Dear Alexandre,
 
 
If a oxidation tube looses combustion capacity, a dead certain give-away are
d13C-values becoming more negative due to incomplete combustion. If you
don't do this already, you should be running a quality control (better two)
of known 13C isotopic composition between samples (say, every 15) to monitor
system performance.
 
You see, if you are running a CuO tube at 820-825 C your tube will "age"
even if you don't run samples for the simple reason that CuO decomposes at T
> 820 C according to 2 CuO ---> O2 + 2 Cu.
 
To answer your second question, it is possible to re-oxidise oxidation
tubes. My recommendation would be 12 hours at 600 C followed by another 12
hours of purge during which time you should bring up your furnace to your
standard combustion temperature.
 
You can also try to rig your system so a constant bleed flow of oxygen
enters the combustion tube all the time. This approach however requires you
to monitor the O2 signal in your IRMS quite religiously (even with a
reduction tube in situ) since you don't want to kill of your filament
prematurely and neither do your wish to spawn strange reactions between your
analyte gas and concurrently admitted O2 (or at the very least quench
ionisation).
 
 
Cheers,
 
Wolfram
 
 

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Alexandre Myre
Sent: 29 April 2005 16:24
To: [log in to unmask]
Subject: Aging carbon furnace tube in GC-C-IRMS



Hi everyone !

Our CuO frunace tube has combusted a lot samples during the past few months.
We havn`t noticed any chromatoragraphic problem yet and delta values for our
samples seem reasonable. Although we could have a problem of a loss of
discrimination potential between samples having similar but statistically
different 13C values. Is it possible that a too much oxydised CuO tube could
<<buffer>> the isotopic variations between different samples due to
incomplete oxydation ?

It this is so, I would like to know of any tricks on how to rejuvenate our
CuO tube. From Merritt et al. (1995), a 4-6 hours of O2 flushing at 500oC
should be sufficient... Does anyone have another idea ?

Many thanks !

Alexandre Myre

Research officer, University Laval, Animal Science Department


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