MASS SPECTRA OF DMOX DERIVATIVES
Part 2. Monoenoic Fatty Acids
Straight-Chain Monoenoic Fatty Acids
The mass spectra of DMOX derivatives of monoenoic fatty acids tend to be distinctive and permit facile location of the double bond, especially when the double bond is located centrally. When it is near either end of the molecule, interpretation of spectra can be more difficult, especially as until recently spectra of relatively few of the possible isomers had been published. However, we have access to all the positional isomers of octadecenoic acid (2- to 17-18:1), and details have now been published (Christie et al., 2000). To begin, the spectrum of the most common natural monoene, the DMOX derivative of octadec-9-enoate (oleate) is illustrated -
DMOX derivatives of unsaturated fatty acids tend to give more abundant ions in the higher mass range in comparison to saturated. Zhang et al. (1988)) formulated an empirical rule:
"If a mass separation of 12 instead of the regular 14 amu is observed between two neighbouring even-mass homologous fragments containing n-1 and n carbon atoms of the original acid moiety, a double bond exists between carbons n and n+1 in the chain".
In this instance, the gap of 12 amu between m/z = 196 and 208 locates the double bond in position 9. It should be noted that this rule was first formulated for pyrrolidides, which have very similar mass spectra (see the webpage dealing with pyrrolidides of monoenoic fatty acids).
The two prominent ions at m/z = 236 and 250 are formed in rearrangements that give stable conjugated double bond systems, and these and the analogous ions for other isomers can be useful diagnostic indicators when fatty acids are not fully resolved by gas chromatography or when the background noise level is high.
However, as more information has become available, it has become apparent that the above rule only applies from about position 6 onwards, but is only really clear for the 8-18:1 to 15-18:1 isomers. Nonetheless, when the double bond is closer to the carboxyl group (or rather the heterocyclic ring), each isomer gives distinctive fragmentation patterns or fingerprints so compounds can be identified by comparison with authentic spectra.
DMOX of 2-octadecenoate (2-18:1) -
Note that this spectrum differs appreciably from that published by Lamberto and Ackman (1995)), and it now seems certain that they misidentified the spectrum of the DMOX derivatives of 2-octadecenoate as the 3-isomer and vice versa (see - Christie et al. (2000) - for a full explanation including a mechanistic discussion). Note that the [M-15]+ ion (m/z = 320) is the base peak, while the usual ions at m/z = 113 and 126 are inconspicuous. Similarly, a reported spectrum for the DMOX derivative of 3-decenoate is probably that of 2-decenoate having isomerized during derivatization (Luthria and Sprecher, 1993).
DMOX of 3-octadecenoate (3-18:1). The ions at m/z = 152 (the base peak) and 166 in particular are key components of the distinctive fingerprint. The mechanism for their formation is discussed in the paper by this author and colleagues cited above.
DMOX of 4-octadecenoate (4-18:1) -
This spectrum is superficially similar to the previous with the abundant ions at m/z = 152 and 166 (the base peak) being especially distinctive. The ion at m/z = 139 is less abundant than with the 3-isomer.
DMOX of 5-octadecenoate (5-18:1) -
Here the base peak is the McLafferty ion at m/z = 113, but the odd-numbered ion at m/z = 153 is a useful diagnostic guide. With this and DMOX derivatives of other fatty acids with an isolated double bond in position 5, the ions in the higher mass range are or low abundance relative to the base peak at m/z = 113.
DMOX of 6-octadecenoate (6-18:1) (Zhang et al., 1988) -
Here the base peak is at m/z = 126 (as opposed to 113), and the odd-numbered ion at m/z = 167 (or the triplet at 167, 180 and 194) is a further useful diagnostic guide.
DMOX of 7-octadecenoate (7-18:1)-
It requires some imagination to see the gap of 12 amu for the double bond in position 7 between m/z = 168 and 182, but the fingerprint spectrum is distinctive. However, the diagnostic gap of 12 amu is as expected in the following isomers until the double bonds are near the terminal end of the molecule.
DMOX of 8-octadecenoate (8-18:1) -
DMOX of 9-octadecenoate (9-18:1) - see start of this section. Many of the following spectra are illustrated without further comment.
DMOX of 10-octadecenoate (10-18:1) -
DMOX of 11-octadecenoate (11-18:1) (see also Zhang et al. (1988))-
DMOX of 12-octadecenoate (12-18:1) -
DMOX of 13-octadecenoate (13-18:1) (see also Zhang et al. (1988)) -
DMOX of 14-octadecenoate (14-18:1) -
Note that although the ions for cleavage of the double bond (gap of 12 amu) are becoming less distinct, the two prominent ions at m/z = 306 and 320 in this example, formed in rearrangements that give stable conjugated double bond systems, remain useful diagnostic indicators.
DMOX of 15-octadecenoate (15-18:1) -
DMOX of 16-octadecenoate (16-18:1) -
In this instance, although the rule to locate the gap of 12 amu may still operate, it is disguised by other fragmentations, and it would be easy to interpret this spectrum incorrectly as that of the 15-isomer.
DMOX of 17-octadecenoate (17-18:1) -
The [M-15]+ ion (m/z = 320), representing loss of a methyl group from the heterocyclic ring (Hamilton and Christie, 2000), is the most abundant ion in the higher mass range adding confusion to the ions that might locate the double bond. However, both this and the previous isomer have distinctive fingerprint spectra if standard spectra are consulted.
We have unpublished mass spectra of the DMOX derivatives of many more monoenoic fatty acids with a range of chain-lengths, and they are available in the Archive section of these web pages, but without interpretation.
Branched-Chain Monoenoic Fatty Acids
We have mass spectra of the DMOX derivatives of two methyl-branched monoenoic fatty acids, starting with that of 15-methyl-hexadec-9-enoate, detected in a sponge -
The double bond position is determined by the interval of 12 amu between m/z = 196 and 208, while the branch point is revealed by the gap of 28 amu between m/z = 278 and 306 for the loss of carbon 15 and its methyl group. It is noteworthy that an iso-methyl group is not easily distinguished with saturated analogues in this way with DMOX derivatives.
The DMOX derivative of 7-methyl-hexadec-6-enoate -
We would not have been confident of the identification of this fatty acid from tropical marine sources from its mass spectrum, if we had not had complementary information.
References
- Christie, W.W., Robertson, G.W., McRoberts, W.C. and Hamilton, J.T.G. Mass spectrometry of the 4,4-dimethyloxazoline derivatives of isomeric octadecenoates (monoenes). Eur. J. Lipid Sci. Technol., 102, 23-29 (2000).
- Hamilton, J.T.G. and Christie, W.W. Mechanisms for ion formation during the electron impact-mass spectrometry of picolinyl ester and 4,4-dimethyloxazoline derivatives of fatty acids. Chem. Phys. Lipids, 105, 93-104 (2000).
- Lamberto, M. and Ackman, R.G. Positional isomerization of trans-3-hexadecenoic acid employing 2-amino-2-methylpropanol as a derivatizing agent for double bond location by GC/MS. Anal. Biochem., 230, 224-228 (1995).
- Luthria, D.L. and Sprecher, H. 2-Alkenyl-4,4-dimethyloxazolines as derivatives for the structural elucidation of isomeric unsaturated fatty acids. Lipids, 28, 561-564 (1993).
- Zhang, J.Y., Yu, Q.T., Liu, B.N. and Huang, Z.H. Chemical modification in mass spectrometry IV. 2-Alkenyl-4,4-dimethyloxazolines as derivatives for double bond location of long-chain olefinic acids. Biomed. Environ. Mass Spectrom., 15, 33-44 (1988).
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Updated: 6/5/2008 |
Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland
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