Measurement of salicylsalicylic acid and salicylic acid in plasma by high pressure liquid chromatography.

Salicylsalicylic acid (SS) and salicylic acid (SA) both appear in plasma after the oral ingestion of the former. They can be estimated in the presence of each other after extraction into dichloromethane using high pressure liquid chromatography. SS is unstable in plasma, being converted to SA in vitro. Conversion can be prevented by using ethylene diaminetetraacetic (EDTA) as an anticoagulant and carrying out the extraction in the cold. An aliquot of the extract is dried and dissolved in methanol. Weak solutions of SS in methanol are unstable at room temperature. Methanolysis can be prevented by the addition of dilute hydrochloric acid. By adding phenylbutazone to the plasma as an internal standard, variations in extraction and sample application are nullified. Quality control is achieved by concurrent extraction of two different plasma standards containing SS and SA in known amounts. About 40 ng of SS and SA can be detected; reproducibility at 200 ng and above is better than 6.5 percent.