Reducing fragmentation observed in the matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of triacylglycerols in vegetable oils.

Edible oils consist primarily of triacylglycerols (TAGs). Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra of the oils are typically dominated by sodium adducts of these TAGs but also show prominent fragment ions (that do not contain sodium), which can interfere with analytical measurements of other components in oils. The fragments seemingly correspond to the loss of a fatty acid moiety from the sodiated TAGs as a sodium salt: RCOONa. However, a previous study suggested that the fragments actually arise from nearly complete fragmentation of unseen protonated TAGs. These authors suggested that the fragmentation occurs so rapidly and completely that protonated TAGs are not normally observed in the spectra of these oils. In this paper, we present evidence to support their theory and also demonstrate an approach to eliminate these interfering ions from the MALDI-TOF mass spectra via addition of a base to the matrix/sample mixture. The added base does not impede formation of the sodiated TAGs, but does significantly reduce the amount of fragments observed. We propose that this occurs by depleting the H+ ions from the matrix, thus preventing the formation of significant numbers of protonated TAGs in the first place. For measurements by MALDI-TOF, the relative abundances of the fragment ions are related to the strength of the base, and can be almost completely eliminated. However, in longer time-scale experiments such as in post-source decay and Fourier transform mass spectrometry, sodiated and non-sodiated diacylglycerol (DAG)-like fragments are present in spectra, regardless of whether or not a base is added to the sample.