Identification of cis/trans isomers of methyl ester and oxazoline derivatives of unsaturated fatty acids using GC–FTIR–MS

Abstract

SummaryIdentification of cis/frans isomers of unsaturated fatty acids cannot usually be achieved by GC-MS (gas chromatography-mass spectrometry) without reference substances. In this study a GC-FTIR-MS system (gas chromatography-Fourier transform-mass spectrometry) was used to identify fatty acid methyl esters (FAMEs) and differentiate between the cidtrans isomers. Besides methyl esters, 2-alkenyl-4,4-dimethyloxazoline derivatives (DMOX), which have been used to locate double bond positions of unsaturated fatty acids, were examined with respect to their suitability for cis/trans differentiation.A combined GC-FTIR-MS system with a wide band (4000-550 cm-') mercury cadmium telluride (MCT) detector was used in series and parallel to identify 31 reference unsaturated fatty acids, including 7 pairs of ciskrans isomers. Serum samples of healthy persons and commercially available fish oil were analyzed as examples of complex mixtures. Using splitless injection the detection limit for the less sensitive IR detector was 25 ng/bl in case of the weak cis and trans bands.In the FTlR spectra ciskrans isomers were identified by analysis of bands arising from C-H out-of-plane (oop) bending: for both the FAME and DMOX derivatives cis-l,2-disubstituted double bonds give a strong band near 720 cm-' and the corresponding trans isomers near 967 cm-'. cis Isomers could be identified further by a band at 3012 cm-'. With the combined data of the GC-FTIR-MS system it is now possible to identify polyunsaturated fatty acids with regard to the discrimination of ciskrans isomers.

Determination of trans double bonds in polyunsaturated fatty acid methyl esters from their electron impact mass spectra

“…The fragmentations most relevant for this work are briefly summarised below. EI spectra of saturated FAME are dominated by the ion [CH 2 C(OH)OCH 3 ] +• at m/z 74 [18] caused by McLafferty rearrangement [19]. Losses of neutral aliphatic radicals give rise to a series of ions, [(CH 2 ) n CO 2 CH 3 ] + , where m/z 87 is usually the most abundant [18,20].…”

“…Trans isomers of fatty acids analysed by gas chromatography are usually identified from relative retention times or occasionally by application of infrared detectors (GC-IR) [1][2][3][4][5]. Several reviews have been published on the topic [6][7][8].…”

Mass spectrometry of fatty acid derivatives

“…GC-MS of any derivative has its limitations, most notably in the inability to distinguish between geometrical isomers, and complementary approaches, namely GCFourier transform infrared (FTIR) Spectroscopy, are then required. In this regard, double bond position and geometry can be determined by a combined GC-FTIR-MS method [26], although such instruments are not routinely accessible.…”

Analysis of conjugated and trans fatty acids after derivatization (in French)

“…Ceci n'est pas possible avec les diènes mono-t ra n s ou di-t ra n s n o n conjugués dont les spectres ne présentent qu'une seule bande d ' ab s o rption pour la double liaison t ra n s dans la région 965 -972 cm -1 . Enfin, Wahl et al [52] ont utilisé la chromatographie en phase gazeuse couplée à la spectrométrie de masse et à l'inf ra -ro u ge à tra n s fo rmée de Fo u rier (CPG-SM-IRTF) pour l'identification d'isomères trans d'esters méthyliques et de dérivés DMOX d'acides gras à partir de matrices complexes. Cette méthode permet d'obtenir directement le spectre IR à partir des dérivés DMOX, ce qui pourrait éviter tout risque de confusion lors de l'identification de composés dans une forêt de pics, entre d'un côté les esters méthyliques en IRTF et de l'autre les dérivés DMOX en CPG-SM.…”