Matrix ions as internal standard for high mass accuracy matrix-assisted laser desorption/ionization mass spectrometry imaging

Abstract

Rationale: High mass accuracy is indispensable for reliable identification in matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) imaging. Ubiquitous matrix ions can serve as reference masses for mass calibration if their sum formula is known. Here we report an overview of ions generated on tissue by 11 common MALDI matrices for use in internal or external mass calibration. Methods: Matrices covered in this study were applied onto coronal mouse brain sections using a pneumatic sprayer setup. MALDI imaging was performed on a Q Exactive HF orbital trapping mass spectrometer coupled to an AP-SMALDI 10 source. Measurements were conducted with high mass resolution (240 k full width at half maximum at m/z 200) and high mass accuracy with a root mean square mass error of better than 1.5 ppm achieved via internal mass calibration using matrix ions. Results: MALDI MS imaging was used to investigate ions generated on tissue by 11 common MALDI matrices. An example of using matrix ions for internal mass calibration in MALDI imaging of drug substances and lipids in murine lung sections is presented. Tables containing the cluster composition, sum formulae, and the measured and theoretical m/z ratios of the identified ions were compiled for each matrix. Conclusion: Using matrix ions as reference masses for internal and external mass calibration in MALDI MS imaging is an effective and elegant way to achieve sub-ppm mass accuracy as it makes use of ubiquitous signals present in every MALDI MS spectrum without the need for an additional calibration standard.