Ambient lipidomic analysis of brain tissue using desorption electrospray ionization (DESI) mass spectrometry

Abstract

Desorption electrospray ionization (DESI) is a spray-based ambient ionization method for mass spectrometry (MS) which generates ions in native atmospheric conditions (e.g., pressure and temperature). Ambient ionization allows in situ analysis of unmodified biological samples by eliminating analyte extraction and separation steps before MS analysis. DESI-MS has been extensively used to analyze organ tissues both in humans and in vertebrate animals, focusing on the detection of small molecules (e.g., oncometabolites, xenobiotic drugs, hormones, etc.) and lipids. DESI-MS lipidomic analysis workflow involves (1) detection of lipids from intact biological material, (2) detection and identification of lipids in complex mixtures, and (3) discrimination between similar lipids, e.g., isomeric lipids. DESI-MS can provide lipid profiles using a relatively fast and simple workflow in which low-resolution single-stage mass spectra are recorded during 2D or 3D image analysis (i.e., mapping the distribution of lipids within the sample). Such DESI-MS lipid profiles include many classes of lipids, such as phosphatidylcholines (PC), triacylglycerols (TGs), free fatty acids (FFAs), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylserines (PSs), diacylglycerols (DGs), ubiquinone, and cholesterol derivatives (e.g., cholesterol sulfate and cholesterol esters). Depending on the mass spectrometer used, there is the additional possibility of obtaining structural information of lipids via MSn, and molecular formulae via high resolution mass spectrometry (HRMS). Focusing on the analysis of human brain, here we summarize the DESI-MS experimental workflow for tissue analysis, data collection, and processing using low and high-resolution mass spectrometers, emphasizing the strategies for structural identification of lipids.