Comparison of separation selectivity in capillary electrokinetic chromatography using a cationic linear polymeric pseudo-stationary phase or monomeric additives of similar structure

The retention properties in electrically driven systems with monomeric addi tives were compared to an electrokinetic chromatographic system with a line ar, charged polymer of similar chemical structure (all additives are quater nary tetraalkyl ammonium ions). The monomeric additives were tetramethylamm onium (TMA), tetraethylammonium (TEA) and dimethylpyrrolidinium (DMP), resp ectively, the polymeric additive was poly(diallyldimethyl)ammonium (PDADMA) . The additive concentration in the background electrolyte was 2 and 4% (w/ w). The retention characteristics were based on the apparent mobilities of 10 non-charged analytes with different chemical functionality, which were t ransported by the anodic electroosmotic flow in the dynamically coated capi llary, and retained by the counter-flowing cationic additives. From these d ata capacity factors were derived, which ranged up to 0.8. Association cons tants were calculated, and were found between 10 and 170. Roughly, the asso ciation constants increased for a given analyte in the sequence TMA<TEA<DMP < PDADMA. However, changes in the retention order were observed for some ca ses, reflecting the different selectivity of the particular systems for cer tain pairs of analytes. A general advantage of polymeric pseudo-stationary phases compared to monomeric additives is given by the negligible reduction of the mobility of the analyte-polymer associate in relation to the free a dditive ion, resulting in a broader retention window under most practical c onditions. (C) 2000 Elsevier Science B.V. All rights reserved.