Nonaqueous capillary electrophoresis. Application to the separation of complex mixtures of organic acids by ion-pairing mechanism

Separation of a ten-membered model mixture of aromatic compounds possessing a carboxylic moiety along with other functional groups was investigated in the pH range 4.5-8.5, Methanol-acetonitrile (1:1) containing 10 mmol/l sodium acetate and an equal concentration of Tris was used as the background electrolyte. It was demonstrated that within the model set three categories of acids were present, namely those which moved nearby the endoosmotic flow (on the anodic side) and increased slowly their anodic mobility with the increasing apparent pH (1), those which revealed a strong increase with increasing pH and could be best discerned at apparent pH 7-7.5 in the negative operation mode (2) and those (a single member only) which possessed a strong anodic mobility even at low pH values and could be revealed within reasonable time in the negative operational mode only (3). The mobility of the latter was only slightly affected by the pH change of the background electrolyte. Further addition of hydrophobic moiety possessing ion pairing reagents (typically trimethyloctadecyl ammonium bromide) helped to refine the resolution at neutral pH. The results demonstrate clearly the wide differences in selectivity that it is possible to obtain by running electrophoretic separations in nonaqueous buffers. (C) 1998 Elsevier Science Ireland Ltd.