A rapid method for the analysis of hydrogenated fats by GC with IR detection

Abstract

A rapid method for analysis of trans and cis FA in hydrogenated fats has been developed. The method is based on a single analysis by GC with IR detection. Multivariate partial least squares regression is applied on the IR spectra to predict the number of cis and trans double bonds. For each chain length the method provides information about the amount of the saturated FA, the amount of trans monoenes, the amount of cis monoenes, the amount of PUFA, and the average number of cis and trans double bonds in PUFA. The method has been validated by summing the values to a total trans value and total unsaturation. These sum values were compared with total trans unsaturation, as determined by AOCS method Cd 14-95, and iodine value, as determined by AOCS Cd 1d-92. Paper no. J10488 in JAOCS 80, 839-846 (September 2003).KEY WORDS: Gas chromatography, hydrogenated fats, hydrogenation, infrared spectroscopy, partial least squares regression, trans fatty acids.Trans double bonds (DB) in oils and fats can be quantified by IR spectroscopy by measuring the absorption at approximately 967 cm −1 (1,2). This principle is used in several standard procedures (3-5). Methyl elaidate (trans 18:1n-9) or glyceryl trielaidate is usually applied as external standard in these methods. Several reports have shown that the absorption at 967 cm −1 from trans DB in geometric isomers of linoleic acid (18:2n-6) (6-8), linolenic acid (18:3n-3) (9), and trans FA found in hydrogenated fats (8,10) is weaker than the absorption from the trans DB in elaidic acid (trans 18:1n-9). Underestimation of the amount of trans DB may therefore occur in some samples.The IR spectra also contain information about the chain length and the number of cis DB. The most important signals for the determination of FA structure have been summarized by Guillén and Cabo (11). The number of cis DB in a FA can be calculated from the signals at 3020 and 2920 cm −1 (12). Iodine value (IV) also can be calculated from the ratio between the signals at 3030 and 2857 cm −1 (13). Van de Voort et al. (14) obtained good correlation between IR spectra and IV using whole spectra as input in partial least squares (PLS) regression.Trans FA also are analyzed by GC using long polar capillary columns. The GC methods provide more detailed molecular information than IR analysis and have lower detection limits. The major drawback of the GC methods is overlap between the chromatographic zones of cis and trans FA. This overlap may lead to severe errors in quantification, especially when complex samples such as hydrogenated fats are analyzed.Attempts have been made to combine the two techniques using GC with IR detection, often in combination with FID or MS detectors. Using a lightpipe GC-IR interface, Wahl et al. (15) were able to identify 31 unsaturated reference FA including both cis and trans isomers, and reported the detection limit for single isomers to be about 25 ng. GC-MS combined with GC-IR equipped with a direct deposition interface has been used to identify individual isomers in hydrogen...

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