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
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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