The Lipid Library

MASS SPECTRA OF FATTY ALCOHOLS

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. It may then be necessary to analyse them in the free 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. 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.

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. In the spectrum of tetracosanyl acetate, illustrated below, this is at m/z = 336. Little further useful information can be obtained from the spectrum.

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. 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.

The TMS ethers of aliphatic secondary alcohols have mass spectra that clearly distinguish them from the primary alcohols and that of the TMS ether of octadecan-2-ol is illustrated.

The molecular ion is more abundant than with primary alcohols. There are abundant fragments (including the base ion) from cleavage on either side of the hydroxyl group, at m/z = 117 and 327 in this instance. All aliphatic alcohols with the –OTMS group in position 2 should have the distinctive ion at m/z = 117.

Further spectra of TMS ether derivatives of aliphatic primary and secondary (position 2) alcohols (saturated only) are available in our Archive pages, but without interpretation.

Aliphatic 1,2-Diols

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.

Glycerol Ethers

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. 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.

Updated: 7/9/2007 Credits/disclaimer	Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland