Journal of the Mass Spectrometry Society of Japan

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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) MALDI-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 different conditions; dilution at different volumes, at different concentrations, and in two types of solvents. We also measured the matrix weight deposited on samples and determined the optimum condition with respect to matrix weight and signal intensity. The 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 identified 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 negative ion modes. This 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 sufficiently high sensitivity for tandem mass spectrometry and imaging at a spatial resolution of 5 μm.

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