Anal. Chem. 1994,66,4466-4470
Prediction of Retention Characteristics of Multiprotic Anions in Don Chromatography Dennis R. Jenke* Baxter Healthcare Corporation, William B. Graham Science Center, Round Lake, Illinois 60073
The retention characteristicsof several multiprotic anions on an ion chromatographic column were studied as a function of changing mobile phase pH. The changes in mobile phase pH were such that the analyte and mobile phase speciationwas materially impacted. These changes in turn had a significantimpact on the a n w s retention characteristics. Multiprotic species examined included the following anions: citrate, phosphate, malonate, tartrate, and succinate. Chloride and sulfate were also examined. Aretention model, based on the concept of a multiprotic a n w s effective charge and consideration of the various ion exchange equilibria which are established between the chromatographic column and the mobile phase, is developed. The developed model effectively mimics the behavior of all species examined in this study and thus can serve as a tool with which to optimize practical separations. Since its introduction in 1975,' ion chromatography (IC) has evolved into a routine, widely used analytical procedure. In its most general application, the technique utilizes mobile phases containing multiprotic eluents, including phthalate, benzoate, and Utilization of eluents containing W-absorbing species (e.g., phthalate) allows the analyst to exploit indirect UV detectio~-~ Several investigators have developed IC retention models which consider the ion exchange equilibria which occur between the column and the mobile phase. In the simple case of a mobile phase containing a single eluent species (e.g., hydroxide), the model is represented as the linear relationship between the logarithm of an analyte's adjusted retention time and the logarithm of the eluent ion's ~oncentration.2~~~~ In the more complicated case of a mobile phase containing a multiprotic eluent, models have been developed which consider the various ion exchange equilibria which occur at each exchange site on the analytical column.10-13 Most applications of the various IC retention models consider the elution of species which maintain a single charge over the * FAX: (708) 27C-5897. (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975,47,1801. (2) Gjerde, D. T.; Schmuckler, G.; Fritz, J. S. j . Chromatogr. 1980,187,35. (3) Gjerde, D. T.; Fritz, J. S. Anal. Chem. 1981,53, 2324. (4) Weiss, J. Handbook of Ion Chromatography; Dionex: Sunnyvale, CA 1988. (5) Small, H.; Miller, T. E., Jr. Anal. Chem. 1982,54,462. (6) Jenke, D. R Anal. Chem. 1984,56,2468. (7) Foley, R C. L.; Haddad, P. RJ. Chromatogr. 1986,366, 13. (8)Strelow, F. W. E.; Walt, T. N. Anal. Chem. 1975,47,2272. (9) Gjerde, D. T.; Fritz, J. S.; Scmuckler, G. J. Chromatogr. 1979,186, 509. (10) Hoover, T. B. Sep. Sei. Technol. 1982,17,295. (11) Jenke, D. R; Pagenkopf, G. K. Anal. Chem. 1984,56, 85. (12) Jenke, D. R; Pagenkopf, G. K. Anal. Chem. 1984,56, 88. (13) Maruo, M.; Hirayama, N.; Kuwamoto, T. J. Chromatogr, 1989,481,315. I
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Analytical Chemistry, Vol. 66, No. 24, December 15, 1994
applicable range of the model. However, many multiprotic species of analytical interest have a variable speciation in the pH range used, particularly for the phthalate-based eluents (typically between 4 and 7). Thus, as the mobile phase pH is changed, the elution characteristics of the multiprotic species change in response to both the changing character of the mobile phase and their own changing effective charge. Few practically useful attempts have been made to model the elution characteristics of multiprotic species in IC. While Hajos, Horvath, and Denke have expanded existing multiple eluent species models to consider the case of multiprotic analytes in a mixed carbonate buffer,14 their evaluation of the resulting model is limited by the few analytically significant analytes which exhibit variable speciation at the pH range commonly encountered with mixed carbonate mobile phases (typically pH 9 and greater). The purpose of this research was to model the elution of multiprotic analytes with a multiprotic mobile phase. While the model developed herein is applicable to a specific column type and mobile phase couple, the general approach can be utilized to characterize any appropriate elution system. Once such a model is developed, its predictive capabilities can be used to simplify the separation development and optimization processes. THEORY
The elution of an analyte species with a charge n via a diprotic eluent species E can be modeled by the expression15
Dn = &(factor l)"((factor 2)'"
-
1)"
(1)
where factor 1= [E11/4Xzl[E21 and factor 2 = 1+ { (S[EZIXZIQ}/ [Ell2. In this equation, [El] and [E21 refer respectively to the activity of eluent species of charge 1- and 2- in the mobile phase, Q is the column capacity, Xzl is the selectivity coefficient describing the column's relative affinity for the two eluent species, K,,1 is the selectivity coefficient describing the column's relative affinity for the analyte and the E1 eluent species, and D, is the analyte's distribution ratio between the column and the mobile phase. Practically, D, can be directly related to retention properties of the analyte including its capacity factor, k'. For a multiprotic analyte, the charge n in eq 1can be replaced by the species's net effective charge, e, which is affected by the analyte's speciation in the mobile phase. This speciation, a function of the eluent pH and the species's acid dissociation constants, can be expressed via a notation.16 For example, for a diprotic species H A the fractions of H A which specifically exist (14) Hajos, P.; Horvath, 0.;Denke, V. Anal. Chem., in press. (15) Jenke, D. R; Pagenkopf, G. K. In Chromatographic Theoly and Basic Principles; Jonsson, J. A, Ed.; Dekker: New York, 1987; pp 313-346. (16) Pagenkopf, G. K Introduction to Natural Water Chemistry; Dekker: New York, 1978.
0003-2700/94/0366-4466$04.50/0 0 1994 American Chemical Society
in the forms H A HA-, and A2- (a~, al,and a2) can be expressed as a function of the hydrogen ion activity of the mobile phase and the analyte’s acid dissociation constants (kal and kQ) as follows:
Table 1. Properties of the Species Studied
A. Divalent Species eluent a0
3.8 4.0 4.5 5.0 6.0 7.0 8.0
0.274 0.187 0.058 0.013
