Mass Spectra of Derivatives of Dicarboxylic Fatty Acids
Dicarboxylic (dibasic) fatty acids are not often found as such in nature, although they are occasionally found in plants and especially in the suberin in plant cuticles. For example, they are major components of cork (Quercus suber). Short-chain dicarboxylic acids may be encountered in the oxidative degradation of lipids. Commercial standards were used to obtain most of the spectra that follow, though some were obtained from cutin hydrolysates. Few if any of these have been published elsewhere.
1. Methyl Esters
Dimethyl esters of saturated dicarboxylic acids have characteristic mass spectra (Ryhage and Stenhagen, 1959; McCloskey, 1970). These show a distinctive pattern in the high mass range, which typically consists of a small molecular ion peak, a significant ion equivalent to m/z = [M−31]+, and others representing m/z = [M−64]+, [M−73]+, [M−92]+, [M−105]+ and [M−123]+.
|[M−31]+ = loss of CH3O|
|[M−64]+ = loss of 2 x CH3O|
|[M−73]+ = loss of CH3OCOCH2 (McLafferty ion)|
|[M−92]+ = loss of CH3OCO + CH3O + 2H|
|[M−105]+ = loss of CH3OCOCH2 + CH3O + H|
|[M−123]+ = loss of CH3OCOCH2 + CH3O + H2O + H|
A second series of abundant ions is found at m/z = 84 + 14n, which is not present in the mass spectra of methyl esters of equivalent monobasic acids. These are believed to be cyclic enols formed by rearrangement processes. The relative abundances of these two series appear to depend on chain-length. The McLafferty ion at m/z = 74 is usually prominent, but the series [CH3OOC(CH2)n]+ tends to peter out quickly as the chain length increases.
For example, the mass spectrum of the dimethyl 1,9-nonanedioate (dimethyl azeleate) is -
The most abundant ions are in the high mass range, i.e. m/z = 185 ([M−31]+), 152 ([M−64]+), 143 ([M−73]+), 124 ([M−92]+), and 111 ([M−105]+). Ions at m/z = 83, 97 and 111 (in part) are presumably those expected of the m/z = 83/4 + 14n series. Ions equivalent to [M−60]+ (m/z = 156 in this instance for loss of CH3OCO + H) seem to be characteristic of shorter-chain dibasic esters.
The mass spectrum of dimethyl 1,18-octadecanedioate is –
Here, the ions of the m/z = 84 + 14n series are most abundant with the base peak at m/z = 98. The ions representing [M−64]+, [M−73]+, [M−92]+, [M−105]+ and [M−123]+ are all easily recognized. The molecular ion is not seen in this example without magnification of the appropriate region of the spectrum.
Unsaturated dibasic acids are not often encountered in nature, but the mass spectrum of dimethyl 1,18-octadec-9-enedioate is appreciably different from those of saturated esters as might be expected –
In this instance, there is a distinct molecular ion and ions at m/z = 309 ([M−31]+), 276 ([M−64]+), and 248 ([M−92]+), equivalents of which were present in the spectrum of the analogous saturated fatty acid, though very different in magnitude. The McLafferty ion (m/z = 74) is still abundant, but ions for the series m/z = 84 + 14n tend to be less abundant than those of the m/z = 81 + 14n series.
There are more spectra of dimethyl esters of dicarboxylic acids in our Archive section.
2. 3-Pyridylcarbinol (‘picolinyl’) Esters
In contrast to methyl esters, the mass spectra of picolinyl esters have good molecular ions, although the rest of the spectra are relatively uninteresting, since they are dominated by ions related to the pyridine ring. The mass spectrum of di-(3-pyridylcarbinyl) 1,9-nonanedioate (azeleate) is -
Thus, in the low molecular mass range, the main ions are at m/z = 92, 108, 151 and 164 (see the web page on 3-pyridylcarbinol esters of saturated fatty acids for interpretation). At higher masses, the ion representing [M−92]+, [M−150/1]+, and [M−164]+ are most abundant. Harvey (1984) has discussed the mass spectrum of di-(3-pyridylcarbinyl) 1,12-dodecanedioate.
Because of having two nitrogen atoms, the molecular ion is now even numbered (as are those with all the widely used nitrogen-containing derivatives). However, the additional molecular weight and polarity increases the difficulties for GC-MS in that relatively high column temperatures are required. There are more spectra of di-(3-pyridylcarbinol) esters of short-chain saturated dicarboxylic acids in our Archive section (but without interpretation).
To my knowledge, mass spectra of di-pyrrolidides of dicarboxylic acids have not been published to date. Initially we found that pyrrolidides of dicarboxylic acids were difficult to obtain by our usual method (especially those of shorter chain-length), as they appeared to be too polar for extraction from the reaction medium. However, by using diethyl ether alone or diethyl ether-chloroform mixtures as extractant solvents, we obtained good yields of products. Because of the increase in polarity, relatively high GC column temperatures are required, and analysis is easier if non-polar phases can be used.
With the modified derivatization procedure, we were able to obtain the mass spectra of pyrrolidides of a comprehensive series of dibasic acids, including that of the di-pyrrolidide of 1,9-nonanedioic (azelaic) acid illustrated next.
There is a respectable molecular ion (even-numbered at m/z = 294), but the spectrum is dominated by ions associated with the pyrrolidine ring. For example, the base ion at m/z = 113, equivalent to the McLafferty ion, tends to be the most abundant ion in the spectra of most pyrrolidides. It is accompanied by the expected ions at m/z = 98 and 126. Our web page on the mass spectrometry of pyrrolidides of normal saturated fatty acids has a more detailed explanation for these ions.
In addition in the higher mass range, there are ions for loss of 97/8 and 112/3 from the molecular ion at m/z = 197 and 182, respectively. More of a surprise was to find a significant ion at m/z = 70 and one representing [M−70]+ (m/z = 224), which may represent ions consisting of the pyrrolidine ring and loss of this, respectively. On the other hand, , ions in similar positions are present in spectra of DMOX derivatives, where the origin must be somewhat different. Both of these ions are usually present in the spectra of pyrrolidides of mono-carboxylic acids, but with lower relative abundance.
The mass spectrum of the di-pyrrolidide of 1,18-octadecanedioate shows essentially the same features, although the ion at m/z = 308, equivalent to [M−112]+, is now the base ion.
The intermediate ions (m/z = 140 to 294) represent radical induced cleavage at each successive methylene group as in the spectra of more conventional pyrrolidides.
The mass spectrum of the dipyrrolidide derivative of 1,18-octadec-9-enedioate is not too dissimilar to the last, but it does have the gap of 12 amu between m/z = 196 and 208 expected for a double bond in position 9 (see our web-page on pyrrolidides of monoenes).
More spectra are available on our Archive page.
4. 4,4-Dimethyloxazoline (DMOX) Derivatives
Di-DMOX derivatives of dicarboxylic acids would be expected to have spectra very similar to pyrrolidides, as is the case with normal mono-carboxylic acids (by coincidence, DMOX derivatives and pyrrolidides have the same molecular weight despite the difference in structures, and the mass spectrometric fragmentation mechanisms are very similar). DMOX derivatives were easy to prepare and to subject to gas chromatography, but some aspects of their mass spectra are puzzling. For example, the mass spectrum of the di-DMOX derivative of 1,9-nonanedioate is -
The molecular ion is protonated and so is odd-numbered. The base peak at m/z = 182, equivalent to [M−112]+, represents the loss of the McLafferty ion as in the previous spectrum. However, there is no ion for [M−97]+ as in the spectrum of the analogous pyrrolidide.
The ion at m/z = 279 represents the loss of a methyl group from the ring structures, and can be a source of confusion when fatty acids have terminal functional groups (Hamilton and Christie, 2000). Ions at m/z = 207 and 223, representing losses of 87 and 71 amu from the molecular ion, respectively, are not normally found in the spectra of DMOX derivatives, and they must be formed by complex rearrangements involving the ring structures, which have yet to be explained.
Similar features are found in the spectra of the DMOX derivatives of all the other saturated dicarboxylic acids in our Archive pages, as illustrated for the di-DMOX derivative of 1,18-octadecanedioate –
The same is true for the the di-DMOX derivatives of some 1,8-octadecenedioates, (described by Luthria and Sprecher (1993)), although these also have features that permit the location of the double bonds. DMOX derivatives of longer-chain monoenoic dibasic acids have additional unusual features, and the spectrum of the di-DMOX derivative of 1,18-octadec-9-enedioate is -
The ion at [M-112]+ at m/z = 306 in this instance, representing the loss of the McLafferty ion, is still very prominent. The gap of 12 amu between m/z = 196 and 208 is expected for a double bond in position 9 (see our web-page on DMOX derivatives of monoenes). However, the ions at m/z = 347, 361 and 375, equivalent to [M−71]+, [M−57]+, and [M−43]+, respectively, appear to be derived from rearrangements involving expulsion of hydrocarbon fragments from the chain, and that at [M-87]+ remains a puzzle.
The mass spectrum of the di-DMOX derivative of 1,20-eicos-9-enedioate (a natural constituent of cork) has one interesting additional feature. The double bond is in position 9 relative to one of the carboxyl groups and position 11 relative to the other, so it would be expected to have diagnostic ions at m/z = 196 and 208 for the former, and at m/z = 224 and 236 for the latter. These ions are in fact present but largely cancel each other out and a gap of 12 amu appears between m/z = 210 and 222.
The mass spectrum of the di-DMOX derivative of 1,18-octadeca-6,9-dienedioic acid -
This fatty acid has presumably been formed by omega oxidation of linoleic acid. The double bond in position 6 is defined by characteristic ions at m/z = 166 and 180, while that in position 9 by the gap of 12 amu between ions at m/z = 194 and 206 (see our web page on DMOX derivatives of dienes). The ion representing the loss of the McLafferty ion ([M-112]+) is now at m/z = 304. The ions at m/z = 234 and 236 are of interest, but I will leave speculation on their identity or origin to others.
- 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).
- Harvey, D.J. Picolinyl derivatives for the structural determination of fatty acids by mass spectrometry. Applications to polyenoic acids, hydroxy acids, di-acids and related compounds. Biomed. Mass Spectrom., 11, 340-347 (1984) (DOI: 10.1002/bms.1200110705).
- 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) (DOI: 10.1007/BF02536089).
- McCloskey, J.A. Mass spectrometry of fatty acid derivatives. In: Topics in Lipid Chemistry. Volume 1, pp. 369-440 (ed. F.D. Gunstone, Logos Press, London) (1970).
- Ryhage, R. and Stenhagen, E. Mass spectrometric studies. III. Esters of saturated dibasic acids. Arkiv Kemi, 4, 497- 509 (1959).
Updated February 7, 2014