[Elution behaviors of cations on the serial columns combining a cation exchange column with an ion exclusion column]

The elution behaviors of cations (Na+, NH4+, K+, Ca2+ and Mg2+) on the serially connected cation exchange column (ICS-C25) and ion exclusion column (TSKgel OApak-A) were investigated. As shown in Figure 1, three peaks were observed for each cation when a mixture of tartaric acid (TA) and 2,6-pyridinedicarboxylic acid (PDC) was used as eluent. In ion exchange chromatography, the background conductivity (G0) of carboxylic acid (HR) eluent which is based on the dissociation of HR, can be expressed as follows: G0 = C0H lambdaH + C0R lambdaR (1) where C0H and C0R are the initial concentration, lambdaH and lambdaR are the equivalent concentration of H+ and R-, respectively. The concentration of eluent anion R- in mobile phase is reduced when the cations (M) are eluted from cation exchange column, because the R- forms some complexes with the cations. At this time, the conductivity (G) of the eluent will be G = CM lambdaM + (C0R - C'R) lambdaR + (C0H - CM) lambdaH (2) where CM is the concentration of cations, and C'R is a reduce of R-. The peaks on chromatogram results from a change in conductance (deltaG), and in the conditions of the paper. DeltaG = G - G0 = CM (lambdaM - lambdaH) - C'TA lambdaTA - C'PDC lambdaPDC (3). On the ion exclusion column, the eluent anion TA and PDC were retained at different capacity factor by an ion exclusion mechanism while the cations have not any retention. That is to say, the first peak of each cation is caused by elution of the cation and the depletion of eluent ion (H+) in the mobile phase. The second and third peaks have appeared corresponding to the depletion of TA and PDC anion in mobile phase, respectively.