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Issues with Permanent Gas Determination by GC-TCD

Question asked by sara.f on Jun 9, 2020
Latest reply on Jun 18, 2020 by sara.f

Hi all, 

I'm having an issue with the determination of Ar, O2 and N2 by GC-TCD and I'm hoping for any insight into what might be going on. 

First off, here's some info on the instrument/analysis method:

-Agilent 7890B GC with TCD detector

-molsieve column

-helium carrier gas

-cryo method using liquid CO2, where Ar/O2 are separated at 10 degrees C and N2 is detected at 25 degrees C

-gas samples are collected from field sites in pre-evacuated 12mL exetainer vials and loaded directly onto the autosampler 

-we calibrate using certified gas standards, which are injected into evacuated 12mL exetainer vials, just like the samples

 

My problem is I can't seem to get an accurate reading for O2, specifically for field samples and ambient air samples. The GC calibrates well with the certified standards (1% O2, 5% O2, 22% O2), O2 and Ar are well separated, but I'm getting over-estimates of O2 concentrations in my samples. We're working off of the premise that O2, at least in our systems, should never be higher than 20.9%, which is the percent composition of O2 in air. When I run ambient air on our GC using the method described above, I consistently get an O2 concentration of 23%, which shouldn't be possible. I appreciate that being off by 2% is usually not a huge crisis, but we're unable to report O2 concentrations that are higher than atmospheric values - it just doesn't make any sense. I've checked the peak integration, and to my eyes everything seems good. At this point I'm not sure if this boils down to a sample collection issue or an analysis issue. 

 

If anyone is familiar with this method for permanent gas analysis, or has any troubleshooting suggestions, I would really appreciate it. 

 

Thanks!

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