Micro-determination of plasma diphenylhydantoin by gas-liquid chromatography.

A selective, sensitive and precise gas-liquid chromatographic method for the determination of diphenylhydantoin in micro samples of blood plasma is described. After a double extraction with chloroform containing an analogue of diphenylhydantoin as an internal standard, the drug and standard are N,N-dimethylated in alkaline aqueous solution with methyl iodide followed by extraction into acetone. The methylated derivatives are separated gas chromatographically and measured using a flame-ionization detector. The lowest concentration of diphenylhydantoin in plasma which can be measured in a 100 mul sample is 1 mug/ml, which is well below the normal therapeutic concentration of 10-20 mug/ml in plasma. The methylated derivatives of diphenylhydantoin and the internal standard have been identified by their proton magnetic resonance spectra and mass spectra.