Superior spectrophotometric method for determination of a ternary mixture with overlapping spectra
Ambroxol hydrochloride (AMB), guaifenesin (GUF) and theophylline (THP) are co-formulated together for treatment of common cold and influenza. A newly developed spectrophotometric method; namely; ratio isoabsorptive point and ratio difference in subtracted spectra (RIDSS) spectrophotometric method was applied for resolving this ternary mixture with overlapping spectra in two steps. The proposed method is based on dividing the ternary mixture of the studied drugs by the spectrum of AMB to get the division spectrum, from which concentration of AMB can be obtained by measuring the amplitude values in the plateau region at 310 nm. Then the amplitude value of the plateau region at 310 nm was subtracted from the division spectrum and THP concentration was obtained by measuring the difference in amplitude values at 235 and 268 nm (corresponding to zero difference of GUF) while the total concentration of THP and GUF in the mixture was measured at their isoabsorptive point in the division spectrum at 235 nm (Aiso). GUF concentration is then obtained by subtraction. Validity of the method was tested by application to laboratory prepared mixtures and pharmaceutical formulations containing the studied drugs. Statistical comparison between the results of the proposed method and those of the reported HPLC method showed no significant difference.