Confirmatory mass‐spectral data for cyclic fatty acid monomers

Abstract

Cyclic fatty acid monomers (CFAM) are degradation products found in heat‐abused edible oils. This study confirms previously published data and reports the structural elucidation of hydrogenated and deuterated monocyclic and bicyclic CFAM prepared from the corresponding unsaturated species that were previously isolated from heated flaxseed (linseed) oil. CFAM structures were determined as 2‐alkyl‐4,4‐dimethyloxazoline derivatives by using gas chromatography‐electron ionization mass spectrometry. The observed retention times for the deuterated CFAM were about 0.1 min shorter than those of the corresponding hydrogenated species. For two minor six‐membered ring CFAM components, an increase in the mass of the unsaturated ring by eight mass units upon deuteration indicated the addition of four deuterium atoms to two double bonds in that ring. These data unequivocally confirmed the identity of cyclohexadienyl CFAM species in the original CFAM mixture that was isolated from heated linseed oil. The observed electron ionization mass spectrometic data for minor hydrogenated and deuterated CFAM species, which correspond to the last two eluting monounsaturated species, were consistent with CFAM having bicyclic (fused 5‐ and 6‐membered rings) structures. The location of the ring along the fatty acid chain was also confirmed for all saturated CFAM mixture components. The presence of a pair of deuterium atoms on two adjacent carbon atoms further confirmed the previously determined double‐bond position along the fatty acid chain of the corresponding parent (unsaturated) compound.

Structural Analysis of Fatty Acids

“…Both DMOX and picolinyl ester derivatives were used for characterization purposes, although the former seemed preferable in some instances. With these same samples, DMOX derivatives were used in another laboratory for structural studies [145][146][147][148]. A comparison of the mass spectra of picolinyl and DMOX derivatives of some cyclic dienoic fatty acids derived from linolenate are illustrated in Figure 4.14 [83].…”

Gas chromatography‐mass spectrometry methods for structural analysis of fatty acids

“…When a double bond occurred also in the aliphatic chain, the expected mass spectral features were present and located its position, but there were no ions to indicate the position of a double bond in the ring and this remains an unsolved problem in MS. Similarly, the complex range of cyclic fatty acids, with internal 5-and 6-membered rings and double bonds in many different positions, which are formed by free-radical mechanisms when vegetable oils are heated to high temperatures as during frying of food or the commercial physical refining process, has been characterized both as DMOX and picolinyl ester derivatives (36)(37)(38)(39)(40)(41)(42). Here DMOX derivatives may be marginally better.…”

Fatty acids, fatty acid analogues and their derivatives

“…457 Cyclic fatty acid monomers found in heat-abused edible oils have been confirmed by mass spectral analysis. 458 Electron impact and chemical ionization mass spectral analyses of methyl ester species of free mycolic acid from Rhodococcus lentifragmentus have been compared. 459 The presence of unsaturation in the meroaldehyde subunit of methyl mycolate was reflected by the appearance of dehydration fragment ions under chemical ionization.…”