Improved ion detection sensitivity in mass spectrometry imaging using tapping-mode scanning probe electrospray ionization to visualize localized lipids in mouse testes

Mass spectrometry imaging (MSI) is a promising analytical method to visualize the distribution of lipids in biological tissues. To clarify the relationship between cellular distribution and lipid types in a tissue, it is crucial to achieve both an improvement in ion detection sensitivity and a reduction in the ionization area. We report methods for improving the efficiency of ion transfer to a mass spectrometer and miniaturizing the extraction area of a sample for tapping-mode scanning probe electrospray ionization (t-SPESI), atmospheric pressure sampling, and ionization methods. To verify the efficacy of the new t-SPESI measurement system, MSI was performed on mouse testes with a pixel size of 5 µm. Lipid images of the testes from wild-type (WT) and lysophospholipid acyltransferase 3 (LPLAT3) knockout mice revealed the characteristic distribution of docosahexaenoic acid-containing phospholipids (DHA-PLs). A comparison of the ion images obtained by MSI and optical images of the same tissues stained with hematoxylin and eosin suggested that the distribution of DHA-PLs was significantly altered by spermatogenesis in the WT mouse testes.