Simultaneous RP-HPLC estimation of tizanidine and valdecoxib in tablets

A reverse phase liquid chromatographic method was developed for the simultaneous estimation of tizanidine and valdecoxib in tablets. This method is based on using a Hypersil BDS C₁₈ column using a mobile phase of 10 mM octane sulphonic acid sodium salt and 0.3% triethylamine (pH adjusted to 3.5±0.1 with orthophosphoric acid) and acetonitrile in the ratio of 70:30 v/v. Rofecoxib was used as an internal standard. The retention time of tizanidine, valdecoxib, and rofecoxib were 3.15, 10.92, and 16.24 min respectively. The method was found to be linear (correlation coefficient r>0.999), precise (Residual standard deviation: 0.62% for tizanidine, 0.86% for valdecoxib), accurate (overall average recovery yields: 98.7% for tizanidine, 99.2% for valdecoxib), and selective.