Part 2. Derivatives Other than Nicotinates
While most structural information is obtainable from fatty alcohols as the nicotinate derivatives, there are many circumstances when this is not practicable or necessary. When samples are already well characterized, it is simpler to analyse them in the underivatized state or as trimethylsilyl (TMS) ethers or as acetates. For example, waxes contain many different types of aliphatic constituents, and the best means of characterizing the complex mixtures can often be to hydrolyse then silylate for analysis by GC-MS. Fatty alcohols in such samples tend to have relatively simple compositions, being mainly saturated or monoenoic, with occasional iso- and anteiso-methyl branches, so sufficient information may be obtainable by this means to identify a high proportion with reasonable certainty. In addition of the spectra of free and derivatized aliphatic alcohols, those of glycerol ether derivatives are described below for convenience. Access to GC retention data will aid identification greatly. As with other documents on this website, only spectra encountered in our own research work can be illustrated.
No references are cited below, as establishing priority from the early literature is now difficult. Many of the mass spectra that follow will not have been published formally elsewhere. I am not aware of a suitable review article.
Underivatized Fatty Alcohols
Mass spectra of underivatized primary fatty alcohols contain little structural information. As an example, the mass spectrum of hexadecan-1-ol is -
The molecular ion is rarely seen without substantial magnification, and the first significant if small ion in the high mass region is that representing the loss of the elements of water ([M-18]+), in this instance at m/z = 224. The remaining ions are hydrocarbon fragments.
Essentially the same features are seen in the mass spectrum of eicosan-1-ol next -
The mass spectrum of 15-methyl-hexadecan-1-ol, i.e. with an iso-methyl group, is similarly undistinguished, although an ion at m/z = 223, which may represent loss of one of the terminal methyl groups following the loss of water could be diagnostic. However, it would be necessary to have access to spectra of more isomers to be sure of this.
The mass spectra of monoenoic primary alcohols are very similar, except that many of the main ions are 2 amu lower than in the corresponding saturated alcohol. This can be seen from the spectrum of hexadec-9-en-1-ol next -
Further spectra are available in the Archive section of this site, but without interpretation.
Acetate Derivatives of Alcohols
As the molecular ion is absent for all practical purposes, the first significant ion in the high mass region of the mass spectrum of an acetate derivative is that for loss of acetic acid ([M-60]+). In the spectrum of 1-tetracosanyl acetate, illustrated below, this is at m/z = 336. A distinctive ion at m/z = 61 in the spectra of acetates is presumably a protonated acetate moiety - [CH3COOH2]+. Little further useful information can be obtained from the spectrum.
It should be noted that the base ion in the spectra of acetates is often at m/z = 43 (or 44), representing the [CH3CO]+ moiety, but ions below m/z = 50 are automatically edited out when the spectra illustrated here are prepared.
The mass spectrum of 1-octadec-9,12-dienyl acetate –
In this instance, the molecular ion is now discernible, together with the ion for [M-60]+ at m/z = 248, although that at m/z = 61 is smaller than in the spectra of saturated analogues.
Trimethylsilyl Ether Derivatives of Alcohols
Trimethylsilyl (TMS) ether derivatives are probably used more widely than any other for the gas chromatographic analysis of hydroxy compounds in general and for aliphatic alcohols in particular. Their main value lies in increasing the volatility and reducing the polarity of the parent molecules, so ensuring sharp symmetrical peaks on GC analysis. On the other hand, the mass spectrometric properties of the saturated primary alcohols are not exceptional, although they usually do permit the molecular weight to be determined at least. even if the molecular ion per se is small and often not detectable. For example, in the mass spectrum of the TMS ether of eicosan-1-ol, the base peak (m/z = 355) represents the loss of a methyl group from the TMS ether moiety. They do have a characteristic fingerprint at least.
Mass spectra of the TMS ethers of unsaturated alk-1-ols are rather are rather different, and that of the TMS ether of eicos-11-en-1-ol is illustrated –
In this instance, there is an adequate molecular ion, and the base peak at m/z = 75 represents the loss of the trimethylsilyloxygroup. There are no ions that serve to locate the double bond of course. Further spectra of TMS ether derivatives of aliphatic primary alcohols are available in our Archive pages, but without interpretation.
The mass spectrum of the trimethylsilyl derivatives of a secondary alcohol is somewhat different, and that of the TMS derivative of hexadecan-2-ol is illustrated. In this instance the base ion at m/z = 117 represents cleavage beta to the hydroxyl moiety.
Aliphatic I,2-diols are occasional components of waxes, and the spectra recorded here were obtained from constituents of beeswax. Isopropylidene derivatives are useful for characterization purposes in general, as they are only formed from compounds with hydroxyl groups on adjacent carbon atoms. However, they do not give mass spectra that are very informative. For example the mass spectrum of the isopropylidene derivative of hexadecane-1,2-diol is -
The molecular ion is essentially absent, but the base peak is an ion representing loss of one of the methyl groups from the isopropylidene moiety, at m/z = 283 in this instance. This enables precise determination of molecular weight, always an important parameter. The ion at m/z = 101 confirms that the diol moiety is in the 1,2-position, as opposed to 2,3- or other.
The mass spectrum of the isopropylidene derivative of tetracosane-1,2-diol, which follows, is entirely analogous to this.
Mass spectra of bis-TMS ethers of 1,2-aliphatic diols tend to be more informative, and as an example, the mass spectrum of the bis-TMS ether of hexadecane-1,2-diol follows -
The molecular ion is not apparent, but an ion at m/z = 387 for loss of a methyl group enables determination of the molecular weight. The main cleavage is at the centre of the diol system yielding two ions at m/z = 102 and 299 (the base peak) that clearly define the structure of the molecule.
Similarly with the mass spectrum of the bis-TMS ether of tetracosane-1,2-diol -
Further spectra of aliphatic diols in the form of both types of derivative are available in the Archive section of this site, but without interpretation. These include a number of saturated isomers with iso- and anteiso-methyl branches, but I am unable to find any features in their mass spectra that distinguish these.
Monoacylglycerols are analysed most conveniently as the trimethylsilyl ether derivatives as the (1/3)- and 2-isomers are easily separated by gas chromatography and each has a characteristic mass spectrum (Johnson, C.B. and Holman, R.T., 1966; Myher J.J. et al., 1974).
The mass spectrum of the trimethylsilyl ether derivative of 1-monoheptadecanoin is –
The molecular ion is not normally significant with saturated isomers, but is more abundant with unsaturated analogues. An ion representing [M-15]+ at m/z = 473 in this instance serves to determine the molecular weight. The base ion at m/z = 385 or [M-103]+ represents cleavage between carbons 1(3) and 2 of the glycerol moiety as illustrated. An ion at m/z = 253 for the acyl moiety is of moderate intensity (10-20%) and is in a distinctive region of the spectrum. Ions at m/z = 129 and 147 are useful diagnostic markers.
The mass spectrum of the trimethylsilyl ether derivative of 2-monoheptadecanoin is very different –
Ions containing one or both of the silicon atoms dominate the spectrum, e.g. m/z = 103, 129, 147 and 218, and depending on the molecular weight and degree of unsaturation of the fatty acid chain that at m/z = 129 or 218 can be the most abundant. The second of these represents ion remaining after the loss of the acyloxy moiety. The ion at m/z = 253 for the acyl moiety is present but of low intensity.
Glycerol Ethers (1-Alkylglycerols)
Glycerol ether lipids are common in nature and can be major components of some marine oils, especially those from sharks, from which the following spectra were obtained. They are obtained usually by hydrolysis of the lipids and isolation of the non-saponifiable fraction. For analysis by GC-MS, as with the aliphatic 1,2-diols, they can be converted to either the isopropylidene derivatives or the TMS ethers. Neither of these may give a recognizable molecular ion, and an [M-15]+ ion (loss of a methyl group from the derivatizing moiety) is often that of highest mass to be detected. Unsaturated bis-trimethylsilyl ether derivatives may give a respectable molecular ion, however. In general, these spectra are not very interesting, as the main cleavage occurs between carbons 1 and 2 of the glycerol moiety, so that the base peak is for the fragment with the derivatizing group (Myher, J.J. et al., 1974). Some representative examples are illustrated below without further discussion.
Mass spectrum of 1-O-hexadecylglycerol - isopropylidene derivative -
Mass spectrum of 1-O-octadec-9-enylglycerol - isopropylidene derivative -
Mass spectrum of 1-O-hexadecylglycerol - bis-trimethylsilyl ether derivative -
Mass spectrum of 1-O-octadec-9-enylglycerol - bis-trimethylsilyl ether derivative -
Further spectra of glycerol ethers in the form of both types of derivative are available in the Archive section of this site, but without interpretation.
- Johnson, C.B and Holman, R.T. Mass spectrometry of lipids. II. Monoglycerides, their diacetyl derivatives and their trimethylsilyl ethers. Lipids, 1, 371-380 (1966) (DOI: 10.1007/BF02532539).
- Myher, J.J., Marai, L. and Kuksis, A. Identification of monoacyl- and monoalkylglycerols by gas-liquid chromatography-mass spectrometry using polar siloxane liquid phases. J. Lipid Res., 15, 586-592 (1974) (www.jlr.org/content/15/6/586.abstract).
Updated February 27, 2013