Spectral confirmation of trans monounsaturated C₁₈ fatty acid positional isomers

Abstract

The trans octadecenoic acid methyl ester isomers were obtained from a partially hydrogenated soybean oil sample and isolated by silver-ion high-performance liquid chromatography. The double bond configuration was confirmed to be trans by using gas chromatography-direct deposition-Fourier transform infrared spectroscopy. The double bond positions for nine individual trans octadecenoic acid positional isomers were confirmed by gas chromatography-electron ionization mass spectrometry after derivatization to 2-alkenyl-4,4-dimethyloxazoline. These nine trans positional isomers were resolved on either one of the two polar 100% cyanopropylpolysiloxane 100-m capillary columns, SP 2560 and Cp-Sil 88, at an isothermal temperature of 140°C. These nine isomers were confirmed to have double bonds at carbons C-8 through C-16. The pair of trans octadecenoic acid positional isomers with double bonds at C-13 and C-14 are reported for the first time to be resolved by gas chromatography. JAOCS 74, 125-130 (1997).KEY WORDS: 4,4-Dimethyl oxazoline (DMOX), direct deposition, trans double bonds, fatty acid methyl esters (FAME), gas chromatography, infrared spectroscopy, mass spectrometry, monounsaturated fatty acids, positional isomers. FIG. 4. GC-EIMS spectra observed for the ∆8, ∆9, and ∆10 trans 18:1 DMOX positional isomers; a gap of 12 mass units was observed between the pairs of adjacent ions at m/z 182 and 194; 196 and 208; and 210 and 222, respectively; the ions due to allylic cleavage were found at m/z 168, 222, and 236; 182, 236, and 250; and 196, 250, and 264, respectively. See Figure 3 for abbreviations. 130 M.M. MOSSOBA ET AL. JAOCS, Vol. 74, no. 2 (1997) FIG. 6. GC-EIMS spectra for ∆6 and ∆7 trans 18:1 DMOX positional isomer standards. The inset shows the m/z 113 chromatographic trace observed for a co-injected qualitative test mixture of these two standards. See Figure 3 for abbreviations.

Quantitative characterization of unsaturated and trans fatty acids in ewe’s milk fat

“…This spectrum forms the pattern predicted from the work of Zhang et al [31] for trans-8 C18:1 and is nearly identical to that reported for this isomer by Mossoba et al [14], with two prominent ions (m/z 168 and 222) and a gap of 12 atomic mass units between m/z =182-194 to locate the double bond. Spectra from trans-6 and trans-7 C18:1 DMOX exhibit different ion profiles [3,14]. The pattern observed in Figure 1 was similar for the same peak in the five ovine herds studied.…”

Conjugated linoleic acids and octadecenoic acids: Analysis by GC

“…The ∆13t/∆14t pair was finally resolved in 1997 independently by two groups [9,37] by lowering the isothermal temperature to 140°C for the higher molecular dimethyloxazolidine (DMOX) derivatives [37], and to 125°C for the FAME derivatives [9]. Naturally these conditions greatly increased the analysis times to nearly 3 h for the separation for the C18:1 [7,9,37] and 6 h for the C20:1 isomers [7]. Fig.…”

Mass spectrometry of fatty acid derivatives

“…Using the normal high temperature method for forming DMOX derivatives, the C-2 isomer can isomerize to the C-3 isomer and vice versa, but a relatively mild method minimizes isomerizaton [12]. GC-MS of DMOX derivatives was used to characterize the trans monoenoic positional isomers in partially hydrogenated soybean oil [13] and, in combination with GC of methyl esters, the trans and cis monoenes in foods that contained hydrogenated oils and ruminant fats [14].…”