Dear Richard,

> As there is no CO2 peak for KNO3 I can't check on this. I added a little
> carbon to the IAEA-NO3 and then the peak looks a lot better. So the cause
> may be a too slow burning of the KNO3. Does anyone have experience with this?
> Als suggestions are welcome.

The problem is that NO3- is the highest oxidation state for nitrogen.
So, throwing it on to a combustion furnace doesn't really achieve
anything in terms of sample conversion into N2, since your IRMS is
set to measure 29/28 (and 30/28) ratios for d15N analysis.

That is also the reason why the addition of carbon improved
matters. What happens is the creation of a localized reduction
reaction in which the carbon acts as reduction agent for NO3,
taking the oxygen from the NO3 rather than the CuO (or whatever
act as oxygen sink/donor in your elemental analyser) to form CO2.

Adding an excess of carbon, stoechometrically speaking, in form of
e.g. graphite (I think Finnigan suggests the use of sugar) should
improve things tremendously. A slight excess should almost
guarantee that all your NO3 is reduced to N2 according to:

2 NO3 + 3 C ---> N2 + 3 CO2

Hope this helps.


Best wishes,

                         Wolfram



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Dr. W. Meier-Augenstein, CChem MRSC
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University of Dundee, Dept. of Anatomy & Physiology,
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