Assay of mixtures of padimate-O and oxybenzone in sunscreen formulations by high-performance liquid chromatography

A simple assay method for the quality control of some sunscreen products containing padimate-O and oxybenzone has been developed. A methanolic extract of the product containing sulfathiazole internal standard was subjected to reversed-phase high-performance liquid chromatography on a 10-micron Partisil ODS-2 column with methanol-acetonitrile (90:10, v/v) mobile phase. The drug-sulfathiazole peak height ratio was linear between 0.04-2.68 micrograms of padimate-O (r = 1.0003) and 0.02-1.05 micrograms of oxybenzone (r = 0.9997) injected. All peaks were well-resolved. Approximate retention times for sulfathiazole, oxybenzone and padimate-O were 3.9, 5.7 and 7.4 min., respectively. The height equivalent to a theoretical plate (+/- S.D.) were (n = 10) 0.79 +/- 0.07, 0.53 +/- 0.06 and 0.26 +/- 0.04 mm, for sulfathiazole, oxybenzone and padimate-O, respectively. Average percent recoveries (+/- S.D.) (n = 3) from simulated lotions containing 7% padimate-O and 3% oxybenzone were: padimate-O, 101.4 +/- 1.5%; oxybenzone 99.9 +/- 1.9%; from simulated lipsticks containing (a) 7% padimate-O and 3% oxybenzone: 103.8 +/- 1.2% and 100.1 +/- 0.9%, respectively; and (b) 7% padimate-O and 0.5% oxybenzone: 99.4 +/- 0.6% and 99.3 +/- 2.4%, respectively. The method was successfully applied to marketed products.