Dear Jean
One way to tell if CO is present with your N2 is the value of the isotopic composition relative to N2 ref gas. A pure CO signal will give between 500 to 600 per mill relative to N2 air. Depending on the amount of crossover, your N2 value will have a larger relative isotopic value. In addition, the N2 peak tends to have some appearance of being deformed (e.g. double peak, deformed shoulder)
Graphite is actually a difficult product to combust and usually needs a combustion aid such as Vanadium pentoxide. When analysing very large quantitative differences between N & C, the GC column in the EA may not be able to efficiently separate the two gases due to column overloading because of the large sample size. It may be necessary to have a CO2 trap to analyse N and do a separate analysis for C. Many labs do this on all their samples regardless of C/N ratio.
Regards
Gilles
Gilles St-Jean
Chercheur / Research Scientist
Université d'Ottawa / University of Ottawa
Sciences de la Terre / Earth Sciences
140 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
Tel: 1-613-562-5800 xt 6830 (Bureau / Office)
xt 6839 (Bureau / Office Lab)
xt 6836 (IRMS lab)
Téléc. / Fax: 1-613-562-5192
Courriel / E-mail: [log in to unmask]
Toile / Web: www.isotope.uottawa.ca
-----Message d'origine-----
De : Stable Isotope Geochemistry [mailto:[log in to unmask]] De la part de Jean Chen
Envoyé : 06 novembre 2006 01:33
À : [log in to unmask]
Objet : [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
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Jean Chen
Research Assistant
Research Center for Environmental Changes
Academia Sinica
Taipei, Taiwan
email: [log in to unmask]
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