Hi Marcelo,


My hunch is you may be looking at more than one cause to your problem.

The fact that you are "loosing" the S-shape swing of your CO2 peak plus the
observed peak broadening tends to suggest that loss of chromatography is at
least part of the problem.

What kind of columns (stationary phase) do you use in your set-up? That
aside, when's the last time you baked out your GC columns?

Apart from that, re-oxidising your combustion tube on a regular basis (or
replacing it) can't hurt. Even so NiO is "superior" to CuO as oxygen sink
thanks to its higher decomposition temperature, it is still only a finite
reservoir of oxygen.


Cheers,

Wolfram



> -----Original Message-----
> From: Stable Isotope Geochemistry
> [mailto:[log in to unmask]] On Behalf Of Marcelo Z. Moreira
> Sent: 03 May 2005 12:48
> To: [log in to unmask]
> Subject: Re: aging of Cu-tube: re-oxidising
>
>
> Hi folks,
>
> My problem is quite similar to yours on the oxidizing
> furnace. In my case, the furnace contains oxidized Ni wires
> to convert methane to CO2. The system is a modified PreCon
> where injection thru the GC box goes directly cryo-trap, to
> GC column, to the furnace (1050C, verified) and finally, to
> the open split. After ~2000 of high methane concentration
> samples (>60%) the system has no regular "S-shape" ratio
> trace (45/44) for the produced CO2, making the delta more
> than 10 per mil higher than expected and the reaction yield
> is not controlled anymore. We use to adjust injection to
> produce ~4-7V; now it doesn't go further than 2V, but the
> peak has spread a little bit. In relation to the background,
> the ratio trace has an small initial "enriched" peak 10s
> before the CO2, which shows a wide negative belly. We have
> checked flow rate (~1ml/min), and traces of CO2 in the
> standards released from the cryo-trap at the end of analysis
> shows regular s-shaped ratio traces.
>
> Any suggestion would be appreciated.
>
>
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