ONLINE CHROMATOGRAPHIC PRETREATMENT OF SAMPLES WITH VARYING SALT CONCENTRATIONS FOR CAPILLARY ELECTROPHORESIS

Various methods are currently under investigation to improve concentra tion detection limits in capillary electrophoresis (CE). Stacking and isotachophoresis coupled in-line with free-solution electrophoresis ar e the techniques most frequently used. Samples containing a high and/o r varying salt concentration, such as urine and serum, are difficult t o handle: usually a significant loss in electrophoretic efficiency is observed compared with samples possessing a low electric conductivity. In the present paper an alternative approach is developed, i.e. a liq uid chromatographic (LC)-type of sample pretreatment is coupled on-lin e with CE. To demonstrate the feasibility of this approach, the separa tion of three model compounds (benzoates) in water containing up to 40 0 mM of sodium chloride is studied using a 50 mM berate CE buffer of p H 9.5. The direct injection of samples with high salt concentrations i n CE results in peak splitting and/or serious band broadening. These p roblems are not encountered when using the present LC-CE system. In ad dition, the detection limits are hardly influenced by the salt concent ration of the sample; this underlines the robustness of the system.