Simultaneous determination of oxybenzone and 2-ethylhexyl 4-methoxycinnamate in sunscreen formulations by flow injection-isodifferential derivative ultraviolet spectrometry

A flow injection procedure was developed for the simultaneous determination of two UV filters (oxybenzone and 2-ethylhexyl 4-methoxycinnamate) in suns creen formulations, based on the isodifferential approach. The use of the s econd derivative spectra allowed a selective determination of the analytes free from matrix interferences and without pretreatment of samples. Standard solutions of the analytes were analyzed by the proposed procedure in order to select the appropriate wavelengths to solve the mixture. After this, the procedure was validated using commercial sunscreen formulations t he concentrations of which were determined by an HPLC procedure. The two pr ocedures gave comparable results. The sensitivity for oxybenzone determination by the proposed method was (9. 2 +/- 0.1) x 10(-5) ml mug(-1), and the limit of instrumental detection was 0.5 mug ml(-1). Determination of 2-ethylhexyl 4-methoxycinnamate had a sen sitivity of (4.6 +/- 0.1) x 10(-5) ml mug(-1) and a limit of detection of 2 .1 mug ml(-1). The proposed flow injection procedure allowed on-line diluti on of samples, and a sampling frequency of 360 h(-1). The accuracy, speed a nd selectivity of the procedure make it suitable for this type of formulati ons. (C) 2001 Elsevier Science B.V. All rights reserved.