Obviously its important to firstly establish your instrument performance for
'known', 'pure' nitrate. How does chemically pure silver nitrate or
potassium nitrate (e.g. the IAEA N3 standard) run for d18O on your TC/EA? On
our own system these materials yield smooth N2 and CO peaks with centres
about 70 seconds apart, and the CO peak about 3 times the amplitude of the
N2 peak. The CO peak starts about 20 seconds after the finish of the N2
tail. Consequtive samples of IAEA N3 replicate to a 1 S.D. of about 0.3 to
0.4 per mil.

Tim H.E. Heaton

NERC Isotope Geosciences Laboratory
British Geological Survey
Keyworth, Nottingham NG12 5GG, England

Tel. +44(0)115 936 3401
Fax +44(0)115 936 3302
E-mail: [log in to unmask]

-----Original Message-----
From: Eddy Minet [mailto:[log in to unmask]]
Sent: Tuesday, May 20, 2003 6:24 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] pyrolysis of silver nitrate for 15N and 18O ratios


Here are some of my recent problems, and I hope you may bring me some

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]

This e-mail message, and any files transmitted with it, are
confidential and intended solely for the use of the addressee.
If this message was not addressed to you, you have received
it in error and any copying, distribution or other use of any
part of it is strictly prohibited. Any views or opinions presented
are solely those of the sender and do not necessarily represent
those of the British Geological Survey. The security of e-mail
communication cannot be guaranteed and the BGS accepts
no liability for claims arising as a result of the use of this medium
to transmit from or to the BGS. The BGS cannot accept any
responsibility for viruses, so please scan all attachments.