On‐line partial filling micellar electrokinetic capillary chromatography‐electrospray ionization‐mass spectrometry of corticosteroids

The one‐line combination of micellar electrokinetic chromatography (MECC) with electrospray ionization‐mass spectrometry (ESI‐MS) was investigated using some corticosteroids as model components. The on‐line technique is difficult because the micelles in the electrolyte solution tend to soil the mass spectrometer and lower the sensitivity of the spectrometer. To prevent the micelles from reaching the mass spectrometer, several techniques have been developed among which is the partial filling (PF) technique. In this study the PF‐MECC technique was investigated in an on‐line MECC‐ESI‐MS study of mixtures of corticosteroids. Because the compounds are uncharged, partitioning or interaction with micelles is required to achieve separation. Surfactant solutions of sodium dodecyl sulfate (SDS), sodium cholate (SC), and SDS/SC mixtures were used as micellar phase. Good MECC separation was achieved after optimization of the surfactant concentrations and the length of the injected micellar plug. Both hydrodynamic and electrokinetic injections of the micellar solutions were tested. A basic ammonium acetate solution was used as the CE electrolyte solution. The ESI‐MS analysis of the compounds was performed in the positive ionization mode, using an acidic sheath liquid. Because of the low MS intensities of the corticosteroids, the peaks were isolated during the MS runs. MS‐MS and MS‐MS‐MS data on the corticosteroids were obtained by off‐line injection of the compounds.