Dear Andrea,
there are 3 problems I see associated with your 15N measurements: 1.
there is possible contamination of your instrument, 2nd your instrument
might need modification, and 3rd there is no suitable reference at 10%
15N (this is about 30000 per mill).
1. Even with the elemental analyzer setup we see little contamination
inb our lab. There is a memory which can alter the results of up to 10
samples directly following the enriched sample. The number depends upon
the enrichment and upon the amount of material. Recommendation:
following measurement of enriched material condition the EA with a
number of combustions of natural material before making further
analysis. The mass spectrometer inlet should not exhibit significant
memory. nitrogen can be pumped out very efficiently. Even when switching
from CO2 to N2 you probably have no difficulties despite the fact that
CO2 is not as easy to pump away and despite the apparent +450 per mill
29/28 value of CO. Beware of your laboratory equipment (glass ware,
balance, tweezers etc), when working with such high enrichments.
2. The resistor for measuring mass 29 needs to be changed to an
appropriate value in order to reflect the ratio of the 29/28 ion
currents expected for your samples. If your samples have a 15N/14N ratio
of 1/10 the 29/28 ion currents will be 2/10. So, for a 1e9 Ohm resistor
(corrrect for the SIRA?) I would put in a 5e9, which might not  be
available. A 1e10 resistor would be available and a good compromise.
However,
3. There is no suitable reference for this enrichment. Instead of
measuring the highly enriched material direcvtly I would recommend to
dilute the sample by precisely weighing it with a material with the
isotopic composition well known. This could bring the isotope values of
your samples down to something less than 100 per mill. The referencing
problem would be solved and you could recalculate the original value
from the mass balance relation with high precision.
Best regards    Willi

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Willi A. Brand, Stable Isotope Laboratory
Max-Planck-Institute for Biogeochemistry
Tatzendpromenade 1a, 07745 Jena, Germany
P.O.Box 100164, 07701 Jena, Germany
Tel +49-3641-643718
Fax:  +49-3641-643710
e-mail: [log in to unmask]