Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) can simultaneously record the lateral distribution of numerous biomolecules in tissue slices, but its sensitivity is restricted by limited ionization. We used a wavelength-tunable postionization laser to initiate secondary MALDI-like ionization processes in the gas phase. In this way, we could increase the ion yields for numerous lipid classes, liposoluble vitamins, and saccharides, imaged in animal and plant tissue with a 5-micrometer-wide laser spot, by up to two orders of magnitude. Critical parameters for initiation of the secondary ionization processes are pressure of the cooling gas in the ion source, laser wavelength, pulse energy, and delay between the two laser pulses. The technology could enable sensitive MALDI-MS imaging with a lateral resolution in the low micrometer range.

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Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) visualizes the distribution of phospho- and glycolipids in tissue sections. However, C=C double-bond (db) positional isomers generally cannot be distinguished. Now an on-tissue Paternò-Büchi (PB) derivatization procedure that exploits benzaldehyde as a MALDI-MSI-compatible reagent is introduced. Laser-induced postionization (MALDI-2) was used to boost the yields of protonated PB products. Collision-induced dissociation of these species generated characteristic ion pairs, indicative of C=C position, for numerous singly and polyunsaturated phospholipids and glycosphingolipids in mouse brain tissue. Several db-positional isomers of phosphatidylcholine and phosphatidylserine species were expressed with highly differential levels in the white and gray matter areas of cerebellum. Our PB-MALDI-MS/MS procedure could help to better understand the physiological role of these db-positional isomers.

Coupling laser post-ionisation with a high resolving power MALDI Orbitrap mass spectrometer has realised an up to ∼100-fold increase in the sensitivity and enhanced the chemical coverage for MALDI-MS imaging of lipids relative to conventional MALDI. This could constitute a major breakthrough for biomedical research.

Matrix-assisted laser desorption/ionization combined with laser-induced postionization (MALDI-2) is a recently introduced method for enhanced mass spectrometry imaging of numerous classes of biomolecules, including phospho- and glycolipids in tissue sections at high lateral resolution. Here we describe the first adaptation of the technology to a Bruker timsTOF fleX mass spectrometer. Upon use of a 1 kHz postionization laser, MALDI-2 produces a sizable increase in the number of detected features as well as in ion signal intensities. This enhancement is similar to that described previously for low repetition rate MALDI-2 systems, but now enables substantially enhanced measurement speeds. In our proof-of-concept study, we furthermore demonstrate, on examples of rat brain and testis tissue sections, that the combination of MALDI-2 with the trapped ion mobility spectrometry (TIMS) functionality of the instrument can crucially support unravelling the complex molecular composition of the lipidome. Numerous isomeric/isobaric ion species are successfully separated upon using the collisional cross section (CCS) as additional specific physical property. With the possibilities of high data acquisition speed or high separation powers in combination with the increased sensitivity of MALDI-2 available in one instrument, the described methodology could be a valuable tool in many areas of biological and medical research.