Peak purity of the samples resulted from stress degradation was checked using an Agilent 1100 HPLC system (Agilent Technologies, USA), equipped with a quaternary pump, an on-line degasser, an auto-sampler, a column oven, and a Photodiode Array (PDA) detector coupled with a ChemStation Software version B.04.01 (Agilent Technologies, USA). For the peak purity analysis, the spectra were corrected for background absorption caused by the mobile phase or matrix compounds, by subtracting the appropriate reference spectra. The spectra were assessed in the range of 225–254 nm and the mode of “best possible match of the entire spectrum” was selected as the mode of normalization.