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Teppei

How about giving a reference for the benefit of all on that "microbial
method"???

Rodrigo

Rodrigo Maia
Lab Tech
Stable Isotopes Lab - ICAT Lisbon
Portugal
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-------Original Message-------
 
From: Stable Isotope Geochemistry
Date: terça-feira, 20 de Maio de 2003 19:24:33
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] pyrolysis of silver nitrate for 15N and 18O ratios
measurements
 
Hello,
 
How about measuring the gas product by gas chromatograph?
How do you eliminate organic matters in the samples?
Have you checked the Cecily Chang's method? Have you washed
Ag2O before using it? 
How about eliminating anions using BaCl2 before the anion
exchange NO3 absorption?
 
How about the new microbial method?
 
 
Regards,
 
 
Teppei
----- Original Message ----- 
From: Eddy Minet 
To: [log in to unmask] 
Sent: Tuesday, May 20, 2003 6:24 PM
Subject: [ISOGEOCHEM] pyrolysis of silver nitrate for 15N and 18O ratios
measurements


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]