Volatile p-Nitroaniline as Matrix for High Spatial Resolution Imaging of Phospholipids in Both Ion Modes by AP-MALDI-IMS

Matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) is a useful techniques that allows to derive the spatial distribution of numerous molecules ionized on a tissue section. Selection of the suitable matrix is the key to ionize molecules of interest in MALDI. Each matrix has its own characteristics such as being proper for ionizing molecules in either positive or negative ion mode, different crystallization, suitability for specific biomolecules, and so on. 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9AA) are conventionally used as matrices in MALDI. These matrices are used for particular ion mode in MALDI-IMS, DHB for positive and 9AA for negative ion mode. Therefore, we prepared two serial sections to reveal the distribution of biomolecules in both ion modes. It is desirable to find a new matrix which is suitable for general purpose, for example is applicable in both ion modes. Such a matrix can reduce the loss of rare or hard to obtain samples, such as clinical ones. Recently, volatile materials gathered interest as MALDI matrices. Although volatile matrices are expected to be useful for detecting new molecules, they are unstable in vacuum condition. Here, we utilized a volatile matrix for atmospheric pressure (AP)-MALDI-IMS instrument, Mass Microscope (Shimadzu). The instrument had been developed in our laboratory in collaboration with Shimadzu. We selected p-nitroaniline (NIT) as a volatile matrix that is expected to be applicable for IMS in both ion modes because previous reports show that NIT can be used for MALDI-MS analysis in both ion modes. In this study, we optimized the pretreatment using NIT in order to perform high spatial resolution ion imaging in both ion modes by using Mass Microscope.