Ion-exchange capillary electrochromatography of inorganic and small organic anions

This work presents a systematic study on the use of packed column ion-exchange capillary electrochromatography (IE-CEC) for the separation of ionic analytes. Methods have been developed for preparing packed capillary columns utilising either silica-based or polymer-based stationary phases. The stationary phase is retained by futs prepared by sintering either the silica-based packing material or pure silica. The electroosmotic and pressure-driven properties of frits prepared from bare or functionalised silica were investigated. Despite the high temperature of heating of the packing, sufficient residual functional groups remained on the fut such that distinctively different EOF behaviour was observed for each type of packing. Frits made from materials providing negative surface charges increased the magnitude of the cathodic EOF compared to the open capillary, but a substantial anodic EOF of -1.88 10-8 m2 v-1 s-1 was produced by introduction of a single fut made from SAX material. An explanation of this behaviour is based on the hypothesis that the EOF generated by the frit determines the overall flow in the whole open capillary. The frit is considered to work as a pump and overrides the EOF generated at the capillary wall. IE-CEC was used for selectivity manipulation in the separation of a number of inorganic anions using a silica-based anion-exchanger. Changing the type and concentration of the competing anion in the eluent mediated the ion-exchange (IE) contribution to the separation mechanism. The use of polymer-based anion exchangers and a combination of hydrodynamic and electroosmotic flow allowed both the electrophoretic and/or chromatographic components of the separation mechanism to be varied and considerable changes in the separation selectivity could be obtained. With a short packed bed the separation of 8 test analytes in under 2.2 min was possible using pressure-driven flow and a simple step voltage gradient. Using only the application of high voltage allowed many of the same analytes to be separated in less than 20 s with different separation·selectivity. A theoretical model was derived from IE and capillary electrophoresis (CE) theory, which describes the mobility of inorganic anions in IE-CEC. The model was verified using a mixture of UV absorbing inorganic ions in eluents of differing eluotropic strength with excellent agreement (r2 > 0.99) obtained. Values of constants in the model equation determined by non-linear regression were used to estimate the relative strengths of the interactions of different analytes with the stationary phase and were found to agree well with elution orders observed in conventional IE. A mixed-mode C6/SAX stationary phase was used for the simultaneous separation of acidic, basic and neutral organic compounds and inorganic anions. The analytes were separated by a mechanism that comprised chromatographic interactions (hydrophobic interactions, ion-exchange) as well as electrophoretic migration. The influence of ion-exchange and hydrophobic interactions on the retention/migration of the analytes could be manipulated by varying the eluent composition. Conductivity detection was applied to IE-CEC using a capacitively coupled contactless conductivity detector (C4 D) with detection occurring through the packed bed. A systematic approach was used to determine suitable eluents for IE-CEC separations with simultaneous indirect UV and direct conductivity detection. Homogenous column packing was found to be imperative, and monitoring of the baseline could be used to assess the homogeneity of the packed bed. Direct conductivity detection was found to be superior to indirect UV detection with regard to both baseline stability and detection sensitivity with detection limits of 4-25 µg/L being obtained.