Orthophosphoric Acid Solution

A mobile phase system consisting of acetonitrile and 25mM phosphate buffer of pH 3(sodium dihydrogen phosphate monohydrate adjusted with orthophosphoric acid) in a ratio 60:40 (v/v) were used. The mobile phase was degassed and filtered by passing through 0.45 µm pore size membrane filter (Millipore, Milford, MA, USA) prior to use.
The flow rate was 1.0 mL min-1 all over the run. The injection volume was 20 µL. The eluent was monitored by the diode array detector (DAD) which was set at 250 nm for the quantitation of both VAL and SAC. All determinations were performed at ambient temperature.

Standard Solutions

Stock solutions of either VAL or SAC 1000 µg/mL were prepared in HPLC-grade methanol. The solutions were stored in refrigerator at

HPLC-grade methanol (15 mL) was added to a quantity of the powdered tablets equivalent to one tablet. This solution is sonicated for 20 min. Filtration through Whatman No. 1 filter paper followed by quantitative transfer of the filtered solution into 25 mL volumetric flask and then dilution was made to volume with methanol. A further dilution was made to reach final concentration of 102.8 µg/mL, 97.2 µg/mL for VAL and SAC respectively. Four aliquots (0.1 ,0.5, 1 and 2 mL) of the diluted solution were transferred to 10 mL volumetric flask and diluted with distilled water to obtain concentrations within the linear range of each studied drug then treated as under General Procedure and the corresponding regression equations were used to calculate the recovered

The flask was heated in a water bath at 40 ˚C for 30 min. After the specified time, the flask was cooled to room temperature, diluted to volume with distilled water to obtain a concentration of 20 µg/mL, filtered and injected into the HPLC system.

Photolytic Degradation

Photo- stability study was performed by exposing 10 mL volumetric flasks containing aliquots from VAL and SAC stock solutions to sunlight during day time for 4 hours. Moreover, in order to study stress degradation in direct UV radiation, a similarly prepared flask was subjected to UV radiation at 254 nm for 4 hours at ambient temperature. After the specified time, the solutions were completed to mark by using distilled water to achieve a final concentration of 20 µg/mL each, filtered and injected into the HPLC system.

Dry Heat Degradation

For thermal stress, 10 mg portions of each of VAL and SAC dry powder were placed in porcelain dish in a controlled-temperature oven at 100˚C for 4 hours. After the specified time, the content of the porcelain dish was transferred quantitatively with HPLC-grade methanol into a 10 mL volumetric flask and the volume was made to the mark by using the same solvent. Then, an aliquot of this methanolic stock was diluted to volume with distilled water to obtain a final concentration of 20 µg mL-1. After the preceding treatments, solutions were filtered and injected into the HPLC