Chromatography Forum

What is the difference between the potency of a ref std and the purity?

Usually:

Potency = [100 - Water content (%)] x purity


Generally on a CoA, purity is measured by a technique (HPLC, TLC etc) which does not account for (measure) the water content of the material.

Water content will be documented elsewhere on the form.

I just wish there was a "Standard Template Style" that all CoAs adhered to, life would be so much easier

Potency will typically adjust for the purity based on the primarry method (eg LC or GC) and impurities not determined by that method. These include residual solvent, residual inorganic content, water and even impurities that are determined by an alternate method. Potency will let you calculate amount/mg of the substance.

Potency can change significantly over time if the material is hygroscopic, while the purity may remain the same.

potency often means the amount of the desired compound in the sample: when you weigh 100 mg of a standard with 90% pontency, there's only 90 mg of the targeted compound in there.

in most cases, purity = chemical purity. that is, percentage of the target compound as determined by the primary method. if HPLC with UV detection is used, then purity is only based on components detected by this method. a standard with 99.5% purity may still have only 90% potency.

in the above example, the remaining 10% may be water, residual solvents, counter ion (if the compound is a salt), other residual inorganic content, and organic impurities determined other than the primary method.

In some cases, you can not simply use the equation listed by JGK to calculate potency. it assumes the only thing not detected by the primary method is water.

Strange nomenclature. I could understand it if one talked about a substance which is chemically undefined and activity units instead of weight is used to have a handle on amount. If a substance contains H2O, etc., it isn´t pure in all the definitions I ever saw. (I am just wondering about the legal ramifications if a company presented their compound in a catalog as 99% pure, but in actuallity it was only 49% + 50% water).
Also, why would chromatography not indicate the presence of water. etc., if one did everything correctly?

This is generally useed in cases where chromatography is used for area percent detection of the main coumpound and impurities in the absence of a reference std against which to assay the material.
potency = 100-%water-%solvent-%related substances (by chromatograpy)-%inorganics etc

It seems understandable if you called this a "blind value" instead of "purity".

Unfortunatley, you will have a better chance of winning the lottery than ever getting consistent terminologies on certificates of analysis