Could someone tell me that the sample (raw material or dosage form) must be dissolved in a sovlent during stress study by acid, base, UV, etc.? I have a compound which will degrated very fast under UV (254nm)if it is dissolved in methanol or ACN but not in the formulation or in raw material form. To disoolve the formulation ranther than it original form (such as lotion) may not represent real eviroment as the formulations would have. Should one use the original form of the sample such as solid or semi-solid for stress study? The sample eventually will be dissolved in prooer solvent for HPLC.

One would typically form a thin layer of drug substance in a petri dish in the light chamber in order to determine the propensity of the solid to degrade. With respect to the dosage form I would suggest to perform your study according to the ICH guidance on photostability with actinometry. If this is just a scientific exercise, the prudent thing would be to suspend the drug substance in a liquid which does not dissolve the drug substance and stir the suspension for the entire course of the study. I hope this gets you started.

It isn't neccessary to dissolve your sample into a solvent, sometimes it will give you a better peakshape. When you are talking about the original form, like lotion you should aware that you dont bring to much dirty on your column (like lotion oil, fat and many more), then its more comfortable to extract your sample