RAPID-DETERMINATION OF THE TOTAL TRANS CONTENT OF NEAT HYDROGENATED OILS BY ATTENUATED TOTAL-REFLECTION SPECTROSCOPY

A Fourier transform infrared spectroscopy procedure is described for q uantitating the levels of total trans triglycerides or their fatty aci d methyl ester derivatives in neat fats and oils. It requires either w arming or no preparation of the laboratory sample, and about 5 min for spectroscopic measurement, band area integration, and calculation of the trans content from a linear regression equation. To eliminate the strongly sloping background of the 966-cm(-1) trans band, the single-b eam spectrum of the trans-containing fat is ''ratioed'' against that o f an unhydrogenated oil or a reference material that contains only cis double bonds. Thus, a symmetric absorption band on a horizontal backg round is obtained. The area under the trans band can then be accuratel y integrated between the same limits, 990 and 945 cm(-1), for all tran s levels investigated. To speed up the analysis, an attenuated total r eflection liquid cell was used, into which oils, melted fats or their methyl esters were poured without weighing or quantitative dilution wi th the toxic and volatile carbon disulfide solvent. The trans levels d etermined by attenuated total reflection were closer to those found by capillary gas chromatography when the hydrogenated far was measured a gainst the corresponding unhydrogenated oil than when it was measured against a cis reference material. Small differences were found between trans levels in hydrogenated fat test samples and the corresponding m ethyl ester derivatives (9.3 and 2.2% at about 2 and 41% trans, respec tively). The lower limits of identification and quantitation were 0.2 and 1%, respectively.