Anal. Chem. 1983, 55, 1168-1169
1168
-
and the favorable quality and structure of the ion-selective membrane surface (nbt 10-7 mol). The evident superiority of precipitate-based silicon-rubber-supported membranes relative t o other membrane configurations was also documented by Pungor's group (12). Registry No. AgC1, 7783-90-6; AgI, 7783-96-2.
LITERATURE CITED Pungor, E.; TBth, K. Anal. Chlm. Acta Ig69, 4 7 , 291. Buck, R. P. Anal. Chem. 1968, 4 0 , 1432. Wuhrmann, HA.; Morf, W. E.; Simon, W. He&. Chlm. Acta 1973, 56,
1011. Morf, W. E.; Kahr, G.; Simon, W. Anal. Chem. 1974, 46, 1538. Hulanickl, A.; Lewenstam, A. Talanta 1977, 24, 171. Frelser, H., Ed. " Ion-Selective Electrodes in Analytical Chemlstry"; Plenum: New York, 1978;Chapters 1 and 2. Rhodes, R. K.; Buck, R. P. Anal. Chlm. Acta 1980, 113, 67. Morf, W. E. "The Prlnciples of Ion-Selectlve Electrodes and of Membrane Transport"; AkadBmlal Kladb: Budapest; Elsevler: Amsterdam,
(12) Llndner, E.; TBth, K.; Pungor, E. Anal. Chem. 1982, 5 4 , 202. (13) Nicolsky~B. p. zh. F1z. Khlm. l9s79 ' 0 , 495. (14) IUPAC Recommendations for Nomenclature of Ion-Selective Electrodes Pure ADD/.Chem. 1978. 48. 127. (15) Huianicki, A,; kwandowski, R. C h e k . Anal. (Warsaw) 1974, 19, 53. (16) Jyo, A.; Ishibashl, N., ref 8, pp 246-258. (17) Pungor, E., BuzSs, I., Eds. Ion-Selective Electrodes"; AkadBmiai KladB: Budapest, 1981. (18) Morf, W. E., ref 17, p 267. (19) Senkj?, J.; Petr, J., ref 17. (20) Hulanickl, A.; Lewenstam, A. Talanta 1978, 23, 661. (21) Schwab, G.-M. Kolloid-2. 1942, 101, 204. (22) Jaenicke, W. 2.Nektrochem. 1953, 57, 843. (23)Jaenicke, W.; Haase, M. 2.Nektrochem. 1959, 63, 521. (24) Morf, W. E. Anal. Lett. 1977, 10, 87. (25) Lindner, E.; TBth, K.; Pungor, E. Anal. Chem. 1982, 5 4 , 72.
Werner
E. Morf
1981.
Department of Organic Chemistry Swiss Federal Institute of Technology CH-8092 Zurich, Switzerland
Hulanicki, A.; Lewenstam, A. Anal. Chem. 1981, 53, 1401. Sandifer, J. R. Anal. Chem. 1981, 53,312. Sandlfer, J. R. Anal. Chem. 1981, 53, 1164.
RECEIVEXI for review July 23,1982. Accepted January 27,1983.
Behavior of Cations in Nonsuppressed Anion Chromatography Sir: The behavior of cationic species during anion analysis of aqueous samples by nonsuppressed chromatography has received little attention. The presence of selected cations could affect the stability and performance of the columns and through interaction with the column materials coelute with the anions of interest. The interaction of alkali and alkaline earth metals with separator columns used in suppressed ion chromatography has been reported (1). For nonsuppressed systems, where silation is not 100% efficient, there are sufficient SiOz.aq sites available to interact with the metals (2-4). The separation of Au3+, Hg2+,and Cu2+by quaternary ammonium based anion separator columns has been reported (5) and presumably is due to the formation of amine complexes. This paper presents the interaction behavior of selected cations with a nonsuppressed anion separator column, and the analytical implications are discussed. EXPERIMENTAL SECTION Chromatographic Procedure. The chromatographic system employed consisted of a Perkin-Elmer Series 3B liquid chromatography, a Vydac Model No. 3021 C4.6 anion separator column, a Vydac Model 6000CD conductivity detector, a Sargent Welch XKR strip-chart recorder, and a Hewlett-Packard Model 3390A integrating recorder. The column separator group was amine based. Injector sample loop volume was 0.100 mL, and samples of 0.5 mL were introduced with a Hamilton Co. Model No. 750 microliter syringe. Laboratory temperature was maintained at 22.5 & 2.0 "C. Phthalate eluents were prepared by dissolving potassium hydrogen phthalate (KHP) in 900 mL of doubly distilled water, adding sufficient 0.1 M KOH to obtain the desired pH, and diluting to a final volume of 1.0 L with the distilled water. Distilled water eluents had their pH adjusted with either 0.01 M KOH or concentrated "0% Cation stock solutions, 0.10 M, were prepared by dissolution of their nitrate salts in distilled water. Standard solutions were prepared for analysis by dilution of the appropriate stock with the eluent of interest thus minimizing the magnitude of the solvent response in the chromatogram. All salts used were reagent grade. Fraction Analysis. In some cases, eluent fractions were collected for further chemical characterization. Fractions were collected manually for each 20-9 period over the entire course of the determination. All analyses were repeated a minimum of three times and all equivalent fractions were combined prior to analysis. The fractions were stored in closed Pyrex test tubes for less than 24 h prior to analysis. Cation content of the fractions was obtained by standard flame atomic absorption. Sample pH was measured
Table I. Retention Characteristics of Ionic Speciesa anions
3.6
Br NO;
4.7
so,2-
s,o,za
RT,min
c1-
5.4 9.4 13.1
cations Pb2+ Znz+ cu2+
R,, min
3.5 4.4
5.1
Eluent is 2.5 mM KHP; pH 5.0; flow rate, 2 mL/min.
with a Microelectrodes, Inc. Model MI-I110 combination pH probe, standardized at pH 4 and 10, and monitored with a Radiometer Model 26 pH meter. Total phthalate in the fractions was determined spectrophotometrically at 281 nm in a 1/50 (v/v) HCl matrix with 1-cm quartz cells (6). RESULTS AND DISCUSSION During a chromatographic study of factors that control the separation of anions, it was observed that certain cations were retained by the anion exchange column. Of the cations studied there appear to be three general groupings. The first of these includes the cations that do not interact with the column, Na+, K+, Ca2+ Mg2+9 Ni2+, Mn2+, and Cd2+. The second group interach to such a degree as to be chromatographed at times similar to those observed for certain anions, Cu2+,Pb2+,and Zn2+. The third group, Fe3+,A13+,and Hg2+,is strongly retained by the column and did not elute. Figure 1 presents a typical chromatogram for NaN03 (graph A) and what is observed when C U ( N O ~and ) ~ Pb(N03) are chromatographed (graph B). Atomic absorption spectrophotometry was necessary to identify the analyte for each metal peak. At low pH values, -4, the cations elute more rapidly than at higher pH and conversely anions elute less rapidly since there is a smaller fraction of the completely deprotonated phthalate anion present. The retention times demonstrated by the cations are comparable to those observed for some anions and thus incorrect peak assignments could be made. This problem is demonstrated in Figure 2 where chromatograms of copper, lead, and zinc nitrate are superimposed upon a chromatogram of NaC1, KBr, and NaN03. Table I summarizes retention times for the analytes. As the pH of the eluent changes from 4 to 5, the retention time of the cations increases by approximately 25% whereas the retention time of the anions decreases by 9
0003-2700/83/03551 168$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
1
JNKNIIWN
0 i
NO
A -
1
1
l
2
I
3
I
I
4
I
I
5
RETENTION
6
TIME
7
8
9
(rnin.)
Figure 1. Typical chromatogram using phthalate eluent: eluent = 2 X lo3 M KHP, pH1 4.10, flaw rate = 2 mL/min; anawe concentratlons, (sample A) [Na’], [NO3-] = 3 X M, (sample B) [Cu], [Pb] = M; peak identificatlons for cation M, [NO,-] = 12 X 3 X confirmed by atomic absorption analysis of collected fractions. UNKNOWN
/ I
I
2
3
4
I
I
I
I
RETENTION
TIME
(m1n.l
Figure 2. Potential cation-anion interference due to overlapping peaks: eluent, 2 X M KHP, pH 4.9, flow rate = 3 mL/min. For sample M and in sample B the A the concentration of each anlon Is 2 X concentration of each cation is 1 X M.
approximately 50%. As a consequence there is more pronounced overlap of the peaks (see Figure 2 as compared to Figure 1). The phthalate total concentrations used are sufficient to complex sizable fractions of the copper, lead, and zinc present in solution. Variation of total phthalate from 0.5 X lo9 to 4.0 X M does not affect the retentnon time; however, it does change the speciation. The metal speciation is dominated by M2+,MP, and MP$-, where M2+designates Cu, Pb, and Zn and P2-represents the doubly deprotonated phthalate anion. Under the conditions where the 1:l species dominates, the interference of the metals will be minimized since these species will not produce a conductivity detector response. Species calculations (7) using [PIT = 2.0 X M, [CUI, = 1.0 X M , and pH 4.0 indicate that Cu2+is dominant (81.7%), 17.5% is present as CUP, and the CuPz2-complex provides 0.8%. At pH 6.0 the distribution is much different;
1169
Cu2+ = 16.0%, CUP = 80.0%, and CuPz2- = 4.0%. As a consequence the magnitude of the cation interference is reduced by a factor of 4.1 at pH 6.0. No CUP^^- peak could be identified, and apparently this species is masked by one of the other peaks. Quantitative chromatographic analysis of the three metals may be achieved; however, since their speciation will change with total metal content, the results have to be interpreted with caution. This is due to the fact that the stability constants for the metals are different and thus the ratios of the metals used to generate the standard curves will dictate the speciation and consequently the detector response. While the elution of the cations does not respond to total phthalate concentration, it does respond to changes in solution pH. For example RT for Cu at pH 4 is 3.5 min whereas at pH 6.5 it equals 8.0 min ([PIT = 2.5 mM; flow rate, 2 mL/min). Similar trends are also observed for Pb and Zn; these results indicate that H+ is the active eluent for these cations, Continued utilization of the anion column for cation separation at high analyte loadings produced a noticeable decrease in the column capacity. In addition a peak is observed early in the chromatograms of the metals that interact with the column. The size of this peak is directly related to the concentration of the cation in the sample. The collected fractions of this peak do not contain the cation nor are they significantly enriched in Kf, H+, total phthalate, or Si when compared to other fractions of the eluent. It is proposed that the column/cation interaction causes hydrolysis of the carbon-silicon bonds and the unknown peak is probably an amine cation. While the exact nature of this decomposition is unknown, it apparently does not include degradation of the silica support. Spot tests were conducted for anions, but none was observed. When dilute acid is used as the eluent, similar cation chromatographic behavior is observed. However the column does not appear to degrade since no “unknown” peak is observed. As with the phthalate system the cations are eluted more rapidly when the pH of the solution is near 4 as compared to a pH of 6. With a dilute acid eluent there is no anion chromatography and thus no cation-anion interference. Also, eluent complexation does not change the net charge of the cations. At pH 5 . 3 , l Fg of Cu, Pb, or Zn can be detected and calibration curves are linear for 2 orders of magnitude. The retention times for the three cations are 2.9 min, 4.1 min, and 3.8 min, respectively, at a flow rate of 2.5 mL/min. Registry No. Na, 7440-23-5;K, 7440-09-7;Ca, 7440-70-2;Mg, 7439-95-4; Ni, 7440-02-0; Mn, 7439-96-5; Cd, 7440-43-9; Cu, 7440-50-8;Pb, 7439-92-1;Zn, 7440-66-6;Fe, 7439-89-6;Al, 742990-5; Hg,7439-97-6.
LITERATURE CITED (1) Wlmberley, J. W. Anal. Chem. 1981, 53, 1709-1711. (2) Vydra, F. And. Chim. Acta 1967, 38, 201-205. (3) Schlndler, P. W.; Fuerst, B.; Dick, R.; Wolf, P. U. J . Colloid Interface SCi. 1976, 55, 469-475. (4) Strazhesko, D. N.; Strelkp, V. 6.; Belyakov, V. N.; Rabavlk, S. C. J. Chromatogr. 1974, 102, 191-195. (5) Egawa, H.; Saekl, H. Kogyo Kagaku Zusshi 1971, 74. 772-775; Chem. Abstr. 1971, 75, 64935a. (6) Snell, F. D.;Snell, C. T. ”Colorlmetrlc Methods of Analysis, Volume 3”;
D. Van Nostrand: New York, 1961, pp 398. (7) Pagenkopf, G. K. “Introductlon to Natural Water Chemistry”; Marcel Dekker; New York, 1978; 272 pp.
Dennis R. Jenke Gordon K. Pagenkopf* Department of Chemistry Montana State University Bozeman, Montana 59717 RECEIVED for review December 9,1982. Accepted February 1, 1983. This work was supported in part by the Office of Water Resources Research, U.S. Department of Interior, through Project A-0138 MONT.
