Dear Jean
There maybe another factor you wish to consider, being carryover/tailing
of the CO2 into the following samples N2 peak. The CO2 will be ionised
to CO2+ and decay to CO+. This CO+, as mentioned by Gilles will have a
very high 29/28 ratio.
Brooks et al (see below) published an article several years ago
describing the installation of a second CO2 trap that is positioned
after the GC column which removes CO2 that may coelute with the N2.
Improved precision of coupled 13C and 15N measurements from single
samples using an elemental analyzer/isotope ratio mass spectrometer
combination with a post-column six-port valve and selective CO2
trapping; improved halide robustness of the combustion reactor using
CeO2
Rapid Communications in Mass Spectrometry
Volume 17, Issue 16, Date: 30 August 2003, Pages: 1924-1926
P. D. Brooks, H. Geilmann, R. A. Werner, W. A. Brand
Cheers
Jason
Jason Tranter
Scientific Officer
Building 19 Room 242
School of Natural Sciences
Edith Cowan University
100 Joondalup Drive, Joondalup
Western Australia
Australia
(Ph) +61 8 6304 5769
(Fax) + 61 8 6304 5842
(Email) [log in to unmask]
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Jean Chen
Sent: Monday, 6 November 2006 2:33 PM
To: [log in to unmask]
Subject: [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]
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