Measuring Ethylene in 7890A

Question

I am currently about to measure a mixture of ethylene and ambient air with an Agilent 7890A. I want to inject it manually with a syringe in to the front inlet, following to this method

https://www.agilent.com/cs/library/applications/5991-4873EN.pdf

The columns are 19095P-Q04 and 19095P-MS0.

No matter what I changed, I see only 2 peaks, which I would identify as Argon and Nitrogen with reference measurements. How is this possible?

Thanks and best regards

Robin

hoonte · Accepted Answer

Indeed, as mentioned above the ethylene needs to bypass the molsieve column otherwise it gets stuck, so to check for the correct switching moment of the valve; set the valve in the bypass mode, run a sample over the porabond Q column to the TCD, you will see 2 peaks, 1 for Ar+N2 and the 2nd for ethylene, then you define the time inbetween those peaks as the switching time for the bypass valve and run with the Ar+N2 peak going to the molsieve, switch the ethylene to the bypass and back to the molsieve after that peak has reached the TCD, so the Ar and N2 peaks can elute from the molsieve. Important to have a needle valve restriction in the bypass which gives the same restriction as the molsieve column, so you can keep the expected retention when switching the valve.

Lance Nelson · Answer

Do you have a valve configuration on your GC similar to that shown on page 2 of the application note? If the columns are simply connected in series to the inlet, you may not see anything more than the O2 and N2 peaks, as the other components would be trapped by the Molesieve PLOT column ( 19095P-MS0).

Measuring Ethylene in 7890A

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