Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry of lipids: ionization and prompt fragmentation patterns

Abstract

Ionization and prompt fragmentation patterns of triacylglycerols, phospholipids (PLs) and galactolipids were investigated using matrix-assisted laser desorption/ionization (MALDI). Positive ions of non-nitrogen-containing lipids appeared only in the sodiated form, while nitrogen-containing lipids were detected as both sodiated and protonated adducts. Lipids containing acidic hydroxyls were detected as multiple sodium adducts or deprotonated ions in the positive and negative modes, respectively, with the exception of phosphatidylcholines. The positive MALDI spectra of triacylglycerols contained prompt fragments equivalent to the loss of RCOO(-) from the neutral molecules. Prompt fragment ions [PL-polar head](+) were observed in the positive MALDI spectra of all phospholipids except phosphatidylcholines. The phosphatidylcholines produced only a minor positive fragment corresponding to the head group itself (m/z 184). Galactolipids did not undergo prompt fragmentation. Post-source decay (PSD) was used to examine the source of prompt fragments. PSD fragment patterns indicated that the lipid prompt fragment ions did not originate from the observed molecular ions (sodiated or protonated), and suggested that the prompt fragmentation followed the formation of highly unstable, probably protonated, precursor ions. Pathways leading to the formation of prompt fragment ions are proposed.

Structural analysis of phosphatidylcholines by post-source decay matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

“…In contrast to other PL classes, PC cannot be detected in the negative ion mode as deprotonated molecule, and its positive reflector MALDI spectrum lacks the prompt fragment corresponding to PC minus the polar head group [50]. This is attributed to the unique structure of PC, which contains a positive quaternary nitrogen atom.…”

“…The ionization patterns of PC, relative to other PL classes, have been discussed previously [50]. In this work, we examine the utility of post-source decay (PSD) coupled with MALDI-TOF for the structural identification of PC and propose pathways for the formation of the fragments in the PSD spectra.…”

“…Recently, matrix-assisted laser desorption/ionization (MALDI) interfaced with FTICR [42] and time-offlight [43][44][45][46][47][48][49][50] was introduced as an alternative soft ionization technique for mass analysis of PL. In comparison with the other soft ionization sources (FAB and ESI), MALDI analysis of lipids is more sensitive, less affected by impurities and offers rapid sample preparation, making it an excellent analytical approach for rapid screening of lipid components in biological matrices.…”

The renaissance of high-energy CID for structural elucidation of complex lipids: MALDI-TOF/RTOF-MS of alkali cationized triacylglycerols

“…So far, MALDI-RTOF-MS was primarily utilized in combination with PSD yielding product ion spectra with only two types of product ions, the B-and the C-type ion series [e.g., [33][34][35]. Only in one publication, a CID-spectrum of a synthetic triacylglycerol exhibiting poor spectral quality was shown [29].…”

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

“…For instance, phosphatidylcholines (PCs) analyzed in positive ion mode with a soft ionization source typically form protonated [L+H] + or sodiated [L+Na] + ions [6][7][8]. PCs are zwitterionic and possess a permanent positive charge on the nitrogen atom of the phosphocholine head group.…”