Schematic illustration of PA-like artifact generated from in-source fragmentation of sodium adduct of PC in MALDI-MSI. (A) The post-source decay fragmentation spectrum of sodiated di16:0 PC (m/z 756.5) displays three predominant headgroup-related fragment ions resulted from the loss of trimethylamine (m/z 59) cholinephosphate (m/z 183), and sodiated cholinephosphate (m/z 205) (Reprinted from ref. (Stübiger, Pittenauer, & Allmaier, 2008) with permission from the American Chemical Society, Copyright 2008). (B) The fragmentation pathway proposed for formation of the sodiated cycloethylene PA (m: n) diester ion, which is the isomer of sodiated PA (m+2: n+1) ion (Hsu & Turk, 2003).