Tandem MS and C₆₀ SIMS for the identification and characterization of lipids

The tandem MS capability of a C₆₀-Qstar hybrid instrument was utilized for the characterization of glycerophosphocholine (GPC) lipids in positive ion mode SIMS. In general, the tandem MS of lipids is facilitated by cationization or reaction with alkali metal salts. Thus, the lipids were treated with LiTFA salt (lithiation) prior to mass spectrometric analysis. As a result of the sample treatment, cationized lipid molecular ions with lithium adducts were formed and were revealed by mass shifts of 6Da relative to unreacted species in the SIMS spectra. The lithiated molecular ions of GPCs were then subjected to collision-induced dissociation (CID) in tandem MS mode. Upon CID, fragments specifying the fatty acid and head group composition of the GPC lipid were generated and could be used for its structural characterization and identification. In addition, a more significant observation was revealed in the time-of-flight-SIMS spectral data of the lithiated GPC. Here, we found that fragmentation was enhanced, and fragment ions similar to those produced by CID were present in the SIMS spectra. The implications of these results are discussed.