Use of large-volume sample stacking in on-line solid-phase extraction-capillary electrophoresis for improved sensitivity

We present a new system for the sensitive analysis of cephalosporins by CE using both online SPE and large-volume sample stacking (LVSS). Sample volumes of 250 mu L were loaded onto the SPE microcolumn which was then desorbed with 426 nL of ACN. The SPE elution plug was injected into the CE system via an in-line valve interface filling approximately 60% of the volume of the separation capillary. Subsequently, LVSS was performed by applying a voltage of -5 kV which resulted in the simultaneous removal of the elution solvent and the preconcentration of the analytes in a narrow zone. This way the amount of analyte loaded into the capillary could be considerably increased without serious loss of CE separation efficiency. LODs for cefoperazone and ceftiofur were in the ng/L range which represents an improvement of a factor of 8450 and 11450 when compared with direct CE injection. The cephalosporin test compounds presented a good linear response (corrected peak area) between 0.5 and 10 mu g/L with correlation coefficients higher than 0.995. The final method is compared with previously reported LVSS-CE and SPE-CE systems for the analysis of cephalosporins.