MALDI in-source decay, from sequencing to imaging.

Matrix-assisted laser desorption/ionization (MALDI) is now a mature method allowing the identification and, more challenging, the quantification of biopolymers (proteins, nucleic acids, glycans, etc). MALDI spectra show mostly intact singly charged ions. To obtain fragments, the activation of singly charged precursors is necessary, but not efficient above 3.5 kDa, thus making MALDI MS/MS difficult for large species. In-source decay (ISD) is a prompt fragmentation reaction that can be induced thermally or by radicals. As fragments are formed in the source, precursor ions cannot be selected; however, the technique is not limited by the mass of the analyzed compounds and pseudo MS3 can be performed on intense fragments. The discovery of new matrices that enhance the ISD yield, combined with the high sensitivity of MALDI mass spectrometers, and software development, opens new perspectives. We first review the mechanisms involved in the ISD processes, then discuss ISD applications like top-down sequencing and post-translational modifications (PTMs) studies, and finally review MALDI-ISD tissue imaging applications.