Electro-osmotic and pressure-driven flow properties of frits for packed column capillary electrochromatography prepared from functionalised and bare silica packings

Abstract

Changes in electro-osmotic flow (EOF) induced in an open fused silica capillary by the introduction of a single packing retainer (referred to as a ‘frit’ throughout the article) are investigated. Frits located in a 34.5 cm long capillary close to the detection window, as used commonly in packed column capillary electro-chromatography, were made by sintering silica based packing materials having varying functionalities (bare silica, ODS-silica, a strong cation exchange packing (SCX) and a strong anion exchanger (SAX) at 430 °C for 15 s. The contributions of such frits to the total EOF measured in the capillary are discussed. Despite the high temperature of heating of the packing, sufficient residual functional groups remained on the frit such that distinctively different EOF behaviour was observed for each type of packing. Frits made from materials providing negative surface charges, such as silica, ODS-silica and SCX, increased the magnitude of the cathodic EOF compared to the open capillary. On the other hand, a substantial anodic EOF of −1.88 × 10⁻⁸ m² V⁻¹ s⁻¹ was produced by introduction of a single frit made from the SAX material. An explanation of this behaviour is given, 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.