Determination of double bond positions of unsaturated fatty acids by a chemical ionization mass spectrometry computer system.

After stereospecific oxidation, trimethylsilylated methyl esters of mono- and diunsaturated fatty acids were analyzed by combined gas-liquid chromatography-chemical ionization mass spectrometry. The positions of original double bonds were deduced from the fragment ions produced by the cleavage of the carbon-carbon bond between two trimethylsilyl ethers. These fragment ions were recorded at m/e 187 and 259 in the case of 16:1(n-7), at m/e 187 and 287 in the case of 18:1(n-7), at m/e 215 and 259 in the case of 18:1(n-9), and at m/e 172 and 259 in the case of 18:2(n-6), respectively. The diastereoisomers of monounsaturated fatty acids can be discriminated by comparing the intensities of the fragment ions at m/e 253 and 285 in the case of 16:1 and at m/e 281 and 313 in the case of 18:1. The diastereosiomers of diunsaturated fatty acids may also be distinguished from each other by comparing the intensities of the fragment ions formed by the loss of trimethylsilyl function from the characteristic ions. Mono- and diunsaturated fatty acids in liver may be dominantly in cis- 18:1(n-1) and cis-cis-18:2(n-6) as determined by a mass chromatographic technique.