USP Related Compounds repeatability issues

[shaorinor]

Hello all,

I was wondering if anyone out there had experience running the USP Related Compounds test per the Glycerin monograph. I ask because we are trying to get some semblance of repeatability from replicate injections and there really is very little.

This could probably be remedied by an internal standard, but since the monograph doesn't include one, we're stuck tooling around with the actual method. I'm thinking that the configuration of the liner, inverted cup/spiral structure, could be one of the main contributors in addition to the split flow. But, even if we reduce the split to nothing, we still see high RSDs.

Anyone out there have experience with this and have a remedy? Worst case, I was going to just write a huge RSD limit into our method.

Thanks for any help!

[shaun78]

Glycerin is one of those highly reactive compounds for a GC.

Use inert everything that you can get your hands on.

Perform conditioning injections of a standard ~10x more concentrated than that required for analysis.

Make injections of the 10x standard until peak areas become consistent; when this happens you have bound all of the active sites in the GC.

Continue with your run immediately after the peak areas of the 10x standard are consistent.

If there is any stop or pause for any reason, you are better off starting the entire process over.