An attenuated total reflection infrared spectroscopy procedure was collaboratively studied among two sets of five laboratories for quantitating the total trans fatty acid levels in neat (without solvent) hydrogenated vegetable oils, measured as triacylglycerols in one study, and as fatty acid methyl ester derivatives in the other. Unlike the fatty acid methyl esters, the triacylglycerols required no derivatization but had to be melted prior to measurement. To obtain a symmetric absorption band at 966 cm −1 on a horizontal background, the singlebeam spectrum of the trans-containing fat was "ratioed" against that of a refined oil or a reference material that contained only cis double bonds. A single-bounce horizontal attenuated total reflection cell that requires 50 µL of undiluted test samples was used for oils, melted fats, or their methyl esters. For fatty acid methyl esters, the reproducibility relative standard deviations were in the range of 0.9 to 18.46% for 39.08 to 3.41% trans, determined as methyl elaidate per total fatty acid methyl esters. For five pairs of triacylglycerol blind duplicates, the reproducibility and repeatability relative standard deviations were in the ranges of 1.62 to 18.97%, and 1.52 to 13.26%, respectively, for 39.12 to 1.95% trans, determined as trielaidin per total triacylglycerols. Six pairs of spiked triacylglycerol blind duplicates (quality assurance standards) exhibited high accuracy in the range of 0.53 to 40.69% trans and averaged a low bias of 1.3%. These statistical analysis results were compared to those collaboratively obtained by the recently adopted AOCS Cd14-95 and AOAC 994.34 Infrared Official Methods. JAOCS 75, 353-358 (1998).

Gas chromatography (GC) has been a standard analytical tool in lipid chemistry. The rapid attenuated total reflection (ATR) infrared (IR) American Oil Chemists' Society (AOCS) Recommended Practice (Cd 14d-97) was compared to the capillary GC AOCS Recommended Practice (Ce 1f-97) that was optimized to accurately determine total trans fatty acids on highly polar stationary phases. This comparative evaluation was validated in an independent laboratory. These procedures were used to quantitate the total trans fatty acid levels in partially hydrogenated vegetable oils, measured as neat (without solvent) triacylglycerols (TAG) by ATR and as fatty acid methyl ester (FAME) derivatives by capillary GC. Unlike FAME, TAG determination by ATR required no derivatization, but samples had to be melted prior to measurement. Five blind replicates for each of three accuracy standards and three test samples were analyzed by each technique. The GC and ATR determinations were in good agreement. Accuracy was generally high. The ratios of ATR mean trans values (reported as percentage of total TAG) to the true values (based on the amount of trielaidin added gravimetrically) were 0.89, 0.98, and 1.02 for accuracy standards having about 1, 10, and 40% trans levels. The corresponding GC values, determined as percentage of total FAME, were 0.98, 0.99 and 1.04. The ratios of mean trans values determined by these techniques were ATR/GC 0.85, 1.04, and 1.01 for test samples having trans levels of about 0.7, 8, and 38%, respectively. The optimized GC procedure also minimized the expected low bias in trans values due to GC peak overlap found with the GC Official Method Ce 1c-89. Satisfactory repeatability and reproducibility were obtained by both ATR and GC.For several decades analytical methods, such as gas chromatography (GC) (1) and infrared (IR) spectroscopy (2,3), for the determination of total trans fatty acids in partially hydrogenated vegetable oils have each been widely used (4) and repeatedly modified in order to improve accuracy (5,6). A recent publication by Duchateau et al. (5) detailed optimized capillary GC separations on highly polar stationary phases for the accurate determination of trans fatty acids in partially hydrogenated vegetable oils and in refined (deodorized or stripped) oils. This procedure was adopted by the American Oil Chemists' Society (AOCS) as Official Method AOCS Ce 1f-96 (7). The GC Official Method AOCS Ce 1c-89 (1) for partially hydrogenated vegetable oils underestimates some trans C 18 monoene (18:1) positional isomers in favor of cis 18:1 isomers, with which they overlap. Two relatively minor peaks attributed to the trans-12 and trans-13 plus trans-14 18:1 isomers were resolved by the optimized GC procedure (5), and this reportedly improved the accuracy of the GC determination. These two minor peaks (labeled "valley peaks") emerged between those of the major trans 18:1 isomers and the intense cis-9 18:1 peak. In refined vegetable oils monotrans 18:2 and 18:3 isomers occur at levels that can reach 1-3% of tot...

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