Simultaneous determination of theophylline and guaiphenesin by third-derivative ultraviolet spectrophotometry and high-performance liquid chromatography.

Two methods are described for the simultaneous determination of theophylline and guaiphenesin in combined pharmaceutical dosage forms. The first method depends on third-derivative ultraviolet spectrophotometry, with the zero crossing technique of measurement. Third-derivative amplitudes at 222 and 278 nm were selected for the assay of guaiphenesin and theophylline, respectively. The second method is based on high-performance liquid chromatography on a reversed-phase column using a mobile phase of 0.01 mol dm-3 sodium dihydrogen phosphate-methanol-acetonitrile (8 + 2 + 1) (pH 5.5) with detection at 245 nm. Both methods showed good linearity, precision and reproducibility. The proposed methods were successfully applied to the determination of these drugs in laboratory-prepared mixtures and in capsules or elixir.