Chromatography Forum

I have a headspace GC method for analysis of trace amount of C12 and C14. C14 alwasy gives more variation from injetion to injection.

Any suggestions on how to improve the method. I would like to adjust sample oven temp or injection volume, can someone give me some initial condition I can try?

The fact tahat C14 is the problem - and it's gettign to be a faily good sized molecule - suggests it may be a matter of keeping the molecule off the sides of the vessle used for head space sampling and the transfer components to the GC.

Can you give conditions for sampling, injectin ahd chromatography?

With headspace, you rely on good sample transfer. C14 bis already quite "heavy" and it may not be transported the right way from your headspace unit.
It may be ratained the transferlines, so make sure these are hot;
non reproducibility is often caused by "cold-spots".

On the injection side:
Make sure you have injection port at minimal 150C and use the right liners (1-2mm).
How do you inject? with a loop, a syringe or during a certain time? Normally injecting gases, discrimination effects are minimal, unless there is a cold spot.

Use minimal a split of 1:5.

Also: check your integration parameters on both peaks.

Sounds like a job for SPME. I think you will always struggle with static headspace for these - large by headspace standards - molecules.