Collision-induced dissociation of doubly-charged barium-cationized lipids generated from liquid samples by atmospheric pressure matrix-assisted laser desorption/ionization provides structurally diagnostic product ions

Obtaining structural information for lipids such as phosphatidylcholines, in particular the location of double bonds in their fatty acid constituents, is an ongoing challenge for mass spectrometry (MS) analysis. Here, we present a novel method utilizing the doping of liquid matrix-assisted laser desorption/ionization (MALDI) samples with divalent metal chloride salts, producing ions with the formula [L+M](2+) (L = lipid, M = divalent metal cation). Multiply charged lipid ions were not detected with the investigated trivalent metal cations. Collision-induced dissociation (CID) product ions from doubly charged metal-cationized lipids include the singly charged intact fatty acids [snx+M-H](+), where 'x' represents the position of the fatty acid on the glycerol backbone. The preference of the divalent metal cation to locate on the sn2 fatty acid during CID was found, enabling stereochemical assignment. Pseudo-MS3 experiments such as in-source decay (ISD)-CID and ion mobility-enabled time-aligned parallel (TAP) MS of [snx+M-H](+) provided diagnostic product ion spectra for determining the location of double bonds on the acyl chain and were applied to identify and characterize lipids extracted from soya milk. This novel method is applicable to lipid profiling in the positive ion mode, where structural information of lipids is often difficult to obtain.