Purity of Standards

[Sallybeetle]

I am developing a few HPLC / ELSD methods for compounds that are currently assayed by the USP microbiology methods. The USP micro methods yield incredibly variable results from assay to assay. Therefore, I do not think it a good idea to use the USP designated purity (mcg / mg) to test samples or qualify a secondary standard.

Could I use an alternate method to test the USP or secondary standard to get a more accurate % purity? For example, using LC / MS and peak area ratios of any impurities versus the main analyte peak? Would this be cGMP to use the non-USP method % purity?[/quote]

[tom jupille]

f your compounds are (or will be) labelled as USP grade, they should be assayed by the USP procedure or equivalent.

When you validate your new method, you have to demonstrate accuracy. In general, this can be done by comparing results with a reference technique (in this case, your microbiological assay). Since your old assay is imprecise, it shouldn't be too hard to demonstrate that the new one gives values within the 95% (or whatever) confidence interval of the old method.

To be cGMP compliant, you have to follow your SOPs. Hopefully, there is an SOP in place that specifies what you have to do to demonstrate equivalence. If no SOP, write one.