plays an important role in the chromatographic performance of the weak organic acid salt eluents. A precise control of the mobile-phase pH was found to be crucial to provide elution reproducibility. Okada and Kuwamoto ( 6 ) have studied the elution behavior of tartaric and malic acids for anion IPC. Retention times were found to decrease with increasing pH of the mobile phase. A reverse elution with incomplete separation of some anions at certain pH values can also occur. Jackson and Haddad have also reported a variation in retention times with the p H of 0003-2700/9 1/0363-0699$02.50/0
the mobile phase when using potassium hydrogen phthalate as the eluent (7). Use of aromatic sulfonates instead of carboxylate salts as eluents should alleviate this problem. The use of a weak organic acid salt as an eluent for anion IPC has been usually accompanied by the appearance of one or more system (extraneous) peaks that are not related to the analyte ions. These system peaks can overlap desired analyte peaks particularly at low sample concentrations (7,9). The neutral components of these eluents have been considered to be the responsible species for these extra peaks (10). Chromatograms obtained with strong organic acid salts, on the other hand, have been found to be free of these ghost peaks. Sat0 (11)has also reported the absence of system peaks when using sodium 1,2-dihydroxybenzene-3,5-disulfonate salts as the mobile phase, but a large system peak was observed when the acid form was used as the eluent. This has been attributed to elution of the undissociated form of the acid through a partition chromatography mechanism. Previous work by us using 2-napthalenesulfonate and 1,5-naphthalenedisulfonate salts as IPC eluents has likewise shown no system peak interference (12). The retention time of analyte anions also depends on the concentration of the IPC eluent. Small mobile-phase concentrations are recommended to achieve low background absorbance and good detection limits as predicted by Yeung for all indirect detection techniques using the following formula; where Climis the concentration limit of detection, C, is the concentration of the eluent, DR is the dynamic reserve (ratio of background signal/background noise), and T R is the transfer ratio, which is expressed as the number of mobilephase molecules displaced by one analyte molecule (2). Bulky multicharged eluents should permit the use of very dilute mobile phases without compromising the analysis time, but still providing low detection limits (13). Although a triply charged cation, Ce3+,has been used for IPC (14),we believe a pH-independent triply charged anion has not been employed as an IPC eluent. We are reporting here the characterization of sodium naphthalenetrisulfonate (NTS) as a promising eluent for anion 0 1991 American Chemical Society
700
ANALYTICAL CHEMISTRY, VOL. 63, NO. 7, APRIL
1, 1991
IPC. This triply charged eluent showed selective performance toward the separation and detection of multicharged and/or large anions, particularly the strongly retained sulfur oxides such as dithionite, tetrathionate, and other polythionates. A detection limit comparison of IPC methods using naphthalenesulfonate anion eluents is provided.
l
I
EXPERIMENTAL SECTION Instrumentation. The chromatographicsystem was composed of a Model 510 HPLC pump, a Model U6K injector equipped with a 20-pL sample loop, a Model 490 programmable multiwavelength detector, and an IC-PAK anion-exchange column (4.6 mm i.d. X 5 cm), all from the Waters Chromatography Division of the Millipore Corp. (Milford, MA). Chromatograms were recorded on a Model 5000 Fisher Recordall chart recorder (Austin, TX). Absorption spectra of the mobile phases were taken on a Varian Model DMS 90 UV-vis spectrophotometer (Sunnyvale, CA). Reagents. 1,3,(6or 7)-Naphthalenetrisulfonic acid, trisodium salt hydrate, was purchased from Aldrich Chemical Co. (Milwaukee, WI). This technical grade reagent with purity estimated at 93-95% was used as received. Sodium tetrathionate was purchased from K & K Laboratories, Inc. (Plainview, NY). Sodium dithionite, technical grade, and salts of common anions, reagent grade or better quality, were obtained from different suppliers. Acetonitrile (CH,CN), HPLC grade, was obtained from Fisher Scientific (Fair Lawn, NY). Procedure. A stock solution of 0.01 mM NTS was prepared, and aliquots of this solution were used to prepare more dilute mobile phases with 10% CH3CN. It was found in previous work (15) that the presence of 10% CH,CN was necessary to improve the chromatographic performance of these naphthalene derivatives, by reducing or eliminating the unwanted hydrophobic interaction between the polymeric stationary phase and the naphthalene moiety of the mobile phase. A 1000 ppm stock solution of each of the common anions was prepared, except for sulfite and thiosulfate, which were prepared freshly whenever needed. Stock solutions of 100 ppm containing either dithionite or tetrathionate were prepared. These solutions were used to prepare the more diluted samples and mixtures of them. All solutions were prepared with triply distilled deionized water. The ion-exchange column was conditioned with the desired mobile phase for at least 2 h before analysis. The mobile-phase flow rate was 1 mL/min, generating a column pressure of 1500 psi, and all separations were carried out at ambient temperature. Capacity factors, k’, were calculated in the usual way; the solvent front (injection peak) was taken as the retention time of the unretained compound. Peak areas were calculated manually with the peak height and peak width at half-height method.
RESULTS AND DISCUSSION The UV spectrum from 250 to 350 nm of a NTS solution shows an absorption maximum a t 282 nm with a molar absorptivity of 6860 L/(mol cm). No other peaks were present in the spectrum. Because of the availability of a 280-nm filter for inexpensive UV detectors, we have chosen 280 nm as a representative detection wavelength throughout this work. The molar absorptivity a t 280 nm was 6800 L/(mol cm). The capacity factors of NOz-, Br-, NO3-, SO ,’: I-, and SCNwere measured as a function of the NTS eluent concentration. The retention of smaller anions such as fluoride and chloride was very short and as a result coelute with the solvent (injection) peak. These two anions could be easily determined using sodium naphthalenesulfonate as the mobile phase (12). The capacity factors for most anions studied in this work ranged from 2 to 12 for the concentration range 0.15-0.005 mM NTS, except for thiocyanate, which had an extended range from about 10 to 27. At a mobile-phase concentration higher than 0.02 mM NTS, the capacity factor of ion crosses over that of NO3-, Br-, and NOz-. At mobile-phase concentrations lower than 0.02 mM NTS, separation of all the anions can be easily accomplished in a reasonably short time. The separation selectivity of NTS is limited to only the large and multicharged analytes (SO:-, I-, SCN-) at higher mo-
-3
Log [NTS]
-1
0
Flgure 1. Plots of log k’ for inorganic anions as a function of log NTS concentration. Solute concentration: 10 ppm each of (A) NO2-, (E) Br-, (C) NO3-, (D) S042-, (E) I-, (F) SCN-.
bile-phase concentrations. Consequently, by controlling the concentration of the mobile phase, separation of different groups of analytes can be achieved with high efficiencies. The baseline noise level (expressed as peak-to-peak noise) was 2.8 times higher at 0.15 mM compared to 0.01 mM NTS. The sensitivity was about 3.5 times higher at 0.01 mM than that a 0.1 mM NTS (sensitivity = analyte peak heightlanalyte concentration). These trends are typical and consistent with all indirect detection techniques, due to the fact that the relative change in the baseline absorbance is more pronounced at low background that at high background absorbance levels. Low mobile-phase concentrations provide high sensitivity, good separation power, but relatively longer retention times compared to more concentrated mobile phases. A plot of log k’versus log NTS concentration for these six anions is shown in Figure 1. Good linearity with an average correlation coefficient of 0.998 was noted for all the anions as predicted by the equation log k‘ = - ( y / x ) log E + log B, where y is the charge of the sample anion, x is the charge of the eluent anion, E is the eluent concentration, and B is a constant dependent on the ion-exchange equilibrium constant and the capacity of the resin. This relationship has been demonstrated previously for ion chromatography using either direct (16) or indirect detection (3). A comparison of slope values Cy/x) for the mobile phases sodium 2-naphthalenesulfonate (NMS) and sodium 1,5-naphthalenedisulfonate (NDS) studied in recent work (12) as well as NTS is provided in Table I. As expected, the values for the monovalent ions using NMS are close to 1. The fact that the slopes of the same monovalent ions using NDS are higher than the predicted value of 0.5 has been previously noted when using potassium phthalate (16). The trend of decreasing slope with increasing retention of the monovalent anions is consistently seen for both NMS and NDS. The longer retained ions such as S042-, I-, and SCN- gave slopes close to the theoretical values. This same good agreement is seen for the I- and SCN- slopes of NTS. The other ions have NTS slopes values higher than those predicted by theory. For example, both nitrate and sulfate have y / x values about 0.12 unit too high. In contrast, slope values for bromide and sulfate using trimesate (1,3,5benzenetricarboxylate) salts were lower than theory (3). Apparently for both NDS and NTS, the effective charge of the bulky eluent with two widely spaced sulfonate groups is
ANALYTICAL CHEMISTRY, VOL. 63,NO. 7, APRIL 1, 1991
n
Table I. Slope Values from log k'versus log [eluent] Plots anions
NMS"
NDSb
fluoride chloride nitrite bromide nitrate sulfate iodide thiocyanate
1.00 1.00 0.98 0.97 0.95
0.81 0.67 0.62 0.58 0.57 1.00 0.51 0.51
701
NTSc
0.50 0.47
0.45 0.79 0.31 0.31
'2-Naphthalenesulfonate anion (k' data from ref 12). 1,5Naphthalenedisulfonate anion (k' data from ref 12). c1,3,(6 or 7)-Naphthalenetrisulfonateanion.
1
MINUTES
2
Chromatogram of 10 ppm S20,2-, 34day-old sample. Conditions are 0.02 mM NTS/10% CH,CN as the mobile phase and 0.05 AUFS. See text for peak identification.
Flgure 3.
in aqueous solutions and undergo several decomposition and disproportion reactions, which can make their determination a tedious and time-consuming task. Dithionite ion, S2042-, in solution undergoes several oxidation processes in the presence as well as in the absence of air as shown below (17, 18): 2S,Od2- -,Sz032- + S2052-
--
S2032- SOS2-+ S S032- + (0) S042F
1
E
A
Flgure 2. Separation of a standard mixture of inorganic anions with
IPC. Mobile phase: 0.01 mM NTS/10% CH,CN. Absorbance units full scale: 0.01 AUFS. Peak identification: 1 ppm each (A) NO;, (e) Br-, (C) NO,-, and (D) SO,*- and 5 ppm each (E) I- and (F) SCN-. less than the true charge during the ion-exchange process involving small anions. The separation of a mixture containing 1 ppm each NO,, Br-, NO< and SO?- and 5 ppm each I- and SCN-, using 0.01 mM NTS, is shown in Figure 2. The order of separation is as expected considering retention is inversely proportional to hydration energy and proportional to polarizability (16). The peaks for the first three anions are partially overlapped; however, the last three are as well resolved. By use of 0.1 mM NTS as the mobile phase, the retention times for sulfate, iodide, and thiosulfate are about 1,3,and 5 min, respectively, representing an average reduction of more than 3 times in analysis time as compared to Figure 2. However, using this mobile-phase concentration, it is not possible to completely separate nitrate from sulfate. The strong eluting power of NTS as a mobile phase leads to the analysis of the well-known highly retained sulfur oxides, such as dithionite. These compounds are generally unstable
The above decomposition scheme predicts at least five anions present in the dithionite ion solution, namely, &Os2-, S2042-, S2032-,SO?-, and SO?-.A chromatogram of a 50 ppm fresh solution of dithionite ion showed five peaks consistent with the above decomposition reactions. Peak 1, determined to be SO?-, was the dominant peak in the chromatogram. Peaks 2 and 3 have also been identified as S042-and S2032-,respectively, using freshly prepared solutions. Peaks 4 and 5 were presumed to be S2042- and S202-,respectively; however, no standards were available to confirm their identities. Analysis of a 3-day-old sample of the same solution indicated a decrease in sulfite ion (peak 1)and thiosulfate ion (peak 3) and an increase in the sulfate ion (peak 2), even in a closed sample container. Figure 3 shows a chromatogram representing a 10 ppm 34-day-old dithionite solution. The sulfate ion is now the predominant species, and the status of equilibrium between these sample components has likely been established. Another example of the capability of NTS as a powerful eluent is the determination of other polythionates such as tetrathionate ion, S4O;-, which can have a strong affinity for ion-exchange resins (19). The separation and detection of polythionates can require fairly complicated procedures involving postcolumn derivatization with either Ce(IV) (20,21) or with bromine/iron(III) perchlorate (22). Figure 4 shows a chromatogram of a mixture containing the sodium salts of thiosulfate, thiocyanate, and ktrathionate using 0.1 mM NTS as the mobile phase. These analytes, which are usually found
702
ANALYTICAL CHEMISTRY, VOL. 63,NO. 7, APRIL 1, 1991
n
/I m
U
E
c
F
b
u
Chromatogram of (left)0.05 ppm each (E) I- and (F) SCN(at 0.002 AUFS), and (right) 0.02 ppm SO,'- (D) (at 0.003 AUFS). Conditions are the same as in Figure 2.
Figure 5.
i Figure 4. Chromatogram of (1) 5 ppm thiosulfate ( S 2 0 t - ) ,(2) 10 ppm thiocyanate (SCN-),and (3) 50 ppm tetrathionate (S40e2). Conditions are 0.1 mM NTS/ 10 % CH,CN as the mobile phase and 0.1 AUFS.
Table 11. Detection Limita Comparison for Various Anions anionsb fluoride chloride
nitrite bromide nitrate sulfate iodide thiocyanate
NMS'
NDSc
NTSc
0.2 (4)d 0.1 (2) 0.5 (10) 1 (20) 1 (20)
0.05 (1) 0.05 (1) 0.05 (1) 0.1 (2) 0.5 (IO) 0.1 (2) 1 (20) 1 (20)
0.02 (0.4) 0.05 (1) 0.02 (0.4) 0.02 (0.4) 0.05 (1) 0.02 (0.4)
aSignal-to-noiseratio I 3. *Sample volume 20 rL. 'Using the optimized mobile phases; 0.3 mM NMS, 0.15 mM NDS, and 0.01 mM NTS, all in 10% CH,CN. dConcentration,ppm (ng). in oil shale leachates (23),are well separated in a short time of less than 16 min. It is quite clear from Figure 4 that low ppm detection limits could be easily achieved for the analytes since the absorbance scale is set on 0.1. By use of sulfosalicylic acid with indirect UV detection a t 254 nm, detection limits of only 8 and 40 ppm for S042-and S2032-were cited (24). After HPLC using direct UV detection at 254 nm (20),it was not possible to detect 3.1 and 10.1 ppm S203'- and S4062-, respectively. After separation by a weak anion-exchange column, direct detection at 205 nm of S2032-and S4OS2-was possible a t 3.3 and 0.6 ppm, respectively (24). The short retention time of the tetrathionate ion even a t this low mobile-phase concentration (Figure 4) should make the analysis and detection of higher polythionate ions an easy task particularly when more concentrated mobile phases are used. The quantitation of the analyte anions separated in Figure 2 using NTS as the mobile phase has been studied. Linear least-squares analyses of the common anions showed the linear range extended from at least 100 ppm to the detection limit of each analyte anion, which is low as 0.02 ppm for most anions (Table 11). The relative standard deviation of the slope ranged from 0.41% to 2.2% with an average of 0.95% for all anions. The relative standard deviation in the peak area calculation was 0.85% for an average of four runs. The correlation coefficients were excellent from 0.9996 to 0.9999 for all anions. Figure 5A shows a chromatogram of 0.05 ppm iodide ion and 0.05 ppm thiocyanate ion at or near their detection limits. A 0.02 ppm sulfate ion peak at the detection limit is illustrated
in Figure 5B. All peaks are well defined from the baseline noise ( S I N I 3) using 0.01 mM NTS as the mobile phase. Lower detection limits can be achieved easily using lower mobile-phase concentrations, as described earlier; however, peak broadening and longer retention times are the trade-off. For example, the calculated peak area and the peak width for 10 ppm SO:- were 1.2 and 1.9 times larger, respectively, when using 0.005 mM rather than 0.01 mM NTS at the same detector sensitivity. In addition, the retention times were 8.5 and 5.1 min using 0.005 and 0.01 mM NTS, respectively. The positive peaks appearing a t the early part of the chromatograms are attributed mainly to some sample contamination. However, these peaks were not observed with other analytes and do not interfere with the analysis of mixtures of these anions. Table I1 lists the detection limits of various anions using 0.01 mM NTS as the mobile phase as well as those obtained in recent work (12) using NMS and NDS as mobile phases for anion IPC. The optimized mobile-phase concentrations were 0.30,0.15, and 0.01 mM and the molar absorptivities at 280 nm were 4290, 11250, and 6800 L/(mol cm), for NMS, NDS, and NTS, respectively. NMS was found to offer a good separation performance for the small and singly charged anions such as F-, C1-, NO2-, Br-, and NO3-; however, the retention times for larger and multicharged anions were extremely long. NTS, on the other hand, is better suited for the separation of large and multicharged anions as well as some singly charged anions, such as NOz-, Br-, NO3-, I-, and SCN-. NDS was found applicable for both groups of anions. The detection limits for the early eluted anions ranged from 0.2 to 1 ppm (2-20 ng) using NMS as the mobile phase and from 0.05 to 0.5 ppm (1-10 ng) when using NDS as the mobile phase, for the same group of analytes. However, the detection limits for the other anions studied using NTS are much lower (0.4-1 ng) than those using NDS (1-10 ng). An excellent comparison between these three IPC eluents can be made by using the detection limits of nitrite, bromide, and nitrate ions. The detection limit for nitrite is 2.5 times lower when using NTS than NDS, which is in turn 25 times lower than that found when using NMS. A similar trend is seen with the bromide detection limit being 20 times lower with NTS than NMS and 2 times lower than that of NDS. By use of NTS as the mobile phase, nitrate detection limits are 50 and 25 times lower than those found when using NMS and NDS, respectively.
ANALYTICAL CHEMISTRY, VOL. 63,NO. 7, APRIL 1, 1991
703
respectively. This was found true for all analytes. This is in good agreement with the ratio of the molar absorptivities of the compounds, in which the molar absorptivity of NDS is 1.6 and 2.6 times larger than NTS and NMS, respectively. These napththalenesulfonate anion eluents, especially NTS, will be further studied by using other indirect detection techniques such as conductivity and fluorwence. In addition, IPC of aliphatic organic anions such as surfactants using this class of eluents should be possible.
LITERATURE CITED
0
20
40
60
80
100
120
Conc. (ppm)
Figwe 8. Nitrate ion linearity comparison between (A) NMS, (B) NTS, and (C) NDS.
The effective charge and the molar absorptivity of each mobile phase play an important role in determining the detection limits and sensitivities of these eluents. NTS with -3 charge generated the lowest detection limits compared to NDS and NMS with -2 and -1 charge, respectively, despite NDS having the highest molar absorptivity. The large charge on NTS has resulted in short retention times and consequeuntly sharper peak shapes for all analytes, because the mobile phase is 15 times more dilute than that for NDS. The sensitivity, on the other hand, is more dependent upon the molar absorptivities of these compounds. Figure 6 shows linearity runs for nitrate ion using NTS, NDS, and NMS as mobile phases. The sensitivity using NDS is 1.8 and 3.4 times higher than those found when using NTS and NMS as mobile phases,
Dorsey, J. G.; Foley, J. P.; Cooper, W. T.; Barfwd, R. A.; Barth, H. 0. Anal. Chem. 1990, 62, 324-35813. Yeung, E. S. Acc. Chem. Res. 1989, 2 2 , 125-130. Small, H.; Miller, T. E., Jr. Anal. Chem. 1982. 5 4 , 462-469. Heckenberg, A. L.; Haddad, P. R.; J . Chromfogr. 1884, 299, 301-305. Rapsomanikis, S.; Harrison, R. M. AMI. Chim. Acte 1987. 799, 41-47. Okada, T.; Kuwamoto, T. J . Chromafogr.1984, 284, 149-156. Jackson, P. E.; Haddad, P. R. J . Chromafogr. 1985, 346, 125-137. Small, H. Ion Chromafography; Plenum Press: New York, 1989. Brandt, G.; Vogler, P.; Kettrup, A. 2. Anal. Chem. 1986, 325, 252-254. Sato. H. Anal. Chem. 1890, 62. 1567-1673. Sato, H. Anal. Chim. Acte 1988, 206, 281-288. Maki, S. A.; Danieison, N. D. J . Chromafogr., in press. Yokoyama, Y.; Sato, H. J . Chromafgr. Scl. 1988. 26, 561-565. Sherman, J. H.; Danielson, N. D. Anal. Chem. 1987, 59, 1483-1485. Maki, S . A.; Danielson, N. D. J . Chromafogf. Sci. 1990, 28, 537-542. Gjerde, D. T.; Schmuckler, G.; Frit.?, J. S. J . Chromafogr. 1980, 787, 35. Karchmer, J. H. The Analytical Chemistry of Sulfur and Its a m pounds; Why-Interscience: New York, 1970 Part I. Remy, H. Treafise on I n w a n l c Chemistry: Eisevier Publishing: A m sterdam 1956; Voi. I . Novak. G.; Erdelyi, M.; Vigvari, M. J . Chromafogr. 1980. 207, 313-3 .. . .15. . .. Woikoff, A. W.; Larose, R. H. Anal. Chem. 1975 47, 1003-1008. Wolkoff, A. W.; Larose, R. H. J . Chromatogr. Sci. 1978, 74, 353-355. Story, J. N. J . Chromafogr. Sci. 1983, 2 7 , 272-277. Trujiio, F. J.; Miller, M. M.;Skogerboe, R. K.; Taylor, H. E.; Grant, C. L. Anal. Chem. 1981, 53, 1944-1946. Vins, 1.; Kabrt, L. Collect. Czech. Chem. Commun. 1987, 5 2 , 1167-1 171.
RECEIVED for review September 20,1990. Accepted January 11,1991. We thank the Waters Chromatography Division of Millipore Corp. for their support.
