Construction and testing of an atmospheric-pressure transmission-mode matrix assisted laser desorption ionisation mass spectrometry imaging ion source with plasma ionisation enhancement

Abstract

Matrix assisted laser desorption ionisation mass spectrometry (MALDI-MS) at atmospheric pressure (AP) is, with a few notable exceptions, overshadowed by its vacuum based forms and AP transmission mode (TM) MALDI-MS lacks the uptake its potential benefits might suggest. The reasons for this are not fully understood and it is clear further development is required to realise the flexibility and power of this ionisation method and geometry. Here we report the build of a new AP-TM-MALDI-MSI ion source with plasma ionisation enhancement. This novel ion source is used to analyse a selection of increasingly complex systems from molecular standards to murine brain tissue sections. Significant enhancement of detected ion intensity is observed in both positive and negative ion mode in all systems, with up to 2000 fold increases observed for a range of tissue endogenous species. The substantial improvements conferred by the plasma enhancement are then employed to demonstrate the acquisition of proof of concept tissue images, with high quality spectra obtained down to 10 × 10 µm pixel size.

Effect of MALDI Matrices on Lipid Analyses of Biological Tissues Using MALDI-2 Post-Ionization Mass Spectrometry

“…Briefly, the MALDI-2 process utilizes a secondary laser aligned ~ 400 µm above the primary MALDI-1 ablation surface so that the secondary laser pulses intersect the primary MALDI-1 desorption plume at a given delay time, resulting in ionization of some of the desorbed neutrals. [35][36][37][38][39][40][41][42] The MALDI-2 ionization process is thought to result from a two-photon ionization threshold mechanism that gives rise primarily to protonated species. 34,36,43,44 MALDI-2 has been shown to improve ion yields by more than two orders of magnitude for some lipid classes.…”

“…Thus far, MALDI-2 has proven to improve sensitivity for certain molecular classes including liposoluble vitamins, mono-and oligosaccharides, certain lipid classes, and exogenous drugs. 34,36,[38][39][40][41][42][43]45,46 Lipid classes that have been shown to have dramatically increased ion intensities in positive ion mode include hexosylceramides (HexCers), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylglycerols (PGs), phosphatidylserines (PSs), triacylglycerols (TGs), and cholesterol esters (CEs). 34 Interestingly, other classes of lipids such as phosphatidylcholines (PCs) were not shown to increase significantly with MALDI-2.…”

Effect of MALDI matrices on lipid analyses of biological tissues using MALDI‐2 postionization mass spectrometry

“…Briefly, the MALDI-2 process utilizes a secondary laser aligned $400 μm above the primary MALDI-1 ablation surface so that the secondary laser pulses intersect the primary MALDI-1 desorption plume at a given delay time, resulting in ionization of some of the desorbed neutrals. [35][36][37][38][39][40][41][42] The MALDI-2 ionization process is thought to result from a two-photon ionization threshold mechanism that gives rise primarily to protonated species. 34,36,43,44 MALDI-2 has been shown to improve ion yields by more than two orders of magnitude for some lipid classes.…”

“…Thus far, MALDI-2 has proven to improve sensitivity for certain molecular classes including liposoluble vitamins, monosaccharide and oligosaccharide, certain lipid classes, and exogenous drugs. 34,36,[38][39][40][41][42][43]45,46 Lipid classes that have been shown to have dramatically increased ion intensities in positive ion mode include hexosylceramides (HexCers), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylglycerols (PGs), phosphatidylserines (PSs), triacylglycerols (TGs), and cholesterol esters (CEs). 34 Interestingly, other classes of lipids such as phosphatidylcholines (PCs) were not shown to increase significantly with MALDI-2.…”

Protein ion yield enhancement in matrix‐assisted laser desorption/ionization mass spectrometry after sample and matrix low‐pressure glow discharge plasma irradiation

“…Analysis of molecular standards and murine brain tissue sections was performed by this group. 19 In another study, a microwave argon plasma was employed as an ionization source for analyte particles in ion trap mass spectrometry without any sample preparation steps. Three compounds, including pure analytes, analytes in molecularly imprinted polymers (analyte-MIPs), and bioanalytes (drugs) in medical tablets, were analyzed.…”