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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]