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Abstract
A gas chromatography-mass spectrometry (GC-MS) method for determining methadone in urine and plasma by liquid-liquid extraction with proadifen as internal standard is proposed. The linear concentration range is 0.05–10 μg/mL for urine and 0.05–1 μg/mL for plasma, and correlation coefficients are quite good. The mean recoveries achieved are 64.30% from urine and 79.03% from plasma, with a coefficient of variation less than 1.25%. Samples from patients under a detoxification treatment involving administration of daily doses of 20–200 mg of methadone exhibited concentrations of this drug over the ranges 0.10–13.25 μg/mL (urine) and 0.07–0.42 μg/mL (plasma), which confirm that it is excreted largely via urine.