Isotopers,

I agree with what Dennis Coleman has to say.  The possbility of adsorption
of CO2 and subsequent fractionation occurs all along the gas pathway to the
mass spectrometer. Keep your system as clean as is possible, but remember,
some sources of fraction are impossible to find and/or difficult to remove.

Run lots of carbonate standards to test your prep line, and lots of gas
standards, (across your typical pressures), to test  your mass spec.

Cheers,
Bob Fifer
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Dennis Coleman wrote:
>
>We recently discovered a slight difference in 13C which correlated with
>sample size; small samples where lighter than larger samples of the same
>material.

Edited: Snip, snip!

>It appears that some contaminant in the frit was adsorbing CO2 with a pretty
>substantial fractionation, but the change was only detectable on small
>samples.  The change for 18O was even more than for 13C.  However, when we
>run samples for 18O its always on large samples of  the same size, so we
>didn't notice it there.  The shifts in 13C and 18O were very repeatable, and
>predictable.   One clue that it was an adsorption problem was that the shift
>was independent of the actual value; i.e. regardless of the isotopic
>composition of the sample, small splits were always more negative than large
>splits.
>
>The moral of this story is, don't be too quick to blame your preparation
>system (or the technician running it).

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R.A. Fifer                               E-mail: [log in to unmask]
Brown University                         Geological Sciences