Combining Results of Two GC Separations Partly Achieves Determination of All <Emphasis Type="Italic">cis</Emphasis> and <Emphasis Type="Italic">trans</Emphasis> 16:1, 18:1, 18:2 and 18:3 Except CLA Isomers of Milk Fat as Demonstrated Using Ag-Ion SPE Fractionation |
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Authors: | John K G Kramer Marta Hernandez Cristina Cruz-Hernandez Jana Kraft Michael E R Dugan |
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Affiliation: | (1) Guelph Food Research Center, Agriculture and Agri-Food Canada, 93 Stone Road West, N1G5C9 Guelph, ON, Canada;(2) Department of Animal Science, University of Vermont, Burlington, VT 05405-0148, USA;(3) Lacombe Research Center, Agriculture and Agri-Food Canada, Lacombe, AB, Canada |
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Abstract: | Milk fat is a complex mixture of geometric and positional isomers of monounsaturated and polyunsaturated, including short-,
long- and branch-chain fatty acids (FAs). There has been partial success to resolve this mixture of FAs using different GC
temperature programs, or a combination of GC isothermal and temperature programs. To overcome the problem associated with
overlapping isomers prior silver-ion separation was recommended. However, this procedure is time consuming and not practical
for routine analysis. In addition, previous methods focused mainly on the trans and cis isomers of 18:1. The present method takes advantage of differences in the relative elution times between different types
of FAs. The method involved analyzing each milk fat using the same highly polar 100-m capillary column and GC instrument,
and conducting two separations using temperature programs that plateau at 175 and 150 °C. The relative shift among the geometric
and positional isomers at these two temperature settings was enough to permit identification of most of the trans and cis 16:1, 18:1 and 20:1, the c/t-18:2 and the c/c/t-18:3 isomers found in milk fat. The identity of these FAs was confirmed by prior separation of the total fatty acid methyl
esters (FAMEs) of milk fat using Ag+-SPE columns, and comparing the fractions to the total milk fat. The Ag+-SPE technique was modified to obtain pure saturated, trans- and cis-monounsaturated and diunsaturated FAMEs. By combining the results from these two separate GC analyses, knowing the elution
order, it was possible to determine most of the geometric and positional isomers of 16:1, 18:1, 20:1, 18:2 and 18:3 without
a prior silver-ion separation. Only few minor FAs could not be resolved, notable the conjugated linoleic acid isomers that
still required the complimentary Ag+-HPLC separation. The two GC temperature programs have been successfully used to routinely analyze most FA isomers in total
milk and beef fats in about 200 min without the use of prior silver-ion separations. |
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Keywords: | Gas chromatography Ag-ion SPE Milk fat cis and trans isomers |
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