QUANTITATIVE-DETERMINATION OF HYDROPEROXIDES BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY WITH A DISPOSABLE INFRARED CARD

Disposable polyethylene infrared cards (3M IR cards) were investigated for their suitability for the quantitative determination of peroxide value (PV) in edible oils relative to a conventional transmission flow cell. The analysis is based on the stoichiometric reaction of triphen ylphosphine (TPP) with hydroperoxides to produce triphenylphosphine ox ide (TPPO). Preliminary work indicated that the cards, although relati vely consistent in their pathlength (+/-1%), had an overall effective pathlength variation of +/-similar to 5%, caused by variability in loa ding of the oil onto the cards. This loading variability was reduced t o <0.5% by developing a normalization protocol that is based on the pe ak height of the ester linkage carbonyl overtone band at 3475 cm(-1) w hich allowed one to obtain consistent and reproducible spectra. The st andard PV calibration approach, based on the TPPO peak height at 542 c m(-1), failed because of unanticipated card fringing in the region whe re the measurements were being made. However, the development of a par tial-least-squares (PLS) calibration provided a means of eliminating t he interfering effect of the fringes and allowed the TPPO band to be m easured accurately. An alternate approach to the standardized addition of TPP reagent to the oil was also investigated by impregnating the 3 M IR cards with TPP, thus allowing the reaction to take place in situ. The spectral analysis protocols developed (normalization/calibration) were programmed to automate the PV analysis completely. The 3M card-b ased Fourier transform infrared PV methods developed were validated by analyzing oxidized oils and comparing the PV predictions obtained to those obtained in a 100-mu m KCl flow cell. Both card methods performe d well in their ability to predict PV. The TPP-impregnated 3M card met hod reproduced the flow cell PV data to within +/- 1.12 PV, whereas th e method with an unimpregnated card was accurate to +/-0.92 FV over th e calibrated range 10-25 PV). Our results indicate that, with spectral normalization and the use of a PLS calibration, quantitative PV data, comparable to those obtained with a flow cell, can be provided by the 3M IR card. With the analytical protocol preprogrammed, the disposabl e 3M card provides a simple, rapid and convenient means of carrying ou t PV analyses, suitable for quality control laboratories, taking about 2-3 min per analysis.