A sequential-injection system for spectrophotometric determination of p-aminobenzoic acid in sunscreens

A sequential injection method is proposed for spectrophotometric determination of p-aminobenzoic acid (PABA) in cosmetic formulations. The method is based on diazotization of the analyte, coupling with 8-hydroxy-quinoline, and the subsequent formation of a colored product. The experimental conditions used (coupling reagent, sandwich arrangement, volumes aspirated, propulsion flow rate, reaction coil length) were studied. Response of the sequential injection method were linearly dependent on concentrations up to 25 mug mL(-1) and the detection limit was 2 mug mL(-1). Throughput was 51 measurements per hour and a complete cycle, including three measurement per sample and a washing step, resulted in a sample throughput of 13.8 h(-1). Sunscreen formulations containing PABA were analyzed by the proposed method and validated by use of an HPLC procedure. The two methods gave comparable results. Yields ranged between 97-103% and good precision (RSD between 2 and 6%) was obtained. The accuracy, speed, and selectivity of the method make it suitable for safe application for quality-control of final products.