Figure 2. DESI-CID/OzID mass spectra obtained from [M+Na]⁺ ions of synthetic glycerophospholipids (a) PC 16:0/18:1(9Z) and (b) PC 18:1(9Z)/16:0.

The CID/OzID peaks indicative of the presence of the 18:1 chain at the sn-2 position (*) and 16:0 at the sn-2 position (▪) are marked. The dissociation pathways for the two ionized glycerophospholipid sn-positional isomers are indicated above the corresponding mass spectra. Putative dissociation pathways are based on the study of Pham et al.¹⁹ and full reaction mechanisms for CID and OzID steps are provided as Supporting Information (Scheme S1). Spectra acquired using offline ozone generation (see Methods).