USP

[Consumer Products Guy]

OK, we all know USP test procedures are often quirky and/or outdated. I've come across one which uses a 50 ul injection (on a 4.6 x 250 mm 5u column) after the sample had been diluted a fair amount with HPLC grade water and organic solvent containing buffer, made to a defined pH. All I can think of that the guy/gal who developed the assay had a fixed-loop sampler of 50 ul, so that was used. My company has decided that we would deviate from USP and make the samples and standards up 10x more concentrated and inject 5.0 ul, to get the same amount of ng injected, and that would be equivalent. I'm just getting "entertained" by these quirks in USP offical procedures. For the same material, USP also details an active + degradation product system suitability mixture, and then a separate active solution for quantitation, and I believe my company gave the OK to just use the system suitability solution to assay the active content (the impurity level is determined by a separate procedure).

[Dan]

The method development may have gone as you have guessed; I have seen many methods (both older ones and more recent ones) developed using what the analyst had in the lab at the time.

As to changing the injection volume and sample/standard concentration, what you state sounds valid (at least scientifically), but I have not come across doing that before, so I cannot comment as to what the regulators might say. My guess: you would never get to try it as your QA group would freak out if you tried.

Is the active + degradant solution used for evaluation for the resolution criteria of the system suitability?

I have seen that often. Remember the USP and EP expect to have system suitability criteria for: reproducibility, tailing and resolution (at least) and the FDA and EMEA will also expect that. Although I have seen older methods where a resolution criteria was set for the standard with only the active peak in the chromatogram (it always passed!); you do not see that allowed much these days.

Regards,
Dan