Gas-chromatographic quantification of methylphenidate in plasma with use of solid-phase extraction and nitrogen-sensitive detection.

The gas-chromatographic assay for methylphenidate described here involves isolation by solid-phase extraction and quantification by thermionic nitrogen-phosphorus detection. Methylphenidate and the internal standard, ethylphenidate, are extracted from plasma by partition onto C2 reversed-phase packing. Methylphenidate and ethylphenidate are eluted, dried, derivatized with trifluoroacetic anhydride, and gas-chromatographed, with nitrogen-sensitive detection. The standard curve for the assay is linear in the range 5-100 micrograms/L. The within-run CV is less than 4%, the between-run CV less than 6%. Mean analytical recovery of methylphenidate was greater than 90%. The smallest measurable concentration is 2 micrograms/L. The sensitivity, reproducibility, and economy of this assay make it suitable for clinical monitoring and pharmacokinetic studies.