Simultaneous determination of oxybenzone and 2ethylhexyl 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 sunscreen formulations, based on the isodifferential approach. The use of the second 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 the concentrations of which were determined by an HPLC procedure. The two procedures gave comparable results. The sensitivity for oxybenzone determination by the proposed method was (9.2±0.1)×105mlµg-1, and the limit of instrumental detection was 0.5µgml-1. Determination of 2-ethylhexyl 4-methoxycinnamate had a sensitivity of (4.6±0.1)×10-5mlµg-1 and a limit of detection of 2.1 µgml-1. The proposed flow injection procedure allowed on-line dilution of samples, and a sampling frequency of 360h-1. The accuracy, speed and selectivity of the procedure make it suitable for this type of formulations.