Dear Bill,

Although I have no hands-on experience with running NO3 on an EA-IRMS
system I would surmise that you might stand a better chance getting
reasonable results if you in a way bypass the combustion tube.

The combustion tube is an oxidation reactor and as such does not do
anything for NO3 as NO3 constitutes the highest oxidation state for
N (+5). The addition of excess sugar acts as a carbon source to
reduce NO3 to volatile N-oxides (and ideally some N2) that can
subsequently be reduced to N2 by the reduction reactor.

I would be tempted to swap the oxidation tube for a reduction tube
(giving you two reduction stages) and add some Pd spun fibres to the
Cu at the top of the reactor. Since KNO3 melts at 334 C, it should
run down the tube which is held at 600 C. I realize, that's a lot of
trouble to go through and is probably only worth your while when
running large numbers of sample batches.

Alternatively, you could try adding various carbon sources (other
than sugar) to your sample to afford better reduction in the
oxidation tube, short of producing gunpowder-like mixtures that is.

Lastly, you can reduce your NO3 off-line to NH4 prior to EA-IRMS
analysis by adding AlCl3, a few drops of water, and Manganese grit
(Fluka, #63530) to your samples. I know this works but I have not the
foggiest idea about what kind of mass discrimination to expect. The
reaction is quantitative, even at room temperature although it takes
a day or two, so, from this point of view there should be no kinetic
isotope effects.

Hope this helps.


Cheers,
                  Wolfram



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Dr. W. Meier-Augenstein
Lecturer/Research Fellow

University of Dundee, Dept. of Anatomy & Physiology,
Small's Wynd, DUNDEE  DD1 4HN, United Kingdom

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URL: http://www.dundee.ac.uk/AnatPhys/wma/wolfram.htm
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