Abstract: A gas chromatography-electron impact mass spectrometric(GC-EIMS) method was developed for analyzing the unsaturated fatty acids, while ortho-aminophenol was used for the pre-column derivatization reagent of unsaturated fatty acids so as to modify carboxy1 group into nitrogen-containing heterocycle which effectively suppressed the migration of double bonds of the alkeny1 under the electron impact source and led to regular informative mass spectra regarding the position of the olefinic bonds in the chain .The empirical rule in which the position of double bonds of olefinic acids was located was refered to by analyzing electron impact spectra of the derivatives of the fatty acids modified by ortho-aminophenol reagent. This methodprovides a new technical tool for analyzing the fine structure of unsaturated fatty acids. Ortho-aminophenol is used as derivatization reagent to the fatty acids and 2 unsaturated fatty acids in sunflower oil are identified according to the analysis of GC-EIMS .The position of double bonds of the unsaturated fatty acids from sunflower oil is located and the results are in good agreement with bibliography.

Key words: unsaturated fatty acids, position of double bond, ortho-aminophenol, derivatization, electron impact mass spectrometry