The Lipid Library

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 plant cuticles. For example, they are major components of cork (Quercus suber). They are often encountered in the oxidative degradation of lipids. Commercial standards were used to obtain the spectra that follow.

1.  Methyl Esters

Dimethyl esters of saturated dicarboxylic acids have characteristic mass spectra (Ryhage and Stenhagen, 1959; McCloskey, 1970). These show a characteristic 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.

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.

There are more spectra of dimethyl esters of short-chain dicarboxylic acids in our Archive section.

2.  Picolinl 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-picolinyl 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 picolinyl 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) discussed the mass spectrum of di-picolinyl 1,12-dodecanedioate.

Because of having two nitrogen atoms, the molecular ion is now even numbered (as are 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-picolinyl esters of short-chain dicarboxylic acids in our Archive section (but without interpretation).

3.  Pyrrolidides

To my knowledge, mass spectra of di-pyrrolidides of dicarboxylic acids have not been published to date. We found that pyrrolidides of short-chain dicarboxylic acids were difficult to obtain by our usual method, as they appeared to be too polar for extraction from the reaction medium. No doubt extraction conditions could be devised to solve the problem if required. Because of the increase in polarity, relatively high GC temperatures are required, although non-polar phases can be used. However, we were able to obtain the mass spectrum 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.

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). Presumably, these represent cleavage of the pyrrolidine ring between the nitrogen atom and the carboxyl carbon. The first of these ions is usually found at low abundance in the spectra of mono-carboxylic acids, but not the second.

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. 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 and the size of the rings is the same despite the difference in structures). DMOX derivatives were easy to prepare and to subject to gas chromatography, but their mass spectra are puzzling. For example, the mass spectrum of the di-DMOX derivative of 1,9-nonadecadienoate 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 might be expected from 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 71 and 87 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.

Similar features are found in the spectra of the DMOX derivatives of all the other dicarboxylic acids in our Archive pages, and in that of the di-DMOX derivative of 1,8-octadecenedioates, described by Luthria and Sprecher (1993).

References

• 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).
• 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).
• 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).
• 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: 23/4/2008 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland