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Hi,
Here are some of my recent problems, and I hope you may bring me some
answers.
I work on the measurement of 15N and 18O ratios in silver nitrate.
Nitrates originate from groundwater and have been extracted using the
technique issued in Silva S.R.., Journal of hydrology, 228 (2000), 22-36
(i.e. using anion exchange resin with final neutralization by silver
oxide). The isotopic ratios are measured by pyrolysis, with the furnace
temperature around 1100 degrees for 15N analysis (Roboprep CN) and 1450
degrees for 18O (ThermoFinnigan TC/EA).
The reproducibility on both 15N and 18O has been a bit disappointing so
far.
In relation to 18O analyses, the CO peak looks always okay whilst the N2
peak (that appears for m/z 28 just before the CO peak) can have a very
strange shape, or just trails and even sometimes overlaps the CO peak.
Whenever this first peak (N2) looks funny, and even if it doesn't seem
to overlap the CO peak, the resulting 18O ratios hits the roof and can
rise up to +75 per mil when other replicates would show up around +6 per
mil.
As for 15N analyses, the reproducibility is much better (standard
deviation usually within 0.5 per mil), but results are far from what is
usually achieved when analyzing some organic samples. Addition of some
sucrose seems to improve a bit, but still.
At the end, we are wondering whether the problem is just a matter of
sample homogeneity (my silver nitrate samples look a bit like fibers or
wool). I'll try very soon to grind finely some samples after freezing
them with liquid nitrogen to see if there is any improvement.
Nevertheless I would appreciate to know if anyone of you has already
experienced these problems and managed to work them out.
All the best,

Eddy Minet, Ph.D. student
Centre for the Environment
Trinity College
Dublin 2 (Ireland)
Tel: +353 (0)86 827 4921
Email: [log in to unmask]