PNNL

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) may provide spatially resolved molecular information about a sample. Recently, a post-ionization approach (MALDI-2) has been commercially integrated with MALDI-MSI, allowing for bettered sensitivity and consequent improved spatial resolution. While advantages of MALDI-2 have previously been established, we sought to evaluate potential in-source fragmentation (ISF) effects that result from post-ionization with a commercial instrument. Analysis of rat brain homogenate detected statistically more positive ion mode peaks with MALDI-2 (1090) than with MALDI-1 (719). However, we annotated (using LipidMaps database) several enhanced lipids in MALDI-2 compared to MALDI-1 that were classified as hydrocarbons (415% increase), isoprenoids (100% increase), and fatty acids and conjugates (59% increase). We also annotated relatively more low m/z ions (i.e., m/z 75 – 500) for MALDI-2 compared to MALDI-1. Taken together, these observations may indicate ISF due to post-ionization, as the greatest annotation enhancement was observed for lipids that could be fragments of higher-order species (e.g., hydrocarbon fragments from lipid carbon chains). Via lipid standard analyses, known MALDI ISF fragmentation pathways (e.g., loss of trimethylamine) were statistically increased in MALDI-2 compared to MALDI-1, in line with the rat brain homogenate observations. Finally, gas phase molecular modelling with density functional theory provided rationalizations of increased ISF in MALDI-2. Overall, both experimental and computational datasets suggest ISF is statistically increased in MALDI-2 compared to MALDI-1.