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 [21]. Copyright (2015) Elsevier."