methane, air-peak?

[bhvonkorff]

I’m trying to set up a Shimadu GC-2014 to measure methane, and I am new to using GCs. When we run a pure air sample, two small peaks occur at .546 and .722 minutes. The peak for methane standards (dilutions of 500ppm methane in air) comes off at the same time as the second air-peak (.722). Does this mean there is a small amount of methane in the air? Or, is there poor separation? I assume it wouldn't be quantitative to subtract the air-signal from our samples.

Here are the settings we are using: Porapak N column (80/100 mesh), FID temp (200 C), column temp (50 C), packed flow rate: 10ml/min. The methane peak follows the air-peak even after adjusting the temperatures/flow rate.

Any suggestions? Thanks!

Ben

[Don_Hilton]

A bit of research on line shows: http://www.oar.noaa.gov/spotlite/2006/spot_methane.html methane is found in air, has been measured, and is discussed as related to the environment.

[chromatographer1]

You won't see an air peak with the FID unless you inject a goodly amount of air, enough to change the fuel - air mix of the FID.

You might see an 'air' response then with the FID.

Air normally has methane in it. Go to wikipedia.com to find out how much or search the archives here as I answered that question a few weeks ago.

best wishes,

Rodney George

[AICMM]

Syringe injection or valve injection? The "peak" you see could be a pressure pulse at the detector....

Best regards.

[kltowery]

Hi bhvonkorff,

My project actually centers around measuring atmospheric methane, and there is typically about 2ppm in air.

I'm also pretty new to GC, but I would say if you're looking to measure methane at low levels in this range, you'll want to make sure your standards are made with a different balance (I use N2) so that you're not adding the 2ppm atmospheric in addition to your standards.

[bhvonkorff]

We are using a valve injector. After doing some internet research as suggested, a methane concentration in air around 1.5-2 ppm seems reasonable. The air-peak is about 1/6 the area of the 10 ppm standard, so that makes sense. I appreciate your help!
Ben