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Liquid chromatography-atmospheric-pressure chemical ionization mass spectrometry (LC-APCI-MS) of some plant triglycerols gave mass spectra with the peaks of [M + H] + and [M - R 1 (R 3 ) COOH + H] + ions, in which [M + H] + and R 1 (R 3 )COOH represent, respectively, the protonated molecular ion and fatty acid at the sn-1-(or -3-)position of the triacylglycerol. It was possible to discriminate fatty acids between sn-1-(or -3-) and -2-positions. The ratio of the molecular extinction coefficient (e) of triacylglycerol at 220 nm to that of cholesterol depended on the total number of double bonds in the triacylglycerol. Thus, triacylglycerols in naturally occurring oils could be determined by using LC-APCI-MS with spectrophotometry. LC-APCI-MS of hydroperoxidized triacylglycerols gave characteristic fragment ions [M - H 2 O 2 + H] + , [M - H 2 O + H] + and [M - R 1 (R 3 ) COOH - H 2 O 2 + H] + .