Stoichiometric determination of hydroperoxides in fats and oils by Fourier transform infrared spectroscopy

A primary Fourier transform infrared (FTIR) spec troscopic method for the determination of peroxide value (PV) in edible oils was developed based on the stoichiometric reaction of triphenylphosphine (TPP) with hydroperoxides to produce triphenylphosphine oxide (TPPO). Accurate quantitation of the TPPO formed in this reaction by measurement of its intense absorption band at 542 cm(-1) provides a simple means of determining PV. A calibration was developed with TPPO as the standard; its concentration, expressed in terms of PV, covered a range of 0-15 PV. The resulting calibration was linear over the analytical range and had a standard deviation of +/-0.05 PV. A standardized analytical protocol was developed, consisting of adding similar to 0.2 g of a 33% (w/w) stock solution of TPP in hexanol to similar to 30 g of melted fat or oil, shaking the sample, and scanning it in a 100-mu m KCI IR transmission cell maintained at 80 degrees C. The FTIR spectrometer was programmed in Visual Basic to automate scanning and quantitation, with the reaction/FTIR analysis taking about 2 min per sample. The method was validated by comparing the analytical results of the AOCS PV method to those of the automated FTIR procedure by using both oxidized oils and oils spiked with tert-butyl hydroperoxide. The two methods correlated well. The reproducibility of the FTIR method was superior (+/-0.18) to that of the standard chemical method (+/-0.89 PV). The FTIR method is a significant improvement over the standard AOCS method in terms of analytical time and effort and avoids solvent and reagent disposal problems. Based on its simple stoichiometry, rapid and complete reaction, and the singular band that characterizes the end product, the TPP/TPPO reaction coupled with a programmable FTIR spectrometer provides a rapid and efficient means of determining PV that is especially suited for routine quality control applications in the fats and oils industry.