Determination of ethambutol in plasma using selected ion monitoring

The determination of ethambutol in plasma is described. Using ethambutol-d4 as an internal standard, ethambutol and the internal standard were extracted with chloroform under alkaline conditions, and converted into their trifluoroacetyl derivatives with trifluoroacetic anhydride in benzene-pyridine (4:1). Selected ion monitoring was carried out by monitoring the peaks at m/z 294 and 296 corresponding to the fragment ion [M/2]+ of the derivatives. Ethambutol was determined by use of the peak height ratio of the peak at m/z 294 against that at m/z 296. The method was utilized for studying the bioavailability and pharmacokinetics of the drug.