Single‐Photon‐Induced Post‐Ionization to Boost Ion Yields in MALDI Mass Spectrometry Imaging**

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a rapidly growing method in many elds of the life sciences. For many analyte classes, however, its sensitivity is limited due to poor ionization e ciencies. To mitigate this problem, we here introduce a novel and cost-effective postionization scheme at high repetition rates based on the interplay of single-photon photoionization and subsequent charge transfer reactions. Importantly, the ne vacuum conditions of a dual ion-funnel ion source effectively thermalize the evolving MALDI plume and enable ample gas-phase reactions as well as the addition of chemical dopants that crucially support chemical ionization. Supported by acetone dopant, [M + H] + /[M-H] − signals of numerous glycerophospho-, sphingo-, and further lipids, registered from mammal brain and kidney sections, were boosted by up to three orders of magnitude, similar to results obtained with laser-based postionization (MALDI-2). Experiments utilizing deuterated matrix and dopant, however, indicate complex ionization pathways different from MALDI2.

Mass Spectrometry Imaging of Lipids using MALDI Coupled with Plasma-Based Post-Ionisation on a Trapped Ion Mobility Mass Spectrometer

“…The averaged mass spectrum is shown in Figure 4a and reveals the detection of numerous lipid species with excellent signal-to-noise, and sharing similarities with data obtained using MALDI-2 and other post-ionization methods where protonated PE and HexCer lipids produce the most intense signals 27,30,34 . When analyzed using the METASPACE annotation engine 49 against the LIPID MAPS database, 351 m/z values with distributions specific to on-tissue regions could be assigned to potential lipid species with a 10% false-discovery rate and < 10% off tissue probability (295 peak are annotated at < 1% off tissue probability).…”

“…This suggests some commonalities in the ionization mechanisms of various post-ionization methods in the form of charge transfer to analyte from abundant background reactants. 27,30 An advantage of plasma post-ionisation using SICRIT compared to MALDI-2 is the reduced dependency of alignment issues (e.g., MALDI-2 laser alignment) that makes it easier to acquire optimal data quality and post-ionisation efficiency.…”

“…27,28,29 Vacuum-ultraviolet irradiation from radio-frequency-Krypton lamps has also recently been shown to promote soft postionization of lipids fostered by gas-phase charge transfer from single-photon ionized chemical dopants, producing similar spectral Information as MALDI-2. 30 Dielectric barrier discharge (DBD) is another promising technology to facilitate post-ionization in MSI. DBD devices can be coupled to the AP sampling capillary enabling efficient interaction of collected neutrals with ions formed in the cold plasma.…”

Mass Spectrometry Imaging of Lipids Using MALDI Coupled with Plasma-Based Post-Ionization on a Trapped Ion Mobility Mass Spectrometer

“…Despite technological differences, spectra generated from mouse brain tissue share similarities with MALDI-2 and other postionization approaches in that rich lipid spectra are generated with PE and HexCer lipid classes being well-represented and protonated ions predominating over alkali adducts. This suggests some commonalities in the ionization mechanisms of various postionization methods in the form of charge transfer to analyte from abundant background reactants. , An advantage of plasma postionization using SICRIT compared to MALDI-2 is the reduced dependency of alignment issues (e.g., MALDI-2 laser alignment) that makes it easier to acquire optimal data quality and postionization efficiency.…”

“…29−32 Vacuum-ultraviolet irradiation from Krypton lamps has been used to promote postionization by charge transfer from single-photon ionized sprayed solvent at atmospheric pressure 33,34 or chemical dopants at low pressure, the latter recently demonstrating similarly rich lipidomic spectral information as MALDI-2. 35 Dielectric barrier discharge (DBD) is another promising technology to facilitate postionization in MSI. DBD devices can be coupled to the AP sampling capillary enabling efficient interaction of collected neutrals with reactive species formed in the cold plasma.…”