MASS SPECTRA OF ALKYL ESTERS

Alkyl esters occur can occur naturally in animal and plant tissues, and wax esters, for example, constitute an important natural lipid class. In addition, there may be good practical reasons to prepare alkyl esters other than methyl for chromatographic and mass spectrometric analysis (see our web page on alternatives to methyl esters)). Many of the spectra that follow may not have been published (or at least illustrated) formally elsewhere.


Fatty Acid Ethyl Esters

Fatty acid ethyl esters can be found naturally in animal tissues at very low levels in certain circumstances, and in particular they can be a useful marker for excessive alcohol consumption in humans. I have encountered them as artefacts when methylating reagents have been accidentally contaminated with ethanol, and I am aware that this has happened to others. In addition, purified fatty acid preparations intended for pharmaceutical or nutritional applications are often converted to ethyl esters prior to administration or encapsulation. The mass spectrum of ethyl palmitate (Ryhage and Stenhagen, 1959) is –

Mass spectrum of ethyl palmitate

The spectrum resembles that of the methyl ester, except that the McLafferty ion (the base peak – see our webpage on methyl esters of saturated fatty acids) is at m/z = 88 instead of 74. In the high mass range, following the molecular ion, there is an ion at m/z = 255 for loss of the ethyl group, then one at m/z = 241 from a rearrangement reaction involving expulsion of a three-carbon fragment (C2 to C4). An ion at m/z = 239 adjacent to this represents loss of the ethoxide ion ([M−45]+).

Similar features are seen in the spectra that follow as further examples. As with other simple esters, the McLafferty ion becomes less abundant with increasing unsaturation.

Mass spectrum of ethyl oleate -

Mass spectrum of ethyl oleate

In this instance, the ion for loss of the ethyl group (m/z = 282) is much less abundant than that for the loss of the ethoxide moiety (m/z = 264).

Mass spectrum of ethyl linoleate -

Mass spectrum of ethyl linoleate

Mass spectrum of ethyl 5,8,11,14,17-eicosapentaenoate (20:5(n-3) or 'EPA') -

Mass spectrum of ethyl 5,8,11,14,17-eicosapentaenoate

Note that the ion at m/z = 108, typical of fatty acids of the (n-3) family in the spectra of methyl esters, is independent of the nature of the alcohol moiety so is also prominent in this spectrum, but the position of the α-ion at m/z = 194 is dependent on the nature of the alcohol moiety (see our web page on methyl esters of trienes).

Mass spectra of many more ethyl esters are available in our Archive pages, but without interpretation.


n-Propyl, i-Propyl and n-Butyl Esters

Occasionally, it may be necessary to prepare esters other than methyl for specific purposes, and some examples follow. Propan-2-ol esters may have a slight advantage over methyl esters in mass spectrometric and chromatographic terms as regards geometric isomers and highly unsaturated fatty acids (see our webpage on alternatives to methyl esters).

Mass spectrum of n-propyl hexadecanoate (16:0) -

Mass spectrum of n-propyl hexadecanoate

The ion at m/z = 239 ([M-59]+) reflects the loss of a propyloxy ion, while that at m/z = 257 ([M−41]+ or RCO2H2+) is presumably due to the loss of a propenyl moiety. The latter is often the most abundant ion in spectra of saturated long-chain esters. The McLafferty ion is at m/z = 102.

Mass spectrum of n-propyl 9,12-octadecadienoate (linoleate) -

Mass spectrum of n-propyl 9,12-octadecadienoate

As in the spectrum of methyl linoleate, hydrocarbon ions predominate. The ion representing the loss of the propyloxy ion (m/z = 263) is prominent, but in contrast to the previous spectrum that for loss of a propenyl ion (m/z = 279) is not.

Fatty acid esters of iso-propanol (propan-2-ol) have excellent gas chromatographic properties, and for example they elute appreciable before most higher aliphatic esters in general and the analogous n-propyl esters in particular. They have been recommended for the separation of positional and geometrical isomers of mono- and polyenoic fatty acids (see an an article on this theme in our Topics pages).

Mass spectrum of i-propyl hexadecanoate (16:0) -

Mass spectrum of i-propyl hexadecanoate

The spectrum is somewhat different from that of the corresponding n-propyl derivative, though mainly in the relative abundances of ions. The base peak is at m/z = 256 for loss of a propenyl moiety (m/z = 256 ([M−42]+ rather than ([M−41]+).

Similarly, other than the more abundant ion for loss of a propenyl moiety, the mass spectrum of i-propyl 9,12-octadecadienoate (linoleate) differs in minor ways only from that of the n-propyl ester above -

Mass spectrum of i-propyl 9,12-octadecadienoate

On the other hand, the ion for loss of a propyl unit (now at [M−43]+ or m/z = 327) is apparent even in the spectrum of i-propyl docosahexaenoate, so that the molecular weight can at least be determined. The ω-ion at m/z = 108 is apparent, but the α-ion is barely detectable, possibly because this can undergo elimination of propyl and other moieties too readily.

Mass spectrum of i-propyl docosahexaenoate

In comparison, it is often difficult to determine the molecular weight of methyl esters of polyunsaturated fatty acids. As isopropyl esters are reputed to have excellent GC properties, it might be of value to study them further.

Butyl esters are sometimes favoured for the analysis of short-chain fatty acids such as those in milk fat. The mass spectrum of n-butyl hexadecanoate (16:0) is -

Mass spectrum of n-butyl hexadecanoate

By analogy with the previous spectra, the ion at m/z = 239 ([M−73]+) reflects the loss of a butyloxy ion, while that at m/z = 257 ([M−55]+) is presumably due to the loss of a butenyl moiety. The McLafferty ion is at m/z = 116. The butyl ion at m/z = 56 is the base ion.

Similarly, the mass spectrum of n-butyl 9,12-octadecadienoate (linoleate) has comparable features to that of the n-propyl ester above -

Mass spectrum of n-butyl 9,12-octadecadienoate

In this instance, the first significant ion in the high mass region at m/z = 279 ([M−57]+)represents loss of a butyl rather than a butenyl moiety.

Mass spectra from many more n-propyl, i-propyl and n-butyl esters are available in our Archive pages, but without interpretation.


Wax Esters

Wax esters are among the most abundant lipids in nature as part of the wax components that cover every green leaf. In most instances, the fatty acid and alcohol components are saturated or monoenoic, although polyunsaturated fatty acids are found also in waxes of marine origin. The early mechanistic studies of mass spectral fragmentation of was esters were concerned with fully saturated compounds (Aasen et al., 1971; Ryhage and Stenhagen, 1959), but a more comprehensive study is now available (Urbanova et al., 2012).

The mass spectrum of hexadecanyl tetradecanoate (14:0acid-16:0alcohol) is -

Mass spectrum of hexadecanyl tetradecanoate

For an ester of the type RCOOR’, the important diagnostic ions are for RCO2H2+, i.e. derived from the acid component and at m/z = 229 in this instance, for [R’−1]+ (m/z = 224), and for R’OOC+ (m/z = 269). There is no dubiety about the molecular ion (m/z = 452), but all other ions, including the McLafferty rearrangement ion, are small.

For comparison, the mass spectrum of the isomeric tetradecanyl hexadecanoate (16:0acid-14:0alcohol) is –

Mass spectrum of tetradecanyl hexadecanoate

In this example, the important diagnostic ions are for RCO2H2+ (at m/z = 257), for [R’−1]+ (m/z = 196), and for R’OOC+ (m/z = 241). The McLafferty rearrangement ion at m/z = 312 is just discernable.

The mass spectra of unsaturated wax esters are somewhat different in that ions containing the double bond tend to dominate (Spencer, 1979). That of octadecanyl hexadecenoate (16:1acid-18:0alcohol), i.e. with a double bond in the fatty acid component is -

Mass spectrum of octadecanyl hexadecenoate

The base peak at m/z = 236 now represents [RCO−1]+, while the other diagnostic ions for RCO2H2+ (at m/z = 255), for [R’−1]+ (m/z = 252), and for R’OOC+ (m/z =297), together with the molecular ion, are much smaller than in fully saturated wax esters.

For comparison, the mass spectrum of the isomeric octadecenyl hexadecanoate (16:0acid-18:1alcohol), i.e. with the same molecular weight but with a double bond in the fatty alcohol component is –

Mass spectrum of octadecenyl hexadecanoate

Now the base peak is the [R’−1]+ ion at m/z = 250, i.e. derived from the alcohol component, while other useful ions are for RCO2H2+ (m/z = 257), RCO+ (m/z = 239), and R’OOC+ (m/z =295).

Mass spectra of more wax esters are available in our Archive pages.


References



W. Christie

James Hutton Institute (and Mylnefield Lipid Analysis), Invergowrie, Dundee (DD2 5DA), Scotland.

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AOCS Credits/disclaimer Updated: January 14th, 2013 Author