"I assume that the results you are referring to are from the software that you are using for peak purity analysis. The purpose of this software is to compare the spectra obtained at time points across the peak and detect differences which cannot be observed by eye. Therefore it is possible that there are spectral differences which may be explained by the presence of another peak eluting at a similar time as your drug. When a drug is degraded, the degradation products are often very similar in structure leading to similar retention times and UV spectra. This means that the spectral differences observed during peak purity analysis may be very small.To investigate whether your purity result is actually due to another peak or is just a result of the way the spectra have been compared by the software requires knowledge of how the purity result was generated. I recommend that you read through any available information on how the software compares the spectra.
- What reference spectrum is being used, particularly important if it is a gradient method;
- Compare the peak purity plot observed for your forced deg sample with that for a pure reference standard;
- Loss of linearity can result in spectral differences which are not due to another peak, try diluting your sample and compare the peak purity plot obtained with the original.