Capillary zone electrophoresis in normal or reverse polarity separation modes for the analysis of hydroxy acid oligomers in neutral phosphate buffer

Capillary zone electrophoresis (CZE) with neutral phosphate buffer as the background electrolyte was used to analyse water-soluble oligomers obtained by polycondensation of racemic lactic acid. Two CZE separation modes were tested. The first mode was based on normal separation (injection at the anodic side) using a fused-silica capillary. Eight peaks were observed within a 60-min migration time range. They were ascribed to dimer and higher water-soluble oligomers. Peaks from dimer to tetramer were split due to sensitivity for the fine structures at the level of the distribution of chiral lactic acid moieties in oligomer chains. The second mode was based on reverse separation (injection at the cathodic side) using a fused-silica capillary modified by adsorption of a polycation on its inner wall. Under these conditions, oligomers were rapidly separated without peak splitting. Considering the forces which are involved in CZE, data were plotted as a function of 1/t scale, according to the equation [signal]=f((-1)(k)/t) where: k=0 and k=1 for normal and reverse separation modes, respectively. Such a plot allowed direct comparison between the various runs after a simple translation along the lit axis, regardless of the separation mode and the variation of electroosmotic flow. The second separation mode allowed separation of 3-hydroxybutyric acid and 6-hydroxyhexanoic acid oligomers. For the former series of oligomers, a side reaction generating crotyl bonds was observed due to the high sensitivity of CZE. It was shown that separation was governed by the ratio charge/mass of the oligoesters whatever their structure. (C) 1998 Elsevier Science B.V.