MASS SPECTROMETRY OF FATTY ACID DERIVATIVES
PREPARATION OF METHYL ESTERS
1. Introduction
Although methyl esters are not the best derivatives for the mass spectrometric analysis of lipids, they are so simple in structure and in such wide-spread use for fatty acid analysis in general that they are inevitably much used for mass spectrometry. Without further derivatization, they are of little worth for structural analysis of mono- or dienoic fatty acids, but they can often be of value for polyunsaturated fatty acids, and those with functional groups other than double bonds, especially when spectra of authentic compounds are available for comparison purposes. Following a brief description of the methodology, experimental protocols for the two most important methods are given here. More detailed discussion of the mechanisms of esterification and alternative methods are given in a review article that is now available on this website here...
W.W. Christie, Preparation of ester derivatives of fatty acids for chromatographic analysis. In: Advances in Lipid Methodology - Two, pp. 69-111 (1993) (Ed. W.W. Christie, Oily Press, Dundee).
- or beginners to the subject may prefer some shorter articles on this website here... or the appropriate chapter in 'Gas Chromatography and Lipids'.
- or you may wish to consult my recent book (Christie, W.W. Lipid Analysis (3rd edition) (Oily Press, Bridgwater) (2003)). Other useful derivatization techniques, e.g. hydrogenation, deuteration, etc., are described in the section of these pages dealing with 'Mass spectrometry of methyl esters - further derivatization'.
Full practical details are given elsewhere on this site for preparation of the nitrogen-containing derivatives, i.e. picolinyl esters, 4,4-dimethyloxazoline (DMOX) derivatives and pyrrolidides, which are most useful for mass spectrometric analysis of fatty acids here...
There is no need to hydrolyse lipids to obtain the free fatty acids before preparing esters, as most lipids can be transesterified directly. No single reagent will suffice, however, and one must be chosen that best fits the circumstances. Esters prepared by any of the following methods can be purified if necessary by adsorption chromatography (see below). Care should be taken in the evaporation of solvents as appreciable amounts of esters up to C14 (or even C16) can be lost if this step is performed carelessly, for example by an over vigorous use of a nitrogen stream. The main procedures are acid- or base-catalysed esterification. Diazomethane was once used frequently as a mild method for esterification of free acids, but concerns over the toxicity of reagents now limit its use. The methods described below are suitable for the common esterified lipids and their fatty acid constituents. Special methods are required for amide-bound fatty acids, and those with of short chain-length or with sensitive functional groups, for which the review and books cited above should be consulted.
2. Acid-Catalysed Esterification and Transesterification
Free fatty acids are esterified and O-acyl lipids transesterified by heating them with a large excess of anhydrous methanol in the presence of an acidic catalyst.
If water is present, it may inhibit the reaction. The reagent is most easily prepared by adding acetyl chloride (5 mL) slowly to cooled dry methanol (50 mL). Methyl acetate is formed as a by-product, but it does not interfere with methylations at this concentration. A solution of 1-2% (v/v) concentrated sulfuric acid in methanol transesterifies lipids in the same manner and at much the same rate; it is very easy to prepare whenever it is required (fresh reagent is best). It is usual to heat the lipid sample in the reagent under reflux for about two hours, but equally effective esterification is obtained if the reaction mixture is heated in a stoppered tube at 50°C overnight (also incidentally reducing the glassware requirements). While boron trifluoride in methanol (12-14% w/v) has also been much used as a transesterification catalyst and for esterifying free fatty acids, I have considerable reservations about its use (see a specific web page for detailed discussion).
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Laboratory protocol: The lipid sample (up to 5 mg) is dissolved in toluene (1 mL) in a test tube fitted with a condenser, and 1% sulfuric acid in methanol (2 mL) is added, before the mixture is refluxed for 2 hours (or alternatively the mixture can be left overnight in a stoppered tube at 50°C). Water (5 mL) containing sodium chloride (5%) is added and the required esters are extracted with hexane (2 x 5 mL), using Pasteur pipettes to separate the layers. The hexane layer is washed with water (4 mL) containing potassium bicarbonate (2%) and dried over anhydrous sodium sulfate. The solution is filtered to remove the drying agent, and the solvent is removed under reduced pressure in a rotary film evaporator or in a stream of nitrogen. |
No solvent other than methanol is necessary if free fatty acids alone are to be methylated, or if polar lipids such as phospholipids are to be transesterified. The same method is used to prepare dimethylacetals from aliphatic aldehydes or plasmalogens.
3. Base-Catalysed Transesterification
O-Acyl lipids are transesterified very rapidly in anhydrous methanol in the presence of a basic catalyst. Free fatty acids are not normally esterified, however, and care must be taken to exclude water from the reaction medium to prevent their formation by hydrolysis of lipids.
0.5M Sodium methoxide in anhydrous methanol, prepared simply by dissolving fresh clean sodium in dry methanol, is the most popular reagent, but potassium methoxide or hydroxide have also been used as catalysts. The reagent is stable for some months at room temperature, if oxygen-free methanol is used in its preparation. The reaction is very rapid; phosphoglycerides, for example, are completely transesterified in a few minutes at room temperature, but cholesterol and wax esters take longer. It is commonly performed as follows:
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Laboratory protocol: The lipid sample (up to 10 mg) is dissolved in dry toluene (1 mL) in a test-tube, 0.5M sodium methoxide in anhydrous methanol (2 mL) is added, and the solution is maintained at 50°C for 10 min. Glacial acetic acid (0.1 mL) is then added, followed by water (5 mL). The required esters are extracted into hexane (2 x 5 mL), using a Pasteur pipette to separate the layers. The hexane layer is dried over anhydrous sodium sulfate and filtered, before the solvent is removed under reduced pressure on a rotary film evaporator. |
As with acid-catalysed transesterification procedures, an additional solvent, such as toluene or tetrahydrofuran, is necessary to solubilize non-polar lipids like cholesterol esters or triacylglycerols. Aldehydes are not liberated from plasmalogens with basic reagents.
4. Clean-Up of Methyl Esters
It is sometimes necessary to purify methyl esters after transesterification has been carried out in order to eliminate troublesome impurities prior to analysis by gas chromatography. For example, cholesterol should be removed in this way from animal tissue preparations. This can be accomplished by adsorption chromatography with a short column (approx. 2 cm) of silica gel or Florisil™ in a Pasteur pipette plugged with glass wool, and pre-conditioned with hexane. Methyl esters are eluted with hexane-diethyl ether (95:5, v/v; 10 mL), while cholesterol and other polar impurities remain on the column. Commercial solid-phase extraction columns can be used in the same way. Methyl esters can also be purified by preparative thin-layer chromatography, with hexane-diethyl ether (9:1, v/v) as the mobile phase.
5. Choice of a Column for Gas Chromatography-Mass Spectrometry of Fatty Acid Derivatives
The choice of columns for GC analysis in general is discussed elsewhere on this website. It is not always possible to use the more popular analytical columns for GC-MS analysis of lipids, as bleeding of the stationary phase can lead to troublesome backgrounds. However, a number of manufacturers now make columns with cross-linked highly stable phases specifically for use with mass spectrometry. We have made extensive use of a column of Supelcowax 10™ (25 m in length; from Supelco-Sigma Inc.) in our laboratory. It is typical of columns of the Carbowax type in its elution properties, and can cover with ease a wide range of fatty acid methyl esters or DMOX derivatives (up to C30 in chain-length).
The figure above illustrates a typical chromatogram of the fatty acids of human erythrocytes as methyl esters on a column of the Carbowax type (25 m) with a flame-ionization detector. Excellent separations are achieved of fatty acid esters of a given chain-length that differ both by degree of unsaturation and in the positions of the double bonds. For example, two isomers of 18:1 and of 18:3 are separated, as are three isomers of 20:3, two of 20:4 and two of 22:5. With the methyl esters of the more common families of polyunsaturated fatty acids, the shorter the distance between the last double bond and the end of the molecule, the longer the retention time of the isomer. Even better separations of positional isomers might be obtained with a more polar phase, though the order of elution relative to components of a different chain length might vary and there can be other problems of components overlapping. By a judicious use of two or more columns differing in polarity in this way, most of the fatty acids of metabolic importance can be separated and estimated.
Picolinyl esters and pyrrolidides are more polar or have higher molecular weights, and the Supelcowax column can cope with the common range up to about C22. When a more polar phase is required, we have used a column of BPX-70™ (from SGE Ltd). Of course, other manufacturers will also have columns available that fully meet the requirements for GC-MS, but we no experience of any other than those listed.
It is not possible to expect the same quality of resolution in mass spectrometry applications as with a flame-ionization detector (FID), mainly because of the greater dead volume in a mass spectrometer in comparison with an FID. This depends largely on the specific instrument in use, so further comment is impossible here. Also, it is not possible to expect the same resolution with picolinyl esters and pyrrolidides as with methyl esters and DMOX derivatives, because of the high molecular weight and polarity of the former. However, pyrrolidides do have some interesting GC properties in their own right (see the appropriate web page).
Finally, we have a requirement for a column coated with a low polarity silicone phase, because of its high thermal stability. Such columns lack the resolution of phases of higher polarity, but are adequate for many purposes. We use a DB5TM column for fatty acids of longer than usual chain-length (especially as picolinyl esters), for hydroxy fatty acids, and for other lipids of high molecular weight, including sterols and waxes. It is essential for any fatty acid derivative or other lipid that has been silylated.
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Scottish Crop Research Institute (and MRS Lipid Analysis Unit), Invergowrie, Dundee (DD2 5DA), Scotland. |
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Updated: 14/4/2008 |
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