Stability of β-agonist methyl boronic derivatives before gas chromatography-mass spectrometry analysis

A method for the confirmatory analysis of clenbuterol, mabuterol and salbutamol in bovine urine and feeds by gas chromatography-mass spectrometry (GC-MS) is described. β-Agonist residues were extracted and further purified and concentrated by immunoaffinity chromatography. Methyl boronic acid (MBA) was used for the derivatisation of β-agonist residues and time and temperature of derivatisation were optimised. These derivatives allowed the use of shorter columns while improving the chromatographic resolution. Methyl boronic derivatives were stored either at room temperature or at −20 °C and then injected into the gas chromatograph-mass spectrometer at specific time intervals. Methyl boronic derivatives showed a slight decrease for clenbuterol and a more marked loss for mabuterol when stored at −20° C up to 3 days and then remained stable up to 7 days. However, the stability for salbutamol was poorer as it rapidly decreased at 2 days. When derivatives were stored at room temperature, clenbuterol and internal standard (IS) showed good stability up to 6 h while the stability of mabuterol was progressively reduced. Salbutamol dropped to very low values at just the first hour. Under these conditions it is clear that the analysis should be performed as soon as possible after derivatisation. The combination of immunoaffinity chromatography, MBA derivatisation and gas chromatography-mass spectrometry has shown a high sensitivity, good reproducibility and short chromatography time for the control of the studied β-agonists.