Determination of Optimal Electrospray Parameters for Lipidomics in Infrared-Matrix-Assisted Laser Desorption Electrospray Ionization Mass Spectrometry Imaging
Infrared matrix-assisted laser desorption ionization (IR-MALDESI) is an ambient mass spectrometry imaging (MSI) technique that relies on electrospray ionization (ESI) for ion generation of desorbed neutrals. Although many mechanisms in IR-MALDESI have been studied in depth, there has not yet been a comprehensive study of how the ESI parameters change the profiles of tissue specific lipids. Acetonitrile (ACN)/water and methanol (MeOH)/water solvent systems and compositions were varied across a series of applied ESI voltages during IR-MALDESI analysis of rat liver tissue. Gradients of 12 min were run from 5 to 95% organic solvent in both positive and negative polarities across 11 voltages between 2.25 and 4.5 kV. These experiments informed longer gradients (25–30 min) across shorter solvent gradient ranges with fewer voltages. Optimal ESI parameters for lipidomics were determined by the number and abundance of detected lipids and the relative proportion of background ions. In positive polarity, the best solvent composition was 60–75% ACN/40–25% H2O with 0.2% formic acid at 3.2 kV applied voltage. The best parameters for negative polarity analysis are 45–55% ACN/55–45% H2O with 1 mM of acetic acid for voltages between 2.25 and 3.2 kV. Using these defined parameters, IR-MALDESI positive polarity lipidomics studies can increase lipid abundances 3-fold, with 15% greater coverage, while an abundance increase of 1.5-fold and 10% more coverage can be achieved relative to commonly used parameters in negative polarity.