Hi, Richard,
this is just a simple thought, not backed by intensive investigations:
The EA provides a highly oxidative environment. Hence, everything that 
can be oxidized will easily be converted to the respective product 
gases. For nitrogen containing compounds, the respective gaseous forms 
are N2, but also N2O, NO, NO2 which are reduced to N2 in the second 
furnace. Assuming, this is complete, the column will only see CO2 and 
N2, without having any trouble to separate the two. However, when you 
are combusting nitrates, the situation will change. In this case, the 
nitrate has to be cracked by the high temperature, there is no way to 
oxidize it any further. Hence, when the combustion temperature is not 
very high, which is the case when burning soil and other mineral 
dominated samples, there can be a residue containing nitrate which can 
be a source of nitrogen memory for the next sample.
You can see the converse effect in the TCEA systems. Here, nitrate can 
be reduced easily, but the reduced forms of nitrogen are barely reacting 
(for instance caffeine has only a very small nitrogen peak in front of 
the large CO peak; (CN)2 may be formed instead of N2). I think this 
effect can be used to separate the isotopic information on nitrous 
versus ammonia nitrogen.
Regards   Willi

Richard Doucett wrote:
> Hi All,
>
> I've been running mass specs for over 7 years now, and I have to admit 
> (somewhat embarrassingly) that I still cannot explain to students what 
> exactly causes carry-over inside my EA-IRMS system when running 
> 15N-enriched samples.  If the samples are 100% combusted, how can 
> there be any 15N leftover from the previous sample?  I know that, 
> after running enriched samples, when I replace the reactors with fresh 
> ones and start analyzing natural abundance samples again, all is ok.  
> So I'm guessing that the carry-over problem has to do with either the 
> oxidation or reduction reactor.
>
> Any insight on this would be very much appreciated.
>
> Cheers,
> Rick
>
> Richard (Rick) Doucett, Ph.D., Associate Director
> Colorado Plateau Analytical Laboratory
> Northern Arizona University
> Tel: 928.523.0967 (lab)
> Tel: 928.523.7265 (office)
> Fax: 928.523.7500
>
> Stable Isotope Lab Website: http://www.isotope.nau.edu
>
> Mailing address for packages:
> Dept. Biological Sciences
> Northern Arizona University
> Building 21, South Beaver Street
> Flagstaff, AZ, 86011-5640
>

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Willi A. Brand, Stable Isotope Laboratory      [log in to unmask]
Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
Hans-Knoell-Str. 10, 07745 Jena, Germany      Tel: +49-3641-576400
P.O.Box 100164,      07701 Jena, Germany      Fax: +49-3641-577400
http://www.bgc-jena.mpg.de/
http://www.bgc-jena.mpg.de/service/iso_gas_lab/
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