Dear Jean,

I had a similar problem with charcoal samples which did not combust
entirely even at tiny amounts for d13C measurements only. I doubt that
you will get correct d15N values at all. The N content of such samples
is simply to small !!! However, you can check your system by alternately
analyzing a (graphite) sample followed by a (certified) 15N standard. If
your previous graphite sample was combusted correctly, the subsequent
d15N value of the standard should be correct. Otherwise d15N values of
CO derived from incomplete combustions should generate extremely
positive d15N values.

If you encounter this, you could play around with combustion enhancers
such as WO2 etc. but bear in mind that you have different oxygen sources
then and your d13C values might be erroneous afterwards.

Good luck

Bruno

_____________________________________________
PD Dr. Bruno Glaser
Abteilung Bodenphysik
Universität Bayreuth
Universitätsstr. 30
95440 Bayreuth
Deutschland
Email: [log in to unmask]
Web: http://www.bayceer.uni-bayreuth.de/bophy
_____________________________________________

-----Ursprüngliche Nachricht-----
Von: Stable Isotope Geochemistry [mailto:[log in to unmask]] Im
Auftrag von Jean Chen
Gesendet: Montag, 6. November 2006 07:33
An: [log in to unmask]
Betreff: [ISOGEOCHEM] Analyzing graphite with EA-IRMS

Dear All,

We have an EA-Conflo III-IRMS (ThermoFinnigan 
Deltaplus Advantage) system for carbon & nitrogen isotope determination.


Recently, we have been dealing with graphite samples, which have an 
enormous high CN ratio. That means, a very large sample size is required
to 
put into the combustion chamber in order to obtain nitrogen isotope
value.  
Theoretically speaking, the GC column that comes after the reduction 
chamber will separate the N and C signals. However, in the case of an 
incomplete combustion, if any, due to the large sample size we put it, 
would there be an overlap of CO with N2, since both have a mass of 28?  
Increasing the time on the oxygen injection is one way to prevent 
incomplete combustion. However, we are not sure if the nitrogen signal
we 
obtained is purely nitrogen or a mixture of Co and N2. Maybe someone can

shed us some light on this matter. Thank you very much!!!

Best Regrads,
Jean

----------------------------------------------
Jean Chen
Research Assistant
Research Center for Environmental Changes
Academia Sinica
Taipei, Taiwan
email: [log in to unmask]