Mass spectrometry of food lipids: Negative ion chemical ionization/collision induced dissociation analysis of oils containing γ-linolenic acid as an example

Triacylglycerols (TAG) and phospholipids (PL) may be analyzed by tandem mass spectrometry (MS-MS) in either negative or positive ion mode providing qualitative and quantitative information on: 1) molecular weight species of lipids 2) their esterified fatty acids (FA), 3) combinations of FA, 4) regiospecific positions of FA, and 5) the polar factional groups in PL. The common methods of ionization are fast atom bombardment or electrospray followed by collision induced dissociation of the parent ions Triple quadrupole instruments are increasingly used with Chromatographic intoduction of the samples. The chemical ionization of TAG with ammonia produces stable [M - H]^- ions which may be selected with unit resolution to collide with an inert gas. Abundances of daughter ions RCO₂^- and [M - H - RCO₂H - 100]^- define the fatty acid combinations and, with certain limitations, the regioisomeric structures of TAG (sn-2 vs. sn-1/3 positions of FA). Seed oils of evening primrose (Oenothera biennis), borage (Borago officinalis) and black currant (Ribes nigrum) were analyzed with this method. All the oils contain considerable amounts of γ-linolenic acid but only black currant contains α-linolenic acid. FA compositions calculated from the RCO^2- daughter ions were in accordance with the known composition of the oils. In evening primrose oil FA 18:2 occupied more prominently the sn-2 position than did FA 18:3 and 18:0 which were predominantly in the sn-1/3 positions. Saturated 16:0 and 18:0 as well as C20-C24 fatty acids did not typically occupy the sn-2 position in borage oil. In black currant oil the 18:3 acids were less abundant in the sn-2 position than in the two other oils. Also 18:4n-3 (stearidonic), 20:1 and 20:2 acids were mainly located in the primary positions.