A rapid method has been devised for the GC-MS determination of double bond positions in monounsaturated fatty acids as their dimethyl disulfide adducts.
As a standard procedure for the single-step preparation of adducts, fatty acid methyl esters were incubated with dimethyl disulfide in the presence of I
2 as the catalyst for 30min. After the reduction of I
2 with NaHSO
3, hexane-ether mixture was added to the resulting system, and the upper phase was injected directly into a GC-MS. When the original esters contained a large quantity of polyenoates, TLC purification was used to remove contaminating by-products from the polyenoates before conducting the GC-MS analysis.
The adducts of monoenoates had relatively shorter retention times under the GC-MS conditions, and gave simple mass spectra and easily recognizable molecular ions. The cleavage between the methylthio-substituted carbons gave key fragment ions showing the original double bond positions in the aliphatic chain.
This method was applied to the fatty acid analysis of naturally-occurring lipids, such as chlorella, parsley seed and soybean lipids. The positional isomers, 14:1 (
n-9,
n-7 and
n-5), 16:1 (
n-9,
n-7 and
n-5), 18:1 (
n-9 and
n-7), and 17:1 (
n-8), could be readily detected in chlorella, 18:1 (
n-12,
n-9 and
n-7) in parsley seed, and 18:1 (
n-9 and
n-7) in soybean.
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