Boryana Nikolova-Damyanova

Silver ion chromatography of lipids and fatty acids

Silver ion chromatography as applied to the analysis of lipids is reviewed. Thin-layer, column, high-performance liquid and supercritical fluid chromatography in the silver ion mode are included. The lipid types covered are fatty acids, triacylglycerols and complex lipids. Separations are divided into those according to number, geometry and position of double bonds, as well as acyl positional isomers for triacylglycerols. The mechanism of silver ion chromatography is discussed in relation to recent studies using silver ion high-performance liquid chromatographic methodology.

Stereospecific analysis of triacyl-sn-glycerolsvia resolution of diastereomeric diacylglycerol derivatives by high-performance liquid chromatography on silica

The compositions of positionssn-1, 2 and 3 of triacylglycerols can be determined by partial hydrolysis with ethyl magnesium bromide, derivatization of the total products with (S)-(+)-1-(1-naphthyl)ethyl isocyanate and isolation of the diacyl-sn-glycerol urethane derivatives by chromatography on solid-phase extraction columns containing an octadecylsilyl phase. The diastereomericsn-1,2-and 2,3-diacylglycerol derivatives are separated by high-performance liquid chromatography on silica for determination of their fatty acids by gas chromatography. Each step in the process has been evaluated rigorously. The compositions of all three positions can be calculated with good accuracy from the analyses of these compounds and that of the total triacylglycerols. Although the 1,3-sn-diacylglycerol derivatives can also be isolated easily, they do not give reliable results for the composition of positionsn-2 because acyl migration occurs during their generation. The stereospecific analysis procedure has been applied to some plant and animal triacyl-sn-glycerols of commercial and scientific interest, containing predominantly C16 and C18 fatty acids,i.e. safflower, sunflower, olive and palm oils, tallow, egg and rat adipose tissue. The method is not at present suited to the analysis of more complex triacylglycerols, such as milk fat or fish oils, and problems associated with these are discussed.

Silver ion high-performance liquid chromatography of derivatives of isomeric fatty acids

Abstract Silver ion high-performance liquid chromatography was used to study the retention characteristics of series of monoenoic, dienoic and polyunsaturated esters. Two columns with stationary phases consisting of bonded phenylsulphonic moieties in the silver ion form were used, with a mobile phase consisting of dichloromethane-dichloroethane (1:1) with various proportions of acetonitrile at fixed temperatures. Accurate capacity factors relative to octadecene as an internal standard were obtained. Each column had distinctive characteristics, and methyl ester derivatives were less well resolved than phenacyl derivatives. For example, baseline resolution of three naturally occurring isomers (6-, 9- and 11-18:1) was possible. The distance of the double bond from the car☐yl group was much more important than the nature of the terminal moiety in its effect on retention values. Fixed temperatures were necessary for reproducible retention times, but lowering the temperature improved the resolution merely by extending the time of chromatography. The results are discussed in terms of a hypothetical mechanism involving simultaneous interaction between one silver ion and either two double bonds, or one double bond and one other electron-rich moiety such as the ester group.

Retention of lipids in silver ion high-performance liquid chromatography: Facts and assumptions

The factors affecting the retention of lipids in silver ion HPLC are presented and discussed here, according to the present understanding of the processes that take place in this chromatographic system. The discussion is focused mainly on fatty acids and triacylglycerols.

Mechanistic aspects of fatty acid retention in silver ion chromatography

Abstract The retention properties of various benzyl, phenacyl, and normal- and branched-chain alkyl esters of 6–18:1, 9–18:1 and 11–18:1 fatty acids have been studied by silver ion high-performance liquid chromatography. The weak electron-donating effect of the alkyl substituents on the carbonyl oxygen in the aliphatic alkyl esters ensures baseline resolution of isopropyl and tert-butyl esters, but not of n-alkyl esters. Comparison between benzyl and phenacyl derivatives confirmed that the silver ion interacted simultaneously with the double bond of the fatty acid and the carbonyl oxygen of the phenacyl moiety. This dual interaction enabled the separation of positional isomers. Introduction of electron-withdrawing or electron-donating substituents in the p-position in the benzene ring decreased or increased retention, respectively. The substituents strongly affect the resolution, and p-methoxyphenacyl esters have not only the highest k′ values but also the best resolution of the three positionally isomeric fatty acids, established so far.

Lipid classes, fatty acid composition and triacylglycerol molecular species in crude coffee beans harvested in Brazil

Abstract The lipids of crude coffee beans of Catuai Vermalho, Coffea arabica L. harvested in Brazil have been examined as a part of a broader program on their role for the quality of Brazilian coffee. Triacylglycerols (75%), terpene esters (14%), partial acylglycerols (5%), free fatty acids (1%), free sterols (1.5%), sterol esters (1%) and polar lipids (

The structure of the triacylglycerols of meadowfoam oil

The triacylglycerols of meadowfoam oil have been resolved by HPLC in the silver ion and reversed-phase modes, and by the two techniques used in a complementary fashion. The fractions obtained were collected and quantified by gas chromatography of their methyl esters in the presence of an internal standard. Silver ion chromatography gave a distinctive resolution in which fractions differing solely in the position and chain-length of a single monoenoic fatty acyl group were resolved, the order of elution being 11−20∶1, 5−20∶1, 13−22∶1, 5−18∶1 and 9−18∶1. Reversed-phase chromatography also gave fractions containing single positional isomers, (11−20∶1<5−20∶1<13−22∶1<5−22∶1), but the pattern was more difficult to discern since fractions containing 22∶2 tended to overlap with those containing 20∶1. The species (5−20∶1)(5−20∶1)(22∶2), (5−20∶1)(5−20∶1)(5−20∶1) and (5−20∶1)(5−20∶1)(13−20∶1) were found to be the most abundant, and together comprised 67% of the total. A small but significant trilinolein fraction was detected and its presence may have biosynthetic implications.

Retention properties of triacylglycerols on silver ion high-performance liquid chromatography

Abstract The retention properties of series of synthetic disaturated-monounsaturated (1 to 6 double bonds) triacylglycerols and the common seed oil triacylglycerols on a silver ion high-performance liquid chromatography column have been studied in quantitative terms. Retention factors were found to increase stepwise, with a saturated dimonoenoic species being held ten times and trilinolenin ten thousand times more strongly than a disaturated monoenoic species, for example. Formation of chelate-type of pi-complexes between silver ions and double bonds, with participation of the carbonyl oxygen in the complexation, have been assumed in order to explain the high separation power and specificity of the silver ion column.

Silver ion high-performance liquid chromatography of isomeric cis- and trans-octadecenoic acids: Effect of the ester moiety and mobile phase composition

The effects of the ester moiety and of the mobile phase composition on the resolution of cis and trans positional isomers of octadecenoates have been studied by silver ion HPLC with UV detection. The efficiency of the separation increases in the order phenethyl<phenacyl<p-methoxyphenacyl esters. Retention and resolution are substantially affected by small changes in the proportion of acetonitrile in the mobile phase. Dichloromethane influences retention but has a small effect on resolution. Clear resolution of cis and trans positional isomers of octadecenoic acid in partially hydrogenated sunflower oil has been achieved after conversion into p-methoxyphenacyl esters on a silver ion column by isocratic elution with a mobile phase of hexane–dichloromethane–acetonitrile (60:40:0.2, v/v) in only 22 min.

Determination of Triacylglycerol Classes and Molecular Species in Seed Oils with High Content of Linoleic and Linolenic Fatty Acids

Soya bean and linseed oils were analysed by successive application of silver-ion and reversed-phase thin-layer chromatography and densitometry. Direct quantitative determination of all the 20 possible triacylglycerol (TAG) classes including critical pairs (SD 2 +S 2 T ; M 2 T + D 3 ; SDT + MDT ; ST 2 + MT 2 ) was performed. TAG molecular species down to 0.1% (32 in soya bean oil and 26 in linseed oil) were identified and quantified. The detection limit was 0.2 μg and the standard deviation between 0.1% and 1.0% for silver ion thin-layer chromatography (TLC) and between 0.1% and 3.9% for reversed-phase TLC determinations. The accuracy of the analysis was confirmed by the good agreement between the quantitative results for the fatty acid composition as determined by gas chromatography and as calculated from the densitometric data for the TAG composition.