Chemical stability of ethyl icosapentate against autoxidation. II. Effect of photoirradiation on oxidation kinetics.

The effect of ultraviolet (UV) or visible light (VIS) irradiation on the chemical stability of ethyl icosapentate [ethyl-(all-cis)-5,8,11,14,17-icosapentaenoate] (EPA) was investigated at 45 degrees C by means of HPLC and by measuring the peroxide value (POV). EPA was oxidized to peroxides after an induction period by photoirradiation, and the peroxide subsequently degraded to secondary products. The autoxidation of EPA followed consecutive reaction kinetics including an induction period, and the kinetic parameters of the oxidation were calculated based upon the consecutive reaction model by computer curve fitting. The results of the degradation rate constant, k, and the induction period obtained by HPLC showed that the radical and the peroxide formation rates are affected by UV, but not by VIS light irradiation. The formation rate constant of peroxide, k1, and its degradation rate constant to secondary products, k2, obtained from the POV under UV light irradiation, increased with irradiation intensity, during which the induction period decreased. On the other hand, k1, k2 and the induction period by VIS light irradiation did not change significantly. The relationship between the induction periods obtained by HPLC and POV and the UV light irradiation energy were superimposed in the plots, indicating that these parameters depended on the UV irradiation energy. The relationship between k1/k2 ratio and the UV irradiation energy suggested that the formation of secondary products was more remarkably accelerated by UV energy than that of peroxide.