Atmospheric Pressure MALDI-IMS Using p-Nitroaniline as the Matrix at High Spatial Resolution in the Positive and Negative Ion Modes

Abstract

Matrix-assisted laser desorption/ionization imaging mass spectrometry MALDI-IMS can provide information on the spatial distribution and amount of numerous molecules on a single tissue section. A single sample that can be used in both positive and negative ion analyses is needed, since this can reduce losses of rare clinical samples. Although p-nitroaniline NIT has been shown to be applicable for use as a matrix in MALDI-IMS in the positive and negative ion modes, it is volatile and instable under vacuum conditions. In this study, we applied NIT as a volatile matrix for atmospheric pressure AP MAL-DI-IMS analysis, using a Mass Microscope, because NIT is considered to remain stable on the sample under AP conditions. Our objective was to optimize NIT usage by evaluating peak intensities acquired in the positive and negative ion modes. We evaluated signal intensities under 3 diierent conditions; dilution at diierent volumes, at diierent concentrations, and in two types of solvents. We also measured the matrix weight deposited on samples and determined the optimum condition with re-spect to matrix weight and signal intensity. optimum condition was determined as follows; a solvent volume of 2 mL, an NIT concentration of 30 mg/mL, and 100 methanol as the solvent. Mass spectra obtained using these conditions provided comparable signals with 2,5-dihydroxybenzoic acid and 9-aminoacridine. Using tandem mass spectrometry, the most intense signals in both ion modes were identiied as phosphatidylcholine and phosphatidylinositol. Moreover, the lipid distribution in the mouse cerebellum could be visualized at a high spatial resolution of 5 μm on a single section, in both the positive and nega-tive ion modes. is study shows that optimization of NIT is determined by 3 parameters solvent volume, NIT concentration, type of solvent . Using the optimum condition, it was possible to analyze the same tissue section by AP MALDI-IMS in both the positive and negative ion modes, at a suuciently high sensitivity for tandem mass spectrometry and imaging at a spatial resolution of 5 μm.