Standard

[yxw]

Hi,

Thanks everyone for your help in my previous questions. Now I have a very basic question: I am doing the sterol test and the standards I have got are not pure ones. (For example The standards I have got are 95% and >=75%). I would like to know how you deal with this problem during your test if your standard is not pure. Thank you again.

[ntruong]

Ideally if the standard is pure, you wouldn't have any problem using it as external standard. However, when the standard is not pure (less than 95 or 90%) you might have problem using it for assaying since the concentration of standard should be the same as samples. Let alone unknown peaks possibly got buried under the standard, which might give you the false response. I would purify it further if it were me. If time is of essence and you have to use it...well...ICH guidelines does require you to qualify the standard before using it for material release or method validation. Whether the purity of the reference material critical or not depends on its intended use. In other words, you have to qualify the standard to ensure the accuracy of its assigned purity.
Regards,

[yxw]

Thanks Ntruong. Are you saying I can use it as external standard if a standard is 95% or greater pure. Can we accept this system error? I read some references in which their standards are nor pure (97% or 95%), they did not mention purifying the standards before tests. I am wondering how people do with these standards. Some pure standards are very hard to find or verey expensive.

[ntruong]

When we qualify the material for reference standard use, the minimum specification is usually 95.0%. However, it also depends on how hard it is to synthesize or its intended use. I don't see any problem using it as external standard as long as you characterize it (cGMP compliance).
I don't quite understand what you meant by system error? The purity of the material is assigned and valid after you performed a battery of tests. These include confirmation of identity (NMR, MS, IR, etc.), determination of chromatographic impurities, residual solvents, residual water, inorganic impurity,etc.
I guess I have the luxury of asking the synthetic group to purify it for me because we make most of our standard materials and qualify these in-house. Otherwise, I would have to spend tons of money to buy USP reference standards, which is not available in most cases.[/quote]