Ion-pair association and acid-base equilibria in nonaqueous capillary electrophoresis of weakly basic compounds

CE in nonaqueous media was used to study the migrating behavior of two weakly basic s-triazine pesticides and one of their metabolites. The target pesticides were selected to be representative for each of the two main groups: propazine and deethylatrazine for the chloro-s-triazines group and ametryn for the methylthio-s-triazines group. To elucidate the phenomena involved, systematic studies were carried out in the different organic media studied. Absolute mobilities were determined in 50% v/v methanol (MeOH)/ACN by extrapolation of the effective mobilities to zero ionic strength in the presence of different concentrations of perchloric acid. Conductivity measurements performed in MeOH and 50 and 20% v/v methanol/ACN permitted the evaluation of the associations of the components of the BGE. The effects of ionic strength on the actual mobilities of the compounds were determined in the presence of perchloric acid and SDS in different organic media. Two different ion-pair equilibria were considered: one due to the presence of perchlorate anions present in the BGE and second that from the added dodecyl sulfate anions. Bearing in mind that these weakly basic compounds can exhibit ion-pair and acid-base equilibria, the acid-base and ion-pair parasite reaction coefficients were determined. Finally, the effects of ionic strength, ion-pair interactions and acid-base properties on the effective electrophoretic mobilities of the analytes are discussed.