High mass resolution tissue imaging at atmospheric pressure using laserspray ionization mass spectrometry

• Tissue imaging of lipids, peptides, small proteins, and drugs are demonstrated using laserspray ionization at 100,000 mass resolution.
• Images of low abundance ions show that most originate from the tissue and are not matrix background ions.
• Simultaneous imaging of a peptide, protein, and lipids are achieved with delipified tissue.
• Three lipids are imaged within a mass window of 0.07 Da.

Laserspray ionization inlet (LSII) is a new ionization method in which a laser is used to ablate a matrix similar to atmospheric pressure (AP) matrix assisted laser desorption/ionization, but with LSII, ionization occurs in a heated inlet linking AP and the vacuum of the mass analyzer. LSII produces multiply charged analyte ions similar to electrospray ionization. Here we use LSII to image tissue sections at AP with a mass resolution of 100,000 (50%, FWHH, m/z 200) and <5 ppm mass accuracy. Hydroxycloroquine and metabolites desethylhydroxychloroquine and desethylchloroquine are imaged in mouse kidney tissue. Lipids as well as a +6 charge state ion of m/z 828 for a small protein having a monoisotopic molecular weight of 4961 are imaged in delipified mouse brain tissue simultaneously with the laser focused to ca. 30 μ. Distinct images are obtained for isobaric lipids, and ions just above the noise level produce images of the brain suggesting minimum matrix related background ions. LSII is performed with a single laser shot required for each mass spectrum providing an expedient tissue imaging method.

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