Immunoaffinity screening with capillary electrochromatography

Highly efficient capillary electrochromatographic separations of cardiac glycosides and other steroids are presented. Employing butyl‐derivatized silica particles as stationary phase resulted in a nearly three times faster electroosmotic flow (EOF) compared to capillary electrochromatography (CEC) with octadecyl silica particles. On‐column focusing with a preconcentration factor of 180 was performed and separation efficiencies of up to 240 000 plates per meter were obtained. Using label‐free standard UV absorbance, detection limits of 10–80 nM were reached for all steroids tested. For screening of cardiac glycosides, e.g., digoxin and digitoxin in mixtures of steroids, CEC was combined with immunoaffinity extraction using immobilized polyclonal anti‐digoxigenin antibodies and Fab fragments. Simply adding small amounts of antibody carrying particles to the samples and comparing chromatograms before and after antibody addition allowed screening for high affinity antigens in mixtures with moderate numbers of compounds. Under conditions of competing antigens, affinity fingerprints of immobilized anti‐digoxigenin and anti‐digitoxin antibodies were obtained, reflecting the cross‐reactivity of eleven steroids. The method provides high selectivity due to the combination of bioaffinity interaction with highly efficient CEC separation and UV detection at several wavelengths in parallel. This selectivity was exploited for the detection of four cardiac glycosides in submicromolar concentrations in an untreated urine sample.