the determine of purity and consistency of commercial drug is difficult, please study the method of HPLC you can see more information

Natural product antibiotics are often supplied with bioassay or purity information as a measure of potency. The label will say something like 900 µg/mg, meaning that the product is 90% active ingredient.

This is a thorny question.  HPLC and activity, both answers given above, are good methods to use, and both are probably essential.  

However, it is important to realize that biological drugs can age and go bad with time, so keeping your main supply stored at the lowest recommended temperature and making aliquots that will last a few days or a week can help a great deal with reproducibility (the half-life of the aliquots depends on the specific substance).  That way, only small amounts of material are exposed to room temperature or 37 deg C or whatever your experimental conditions are.

What happens when things "go bad"? For example, glutamine and asparagine can hydrolyze to glutamate and aspartate, respectively, if you have a peptide or protein based based drug.  These changes can be seen in ion exchange chromatography, capillary electrophoresis or gel electrophoresis, depending on the kind of resolution you need.  Some amino acids can epimerize, giving you undesirable isomers. Some non-peptide-based drugs can also epimerize, and this can happen a lot with certain carbohydrates.  

So, keeping an eye on the properties of your drug, especially its activity, throughout the course of a set of experiments is important.  That's part of the role of positive controls (and why they should be run with every experiment).

The source of color in your sample could be a very tiny amount of highly-colored impurity.  Of course, that is speculation on my part.  It could also be a lot of moderately colored impurity. but techniques like evaporative light scattering, coupled with UV/Vis detection and HPLC, can sort that kind of problem out.  You are allowed to purify purchased materials, and sometimes it is necessary.  Published procedures are usually available.

Best of luck!

In general, journals recommend/require 90-95% purity for compounds being used for biological tests, which also means people accept up to 5-10% impurity in testing results.  Not that you shouldn't aim for the purest compound possible, but it's hardly necessary to redo experiments just because you got a new stock of compound, so long as the source/purity is similar.

Second, as was said, coloration can be the result of tremendously small impurities, so while it's not bad to pay attention to color, it's not something you should worry much about if the activity seems similar and the purity is similar.

But if you're worried, yes - HPLC or NMR is probably the simplest way to get a reading on purity, and if they both show a clean/pure compound, you're probably fine moving forward, whatever the compound color may be.

LCMS,NMR,FTIR ARE THE BEST TECHIQUES.COLOUR IMPURITIES U ACN REMOVE BY ANY DECOURISING AGENTS.AND THEN U MIGHT BE GET EXACT PURITY OF DESIRED COMPOUND

In order to determine purity of a particular drug, the best way to analyse is through HPLC - for this however, you need to keep a batch of the drug purchased from a reliable source (i.e. in its purest form - e.g. 99% pure) as the reference standard and compare it to the ones you are not certain of (e.g. another batch from the same manufacturer or from another manufacturer but of different purity).

You will then need to weigh the same amount of drug from each source, run HPLC and compare the peak area between each source with that of the reference standard. There is a formula for this where you will need to plug in the purity claimed for each source. This will indicate to you how pure a particular drug is with reference to your standard.

It is not necessarily true that all the properties of the drug has changed - it is likely that it has degraded due to date of expiry or improper storage/handling - therefore, the main concern would be that the amount of active drug would be considerably less, e.g. 1 mg of drug may not contain 1 mg of active drug any more as it has degraded. This would need to be quantified, otherwise, the safest option would be to purchase a fresh batch of drug and inform the manufacturer of these discrepancies.

Hi,

I prefer LC-MS as a preferred method to determine the purity of the drug. HPLC is also a very good option, but with MS you can get an additional support for the molecular species.