GHG analyses

[NewToTheGC]

Hi all,

I'm interesting in setting up a Varian 2200 GC/MS that we have in the lab to do N2O analyses + ideally, CH4 and CO2. The GC currently has an ECD and headspace autoinjector.

I'll be analyzing both air and water samples, at gas concentrations near atmospheric levels (or above).

I've seen one paper (Loftfield et al 1997) that indicates I can run CO2 analyses using the ECD, but other methods (FID after methanization or TCD?) seem to be more common. Are there any problems I should be aware of running CO2 on an ECD?

As for N2O analyses, can someone direct me to information on detection limits for N2O? Would purge + trap increase these detection limits? How about injecting higher volumes of headspace?

Sorry for the rookie questions...

[MikeD]

I don't do ambient N2O measurements but here are a few performance indicators. The linear range of my PerkinElmer Ni63 ECD is about 2-80 nanograms at the detector (60 m GasPro PLOT capillary, ECD temp at least 300C). The useable dynamic range goes up to 300 ng but non-linear. A 2 ng quantitation limit, assuming splitless, translates to 20 ppm for a 0.1 ml injection volume. Of course other ECDs might be better, but it does seem that ambient 0.3 ppm measurements are out of reach without a pre-concentration step. I use Molecular Sieve 5A. The retention volume for N2O on MS5A is 7 litres/g at 20C. This is the thermodynamic measure Vg. The breakthrough volume will depend on the shape of your trap and its weight. The minimum recommended desorb temperature is 150C. MS5A will adsorb almost all the water vapour in the purge gas so it can be a problem to manage effects at the trap/GC interface.

Can't help with the CO2 but I don't get any interfering ECD signal from CO2 and it adsorbs on MS5A in large amounts.

[MikeD]

Sorry, correction - 2ng N2O in 0.1 ml is 11 ppm by volume at 20C.

[NewToTheGC]

Thanks Mike,
That'll help me get started...
-Helen

[CE Instruments]

I am very puzzled ! You have a GC/MS and headspace system that includes an ECD. You want to analyse N2O, CO2, ME in air and disolved in water ?
If you have a GC/MS why not use this for gases in Air using a gas sampling valve ?
Disolved gases in water are more of a problem as levels will be low and water is a terrible solvent to use with GC. Headspace, not useable unless you can fill the vial under Helium and the vial are capable of sealing in Helium and the gases you are interested in. Purge and trap, I very much doubt you can sufficiently trap these gases well enough even if you can ensure a low enough blank in the system in the first place. I thought that other techniques were much better for dissolved gases.
ECD detectors work by getting compounds to capture an electron, they work well for Halogenated species and certainly detect O2. I have never heard of anyone looking at CO2. ECD does not give the same responce for different capturing species and also gives a different responce for different compounds. Quoted responses with halogenated pesticides are irrelevant in determining the detector sensitivity for other species. Most GC methods I have seen for CO2 are either HWD/TCD at levels above 100ppm or Methanisation and FID for below (old GCs) modern use Discharge Ionisation Detectors that are universal very sensitive gas detectors.
I look forward to responses from those who routinely carry out this type of analysis.

[NewToTheGC]

I have seen one paper (but one only) that suggests I can analyse CO2 using an ECD. However, I anticipate we’d have to add a FID to run CO2 and methane analyses (or, perhaps run them on a second machine). Anyhow, for now my primary concern is the N2O.

I’m interested in concentrations of these gases dissolved in natural waters – so the samples will be water samples, but I will use headspace equilibrium – either using He in the headspace, or air, and subtracting N2O in the air (though, I presume He is better at low concentrations).

I presume I would have to strip the CO2 and H2O out of the gas using a pre-column… Anyhow – I can look into whether any purge and trap methods have been developed for N2O…
Thanks.