Improving detection of low-abundant pharmaceuticals and neurotransmitters via quadrupole isolation in MALDI mass spectrometry imaging.

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a powerful tool for mapping the spatial distribution of pharmaceuticals and metabolites in tissue. However, its effectiveness in the low-mass range (< 400 m/z) is impacted by matrix interference and ion suppression, reducing signal-to-noise (S/N) ratios. To address this challenge, we assessed a targeted quadrupole isolation strategy using a MALDI q-TOF mass spectrometer. This approach selectively isolates narrow windows around individual analytes m/z to reduce background noise and enhance S/N, therefore generating a smaller selective ion packet to the TOF analyzer. We applied this method to detect acetaminophen, paracetamol sulfate, caffeine, paraxanthine, as well as dopamine and its metabolites (HVA, 3-MT, and DOPAC) within murine liver and brain tissue sections, respectively. We utilized a range of isolation windows (e.g., 1, 5, 10, 20, 100 m/z) around each target m/z value. This dynamic isolation significantly decreased chemical background without fragmenting the target ions. As a result, we achieved precise spatial localization of parent drugs and their metabolites. Compared to untargeted acquisition, this method improved S/N by over 50% for all analytes within ≤10 m/z isolation ranges, while preserving tissue morphology. This targeted isolation approach extends the capabilities of MALDI MSI and offers a robust and scalable solution for analyzing low-mass xenobiotics and metabolites in situ.