Analysis of beta-agonists in urine and tissues by capillary gas chromatography-mass spectrometry: in vivo study of salbutamol disposition in calves.

The fate of salbutamol sulphate given orally has been investigated in calves. The urinary excretion rate and the tissue distribution of this beta-agonistic drug were studied by capillary gas chromatography coupled to low resolution mass spectrometry (GC-LRMS) under electron impact (EI) ionization mode, using an hexadeuterated salbutamol analogue as the internal standard. The parent drug and metabolites were extracted via solid phase extraction (SPE) mixed-phase-containing disposable columns and analysed as their trimethylsilyl derivatives. A more efficient clean-up had to be carried out for tissue samples. An acidic precipitation followed by a liquid-liquid extraction were therefore performed before the SPE. Moreover, the problem of tissue digestion was elucidated by means of an ultrasonic probe. Samples were also analysed before and after enzymic hydrolysis using purified beta-glucuronidase and a mixture of beta-glucuronidase and arylsulphatase, to obtain evidence of phase II conjugation mechanisms. Both free salbutamol and conjugated metabolites were detected in urine and tissue samples. Except for liver or kidney, salbutamol was rapidly cleared from most tissues after a withdrawal period. The possible excretion of some phase I metabolites was also investigated, using further analyses under positive chemical ionization LRMS and high resolution mass spectrometry (HRMS).