BIOANALYSIS OF ERYTHROMYCIN 2'-ETHYLSUCCINATE IN PLASMA USING PHASE-SYSTEM SWITCHING CONTINUOUS-FLOW FAST-ATOM-BOMBARDMENT LIQUID-CHROMATOGRAPHY MASS-SPECTROMETRY

A specific method for the determination of erythromycin 2'-ethylsuccinate (EM-ES) in plasma is described. The method involves a liquid-liquid extraction procedure followed by the analysis of extracts using phase-system switching (PSS) continuous-flow fast atom bombardment (CF-FAB) liquid chromatography-mass spectrometry (LC-MS). In PSS EM-ES is enriched after analytical separation on a short trapping column, from which it is desorbed to the LC-MS interface. In this way, favourable mobile phases can be used for the LC separation and for the MS detection. Using the PSS approach a flow-rate reduction from 1.0 ml/min in the LC system to 15-mu-l/min going into the mass spectrometer was achieved without splitting. The determination limit for EM-ES was 0.1-mu-g/ml.