A sensitive liquid chromatographic assay for plasma aspirin and salicylate concentrations after low doses of aspirin

An original, highly sensitive and specific high performance liquid chromatographic method has been developed for the measurement of aspirin and salicylate in plasma. Minimum concentrations of 10 xg/L (aspirin) and 0.5 mg/L (salicylate) can be measured using 1 ml of plasma. After collection, plasma is first treated with physostigmine sulfate to inhibit enzymatic hydrolysis of aspirin to salicylate. Maximal recovery is achieved using an acid extraction into anhydrous diethyl ether with a subsequent drying-down step in an iced water bath. Aspirin and salicylate are separated by elution with a mixture of methanol, 1-butanol, orthophosphoric acid, and water on a reversed-phase octadecyl silane column at 47°C and detected at 234 nm by ultraviolet absorption. Quantitation is achieved using the peak height ratio of aspirin and salicylate to internal standard (m-anisic acid). The assay has been used for the study of simultaneous aspirin and salicylate pharmacokinetics after a single oral dose of 100 mg soluble glycinated aspirin for platelet antiaggregatory therapy in six subjects, one of whom was also studied after receiving a 600 mg dose