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Stable Isotope Geochemistry

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Subject:
From:
John Eiler <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Wed, 25 Feb 2004 08:55:39 -0800
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Many commercial glasses are fused in C-bearing crucibles that can introduce
trace C-bearing contaminants.  The worst is obviously graphite, but a
carbide or even C-bearing metal is a potential source of blank.  Also,
glassing of silicates is often done under a reducing atmosphere; if it is a
CO2/CO mixture, one might imagine introducing a dissolved gas contaminant.
Finally, organics from solvents and fingerprints can be surprisingly
difficult to get rid of.

The only practical advice I can offer is a procedure I used years ago when
attempting to measure isotope exchange equilibria between CO2 vapor and
glasses.  One requirement for maintaining C-isotope mass balance over the
course of the experiment was to bake the capsules (either Pt for high P
work, or Qtz for low-pressures) in an oxidizing atmosphere at 1000 C
overnight, and then to be careful about handling the capsules before use
(e.g., store them in clean foil and only handle them with clean tongs).  I
never established the source of the C blank that was removed by this
procedure.

Good luck,

John

>Has anyone experienced outgassing of CO2 during the heating or sealing
>of quartz or pyrex tubing?
>
>We have recently been trying to seal such tubes and have noted a
>heating-temperature dependent CO2 background in our blanks. We can
>monitor this by other means (flushing the contents of the tube into a
>Licor CO2 meter) and see that if we even just flame the tubes above
>about 500c, we observe a CO2 signal. Repeated heating of the same area
>results in progressively lowered signals, so this is not a leak, but
>rather something coming out of the glass itself. Fully melting the tube
>to seal it generates an unacceptably large blank for our purposes. The
>effect can be somewhat reduced by working with tubes that are
>pre-constricted with a *very* thin wall in the area to be melted.
>
>Thanks for any input you may all be able to offer.
>
>Carl Johnson
>Research Specialist
>Woods Hole Oceanographic Institution
>Woods Hole, MA
>USA
>02543-1543

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