Part 3. Dienoic Fatty Acids
Mass Spectra of DMOX Derivatives
Methylene-Interrupted Dienoic Fatty Acids
The electron-impact mass spectra of 4,4-dimethyloxazoline (DMOX) derivatives of dienoic fatty acids, like those of monoenes, tend to be distinctive and permit location of the double bonds, especially when they are present in central positions. For example, that of the DMOX derivative of 9,12-octadecadienoate (linoleate) is illustrated below (Zhang et al., 1988).
Double bonds are located by finding the gaps of 12 amu as described for monoenoic fatty acids (Mass spectra of DMOX derivatives. Part 2. Monoenoic fatty acids). Thus, in this instance they are located between m/z = 196 and 208, and 236 and 248, for double bonds in positions 9 and 12, respectively. The prominent ions at m/z = 222 and 276 are also useful diagnostic guides.
When the double bonds are located near either end of the molecule, recourse to spectra of authentic compounds may be required for definitive identification. Fortunately, even when all double bonds cannot be located with certainty from first principles, each isomer has a distinctive fingerprint for comparison purposes. Few spectra of this type have been published in the literature. However, we have spectra for the DMOX derivatives of all the methylene-interrupted C18 isomers from 4,7- to 14,17-18:2 on file (not published elsewhere), and these are now reproduced below with brief comments.
DMOX of 4,7-octadecadienoate (4,7-18:2) -
In this instance, the spectrum is dominated by an ion at m/z = 152, which may be diagnostic for the double bond in position 4.The double bond in position 7 is clearly located by the gap of 12 amu between m/z = 166 and 178. Thereafter, the ions are 14 amu apart for each successive methylene group.
DMOX of 5,8-octadecadienoate (5,8-18:2) -
The double bond in position 8 is clearly located by the gap of 12 amu between m/z = 180 and 192, while the ion at m/z = 153 is a diagnostic guide for a double bond in position 5, as is the relatively low abundance of ions in the higher mass range.
DMOX of 6,9-octadecadienoate (6,9-18:2) -
In this instance, the gap of 12 amu between m/z = 194 and 206 locates the double bond in position 9, while the fingerprint ions at m/z = 167, 180 and 194 identify that in position 6. The base peak is at m/z = 126 (as opposed to 113), as with 6-18:1
DMOX of 7,10-octadecadienoate (7,10-18:2). In this and most of the remaining isomers, the double bonds are easily located from the gaps of 12 amu, as highlighted on the spectra. Recourse to the spectra of the monoenes simplifies he task. Many of the following spectra are illustrated without further comment.
DMOX of 8,11-octadecadienoate (8,11-18:2) -
DMOX of 9,12-octadecadienoate - see start of this section.
DMOX of 10,13-octadecadienoate (10,13-18:2) -
DMOX of 11,14-octadecadienoate (11,14-18:2) -
DMOX of 12,15-octadecadienoate (12,15-18:2) -
DMOX of 13,16-octadecadienoate (13,16-18:2) -
DMOX of 14,17-octadecadienoate (14,17-18:2) -
Note that now the first double bond in position 14 can be located by the gap of 12 amu between m/z = 266 and 278, but that for the terminal double bond could be confused for one in position 16. However, identification of terminal double bonds appears to be a problem with all nitrogen-containing derivatives, though there are special problems with DMOX derivatives (see Hamilton and Christie, 2000).
We have unpublished mass spectra of the DMOX derivatives of many more dienoic fatty acids of this type with a range of chain-lengths, and these are available in the Archive section of these web pages, but without interpretation.
Conjugated Dienoic Fatty Acids
DMOX derivatives appear to be better than 3-pyridylcarbinol ('picolinyl') esters (or pyrrolidides) for fatty acids with conjugated double bond systems (see Mass spectra of 3-pyridylcarbinol esters. Part 3. Dienoic fatty acids), although MTAD adducts of methyl esters are also valuable (more here ...) as is the use of acetonitrile-chemical-ionization mass spectrometry. DMOX derivatives usually give interpretable spectra, especially when isomers are well resolved on the GC column, although it is always helpful if authentic standards are available for comparison. The spectrum of the DMOX derivative of 9-cis,11-trans-octadecadienoate (9,11-18:2) follows -
The molecular ion is relatively abundant. The positions of the double bonds are given by the gaps of 12 amu between m/z = 196 and 208, and 222 and 234, for the double bonds in positions 9 and 11, respectively. Also, the prominent ions formed by rearrangements involving formation of further conjugated intermediates are diagnostic, i.e. at m/z = 262 and 276. These have been used, indeed, to quantify mixtures of isomers in commercial conjugated linoleic acid preparations, by means of selective ion monitoring. This and the following spectrum were first published by Spitzer et al. (1994).
DMOX of 10-trans,12-cis-octadecadienoate (10,12-18:2) -
In this instance, the diagnostic ions are shifted up by 14 amu in comparison to the previous.
Bis- and Polymethylene-Interrupted Dienoic Fatty Acids
It is becoming apparent that bis- and polymethylene-interrupted dienoic fatty acids are more common in nature than may have been supposed. In particular, fatty acids with a 5,9-double bond system or their chain elongation products are common in seed oils from Gymnosperms or in certain marine invertebrates such as sponges. The spectra of DMOX derivatives of 5,9-18:2 and of some other natural fatty acids encountered during our research are illustrated below. Most of these will not have been illustrated elsewhere.
DMOX of 5,9-octadecadienoate (5,9-18:2) from Pinus contorta seed oil -
Although it is not possible to locate gaps of 12 amu that define the position of the double bonds, this hardly matters as the abundant ion at m/z = 180 for cleavage at the centre of the bis-methylene-interrupted double bond system is sufficiently characteristic (Wolff and Christie, 2002).
DMOX of 7,11-octadecadienoate (7,11-18:2) from the sponge Haliclona cinerea -
In this instance, the key diagnostic ion has shifted upwards as expected to m/z = 208. As the double bonds move further from the carboxyl group additional diagnostic ions become apparent.
DMOX of 9,13-eicosadienoate (9,13-20:2) from the sponge Haliclona cinerea -
The key diagnostic ion has shifted upwards as expected to m/z = 236, and further diagnostic ions for the double bonds locate gaps of 12 amu between m/z = 196 and 208, and 250 and 262 for positions 9 and 13, respectively.
Fatty acids with more than two methylene groups between double bonds are perhaps less common in nature, but some examples of mass spectra of DMOX derivatives of natural fatty acids of this type are illustrated below. With such compounds, the double bonds are located in the same way as for isolated monoenes, and when they are close to the carboxyl group, recourse to comparison with authentic spectra is essential. None of the following have been published elsewhere.
DMOX of 5,11-eicosadienoate (5,11-20:2) from Pinus contorta seed oil -
The double bond in position 5 is recognized by the diagnostic ion at m/z = 153, while that in position 11 is located by the gap of 12 amu between m/z = 222 and 234 (Wolff and Christie, 2002).
DMOX of 5,13-docosadienoate (5,13-22:2) from meadowfoam oil. In this instance, the double bond in position 13 is indicated by small but distinct ions at m/z = 250 and 262.
DMOX of 7,13-docosadienoate (7,13-22:2) from a marine organism (Rapana thomasiana). In this and the next spectrum, the key diagnostic ions are marked.
DMOX of 7,15-docosadienoate (7,15-22:2) is from the same source as the previous spectrum. However, this spectrum was published earlier by Dunstan et al. (1993).
Many of the above spectra have not been formally published elsewhere. We have unpublished mass spectra of the DMOX derivatives of many more bis- and poly-methylene-interrupted dienoic fatty acids with a range of chain-lengths (including some branched-chain isomers), and these are available in the Archive section of these web pages, but without interpretation.
- Dunstan, G.A., Volkman, J.K. and Barrett, S.M. The effect of lyophilization on the solvent extraction of lipid classes, fatty acids and sterols from the oyster Crassostrea gigas. Lipids, 28, 937-944 (1993) (DOI: 10.1007/BF02537504).
- 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) (DOI: 10.1016/S0009-3084(99)00133-4).
- Spitzer, V., Marx, F. and Pfeilsticker, K. Electron impact mass spectra of the oxazoline derivatives of some conjugated diene and triene C18 fatty acids. J. Am. Oil Chem. Soc., 71, 873-876 (1994) (DOI: 10.1007/BF02540465).
- Wolff, R.L. and Christie, W.W. Structures, practical sources (gymnosperm seeds), gas-liquid chromatographic data (equivalent chain lengths), and mass spectrometric characteristics of all-cis 5-olefinic acids. Eur. J. Lipid Sci. Technol., 104, 234-244 (2002) (DOI: 10.1002/1438-9312(200204)104:43.0.CO;2-H).
- 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) (DOI: 10.1002/bms.1200150106).
Updated September 25, 2012