Application to Analysis of Fatty Acids by Chemical Ionization Mass Spectrometry

Abstract

Chemical ionization mass spectrometry (CI-MS) permits rapid analysis of fatty acids without methyl esterification of samples and even without gas chromatographic separation. It is capable of qualitative determinations with samples of trace quantity level. The quasimolecular ions obtained provide information on the type and the degree of unsaturation of the fatty acid components.
Shimadzu-LKB 9 000 gas chromatograph mass spectrometer combined system was used. with a chemical ionization source. Sample introduction to mass spectrometer were made by the direct sample introducing unit. The mass spectrometric conditions for CI-MS were as follows. The ion source temperature was held at 200°C dualing the CI-MS runs. The mass spectra were all obtained at 500 eV of electron energy, 3. 5 kV of accelerating voltage, and 500 μA of emission current. The scan speed was 6. The pressure in ionization source was 0.5-1 torr.
Methane and iso-butane were used as the reagent gas, and iso-butane proved to be more suitable for fatty acid analysis, because of fewer dehydrated ions and fewer fragment ions from alkanes.
In the CI-MS analysis of fatty acids in coconut oil, nine fatty acids ranging from six to eighteen in the number of carbon atoms were identified. There was a good agreement between these data and the data obtained by GC (FID).
The fatty acids in rapeseed oil were analyzed as the more complicated sample, and C₁₂⁰, C₁₄⁰, C₁₅⁰, C₁₆⁰, C₁₆¹⁼, C₁₆²⁼, C₁₆³⁼, C₁₇⁰, C₁₈⁰, C₁₈¹⁼, C₁₈²⁼, C₁₈³⁼, C₁₉⁰, C₁₉¹⁼, C₂₀⁰, C₂₀¹⁼, C₂₀²⁼, C₂₀³, C₂₂⁰, C₂₂¹⁼, C₂₂²⁼, C₂₄⁰, C₂₄¹⁼, and C₂₆¹⁼ were identified. These data agreed quite well with the result of GC analysis by a BDS capillary column (50 ft×0.01 in.).
As other fatty acid samples, cottonseed oil, corn oil, peanut oil, linseed oil and Chinese paulownia oil were analyzed.