Richard E. McDonald

Rapid determination of the totaltrans content of neat hydrogenated oils by attenuated total reflection spectroscopy

A Fourier transform infrared spectroscopy procedure is described for quantitating the levels of totalrans triglycerides or their fatty acid methyl ester derivatives in neat fats and oils. It requires either warming or no preparation of the laboratory sample, and about 5 min for spectroscopic measurement, band area integration, and calculation of thetrans content from a linear regression equation. To eliminate the strongly sloping background of the 966-cm−1trans band, the single-beam spectrum of thetrans-containing fat is “ratioed” against that of an unhydrogenated oil or a reference material that contains onlycis double bonds. Thus, a symmetric absorption band on a horizontal background is obtained. The area under thetrans band can then be accurately integrated between the same limits, 990 and 945 cm−1, for alltrans levels investigated. To speed up the analysis, an attenuated total reflection liquid cell was used, into which oils, melted fats or their methyl esters were poured without weighing or quantitative dilution with the toxic and volatile carbon disulfide solvent. Thetrans levels determined by attenuated total reflection were closer to those found by capillary gas chromatography when the hydrogenated fat was measured against the corresponding unhydrogenated oil than when it was measured against acis reference material. Small differences were found betweentrans levels in hydrogenated fat test samples and the corresponding methyl ester derivatives (9.3 and 2.2% at about 2 and 41%trans, respectively). The lower limits of identification and quantitation were 0.2 and 1%, respectively.

Confirmatory mass-spectral data for cyclic fatty acid monomers

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.

Identification Of Fatty Acid Isomers By Gas Chromatography / Matrix Isolation / Fourier Transform Infrared Spectroscopy

Geometric and positional isomers of fatty acid methyl esters (FAME) derived from hydrogenated soybean oil and margarines were separated by silver nitrate-thin layer chromatography (AgNO3-TLC) followed by capillary gas chromatography (GC) and identified by matrix isolation / Fourier transform infrared (MI/FTIR) spectroscopyi,2. Because of the high specificity of the MI technique, it was possible to distinguish between different 18-carbon aliphatic chains of FAME positional isomers with cis or trans configuration, and to determine their degree of unsaturation. For the first time mid-IR spectra were observed for methylene-interrupted or isolated trans, trans or cis/ trans C18 FAME positional isomers. These spectra could be readily differentiated based on unique MI/FTIR spectral characteristics.