Kieran,

Hilary and Mihai make some good points, and I won't repeat what they suggest. But I do have another suggestion... 

I'd also add in a note on the silver capsules...they don't combust as well as Tin capsules (ther has been a couple of threads on ISOGEOCHEM this year about capsule combustion). Even though the furnace temperature is up at 1020C, the silver capsules only slowly burn, relative to Tin capsules which show a rapid "flash" combustion. It is worth considering the potential for CO production due to this longer burn uni the Silver capsule, relative to the timing of the O2 pulse. 

You may try further wrapping the silver capsules in tin capsules just before you line them up in the autosampler to improve combustion, or alternatively drop some Tin powder into the dried sample just before you  close up the capsule and place in the autosampler.

My recent publications examined the difference in measured data by looking at capsule combinations for both C and N isotopes: Please see

Brodie et al (2011) Chemical Geology, 282, 67 - 83 (Carbon Isotopes)   and    Brodie et al (2011) Rapid Communications in Mass Spectrometry, 25, 1089 - 1099 (Nitrogen Isotopes)

Also, what sample sizes are you dealing with? is your %N very very low? If so, you may be operating quite close to baseline conditions and the data may nto be reliable....worth consdierign if this is a possibility in your instance.

Hope this is helpful...

Cheers,
Chris

Dr. Chris Brodie
Department of Earth Sciences
James Lee Science Building. 
Hong Kong University
Pokfulam Road
Hong Kong

http://hku-hk.academia.edu/ChrisBrodie
________________________________________
From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Kieran Craven [[log in to unmask]]
Sent: Thursday, June 16, 2011 6:29 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] CO formation using EA

Dear Isogeochem,

A couple of questions about carbon monoxide formation from estuarine
sediment while using an EA. I had a quick trawl through the archives but
didn't come across anything on these.

If CO results from incomplete combusion of a sample can its effects be
picked up? Would CO travel at the same speed as N2 and affect both %N and
d15N, or is it a slower moving molecule and crop up elsewhere?

These questions arise from running silver capsules (following acid fuming to
remove IC) at a temperature of 1020C with a 3sec O2 pulse. Some of the acid
fumed samples have higher %N than non-acidified samples (acid fumed blank
samples show no contamination, and there doesn't appear to be any memory
effects). d15N values don't massively differ between fumed and non-acidified
samples.

Thanks in advance for any comments/suggestions/insights/musings/nods in
right direction.

Cheers,

Kieran


Kieran Craven
Postgraduate Researcher
Department of Geology
Museum Building
Trinity College
Dublin 2
Ireland

Tel: +353 (0)1 896 1363
Email: [log in to unmask]