Fig. (4) MSI ionization sources. The three most common sources for MSI: (a) Desorption electrospray ionization which uses a stream of
solvent ions to desorb and ionize analyte molecules, (b) secondary ion mass spectrometry which uses a beam of ions from an ion gun to sputter
analyte ions off the sample, and (c) matrix-assisted laser desorption/ ionization which uses laser irradiation of a matrix-coated sample for
desorption and ionization. "Reprinted (adapted) with permission from . Copyright (2015) Elsevier."