Mass Spectrometry Imaging Coupled with Pixel-Level Absolute Quantification of Olanzapine and Its Metabolite by Coulometric Mass Spectrometry

Mass spectrometry imaging is a powerful technique for mapping the spatial distribution of endogenous and exogenous molecules in biological tissues, but it is limited to relative quantitation. To address this limitation, we developed an integrated approach that combines desorption electrospray ionization mass spectrometry imaging with coulometric mass spectrometry to achieve the absolute quantification of drug molecules at the pixel level, without the use of external standards or isotopic labeling. We applied this method to quantify olanzapine, an antipsychotic drug, and its minor hepatic metabolite, 2-hydroxymethylolanzapine, in rat liver tissue. CMS enabled the electrochemical redox quantification of both compounds extracted from tissue regions of interest as defined by mass spectrometry imaging. This established a direct correlation between the imaging signal intensity and the absolute analyte concentration. Our results demonstrate the robustness and sensitivity of this combined approach and support its potential as a generalizable platform for standard-free, quantitative imaging of biological tissues, thereby converting relative ion maps into fully quantitative molecular images.