Any chance you have traces of contaminant (ethanol?) in the reference
bellows? With Finnigan-MAT instruments you can compare the 46 background
from the reference and sample bellows. Otherwise you can run a background
scan to see if there are fragmentation peaks (mass 29 for ethanol) or
differences in masses 16 and 17 between the reference and sample
backgrounds. It does sound wild, but similar drifts (and larger) were
observed after using stainless steel connectors that had been rinsed in
ethanol, heated in an oven overnight, and been used on vacuum lines
numerous times prior to sample transfer. The problem is exacerbated if a
cold finger is used-presumably more contaminant (ethanol?) is desorbed from
the stainless steel. The only explanation for the 'unexplained' drift I
could come up with was a difference in polarity between CO2 and
contaminant, where the capillary leaks acted in a manner similar to a
chromographic column, and one gas was admitted at a slightly faster rate
than the other to the source. The result was large jumps or drifts in the
d46 over several runs with little or no change in d45. I never did get the
contaminant analyzed but discussions with Roger Francey at CSIRO who had
observed similar 'drifts' in some samples suggested TRACE ethanol was the
source of this type of problem.
Hope this helps,
Stable Isotope Group
The University of Calgary
> From: Paul Dennis <[log in to unmask]>
> To: [log in to unmask]
> Subject: Dual-inlet bellows problem!
> Date: Thursday, November 18, 1999 8:26 AM
> 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
> drifting reference gas composition. In a clean, baked system, using
> 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
> 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
> shows that the delta 18O changes by nearly 4 per mille, but the delta 13C
> 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
> on either the sample or reference sides. Neither is there any valve
> 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
> correction shows that the carbon isotope composition is invariant and the
> oxygen composition changes. If there were a contaminant gas with a peak
> 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.
> 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
> 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
> 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
> 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
> Tel: 01603 593105
> Fax: 01603 507719
> email: [log in to unmask]