Mass Spectrometry of Methyl Esters
Halogenated Fatty Acids
As with my other documents on mass spectrometry, this is a subjective account that details only those fatty acids relevant to this topic that have been encountered during our research activities and for which we have spectra available for illustration purposes. Regretfully, too few spectra were available for a comprehensive account. Spectra of methyl esters are described here, but those of 3‑pyridylcarbinol esters and DMOX derivatives with pyrrolidides are discussed elsewhere on this site. Key diagnostic ions only are described as general features of the spectra of methyl esters are described in the earlier documents in this series. It is doubtful whether any of these spectra have been published elsewhere.
Fluoro fatty acids were found to be perfectly stable and were easily derivatized, but chloro- and bromo-fatty acids can lose the halogen atom readily unless care is taken during derivatization.
Fluoro Fatty Acids
One natural seed oil contains several fatty acids with a fluorine atom on the terminal carbon, i.e., Dichapetalum toxicarum, and the first two mass spectra illustrated below were from this source (Christie et al., 1998). We also have some mass spectra from several synthetic fluoro-fatty acids kindly donated by Dr David O'Hagan of St Andrews University, and they provided important evidence for some mechanistic concepts, although the main aim of these pages is simply to act as a practical guide.
Methyl esters give distinctive spectra, but information on more isomers would be required to give definitive interpretations. The mass spectrum of methyl 18‑fluoro-octadecanoate is -
In the high mass range, there is an ion at m/z = 296, which presumably represents loss of HF and ions for loss of the methanol moiety from the carboxyl group (m/z = 285), and for the McLafferty ion (m/z = 74) can be recognized. Without more information, there is no feature that locates the fluorine atom on its specific position, and further comment would merely be speculation. The spectrum is appreciably different from that of methyl stearate.
Similarly, in the mass spectrum of methyl 18-fluoro-octadec-9-enoate, there is an ion at m/z = 294, which presumably represents loss of HF, together with the expected ions for the loss of methanol ([M−31/32]+, m/z = 283 and [M−HF−32]+ at m/z = 262).
The following two spectra were published and discussed by Hamilton and Christie (2000). Mass spectrum of methyl 2-fluoro-octadecanoate (synthetic standard) -
The base peak is the McLafferty ion at m/z = 92, which confirms that the fluorine atom is on position 2. Abundant ions at m/z = 147 and 161 indicate preferential cleavages between C-6 and C-7 and C-7 and C-8, respectively. Furthermore, losses of 47 (m/z = 269) and 61 (m/z = 255) amu from the molecular ion indicate the expulsion of fluorinated ethyl and propyl radicals, while other losses of 75, 89, 103, and 117 amu at m/z = 241, 227, 213 and 199, respectively, represent the expulsion of 4-, 5-, 6- and 7-carbon fragments that must contain a CHF group together with an additional hydrogen atom.
A study of the spectrum of methyl 2,2-difluoro-octadecanoate adds clarity to some of the interpretations, but there are some distinctive features.
The ions at m/z = 165 and 179 presumably now contain two fluorine atoms by analogy with the ions at m/z = 147 and 161 in the previous spectrum, while the ions at m/z = 193, 207, 221 and 235 form a continuance of this homologous series. The series of ions at m/z = 199, 213, 227, 241 and 255 remains unchanged. The McLafferty ion (m/z = 110) is small and an expected ion at m/z = 123 is missing (equivalent to that at m/z = 87 in the non-fluorinated analogue).
Mass spectra of further fluoro fatty acids are available on our Archive page, but without interpretation.
Bromo Fatty Acids
Brominated fatty acids have been found in many marine organisms, but we do not have access to these, although we were able to obtain synthetic 2‑bromo-hexadecanoic acid. The mass spectrum of methyl 2‑bromo-hexadecanoate is -
The spectrum is complex, but the expected double molecular ion at m/z = 348 and 350 can be seen (as a reminder, bromine exists as two isotopes in approximately equal amounts of atomic mass 79 and 81). The key diagnostic feature is the fact that the McLafferty ion forms a doublet of ions at m/z = 152 and 154 instead of at m/z = 74. The ion at m/z = 269 is for the loss of the bromine atom.
In this instance, the methyl ester was prepared by reacting the free acid with trimethylsilyl-diazomethane. Our first attempts to prepare derivatives of bromo fatty acids (produced by microbial fermentation of bromo-hydrocarbons) failed when our transesterification and hydrolysis procedures caused elimination of bromine with formation of ethoxy or methoxy fatty acids (see the web page on mass spectra of pyrrolidide derivatives of epoxy, furanoid and alkoxy fatty acids).
References
- Christie, W.W., Hamilton, J.T.G. and Harper, D.B. Mass spectrometry of fluorinated fatty acids in the seed oil of Dichapetalum toxicarium. Chem. Phys. Lipids, 97, 41-47 (1998); DOI.
- 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.
| © Author: William W. Christie |  |
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