09.12.2012 Views

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

Food Lipids: Chemistry, Nutrition, and Biotechnology

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

Figure 5 Separation of the phenacyl esters of 18:3 geometric isomers by silver ion HPLC:<br />

SF, solvent front; 1, all trans-18:3; 2–7, cis,trans-18:3; 8, all cis-18:3. (From Ref. 88.)<br />

used to separate the cis/trans fatty acid isomers of methyl oleate, methyl linoleate,<br />

methyl linolenate, <strong>and</strong> 15 of the 16 cis/trans methyl arachidonate positional isomers<br />

[89]. In other research, this column was used with a 0.15% acetonitrile in hexane<br />

isocratic mobile phase to obtain four fractions from hydrogenated vegetable oil that<br />

were subsequently analyzed by capillary GC [90]. As Figure 6 shows, the silver ion<br />

HPLC column separated the cis <strong>and</strong> trans monoene positional isomers of fatty acid<br />

methyl esters with 0.08% acetonitrile in hexane mobile phase. Quantitation of positional<br />

isomers was obtained with this silver ion HPLC procedure. Comparable<br />

quantitation was also achieved with a 100-m capillary GC column used to analyze<br />

the isolated trans HPLC fraction.<br />

IV. ANALYSIS USING COMBINED TECHNIQUES<br />

The direct quantitation of all fatty acid isomers in hydrogenated oils by gas chromatography<br />

is nearly impossible. There is overlap of cis/trans monoene <strong>and</strong> diene<br />

positional isomers even with the high efficiency of long capillary columns [68,89–<br />

91]. Several analytical techniques have been combined with gas chromatography to<br />

determine accurate fatty acid profiles of hydrogenated oils including ozonolysis <strong>and</strong><br />

infrared spectroscopy.<br />

A. Ozonolysis/Gas Chromatography<br />

Ozonolysis followed by gas chromatography can be used to determine the doublebond<br />

position of cis <strong>and</strong> trans isomers. Ozonolysis cleaves the hydrocarbon chain of<br />

fatty acids at positions of unsaturation. This includes reductive cleavage to form<br />

aldehydes, aldehyde esters, <strong>and</strong> other fragments, which are identified according to<br />

their retention times on GC columns compared with known st<strong>and</strong>ards. Ozonolysis<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!