CO2 causing negative peak

[algboston]

Hi,

I have a question about the CO2 response I am getting on my ECD. A little background about my system: I am running an ambient air sample through a Porapak Q column followed by a molecular sieve 5A column. The ECD is running at 340C.

The question I have is when I run my sample using N2 carrier gas, I see a small negative peak when CO2 elutes. When I add a CO2 dopant to the N2 carrier stream, the size of this negative CO2 peak increases. However, when I run the same system with 5% methane/argon carrier gas, there is no CO2 peak. I have determined that this peak is a result of ambient CO2 because when I scrub the air sample using ascarite, the negative peak disappears. The CO2 dopant is added as a makeup gas and only mixes with the N2 carrier gas right before the detector. I have tried purifying the N2 gas with a hot gas getter but there is no difference so I do not think it is a purity issue.

Can anyone help me figure out what's going on and why the different carrier gases cause different responses? I am trying to eliminate this negative CO2 peak when using N2 carrier gas.

Thanks for all you help!
Anita

[GasMan]

I suggest that you do a Google search on "ECD carrier gas doping", and you will see that this is a known phenomena, and can be used to enhance the sensitvity of certain groups of compounds. Maybe somebody like AICMM who posts here regularly can also throw some light on the issue.

Gasman

[algboston]

Thanks! I am using this dopant to enhance sensitivity for nitrous oxide. I just don't understand why nitrogen carrier gas gives a negative CO2 response while Argon/methane gives no response or why using CO2 as a dopant amplifies this negative response. The ECD chemistry can be very complex!

[nick576]

This article might help,

Quantification of N2O fluxes from soil-plant systems may be biased by the applied gas chromatograph methodology. Plant and Soil (2008), vol:311, issue 1-2, pages: 211-234

let me know if you can't access it,
Nick