Combination of a liquid chromatography–ultraviolet method with a non-volatile eluent, peak trapping and a liquid chromatography–mass spectrometry method with a volatile eluent to characterise erythromycin related substances

The selectivity and sensitivity obtained with volatile liquid chromatographic (LC) methods are often inferior compared to non-volatile ones. However, the buffers often used in the non-volatile system are incompatible to mass spectrometry (MS). So, the characterisation of unknown peaks in a non-volatile system, based on data obtained from a volatile LC–MS method, is problematic. In this study, the unknown peaks in a non-volatile liquid chromatography coupled with ultraviolet detection (LC–UV) system were directly characterised by a volatile LC–MS system using a peak trapping technique. Each peak eluted from the non-volatile system was trapped by a switching valve and sent to a LC–MS system using a volatile mobile phase. Mass spectral data were acquired on an LCQ ion trap mass spectrometer equipped with electrospray ionisation (ESI) operated in the positive ion mode. Using this technique, the fragmentation behaviour of erythromycin and its related substances was studied and the components occurring in commercial samples were investigated. In total 25 compounds mentioned in the literature were traced. Fourteen more unknown impurities were also studied.