Comparison of FTIR and capillary gas chromatographic methods for quantitation oftrans unsaturation in fatty acid methyl esters

Abstract

A computer-assisted method has been developed for estimation of isolated trans unsaturation using the peak area of the trans absorbance band at 966 cm-1 from FTIR spectra of fatty acid methyl esters. Peak areas were used to determine the trans content of weighed standards containing from 0 to 100% methyl elaidate and of hydrogenated soybean oil samples containing up to 36% trans unsaturation. These data for percent trans by FTIR were compared to corresponding data obtained by capillary gas chromatography and the AOCS Official Method 14-61. Determination of isolated trans composition in oils using peak areas gave values with the smallest standard deviation for weighed standards and values within 4% of those obtained by capillary gas chromatography and the AOCS Official Method for hydrogenated samples.Various infrared (IR) spectrophotometric methods for estimation of trans unsaturation in lipids have been suggested and routinely used in the fat industry for many years (1-4). These methods are based on measuring peak height at 965-966 cm 1 from a manually drawn or zero adjusted (where possible} baseline and comparing this absorbance value to that of standards or to other related absorbances.Recent advances in equipment technology have incorporated computer capabilities into FTIR instruments allowing manipulations of the IR spectra, i.e., enlargement, isolation of selected regions, and calculation of peak areas, all of which can be outputted to a plotter or a printer. If output is to a printer, an IR absorbance grid suitable for manual measurement of peak height differences in absorbance units is not normally obtained. Standard peak height procedures for estimating isolated trans unsaturation, therefore, become more tedious and less accurate.A simple computer-assisted procedure has been developed for determining isolated trans unsaturation in fatty acid methyl esters (FAME) based on integration of the area of the absorbance peak due to trans unsaturation. Areas were obtained with a Fourier transform infrared (FTIR) instrument and used to determine percent isolated trans in weighed mixtures of methyl elaidate and methyl oleate (0-100% elaidate) and in six partially hydrogenated soybean oil samples. These data were compared to corresponding data determined by the AOCS Official Method (peak height) and by capillary gas chromatography. MATERIALS AND METHODSMethyl oleate (cis-9-octadecenoate) and methyl elaidate (trans-9-octadecenoate) were purchased from Hormel

Hydrolysis of fatty acid esters in subcritical water

“…Determination of the trans isomer content is not a simple task. To determine the total content of trans acids, IR spectroscopy is widely used, however, without quantification of individual acids [12,13]. It is possible to perform a complete analysis by combining capillary gas chromatography with mass spectrometry [14].…”

Isomerization of 9c11t/10t12c CLA in Triacylglycerols

“…The CH out-of-plane deformation vibration of the trans-trans isomers absorb around 982-988 cm -1 , and cis-cis isomers do not absorb in this region. The =CH deformation vibrations of the methylene interupted cis-trans, trans-cis and trans-trans linoleic acid isomers absorb at approximately 967 at 4 cm -1 resolution at room temperature [19][20][21][22].…”

Analytical methods for determination of trans-C18 fatty acid isomers in milk fat. A review

“…Emergence of Fourier-transform infrared spectroscopy (FTIR) [18,19,20] and the use of computer-assisted spectral subtraction procedures [21,22] allowed for the improved detection efficiency of this method. Unfortunately, results obtained by this new method yielded somewhat greater levels than chromatographic values, even if discrepancies were significantly reduced as a result of using an IR method.…”