Adam Socia, Joe P. Foley

This paper demonstrates that sequential elution liquid chromatography (SE-LC), an approach in which two or more elution modes are employed in series for the separation of two or more groups of compounds, can be used to separate not only weak acids (or weak bases) from neutral compounds, but weak acids and weak bases from neutral compounds (and each other) by the sequential application of either of two types of an extended pH gradient prior to a solvent gradient. It also details a comparison, based on peak capacity and separation disorder, of the probability of success of this approach with the unimodal elution approach taken by conventional column liquid chromatography. For an HPLC peak capacity of 120 and samples of moderate complexity (e.g., 12 components), the probability of success (Rs≥1) increases from 37.9% (HPLC) to 85.8% (SE-LC). Different columns were evaluated for their utility for SE-LC using the following criteria: (1) the prediction of the elution order of the groups based on the degree of ionization of the compounds; and (2) the closeness of the peak shape to the ideal Gaussian distribution. The best columns overall were the Zorbax SB-AQ and Waters XBridge Shield columns, as they provided both between-class and within-class separations of all compounds, as well as the lowest degree of tailing of 4-ethylaniline using the pH 2 to pH 8 gradient.