Natalia, another thought...
Ohlsson and Wallmark published a paper that presents an algorithm to mitigate the mass effect on isotope ratio analyses. That is, the ratio changes as a function of material combusted-- which is why we are fairly careful about weighing standards precisely. In a previous life, I modified their algorithm because we found that isotope ratio dropped as a function of Total N or C combusted. It dropped in a manner that could be approximated by a first order decay curve. We never adopted this in the lab because it requires analysis of standards with a range of masses bracketing the masses of the samples within every run. However, trials showed that this improved precision in analysis of standards by as much as an order of magnitude. If it is important for you to run samples of varying sizes you might give this method a try.
Reference Ohlsson, K.E., and P.H. Wallmark. (1999) Novel calibration with correction for drift and non-linear response for continuous flow isotope ratio mass spectrometry applied to the determination of (delta notation)15N, total nitrogen, (delta notation)13C and total carbon in biological material.
The Analyst 124:571-577.
Good luck
John
John B. Cliff, Ph.D.
Research Scientist
Advanced Radioanalytical Chemistry Group
National Security Division
Pacific Northwest National Laboratory
Richland, WA U.S.A.
(509) 373-9003, fax (509) 376-5021
([log in to unmask])
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Dr. Bruno Glaser
Sent: Tuesday, May 23, 2006 11:00 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
Dear Tiziano,
amount dependence might also be due to the split dilution, especially from the Conlfo II such effects are known and can only be avoided when using samples with the same amount of analyte element or in other words using samples yielding e.g. around 2 V signal intensity.
Kind regards
Bruno
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]]On
Behalf Of Tiziano Boschetti
Sent: Dienstag, 23. Mai 2006 17:31
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] calibration TC/EA for O2
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
--
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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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