Effects of system peaks in a column-switching liquid chromatographic system for bioanalysis

A reversed-phase liquid chromatographic column-switching system was used for the determination of drugs and metabolites in blood plasma with a direct injection procedure. A short precolumn was used for enrichment of the drugs and clean-up from weekly retained plasma components using an aqueous solution. In a second step the drugs were eluted, with a suitable eluent, from the precolumn to the separation column for the final separation. When the aqueous plug on the precolumn, created by the clean-up procedure, was transferred to the separation column a disturbance of the established equilibria in the separation column occurred. This resulted in migrating zones, one of each mobile phase component. If one mobile phase compound gives a detectable signal the zone will be visualized as an extra peak (system peak) in the chromatogram. Deformed peaks of the drug were obtained when the drug was co-eluted with a non-visible system peak. Changes in the retention and selectivity of weakly retained solutes on the separation column were obtained when the pH of the aqueous plug in the precolumn deviated from the pH of the mobile phase. This is explained by the formation of a pH gradient in the separation column.