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.