Ionic matrix for matrix-enhanced surface-assisted laser desorption ionization mass spectrometry imaging (ME-SALDI-MSI)

Abstract

Matrix-enhanced surface-assisted laser desorption ionization mass spectrometry imaging (ME-SALDI MSI) has been previously demonstrated as a viable approach to improving MS imaging sensitivity. We describe here the employment of ionic matrices to replace conventional MALDI matrices as the coating layer with the aims of reducing analyte redistribution during sample preparation and improving matrix vacuum stability during imaging. In this study, CHCA/ANI (␣-cyano-4-hydroxycinnamic acid/aniline) was deposited atop tissue samples through sublimation to eliminate redistribution of analytes of interest on the tissue surface. The resulting film was visually homogeneous under an optical microscope. Excellent vacuum stability of the ionic matrix was quantitatively compared with the conventional matrix. The subsequently improved ionization efficiency of the analytes over traditional MALDI was demonstrated. The benefits of using the ionic matrix in MS imaging were apparent in the analysis of garlic tissue sections in the ME-SALDI MSI mode. To further improve imaging sensitivity, a hybrid ionization approach, matrix-enhanced SALDI (ME-SALDI), has been recently reported in which conventional matrix molecules were deposited atop a porous SALDI substrate to provide a proton-rich environment for enhanced ionization of desorbed species [6]. The porous SALDI substrate in ME-SALDI is believed to effectively reduce laser flux needed for analyte desorption; consequently few matrix ions and fragments are observed in the resulting spectra. Both characteristics are crucial for detection of low-mass species, rendering ME-SALDI an attractive solution for MS imaging (MSI) of metabolites, especially for those where imaging at high spatial resolution under a reduced laser beam size is desired. Improved MS performance over conventional MALDI and SALDI ionization methods has been demonstrated with enhanced detection sensitivity and a broadened detection mass window [6].However, an inherent challenge associated with matrix-based laser desorption ionization sources is also present in ME-SALDI for MSI applications: it is known that most conventional MALDI matrices slowly vaporize under high vacuum (10 Ϫ7 ϳ10 Ϫ8 Torr) due to their low sublimation points. The loss of materials becomes more severe when the matrix is amorphous-a common form observed when the matrix is deposited by sublimation in a solvent-free fashion. While the loss of matrix under vacuum does not necessarily impose a problem in MALDI profiling experiments where most measurements occur in seconds, it cannot be ignored during MS imaging experiments because of extended interrogation time and the preferred solvent-free approach for matrix deposition [7]. For example, for a commercial MALDI-TOF instrument equipped with a 20-Hz N 2 laser, approximately 720 pixels can be imaged in 1 h when 100 laser shots are averaged per pixel, which translates to ϳ55.6 h of instrument time to examine a 1-cm 2 tissue section with spatial resolution of 50 m, not counting the time spent for translational s...

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