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Stable Isotope Geochemistry

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Subject:
From:
Douglas Dvoracek <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Wed, 25 Feb 2004 12:44:39 -0500
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Carl,

I'm curious as to what application you are using here.  For AMS sample
combustion, we use quartz tubing that is treated and handled essentially
as John describes, except we use 900 C for a few hours, and then handle
everything accordingly.  Additionally, we store the foil-wrapped tubing
above a tray of weak NaOH in a dessicator cabinet, in hopes the NaOH
will preferentially adsorb atmospheric CO2.  This allows us to achieve
blank levels of modern carbon comparable to other AMS labs.

Good luck,

Doug


John Eiler wrote:

>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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