Hi Karen!

I can second Jason's remarks.  We, too, use cheap CO2 for
equilibrating water samples.  Our CO2 comes from Air Products and is
called "Anaerobe grade."  We first fill the headspace above the water
samples in our reaction vials in a cumbersome but effective glove bag
and then equilibrate the samples at 40°C for 6 hours in the MultiPrep
before analyzing on our Optima.

For our reference gas, we use a 40 lb tank of "5.0 Research Grade"
CO2 from BOC Gases from which we draw the gas from the headspace
above the liquid CO2.  It has a d13C of about -10 per mil and a d18O
of about -0.5 per mil, which suits our mostly marine carbonate
samples well.  Because we do so few continuous flow analyses, this
same tank has lasted for over 8 years (about 38,000 dual-inlet
analyses) with essentially no fractionation.

Steve


>Dear Karen,
>We use industrial grade CO2 (very cheap) for our water
>equilibration with CO2 using a multiprep and prism II.  I also
>initially thought that better CO2 was necessary but was told but
>someone (Matt Emmon formerly with VG, I think) to just use plain
>CO2.  We have never had any problems in many years.
>
>For reference gas we either use homemade CO2 from carrera marble
>(for deltas close to zero) or 99.99% (HP) for more negative stuff.
>We can get HP CO2 with d13C of about -5 and d18O of about -12
>fairly consistently from our supplier.
>Jason
>
>
>On Mon Apr 18 00:26:33 EDT 2005, Karen Leslie
><[log in to unmask]> wrote:
>
>>Dear all,
>>
>>Many thanks to everyone who replied to my previous queries.
>>
>>I would like to know what grade of CO2 people use when
>>equilibrating waters
>>for subsequent delta 18O analysis on a mass spec. I assume that
>>the purer
>>the CO2 the better as impurities have the potential to be a
>>source of
>>contamination? The system is currently running with Scientific
>>grade but is
>>this necessary? Will UHP CO2 be just as good?
>>
>>Also, what are people using as reference gas? Is this the same
>>CO2 as used
>>for the equilibration or something different, say gas of a
>>special or
>>specific composition? This is for both waters and carbonates.
>>
>>Many thanks in advance,
>>Karen
>>
>>
>>Dr Karen Leslie
>>Earth Sciences
>>University of Queensland
>>St. Lucia, Qld, 4072
>>Australia
>>E-mail: [log in to unmask]
>>www.earth.uq.edu.au
>>Tel:    +61 7 336 51132 / 53281
>>
>
>
>
>--
>Jason Curtis

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