Abstract
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 I2 as the catalyst for 30min. After the reduction of I2 with NaHSO3, 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.
