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Paul Dennis wrote:

> Dear all,
>
> Here is a teaser? For some time now we have had problems with one of our
> dual-inlet mass spectrometers. The problem is best described as a severely
> drifting reference gas composition. In a clean, baked system, using 99.999%
> CO2 as the working reference gas and loading the same gas into the sample
> bellows, the symptoms are as follows:
>
> 1) In a series of 48 measurements of sample versus reference there is a
> drift of nearly -4 per mille in the delta 46 composition of the sample with
> respect to the reference
>
> 2) The delta 45 composition also drifts by about -0.12 per mille.
>
> 3) Carrying out a Craig correction on the delta 45 and delta 46 compositions
> shows that the delta 18O changes by nearly 4 per mille, but the delta 13C is
> constant with an external precision of better than +/-0.01 per mille over
> the 48 analyses.
>
> 4) Checking the purity of the gas by peak jumping shows there to be no leaks
> on either the sample or reference sides. Neither is there any valve x-seat
> leakage.
>
> Observations 1), 2) and 3) seem to infer that there is an oxygen isotope
> exchange between the reference gas and a contaminant, possibly in the
> reference bellows. I suggest an oxygen isotope exchange as applying a Craig
> correction shows that the carbon isotope composition is invariant and the
> oxygen composition changes. If there were a contaminant gas with a peak at
> 46 then the Craig correction would not necessarily convert the delta 45
> compositions to a constant delta 13-C!
>
> As a further test we have pumperd the reference gas out of the reference
> side, incl. bellows and remeasured the gas in the sample side. In this
> second series of experiments we have not used the reference bellows. Rather
> we are expanding gas into the micro-inlet from a large volume behind the
> reference inlet valve. In this series of measurements we find no observable
> drift in the delta 46, and delta 45 measurments, with external precisions
> for delta 46 much less than 0.02 per miile and for delta 45 less than 0.01
> per mille.
>
> So the conclusion is that there is an oxygen exchange process going on in
> the reference gas bellows between CO2 and something.
>
> Has anyone ever observed such a phenomenon? Has anyone any suggestion as to
> what the exact process is? More to the point has anyone found a way of
> cleaning the bellows?
>
> Any suggestions will be very gratefully received!
>
> Best wishes to all,
>
> Paul Dennis
>
> Paul F. Dennis
> Head of Stable Isotope Laboratory
> School of Environmental Sciences
> University of East Anglia
> NORWICH NR4 7TJ
> UK
>
> Tel: 01603 593105
> Fax: 01603 507719
> email: [log in to unmask]

Paul run the reference side with the bellows closed to see if there is a drift
before you get carried away with cleaning the bellows.  Leo