Simultaneous determination of methocarbamol and aspirin binary mixture in their combined tablets by derivative and ratio derivative spectrophotometry

Two simple and rapid spectrophotometric methods were developed for the simultaneous determination of methocarbamol (MET) and aspirin (ASP) in their combined dosage form. The first method, zero-crossing first derivative spectrophotometry, is based on the measurement of the first derivative amplitude values at 279.2 nm and 246.6 nm for the simultaneous determination of MET and ASP, respectively. The second method, ratio first derivative spectrophotometry, is based on the measurement of the peak amplitudes of the first derivative of the ratio spectra at 284 nm for MET and at 238.2 and 246 nm for ASP determination. The calibration graphs were linear over the range of 2.0–30.0 μg mL−1 for MET and ASP by the two proposed methods. The detection limits of the first derivative method were found to be 0.2998 and 0.3227 μg mL−1 and the quantitation limits were 0.9084 and 0.9780 μg mL−1 for MET and ASP, respectively. For the ratio first derivative method, the limits of detection and quantitation were 0.2795 and 0.8470 μg mL−1 for MET, respectively, while for ASP the limits of detection and quantitation were 0.2485 and 0.7530 μg mL−1 at 238.2 nm and 0.4173 and 1.2647 μg mL−1 at 246 nm. The proposed methods were applied successfully in the assay of the studied drugs in their combined tablets. The results compared favorably to those obtained by the reference methods.