On-line coupled reversed-phase high-performance liquid chromatography—gas chromatography—mass spectrometry: A powerful tool for the identification of unknown impurities in pharmaceutical products

An alternative approach to HPLC—MS involving direct coupling of reversed-phase HPLC to GC—MS is described for the identification of unknown impurities in pharmaceutical products. Conventional-sized reversed-phase HPLC (column of 4.6 mm I.D., flow-rate 2 ml/min) was coupled to GCMS. Liquid sample volumes of 500 μl were transferred. In contrast to LC—MS coupling techniques, the proposed method technique experiences no problems with low-molecular-mass solutes or any buffer salts in the reversed-phase LC eluent. An example illustrating the use of this technique in analytical research and development in the pharmaceutical industry is presented. As unknown impurity observed in the HPLC of a stressed sample of a pharmaceutical product was identified by directly transferring the LC fraction of interest to a GC—MS system. The transfer problems arising from the content of water and buffer salts in the LC eluent were circumvented by on-line coupling of liquidliquid extraction by means of a sandwich-type phase separator, which was coupled to a loop-type LC—GC interface. It is shown that this technique can be improved by lowering the extraction temperature below the freezing point.