Determination of urinary metabolites of the UV filter homosalate by online-SPE-LC-MS/MS.

Homosalate (HMS) is an organic UV filter used in sunscreens and personal care products worldwide. It has been detected in various environmental matrices and in humans after application of HMS-containing products. However, sufficient data on the internal HMS exposure in humans is currently not available. Thus, we aimed at providing an analytical method for the sensitive determination of specific HMS metabolites in human urine. We describe the synthesis of analytical standards for the four oxidative HMS metabolites included in this method: 5-((2-hydroxybenzoyl)oxy)-3,3-dimethylcyclohexane-1-carboxylic acid (HMS-CA) and 3-hydroxy-3,5,5-trimethylcyclohexyl 2-hydroxybenzoate (3OH-HMS), as cis- and trans-isomers, respectively. After enzymatic hydrolysis, urine samples were analyzed using liquid chromatography-electrospray ionization-triple quadrupole-tandem mass spectrometry, including turbulent flow chromatography for online sample cleanup and analyte enrichment (online-SPE-LC-MS/MS). Quantification was performed by stable isotope dilution analysis, using deuterium-labeled HMS-CA as internal standards (cis and trans). Limits of quantification of 0.02-0.04 μg L were sufficiently low to quantify the HMS metabolites for up to 96 h (trans-HMS-CA), 48 h (cis-HMS-CA and 3OH-trans-HMS), and 24 h (3OH-cis-HMS) after a pilot dermal application of a commercially available sunscreen in one human volunteer, showing clear elimination kinetics. Furthermore, in a German pilot population (n = 35), HMS metabolites were above the LOQ precisely in those three individuals who had applied sunscreen within the previous five days, thus corroborating the specificity of the identified metabolites as biomarkers of HMS exposure. The method is currently used in a human metabolism study and will be applied in future population-scale human biomonitoring studies.