Naturally occurring epoxy acids: ii. detection and measurement of long-chain epoxy acids by near infrared spectrophotometry*†

Abstract

A new method for the detection and estimation of long-chain epoxy acids in seed oils is described. It depends on the measurement of increased absorption at 2.795 μ in the near infrared spectrum caused by chlorohydrins produced from epoxides by treatment with anhydrous ethereal hydrogen chloride. The method is sensitive to approximately 0.2% of epoxy acid in an oil and is specific for epoxides. Hydroxy components of a sample do not interfere since the strongly associated hydroxyl band of chlorohydrins is normally clearly resolved from other OH absorption. The presence of large amounts of vicinally unsaturated hydroxy acids, however, results in large changes in absorption intensity in the 2.8 μ region on HCl treatment and in these cases epoxide concentration cannot be accurately measured but must be estimated. These reactive hydroxy acids, which lead to spurious epoxide values by the conventional methods, lose hydroxyl during the acid treatment, and measurement of the decrease in their absorption at 2.762 μ means that their concentration may be estimated concurrently with that of epoxy components. Other reactive acids, such as cyclopropenoid acids, which result in high epoxide values by the usual methods, do not interfere. Results obtained by this spectrophotometric method are compared, for some oils, with those obtained by the usual chemical methods of epoxide determination.