A capillary electrophoresis method was developed to quantify caffeine and theophylline, xanthine derivatives with bronchodilator activity. Buffer concentration, pH and applied voltage were optimized using a central composite design-face centred. Separation conditions were: silica capillary tube, 75 μm (i.d.) and 61 cm (total length); absorbance detection, 280 nm; borate buffer, 20 mM, pH 9.0; applied voltage, 25 kV and 1 psi injection/8 s. Validation was performed in blank serum following the International Conference Harmonization guidelines: resolution (peaks without overlapping), linear range (0.125-50 µg/mL; r(2) > 0.9999), limits of detection and quantification (10; 20 and 33; 66 ppb for caffeine and theophylline, respectively), intra- and inter-day precision (Relative standard deviation lower than 1.9%) and accuracy (98-101%). Migration times were <8 min. This method is simple, specific and suitable and reaches high label claims (98.7-100.4%) in pharmaceutical formulations analysis. Moreover, the method was applied to the monitoring of the analytes in serum of patients.
