Hybrid triple quadrupole‐linear ion trap mass spectrometry in fragmentation mechanism studies: application to structure elucidation of buspirone and one of its metabolites

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Abstract
The use of a hybrid triple quadrupole‐linear ion trap (QqQLIT) mass spectrometer system for a comprehensive study of fragmentation mechanisms is described. The anxiolytic drug, buspirone, was chosen as a model compound for this study. With the advent of a QqQLIT instrument, both the traditional quadrupole and the new linear ion trap scans (LIT) could be performed in a single LC run. In the past, a sample had to be run on two different instruments, namely, a triple quadrupole instrument (QqQ) and a 3D ion trap (3D IT) to obtain similar information. With the new QqQLIT technology, collision‐induced dissociation (CID) occur in a quadrupole collision cell, q2, and fragment ions are trapped and analyzed in Q3 operated in LIT mode. In this work, high‐sensitivity product ion spectra of buspirone were obtained from the one‐stage ‘Enhanced Product Ion’ scan (EPI) with rich product ions and no low mass cut‐off. Furthermore, detailed fragmentation pathways were elucidated by further dissociation of each of the fragment ions in the EPI spectrum using MS3 mode in the same run. The MS3 scan was performed by incorporating CID in q2, and trapping, cooling, isolation, and resonance‐excitation in Q3 when operating in LIT mode. This approach allowed unambiguous assignment of all fragment ions quickly with fewer experiments and easier interpretation than the previous approach. The overall sensitivity for obtaining complete fragment ion data was significantly improved for QqQLIT as compared with that of QqQ and 3D IT mass spectrometers. This is beneficial for structure determination of unknown trace components. The method allowed structure determination of metabolites of buspirone in rat microsomes at 1 µM concentration, which was a 10‐fold lower concentration than was needed for QqQ or 3D IT instruments. The QqQLIT instrument provided a simple, rapid, sensitive and powerful approach for structure elucidation of trace components. Copyright © 2005 John Wiley & Sons, Ltd.

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