Poor Reproducibility

Chromatography Forum: GC Archives: Poor Reproducibility
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Friday, January 12, 2001 - 03:59 pm:

I am in the process of getting qualified in a validated GC procedure (with no previous GC experience), but am having problems with the reproducibility in one of the peaks of interest. It is a headspace method used for a limit test for three residual solvents (acetone, DMF, and MeCl2). For the system suitability we inject a solution spiked with the three solvents 3 times. The peak heights for the acetone and MeCl2 are tight between the two methods. However, the peak heights for the DMF vary considerably (~20%RSD). Does anyone have a suggestion on what could be going on here?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By John Hinshaw on Tuesday, January 16, 2001 - 07:47 am:

DMF has a lower vapor pressure than the other solvents -- is its peak size considerably less? RSDs will generally be higher the closer that peaks approach the detector noise level. You should be looking at peak areas: how do the peak integration start and stop points look?

To get more of the DMF into the headspace you may need to run at a higher headspace sampling temperature and you may require a longer equilibration time than called for in, for example, the USP OVI method.

I'd be happy to discuss this further with you offline.

John Hinshaw


Top of pagePrevious messageNext messageBottom of pageLink to this message  By RJG on Tuesday, January 16, 2001 - 11:57 am:

I think it is best to look at both area and height and choose whichever gives the best %RSD. Area is not always better.

It would be helpful to isolate your problems to either the GC or the sampler. If you still have an injection port on the GC, try making some injections using a syringe.


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