Adsorption dynamics of dipeptides in a column with chiral zwitterionic quinine-based stationary phase

Adsorption-desorption kinetics in chromatographic columns is difficult to study by the moment method because of interference with other mass transfer processes. The Eberly-Spencer method provides another way to determine the desorption rate constant. A combination of these techniques was applied to study the adsorption dynamics of almost unretained (Gly-Gly) and retained (L(D)-Leu-L(D)-Leu and L(D)-Leu-L(D)-Phe) dipeptides in a column packed with chiral stationary phase bearing quinine-l-Ala-l-Leu (AK 57) as a chiral selector. It was shown that eddy diffusion and slow adsorption-desorption kinetics are main contributors to the peak broadening. The lack of enantioselectivity in chromatographic efficiency (i.g. in HETP) despite marked enantioselectivity in retention was observed and explained by the non-selective rate-limiting desorption step, while the adsorption step was enantioselective and faster, except for Gly-Gly, in which case adsorption was slower than desorption. It was also shown that eddy diffusion was characterized by unusually high values of the correponding HETP contribution. This phenomenon was explained by interference between eddy diffusion and slow adsorption-desorption kinetics.