Range of concentrations in a purity study?

[MikeT]

Hey guys. Just landed a job as a lab manager for a sports nutrition company. I need to set up a method to confirm that the purity of the compounds shipped to us is reasonable. For example, if L-Arginine comes in from China, I would need to confirm its purity using an external standard purchased from a reputable company (ex. Sigma-Aldrich). The thing is that looking at a nutrition label to decide what concentration of standards to use would be seemingly useless since I want to only know purity? My mind is just puzzled and confused about what to do! Thanks so much guys.

-Mike

[MikeT]

OOOPS....By the way, this is for U-HPLC

[Johnny Rod]

If you're talking about validating the method once it's up and running then see ICH document Q2. Typically for active ingredients you would look at 80-120% of the amoutn you should be finding.

In terms of buying a standard, for your example you'd buy pure solid material and make it up to the right concentration to match your smaple prep. If this doesn't make any sense then you probably need to have someone there with a bit more knowledge. Presumably you have to deal with the FDA so it isn't just a case of a quick check, especially when it comes to imported "active" materials, which is something of a hot topic.

[HPLCaddict]

If you must defend yourself to the FDA or similar then it's not that easy as buying a "reference substance from a reputable company". You must set up a working standard that can be traced back to a Certified Reference Substance (CRS).
Concerning the concentrations in purity determinations, it depends on the specifications for the impurities. We're talking about raw, pure compounds, right? Then it doesn't matter which concentration they will have in the final product. You need to set up reasonable (and probably defendable) specifications for impurities.

That said, I'd suppose the easist way would be to go with a pharmacopoial monograph method, if it exists for your substances to be checked. Arginine should be present in USP.

If you're not regulated (i.e. FDA doesn't impress you ), a quick check for Arginine purity might be just to measure optical rotation and forget that fancy UHPLC stuff...

[MikeT]

Thanks HPLCaddict. You answered my question 100%