Automated theophylline assay using gas chromatography and a mass-selective detector.

An automated gas chromatographic--mass spectrometric assay for theophylline is described. Theophylline is extracted from plasma or urine (50 microliter) and transformed into an N-pentyl derivative. The internal standard used for quantitation is [1,3-15N, 2-13C]theophylline. The detection is performed by monitoring the molecular ions 250 for theophylline and 253 for the internal standard with a quadrupole mass specific detector HP 5790 A. The system has been fully automated: injection, calibration, assay, calculation. The method shows excellent analytical parameters: linearity between 2 and 40 micrograms/ml; day-to-day reproducibility 1.82% for a concentration of 15 micrograms/ml; repeatability 0.75% (15 micrograms/ml) and 0.33% (30 micrograms/ml). Accuracy is also excellent. Due to the use of an internal standard labelled with stable isotopes, the specificity and high analytical quality of the method make it useful as a reference method to compare with routine theophylline assays.