ATR Infrared Spectroscopy of Thin Layers of Carotenoids and Related Compounds

Abstract

The infrared spectra of thin solid films of carotenoids on supports were investigated using multiple attenuated total reflection (ATR) spectroscopy technique. The specimens were obtained by the evaporation of 1 or 2 drops of appropriate solution of carotenoid on the both sides of quickly rotating ATR element. The carotenoids investigated were β-carotene, canthaxanthin, and lycopene. Retinal and retinyl acetate as related compounds were also studied. Multiple ATR infrared spectra of amorphous and crystalline films were recorded and compared with the corresponding spectra of the compounds in solutions and in bulk crystalline state (in KBr disks). The spectra of amorphous films were shown to be almost identical to the spectra of investigated compounds in solutions whereas in the spectra of crystalline (or partly crystalline) films the same band splittings as in the spectra of crystalline specimens appear. The values of the effective film thickness were calculated for all the compounds investigated and it was shown that the films studied were thin surface ones. The ATR technique was applied to the investigation of reactivity of the polyenes, patrticularly with the atmospheric oxygen in the dark and under illumination by visible and ultraviolet light. The oxidation is accompained by the graduate disappearence of the δ=C-H absorption band at 966 cm−1. The consumption of polyenic double bonds is accompained by the formation of carbonyl and hydroxyl absorption bands. Mechanisms of the initial stages of the process are proposed for different polyenes.