Dear Willi and Natalia,
during the analysis of O-isotope I've noted an increase of the background
mass 30 (14N16O, 12C18O), usually the twice of 28. Probably this could be
due to the low purity of CO-reference gas (4.7 grade), in fact I've also
noted an increase of the 32 mass signal (impurity of oxygen in CO?).
Low Purity of reference CO-gas could be resolved with a point-of-use gas
filtration or gas purifier (
http://www.alltechweb.com/productinfo/technical/datasheets/4001u.pdf
http://www.pall.com/microe_26234.asp?level1=0 )
What do you think?
Ciao!
*****
Tiziano Boschetti
University of Parma, Italy
*****
----- Original Message -----
From: Willi A. Brand
To: [log in to unmask]
Sent: Tuesday, May 23, 2006 2:57 PM
Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
Dear Natalia,
I have two comments to your question.
1. With a perfectly tuned ion source (perfect linearity for CO2 and N2) we
very consistently see a dependence of the 30/28 ratio upon signal size. It
is surprizingly constant at +0.3 per mill / Volt (1V = 3 nA 28). We
routinely correct for this, but we do not know the origin. One possibility
would be formation of an extra ion current on m/z 30 much like the H3+. If
small amounts of N2 are present in the reference gas, formation of NO can
occur on the filament, giving rise to NO+ (m/z 30) during reference
measurement. In this case, the correction would have to be applied to the
reference gas only. However, we see the same dependence upon signal size on
the sample peaks.
2. We generally run our reduction at 1400°C (tube-in-tube arrangement with
the He flow reversal installed) and we adjust the sample size to about 1 mg
Oxygen to generate a larger ion current. We found this necessary in order to
be somewhat less dependent upon background and memory effects.
Regards Willi
Sevastyanov wrote:
Hello!
I am a post-graduate student of V.I. Vernadsky Institute of Geochemistry and
Analytical Chemistry, Russian Academy of Science. I would like to ask some
questions about our DeltaptusXP mass spectrometer.
Two organic substances were analyzed for their δ18О values using our
pyrolysis EA-IRMS system: cellulose and benzoic acid (the reaction furnace
is heated to 13500C, silver capsules were used). A various samples amount
were examined, and our results revealed that there is a dependence of the
δ18О values upon peak amplitude. These changes are not great when peak
amplitudes exceed 2 V but increase rapidly when peak amplitudes become less
than 2 V. Thus, in the range from 2 to 1 V δ18О values become about 5 ‰
smaller. We believe such phenomenon is not normal and we would like to know
how this situation may be improved and where this phenomenon comes from?
Where linearity area of our system ends? Is that true, that linearity bounds
by 2V? If this is so, what is the nature of such phenomenon?
Best regards,
Natalia Sokolova
post-graduate student
V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry,
Russian Academy of Science
19, Kosigin st., Moscow, 119991, Russia
--
.....................................................................
Willi A. Brand, Stable Isotope Laboratory [log in to unmask]
Max-Planck-Institute for Biogeochemistry (Beutenberg Campus)
Hans-Knoell-Str. 10, 07745 Jena, Germany Tel: +49-3641-576400
P.O.Box 100164, 07701 Jena, Germany Fax: +49-3641-577400
http://www.bgc-jena.mpg.de/
http://www.bgc-jena.mpg.de/service/iso_gas_lab/
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GASIR 2005 in Jena:
http://www.bgc-jena.mpg.de/service/iso_gas_lab/gasir2005/index.shtml
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