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 additives were compared to an electrokinetic chromatographic system with a linear, charged polymer of similar chemical structure (all additives are quaternary tetraalkyl ammonium ions). The monomeric additives were tetramethylammonium (TMA), tetraethylammonium (TEA) and dimethylpyrrolidinium (DMP), respectively, 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 transported by the anodic electroosmotic flow in the dynamically coated capillary, and retained by the counter-flowing cationic additives. From these data capacity factors were derived, which ranged up to 0.8. Association constants were calculated, and were found between 10 and 170. Roughly, the association constants increased for a given analyte in the sequence TMA<TEA<DMP<PDADMA. However, changes in the retention order were observed for some cases, reflecting the different selectivity of the particular systems for certain 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 additive ion, resulting in a broader retention window under most practical conditions.