Steve,

The Merritt thesis indicates that when there are two phases of CO2 in the
cylinder (liquid and gas) they are isotopically distinct (the gas is
slightly lighter).  Since the analysis is drawing off the lighter gas and
the two-phase system is continuously re-equilibrating, the isotopic
composition of the cylinder can drift as much as +2 ppt from initial charge
to evaporation of last liquid (see Merritt, page 62), with only a +0.4 shift
after that.  She did claim that the liquid disappeared at 500psi, not 800 psi.

We only recently began to use the gas-only tanks, but they seem to work
well.  I think liquid was be fine, but we did re-calibrate it periodically.

Louis


>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
>>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>---------------------------------------------------------------------------
>Stephen S. Howe                         Office:  (518) 442-5053
>Dept. of Earth & Atmos. Sciences        Stable Isotope Lab:  (518) 442-4471
>Earth Science 352B                      FAX:  (518) 442-5825
>University at Albany                    E-mail:  [log in to unmask]
>1400 Washington Avenue        http://www.albany.edu/geosciences/sshowe.html
>Albany, NY  12222-0001
>---------------------------------------------------------------------------
>
>


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~