Stable isotope dilution analysis of salicylic acid and hydroquinone in human skin samples by gas chromatography with mass spectrometric detection

A sensitive and accurate gas chromatographic–mass spectrometric (GC–MS) method has been developed for the quantitative determination of salicylic acid (SA) and hydroquinone (HQ) from human skin samples and cosmetic emulsions. Deuterium labeled SA-d6 and HQ-d6 were used as internal standards (IS). The samples were extracted with methanol, dried under nitrogen and derivatized with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) + 1% trimethylchlorosilane (TMCS). Quantification was performed in SIM mode with a limit of quantification (LOQ) of 50 ng ml−1 for SA and 10 ng ml−1 for HQ. The inter-day variation (R.S.D.) was less than 5% and the accuracy was better than 13.3% for both compounds. The recoveries from the different matrices ranged between 93.1 and 103.3% for SA, and 97.3 and 100.8% for HQ.