Structure Determination of Polyunsaturated Fatty Acids by Gas Chromatography-Mass Spectrometry--A Comparison of Fragmentation Patterns of Various Derivatives

Abstract

GC-MS has been applied to the characterization of mixtures of polyunsaturated fatty acids by way of their isopropylidene, alkylboronate, methyl ether, and trimethylsilyl ether derivatives. In our studies we found a convenient and efficient method for the oxidation of mixtures of polyunsaturated fatty acid methyl esters by osmium tetroxide. The resulting polyhdroxy compounds are easily converted into apolar derivatives for gas chromatographic separation. The mass spectra of these individual compounds were recorded as to their usefulness for complete structure elucidation. Only the trimethylsilyl ethers proved to yield all information necessary for determination of chain length as well as numbers and positions of double bonds of polyunsaturated fatty acids.

Chemical constituents of Dolichos lablab (field bean) pod exudate

“…The spectral characteristics of the two esters along with the mass spectral data of their TMSO derivatives are summarized in Table IV. Recently there have been many reports of the successful application of TMSO derivatives of unsaturated fatty acid esters using mass spectral analysis to locate the double bond position in the fatty acid chain (Capella and Zorzut, 1968;Dommes et al, 1976;Cavalli et al, 1978). The mass spectra of the TMSO derivatives of both C-12:l and C-14:l acid methyl esters show no molecular ion peak, but an M -CH3 peak due to loss of CH3 radical from the trimethylsilyl group.…”

The determination of double bond positions in polyunsaturated fatty acids–‐Gas chromatography/mass spectrometry of the diethylamide derivative

“…However, with fatty acids containing three or more double bonds the molecular weight and GC temperatures tend to become too high to be useful. Trimethylsilyl ethers can be prepared by oxidation of the double bonds to vicinal diols by osmium tetroxide or peracids, followed by further treatment with a silylating agent (Niehaus and Ryhage, 1968;Dommes et al, 1976;Schmitz and Egge, 1979). The former probably gives the most complete reaction but the toxic nature of osmium tetroxide and the fact that the reaction involves hydrogen sulphide makes it less attractive.…”

Derivatization in Mass Spectrometry—5. Specific Derivatization of Monofunctional Compounds

“…26 Most frequently this reaction is accomplished by oxidation with an alkaline solution of KMnO 4 or with a solution of OsO 4 in pyridine. 27,28 Although such reaction "fixes" the site of the double bond and hydroxyl groups direct EI-induced β-cleavage at the bond between the two carbons bearing hydroxyls, diols are rather polar compounds having poor gas chromatographic properties. In addition, their mass spectra are complicated owing to the ease of dehydration under EI conditions.…”

“…34 The diagnostic power of EI mass spectra of methyl ethers, isopropylidene derivatives, cyclic boronates and silyl ethers of diols and polyols prepared by oxidation of mono-and polyenes has been evaluated. 28 It was shown that the main decomposition pathways of methyl ethers are due to the cleavage of bonds between methoxy groups (1) or β-cleavages (2): All the CH 3 O groups retained in the ions can be step-wise eliminated as CH 3 OH. As a result, the mass spectra look very complex and they can hardly be used to obtain structural information.…”