Non-aqueous reversed-phase high performance liquid chromatography (HPLC) has become an established means of separating the triacylglycerols (TAGs) present in natural fats and oils.1 The most efficient separations have been achieved with columns packed with chemically bonded octadecylsilyl phases, employing mobile phases consisting of acetonitrile with acetone, propionitrile or chlorinated solvents. Separation is based on both the combined chain length of the fatty acyl residues and on the total number of double bonds in the molecule. A range of detection methods, including evaporative light scattering, 2,3 refractive index [4][5][6] and flame ionization detection 7 have been employed in the analysis of TAGs. However, identification of components using these methods relies on relative retention times or collection of peaks followed by mass spectrometric analysis. With very complex mixtures such as milk fat or fish oils, identification of TAGs in this way becomes impractical. Consequently mass spectrometric detection, which provides detailed information on the molecular weight and fatty acid composition of TAG molecular species, as well as allowing identification of partially resolved or co-eluting peaks, is becoming increasingly widespread.Whilst + , which allow identification of the fatty acids present in each TAG species. 9 The technique has been applied to the analysis of several vegetable oils. [10][11][12] In addition to determining which fatty acids are present in a TAG, it is also important to be able to determine their positions on the glycerol backbone of the molecule since this has considerable biochemical and nutritional significance.13,14 The ability to unambiguously identify positional isomers of individual components of TAG mixtures, is a long standing problem in lipid chemistry. One method for stereospecific analysis of TAGs involves using enzymatic hydrolysis to sequentially cleave the fatty acids at sn-1 and sn-2. These are then analysed directly and the most probable fatty acid at sn-3 calculated. 15 Christie et al. 16 have developed a method in which TAGs are partially hydrolysed and derivatized to diacylglycerol urethanes which can be analysed by gas chromatography. Both of these methods, however, are time consuming. In enzymatic hydrolysis there may be some selectivity for certain chain lengths and numbers of double bonds, as well as a possibility of acyl migration during the analysis. Similarly, urethane derivatization is not suitable for the analysis of more complex fats and oils. Argentation HPLC, using a silver-ion impregnated column in conjunction with polar solvents, has been demonstrated to separate ABA and AAB type positional isomers, where A and B represent different fatty acyl moieties, 17 but gives no information on the positions of fatty acids in an ABC type TAG. A recent paper described the analysis of a number of γ-linolenic-acid-containing vegetable oils using silver-ion HPLC in conjunction with APCI mass spectrometry. 18 We recently showed that positional isomers of TAGs can be...

The fatty acid compositions and δ 13 C values of the major fatty acids of more than 150 vegetable oils were determined to provide a database of isotopic information for use in the authentication of commercial maize oil. After extraction of oils from seeds, nuts or kernels, and methylation, fatty acid compositions were determined by capillary gas chromatography. All compositions were within the ranges specified by the Codex Alimentarius. Gas chromatography combustion-isotope ratio mass spectrometry was employed to determine the δ 13 C values of the major fatty acids of the oils. A large number of pure maize oils and potential adulterant oils from various parts of the world were studied to assess the sources of variability in δ 13 C values. Such information is vital to establishing the compound specific isotope technique as a reliable means of assessing vegetable oil purity. Variability in δ 13 C values was related to the geographical origin of the oil, year of harvest, and the particular variety of oil. This suggests that the ultimate δ 13 C values of fatty acids are determined by a combination of environmental and genetic factors. JAOCS 75, 371-379 (1998) . FIG. 6. A summary of the variations in δ 13 C values of oleic acid due to plant type, origin, and year of harvest. a Year-to-year variability is shown for variety Idol (winter oilseed rape) grown in five consecutive years. b Variability between varieties for 1992/1993.

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Maize oil commands a premium price and is thus a target for adulteration with cheaper vegetable oils. Detection of this activity presents a particular challenge to the analyst because of the natural variability in the fatty acid composition of maize oils and because of their high sterol and tocopherol contents. This paper describes a method that allows detection of adulteration at concentrations of just 5% (m/m), based on the Mahalanobis distances of the principal component scores of the delta(13)C values of major and minor vegetable oil components. The method makes use of a database consisting of delta(13)C values and relative abundances of the major fatty acyl components of over 150 vegetable oils. The sterols and tocopherols of 16 maize oils and 6 potential adulterant oils were found to be depleted in (13)C by a constant amount relative to the bulk oil. Moreover, since maize oil contains particularly high levels of sterols and tocopherols, their delta(13)C values were not significantly altered when groundnut oil was added up to 20% (m/m) and it is possible to use the values for the minor components to predict the values that would be expected in a pure oil; therefore, comparison of the predicted values with those obtained experimentally allows adulteration to be detected. A refinement involved performing a discriminant analysis on the delta(13)C values of the bulk oil and the major fatty acids (16:0, 18:1 and 18:2) and using the Mahalanobis distances to determine the percentage of adulterant oil present. This approach may be refined further by including the delta(13)C values of the minor components in the discriminant analysis thereby increasing the sensitivity of the approach to concentrations at which adulteration would not be attractive economically.

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