Louis:

I'm not familiar with the Merritt thesis, but based on my experience, the
only time I have ever observed the isotopic composition of CO2 gas from a
liquid-filled tank change by 1-2 per mil was during the last week or two of
the tank's life, when the liquid CO2 was expended and the tank pressure was
dropping rapidly from 800 psi.  Depending on how much the tank is used for
continuous-flow analyses, the tank can last for over a year, more than
offsetting any increased price.  Numerous aliquots of CO2 taken several
months apart to fill smaller reference gas cylinders for dual-inlet
analyses show no more than a few hundredths of a per mil shift in isotopic
composition, except right at the end of the tank's life.

Steve


>Pat,
>
>If I remember correctly, according to Dawn Merritt's Thesis (Indiana), using
>a liquid filled CO2 tank can result in fractionation of 1-2 ppt over the
>life of the tank.  We purchase "gas filled" tanks (which as I understand it,
>means pressure less than about 800 psi) in larger cylinders.  They won't
>last as long, but they cost less and seem to avoid the problem.
>
>Louis Elrod
>
>>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
>>
>>
>
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>Louis W. Elrod
>Research Scientist
>Geotechnology Research Institute
>Houston Advanced Research Center
>4800 Research Forest Drive
>The Woodlands, TX  77381
>
>Phone:     281-364-6061
>FAX:       281-364-6052
>E-mail:    [log in to unmask]
>WebSite:   http://gtri.harc.edu
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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