Determination of acetylsalicylic acid and salicylic acid in blood and plasma by HPLC

A method is described for the determination of acetylsalicylic acid and salicylic acid in human whole blood and plasma which uses liquid chromatography with UV detection. The enzymatic degradation of acetylsalicyclic acid in blood and plasma is examined. Addition of potassium fluoride as enzyme inhibitor and deep freezing after plasma preparation allows storage of plasma for one week. A mixture of acetonitrile and diluted acetic acid as mobile phase give the best peak shape. Traces of iron lead to slight peak broadening; addition of EDTA results in excessive tailing. The detection limit for acetylsalicylic acid and salicylic acid is about 250 ng/ml for a 1.0 ml primary sample. The method is sensitive enough for the monitoring of both drugs in bioavailability studies.