Identification of molecular species of simple lipids by normal phase liquid chromatography–positive electrospray tandem mass spectrometry, and application of developed methods in comprehensive analysis of low erucic acid rapeseed oil lipids

Abstract

Mono-, di- and triacylglycerol (MAG, DAG, TAG), sterol ester (SE), free sterol (S) and free fatty acid (FFA) standards were analyzed in the presence of ammonium ions and ammonia by flow injection MS 2 and MS 3, and by normal phase-liquid chromatography (NP-LC) MS 2 positive electrospray ionization (ESI) mass spectrometry (MS). The MS data recorded for ammonium adducts ([ M + NH 4] +) of TAGs, DAGs, and MAGs were consistent with stepwise fragmentation mechanisms. In the first step, ammonium ion in [ M + NH 4] + donates proton to acylglycerol and ammonia is released. In the second step, FFA is cleaved from protonated TAG, water from protonated 1,3-DAG and MAG, both FFA and water from protonated 1,2-DAG, hence leading to formation of [DAG] + ion from TAG and 1,3-DAG, [DAG] + and [MAG] + ions from 1,2-DAG, and [MAG] + ion from MAG. In the third step, [DAG] + ion of TAG is fragmented to yield [Acyl] +, [Acyl + 74] +, [DAG − 74] + ions, [DAG] ion of 1,3-DAG to [Acyl] + ions, and [MAG] + ion of MAG to protonated FAs, which are decomposed to water and [Acyl] + ions in the fourth step. A stepwise mechanism for fragmentation of FFA was also evident from MS 2 and MS 3 data. Molecular species of low erucic acid rapeseed oil simple lipids were identified from characteristic ions produced in the NP-LC–ESI-MS 2 of [ M + NH 4] + ions. The percentage composition of the molecular species of each lipid class was calculated from integrated extracted ion chromatograms of [( M + NH 4)] + ions of SE, TAG, MAG, and FFA, of the sum of [( M + NH 4)] + and [( M + NH 4) − NH 3 − H 2O] + ions of both regioisomers of DAGs, and of sterol fragment ions of S.