Selected ion monitoring of metoprolol and two metabolites in plasma and urine using deuterated internal standards.

A highly sensitive and specific quantitative assay for metoprolol and two of its metabolites, containing an unchanged 2-hydroxy-4-isopropylaminopropoxy sidechain, has been developed. The compounds are isolated from the alkalized sample (plasma or urine) by extraction with dichloromethane, and converted to trifluoroacetyl derivatives by reaction with methyl-bis-(trifluoroacetamide). The reaction mixture is gas chromatographed on an OV-17 column and each substance is assayed by electron impact mass spectrometry using selected ion monitoring, and quantified by comparing the intensity of fragment ion m/z 266 with the intensities of corresponding fragment ions from the deuterated internal standards (m/z 270 and 271). It is possible to determine concentrations as low as 1 nmol l-1 (0.3 ng ml-1) in 1 ml of sample with relative standard deviation of less than 10%.