Inner surface properties of capillaries for electrophoresis

Abstract

Certain aspects of capillary electrophoresis (CE), such as the electro-osmotic flow and adsorption to the capillary wall, are strongly related to the surface properties of the capillaries used, so surface investigations improve the understanding of these phenomena. Possible ways of opening capillaries, which include cutting, smashing and grinding, are reviewed in detail. The main requirements for suitable surface analysis are robustness, vertical and lateral spatial resolution, and speed. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) are the most useful techniques and provide complementary information. The choice depends on the type of samples. The risks of producing artefacts by sample preparation or microscopic measurements must always be considered. This article reviews the technical state of the art and the most interesting results for capillary surfaces. Capillaries for electrophoresis are available with good, consistent quality. The drawing temperature of the capillary is a major influence on capillary quality and batch-to-batch variations, but, if properly chosen, atomic flatness of inner capillary surfaces can be achieved. Surface hydrolysis is the major effect in ageing capillaries and can be minimized by storing them dry. XPS is an exciting new option for studying the surface chemistry of CE capillaries. Very surprisingly, silicon–carbon bonds have been found using this technique. Their occurrence is not completely understood. Possible explanations are discussed. Future research on these Si–C bonds and investigations on coated capillaries are outlined.