Hello Jennifer,

I would be interested in responses to your question since I have 
mixed gases in a similar way in the past, but assumed the gases would 
act as perfect gases (like I was taught in school) and mix on their 
own in a short amount of time.  My experiments were pretty rough (on 
purpose) but the relative concentrations worked quite well, and the 
resulting calibration curves were linear.

Lola.




.



At 09:44 AM 5/19/2009, you wrote:
>Hello fellow isotopists,
>
>I have an LGR CO2 isotopic analyzer (CRDS) that I am trying to 
>calibrate.  I need
>standards are several different CO2 concentrations (400 ppm - 1000 
>ppm) because the
>isotopic values have a strong concentration dependance.   I have 
>been making a 400 ppm
>CO2 gas standard from a tank of isotopically calibrated CO2 
>(99.99etc.%) by filling a
>smaller gas canister with 400 Torr CO2 and then backfilling to 1000 
>Torr with high purity
>N2.  I am mixing the gases using a recirculating pump.  I don't know 
>how long it takes to
>homogenize the gases.  I am getting weird results when trying to 
>calibrate the CRDS, but
>I'm not sure if that's because of the CO2 concentration dependance 
>of the CRDS or
>because I am isotopically fractionating my gases when I make my 
>standards.  I don't
>know another way of making an isotopic standard with CO2 in such low 
>concentrations,
>and I cannot check the d13C for these standards using dual inlet 
>IRMS because there
>simply isn't enough CO2 in the standard to make that measurement.
>
>Any suggestions from IRMS and CRDS people would be appreciated, 
>especially anyone
>else using a CRDS CO2 isotopic analyzer.
>
>Thanks!
>Jen
>
>Jennifer Stern, PhD
>Space Scientist
>Atmospheric Experiments Laboratory
>Solar System Exploration Division
>Code 699
>NASA Goddard Space Flight Center
>Greenbelt, MD 20771
>301-614-6062
>301-614-6406 (fax)
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