Abstract: A gas chromatography—electron impact mass spectrometry method was developed for analyzing the composition of the fatty acids from Laminaria japonica. Ortho—aminophenol was developed for chemical modifying reagent of fatty acids so as to modify carboxyl group into nitrogen—containing heterocycle which effectively suppresses the migration of double bonds of the alkenyl under the electron impact source. The empirical rule in which the position of double bonds of olefinic acids were located was refered to by analyzing election impact mass spectra of the chemical products modified by ortho—aminophenal of the fatty acids in Laminaria japonica.12 fatty acids were identified according to the analysis of GC—EIMS and the position of double bonds of the fatty acids from Laminaria japonica was located. The contents of the unsaturated fatty acids and the polyunsaturated fatty acids are 57.41% and 34.01%.

Key words: laminaria japonica, fatty acids, chemical modification, ortho—aminophenol, gas chromatography—elec- tron impact mass spectrometry