Use of a five-channel multiplexed electrospray quadrupole time-of-flight hybrid mass spectrometer for metabolite identification

Metabolism data provided with reduced cycle time has become of increasing importance for the early evaluation of DMPK properties of drugs in discovery. In this regard, quadrupole time-of-flight hybrid mass spectrometers (Q-TOF) can provide very reliable metabolite identification via accurate mass measurement of ions and the consequent access to the elemental composition of the metabolite. However, due to their cost, they are often used for drug metabolism studies on later stage drug candidates or to address challenging metabolism questions. A new prototype, consisting of a five-channel multiplexed electrospray ionization (ESI) source on a Q-TOF with one channel used for lock-mass compound infusion, was evaluated for metabolite identification. The goal was to increase the sample throughput of a single ESI-MS system by a factor of 4, while maintaining efficient metabolite separation in high-performance liquid chromatography (HPLC) as well as adequate sensitivity and mass accuracy, and ultimately improve the speed and quality of metabolism studies supporting drug discovery. The analytical performance of the system was assessed by evaluating the sensitivity and mass accuracy (using real in vitro and in vivo samples), inter-channel differences in retention times, MS/UV response, and cross-talk among channels. The sensitivity using the multiplexed ESI source was on average 2-fold lower than with single ESI, correlating well with previous literature data. The mass accuracy was comparable to that obtained using single ESI in both MS and MS/MS modes, making the metabolite identification process using the multiplexed ESI source as reliable as with single ESI. Compound-dependent differences in ionization efficiencies were observed among channels, and were minimized by analyzing related samples on the same channel. Finally, the level of cross-talk among channels was acceptable (around 0.3%) and comparable to levels previously published for quantitative applications using multiplexed ESI. The paper also focuses on the advantages and disadvantages of this new approach compared to other approaches in the literature in the field of metabolite identification.