Poor repeatability of GC quantification analysis

[simonwei]

Hi all:

I'm using GC-FID (model number: HP 5890 series II) for quantification analysis of chloroform vapor in a gas sample. The molar fraction of chloroform is about 20%. Due to limitation of apparatus I have, I use a syringe to manually transfer gas sample from a gas sampling bulb to GC. The repeatability is very low. Run-to-run variation can be above 20%. From textbooks and experienced people, manual injection may introduce some uncertainty, but I doubt it causes that hight RSD. What is RSD you would expect with manual injection? Where could be the error source accounting for this large uncertainty other than injection variables?
I try split and splitless modes, but there seems no difference in terms of repeatability. Which one would be suggested for my case?
Thanks for your time.

Best Regards,
Simon

[chromatographer1]

Manual injection of a gas, sampling from a container is not easy if reproducibility of a high order is desired. It requires great technique and it is rare to achieve this goal without experience and a lot of practice. The syringe itself has to be qualified for the task and still errors can occur. Analysts years ago went to gas sampling valves to achieve high reproducibility and this requires a large amount of gas to purge lines and uniformly fill sample loops.

Your results are not surprising. Practice and more practice may reduce your variability IF your hardware permits it.

Good luck,

Rodney George
consultant

[AICMM]

Simonwei,

The problem could also be in the gas sampling bulb. You should have some glass beads in the bulb to do some mixing after you have prepared the standard. You might also consider a lower concentration standard as a troubleshooting tool. At that concentration, I would be concerned about the chloroform starting to condense on the walls....

Having said that, some other comments. Use a gas tight syringe, use split injection (you can certainly afford to at 20 mole %), use very consistent technique (time you sampling, time your injection, etc...) and minimize your sampling volume.


Best regards.

[Consumer Products Guy]

Does your gas syringe have a valve to keep your sample intact as you move from sampling bulb to GC? Or is it open to the atmosphere.

[Bruce Hamilton]

Badger your boss for a gas sampling valve - spending your life with workarounds isn't a good idea.

If your syringe doesn't have a valve, you can use the two septa technique.

You push the needle through one septum ( soft single layer silicone septum is good ), push needle into the gas sampling flask, obtain the sample.

Then push the septum on the needle hard against the flask septum and withdraw the syringe until the needle tip is in the second septum.

When injecting you just push the septum against the GC inlet and then push syringe needle through both septa.

That technique worked OK for me, but I still highly recommend obtaining a gas sampling valve.

Please keep having fun,

Bruce Hamilton