UV photostability of insect repellents evaluated through Raman spectroscopy
2016
The use of insect repellents either indoors or at places with incidence of solar radiation has been common due to dengue
epidemics in Brazil. The lack of studies on the photostability of these substances has motivated this study, where the
main goal was to verify the photostability and photodegradation of some of the commercially insect repellents available
under the simulated ultraviolet (UV) radiation, by evaluating the molecular changes using dispersive Raman
spectroscopy (830 nm excitation). A laboratory-made chamber was used for irradiating the repellents, where UV-A +
UV-B radiations (UV-A: 5.5 mW/cm 2 and UV-B 1.5 mW/cm 2 ) can be obtained. The chamber internal temperature did
not exceed 31 °C during experiments. The compounds n,n-diethyl-m-toluamide (DEET), IR-3535, andiroba and
citronella oils, used as active ingredients in insect repellents, and commercial formula containing DEET (14.5% in
ethanol and isopropyl myristate) and IR-3535 (16% in carbopol) were continuously irradiated for 8 h. The Raman
spectrum of each sample was obtained before and after UV exposure. The compounds and the commercial formula
containing IR-3535 showed photo-stability when irradiated, since no changes in the peaks were found. The commercial
formula containing DEET showed spectral decrease at 524, 690, 1003 and 1606 cm -1 , assigned to the DEET, and
increase at 884 cm -1 , assigned to the ethanol. These results indicate that the excipient could influence the photostability
of the active ingredient. The Raman spectroscopy can be suitable to monitor the photodegradation under UV irradiation
rapidly and reliably.
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