Determination of guaiphenesin in anti-tussive pharmaceutical preparations containing dextromethorphan by first- and second-derivative ultraviolet spectrophotometry.

Rapid, simple and direct assay procedures based on selective first (D1)- and second (D2)-derivative spectrophotometry, using a zero-crossing technique of measurement at 279.2 and 280.0 nm, respectively, have been developed for the specific determination of guaiphenesin in the presence of dextromethorphan, drugs with closely overlapping absorption spectra, in synthetic admixtures and in pharmaceutical dosage forms (tablets and syrups). The methods do not require extraction with organic solvents and are easier to perform than their conventional counterparts. Calibration graphs were linear (r = 0.99999 for D1 and 0.99969 for D2, respectively). Good selectivity, accuracy and precision were found. However, the performance of the analysis of guaiphenesin by the second-derivative mode deteriorated when the ratio of dextromethorphan to guaiphenesin was greater than one. Thus, the first-derivative spectrophotometry is the method of choice for the assay of tablets and syrups containing the two drugs.