Chromatography Forum

I am looking for information regarding ambient air samples. We use an SRI GC in the field to analyze CH4(FID) and N2O (ECD) with an inhouse injection system.

Occasionally I get sharktooth shaped troughs in the baseline while not sampling or during sample analysis, doesn't matter which. We checked the baseline using a osciloscope with no anamolies. I believe it is a pressure issue but have no real evidence. How can I resolve this? The troughs are often minutes long and tens of mV in amplitude. It is NOT water.

Also, I see a peak resolve just prior to N2O and after the main air peak on the ECD. Any ideas? If you have a chromatogram research library that is accesible or know of one on the web I'd like to know.

Thanks much.
Kevin MSU

I already configured GC for such kind of application. I normally used a micro PED with a molecular Sieve for measurement of N2O and CH4 in air.
The peak previous to N2O could be N2 depending of the column you use. N2 normally elute just befoe N2O in Mol Sieve.
Let me know if you need more details

Could you possibly send me a chromatograph with peaks marked, and maybe method. Where does your large air peak fall out?

kahmark@msu.edu

I don't have chromatograms easily on hand.
However, the method I used was with 2 x 6 ports diaphragm valve. Valve 1 was injection valve with 1ml sampling loop. Valve 2 was a 6 ports heartcut valve.
The pre-column between valve 1 and 2 was a 6' 60/80mesh mol sieve at 80 celcius. The heartcut valve was used to open tight window to allow N2O and CH4 separately to go to the second column. The second column was a 12'long 60/80Mol sieve 60Celcius. The residual Air was eluting first in the plasma and after the N2O was eluting followed by CH4.
I don't have chormatogram but if you need help to configure such kind of system, let me know and I can help you.
I'm system integrator and I also take care of GC maintenance and GC configuration as consultant.
thanks
let me know if you want my contact