Structural analysis of autoxidation products of FAME using HPLC-ESI/MS: Influence of eluent composition

Within the scope of the ESI/MSn studies of structures of Oxopolymers from the oxidation of linseed oil, methylester (FAME) of Oleic acid (0; 18:1), Linoleic acid (L; 18:2) and Linolenic acid (Ln; 18:3) were autoxidized. The oxidized species were separated by means of HPLC using acetonitrile/water as mobile phase. During the separation, the water content was changed from 80 % to 0 % to achieve the necessary change of polarity. In order to examine, whether the absence of water has an influence on the mass spectra of autoxidised unsaturated FAME, two versions of Electrospray-ionization (ESI) have been studied: Version A: ESI with presence of water, Version B: ESI without presence of water. It is shown, that all ESI MS-spectra of the version A show [M+H], as molecular- and base-peak. Additional molecular ions with m/z > [M+H](+) are observed, which can be assigned to autoxidation products and/or their Na-adducts. The ESI-MS-spectra of version B are characterized by the occurrence of strong molecular ions with m/z >[M] and/or m/z > [2M]. It is particularly worth mentioning, that with all three FAME the molecular ion peak [M+H](+) with M = FAME is missing, which with version A represents the base peak. For the formation of the FAME molecular ion [M+H](+) under ESI conditions, the presence of the protondonator water is absolutely necessary. If water (version B) is missing, exclusively Na-adduct ions of oxidized species are observed. However, if the protondonator water is present (version A), besides Na-adducts species also protonated species are observed in the spectra. Comparing the mass ranges of the oxidized species, the same molecular ion masses are observed in principle, independently of the presence of water during Electrospray ionization. The molecular ion- and/or Na-adduct ion-masses, registered in the ESI-MS-spectra of both versions, can be assigned to plausible empirical molecular formulas. With increasing number of double bonds the number of species with m/z > 2[M] increase, i.e. autoxidized dinner, and for LnME also trimer structures. For some selected oxidized species possible structural models are proposed and discussed in detail.