MASS-SPECTROMETRY OF FOOD LIPIDS - NEGATIVE-ION CHEMICAL-IONIZATION COLLISION-INDUCED DISSOCIATION ANALYSIS OF OILS CONTAINING GAMMA-LINOLENIC ACID AS AN EXAMPLE

Triacylglycerols (TAG) and phospholipids (PL) may be analyzed by tande m mass spectrometry (MS-MS) in either negative or positive ion mode pr oviding qualitative and quantitative information on: 1) molecular weig ht species of lipids, 2) their esterified fatty acids (FA), 3) combina tions of FA, 4) regiospecific positions of FA, and 5) the polar functi onal groups in FL. The common methods of ionization are fast atom bomb ardment or electrospray followed by collision induced dissociation of the parent ions. Triple quadrupole instruments are increasingly used w ith chromatographic introduction of the samples. The chemical ionizati on of TAG with ammonia produces stable [M - H](-) ions which may be se lected with unit resolution to collide with an inert gas. Abundances o f daughter ions RCO2- and [M - H - RCO2H-100](-) define the fatty acid combinations and, with certain limitations, the regioisomeric structu res of TAG (sn-2 vs. sn-1/3 positions of FA). Seed oils of evening pri mrose (Oenothera biennis), borage (Borago officinalis) and black curra nt (Ribes nigrum) were analyzed with this method. All the oils contain considerable amounts of gamma-linolenic acid but only black currant c ontains alpha-linolenic acid. FA compositions calculated from the RCO2 - daughter ions were in accordance with the known composition of the o ils. In evening primrose oil FA 18:2 occupied more prominently the sn- 2 position than did FA 18:3 anti 18:0 which were predominantly in the sn-1/3 positions. Saturated 16:0 and 18:0 as well as C20-C24 fatty aci ds 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 tha n in the two other oils. Also 18:4n-3(stearidonic), 20:1 and 20:2 acid s were mainly located in the primary positions.