Micro-scale anticonvulsant assay with use of nitrogen/phosphorus detector and on-column methylation compared with a macro-scale procedure involving flame-ionization detection.

We report a modified micro-scale gas-chromatographic procedure for determining phenobarbital, primidone, and diphenylhydantoin in serum with use of a nitrogen/phosphorus detector. Serum, 20 mul, is extracted into chloroform, the solvent is evaporated, and the residue dissolved in dilute (32 mmol/liter) trimethylanillinium hydroxide, which yields the methyl-derivatives of the drugs upon on-column injection. Within-run precision (CV) for the three drugs was 1.0, 2.1, and 1.4+, respectively, at concentrations of 19.2, 13.2, and 18.9 mg/liter; run-to-run precision at these concentrations was 4.4, 3.8, and 2.8%. Comparison of the 20-mul micro-scale procedure (y) with our conventional, flame-ionization macro-scale procedure (x), in which on-column methylation is used and which requires 1 ml of serum, gave the following respective results: y = 1.01 X -- 0.16 mg/liter, r = 0.992, y = 1.02 X + 0.29 mg/liter, r = 0.973; and y = 0.96x + 0.04 mg/liter, r = 0.996.