Multidimensional On-Line SPE for Undisturbed LC-MS-MS Analysis of Basic Drugs in Biofluids

In bioanalytical LC-MS-MS matrix effects influencing the ionization process are a major concern with respect to the quality of the results obtained. In general such matrix effects are directly related to an insufficient sample clean-up of the biofluids. In order to establish a MS-adequate clean-up procedure for basic analytes present in biofluids (e.g. urine, plasma) which is based on solid-phase extraction (SPE) principles a combination of tailor-made SPE column packings and automated column-switching was developed. This novel, multidimensional (MD) SPE platform relies on the combination of a SPE column packed with a restricted access material (RAM) allowing size-exclusion and reversed phase chromatography (SEC-RPC) and a second SPE column packed with a mixed-mode phase (MMP) allowing ion exchange and reversed phase chromatography (IEX-RPC). For the evaluation of this MD-SPE platform 8 tricyclic antidepressants and two metabolites were chosen as model analytes. In order to monitor matrix effects, i.e. ion suppression, postcolumn infusion experiments were performed and compared with a two-dimensional SPE column mode (SEC-RPC). The MD-SPE platform is highly efficient for removal of low and high molecular weight sample components which suppress ionization to varying extend. In addition electrospray ionization of the model analytes is not affected by inter- or intra-individual variations in the composition of the matrix investigated. It is also independent of the species the biofluids originate from. It was demonstrated that the MD-SPE platform has a generic potential with respect to on-line SPE of basic drugs having a pKa > 6.5 and a moderate to low polarity and being present in different biofluids.