Part 3. Monoenoic Fatty Acids

As cautioned in the 'Introduction' to these documents, the mass spectra of methyl esters obtained with electron-impact ionization often afford limited information only concerning the structures of fatty acids. The molecular weight is usually obtainable, and this is an important piece of information. If chromatographic retention data are added to this, it is often possible to be 90% certain of the identity of a fatty acid. Unfortunately, mass spectra of methyl esters of most monoenoic fatty acids contain no information that helps to locate the position or geometry of double bonds. While there have been suggestions that such information can be obtained from close examination of certain minor peaks in the spectrum, I am doubtful of the value of such techniques with real samples, as small changes in instrumental parameters could mask such effects. However, chemical ionization with acetonitrile as the reagent gas can enable both double bond position and geometry to be determined as described here...

 

Normal Monoenoic Fatty Acids

The mass spectra of methyl esters of monoenoic fatty acids under electron-impact ionization tend to be rather uninformative. However, they do allow the molecular weight to be determined and this combined with GC retention data can be of practical value. The introduction of a double bond changes the spectrum appreciably from that of the corresponding saturated ester.

The mass spectrum of methyl oleate (cis-9-octadecenoate) is illustrated first (to my knowledge it was first published by Hallgren et al., 1959) -

Mass spectrum of methyl oleate 

The molecular ion (m/z = 296) is clearly seen, and ions representing loss of the elements of methanol (m/z = 264 or [M-32]+), i.e. a methoxyl group plus a hydrogen atom, and the loss of the McLafferty ion (m/z = 222) are abundant, as is the McLafferty ion per se (m/z = 74) (see the webpage on mass spectra of methyl esters of saturated acids). A characteristic ion at [M-116]+ (m/z = 180 in this instance), together with homologous ions at 166, 152, etc, are also diagnostic. The first of these ions is formed by loss of a fragment containing the carboxyl group by cleavage between carbons 5 and 6 with addition of a rearranged hydrogen atom. Thus, in contrast to the spectra of methyl esters of saturated fatty acids, hydrocarbon ions (general formula [CnH2n-1]+) dominate the spectrum, with m/z = 55 as the base peak usually. The relative abundances of all of these ions tends to be appreciably greater than in the mass spectra of dienes and polyenes. However, there is no feature that permits location of the double bond, because this can migrate to any position when the alkyl chain is ionized in the mass spectrometer. Thus, nearly all the cis- and trans-18:1 isomers have virtually identical spectra.

The exception is the spectrum of methyl 2-octadecenoate (below) in which the double bond and carboxyl group presumably form a relatively stable resonance structure. Amongst others there is a distinctive ion at m/z = 113, which is believed to occur via a cleavage between carbons 5 and 6 followed by formation of a stable 6-membered ring with the carboxyl group (>Ryhage et al., 1961)).

Mass spectrum of methyl 2-octadecenoate 

This particular fatty acid does not occur naturally to my knowledge, but it can be formed artefactually by over vigorous trans-esterification of the 3-isomer (trans-3-16:1 is an important component of photosynthetic tissues in plants, for example). However, some other natural fatty acids with α,β-unsaturation are known.

As further examples, the mass spectrum of methyl 5-eicosenoate (from meadowfoam oil) is -

Mass spectrum of methyl 5-eicosenoate

- and that of methyl 13-docosenoate (also from meadowfoam oil) is -

Mass spectrum of methyl 13-docosenoate

Both of these show the same features as in the spectrum of methyl oleate, especially ions representing [M-32]+, [M-74]+ and [M-116]+.

The mass spectrum of the shorter-chain methyl 9-dodecenoate (from milk fat) is –

Mass spectrum of methyl 9-dodecenoate

Analogous ions to those in the previous spectra are present, although the relative intensities are somewhat different as might be expected.

Note that the presence of an alicyclic ring reduces the molecular weight of a fatty acid by 2 amu in comparison to the corresponding straight-chain saturated fatty acid, i.e. the same amount as a double bond. Methyl esters of cyclopropyl fatty acids have mass spectra that are virtually identical to those of monoenes with the same total number of carbon atoms (see the web-page on 'Mass spectra of cyclic fatty acids'). Where there is doubt, GC retention data are an important aid to identification.

 

Branched-Chain Monoenoic Fatty Acids

Unsaturated branched-chain fatty acids are often encountered as minor components of marine lipid samples. The mass spectrum of methyl 15-methyl-hexadec-9-enoate (from a sponge) is -

Mass spectrum of methyl 15-methyl-hexadec-9-enoate

The ions at m/z = 235 ([M-47]+) and 227 ([M-55]+) may be related to the presence of the iso-methyl group, but there is presumably nothing to assist location of the double bond (although the origin of the ions at m/z = 177 and 195 is intriguing). Apart from these last two ions, the spectrum is typical of that of most other monoenes.

The mass spectrum of methyl 7-methyl-hexadec-6-enoate (from a jellyfish in this instance, but often present in fish from warm seas) is -

Mass spectrum of methyl 7-methyl-hexadec-7-enoate

Unlike other monoenes, ions representing [M-32]+, [M-74]+ and [M-116]+ are small. It is tempting to suggest that the ions at m/z = 115 and 167 result from the simple fragmentations between carbons 5 and 6 as illustrated, while those at m/z = 138 and 151 result from a rearrangement following a fragmentation of the hydrocarbon portion of the molecule. Evidence supporting this interpretation comes from the spectrum of the ethyl ester, where the ions at m/z = 138, 151 and 167 remain, but that at m/z = 115 is replaced by one at m/z = 129. Subsequent to publication here, this spectrum was illustrated in a paper by Fardin-Kia, A.R et al. (2013), together with that of methyl 7-methyl-octadec-6-enoate. The latter contains ions that support the above interpretations.

The first of these spectra may not have been illustrated elsewhere. We also have the mass spectrum of the isoprenoid methyl 3,7,11,15-tetramethylhexadec-trans-2-enoate (phytenate) on file in the Archive Section.

Dimethyl disulfide adducts: To get round the problem of location of double bonds, it is possible to prepare specific derivatives of unsaturated fatty acids that 'fix' the double bond. Very many have been described, but the only one to have stood the test of time is the dimethyl disulfide adduct, as this has excellent mass spectrometric properties and is prepared in a simple one-pot reaction (see the section on 'Mass spectra of methyl esters of fatty acids - further derivatization'). Alternatively, 3-pyridinylcarbinol (‘picolinyl’) esters or DMOX or pyrrolidine derivatives can be utilized to locate double bonds, as described in some detail elsewhere in these web pages.

Spectra of many more methyl esters of monoenoic fatty acids can be accessed from our Archive pages (without interpretation).

 

References

  • Fardin-Kia, A.R., Delmonte, P., Kramer, J.K.G., Jahreis, G., Kuhnt, K., Santercole, V. and Rader, J.I. Separation of the fatty acids in menhaden oil as methyl esters with a highly polar ionic liquid gas chromatographic column and identification by time of flight mass spectrometry. Lipids, 48, 1279-1295 (2013) (DOI: 10.1007/s11745-013-3830-2).
  • Hallgren, B., Ryhage, R. and Stenhagen, E. The mass spectra of methyl oleate, methyl linoleate and methyl linolenate. Acta Chem. Scand., 13, 845-847 (1959).
  • Ryhage, R., Ställberg-Stenhagen, S. and Stenhagen, E. Methyl esters of α,β-unsaturated long-chain acids. On the structure of C27-phthienoic acid. Arkiv. Kemi, 18, 179-186 (1961).

 Updated March 11, 2014

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