Pat:

Although I run a Carlo Erba EA interfaced with a Micromass Optima mass
spec, I, too, use a tank of carbon dioxide at about 800 psi as my reference
gas, with the delivery pressure to the reference injector box at about 50
psi.  The injector box delivers the gas to the mass spec at 20 psi.  You
should not need to worry about fractionation of the CO2 as long as you are
drawing CO2 gas off from the headspace above the liquid CO2 in the tank.
The liquid CO2 is essentially a very large reservoir, with the pressure in
the tank remaining at 800 psi for months.  It is when the liquid CO2 is
almost gone that you will probably observe a significant shift in the
carbon and oxygen isotopic composition of your reference CO2.  Around this
time, the pressure of your tank CO2 will begin to drop rather rapidly from
800 psi, and you'll know that it's time to buy a new tank.  If you use the
same reference CO2 gas for both dual-inlet and continuous-flow analyses, as
I do, and you switch back and forth between these two modes, you'll
probably notice first that the isotopic composition of the reference gas
has changed during dual-inlet analyses due to the greater precision of
dual-inlet analyses compared to continuous-flow analyses.  This is also one
reason why I prefer to calibrate my reference CO2 by analyzing a range of
carbonate NIST and reagent standards by dual-inlet analysis and then
periodically check the continuous-flow calibration by analyzing organic
carbon standards that usually have carbon isotopic compositions closer to
the usually more depleted organic carbon.

Hope this helps.

Steve


>I am runing continuous flow system with a Carlo Erba EA, ConfloII, and
>Delta S.
>I am setting up a new tank of CO2 reference gas and am not clear on the
>concept of, or need to worry about fractionation in a tank of CO2 at 800
>
>psi. Should I reduce the pressure so that all the CO2 is in the gas
>phase( what pressure would that be ). Or is combination gas/liquid phase
>
>not a problem.
>Thanks in advance for input.
>
>Pat Anderson
>Great Lakes WATER Institute

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