How about analyzing two, known but very different isotopic composition of
carbonates (NBS-18 and NBS_19) determining the slope and additive
correction every day and forget about calibrating the reference gas. That
is what we did Revesz-Landwehr 2002; RCM, 16: 2102-2114.
Kinga Revesz
Chemist, Stable Isotope Laboratory
U.S.Geological Survey
MS 431
Reston, VA 20192
T:703-648-5865
FAX: 703-648-5274
http://www.isotopes.usgs.gov
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| | "Willi A. Brand" |
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| | 07/06/2005 04:21 |
| | AM |
| | Please respond to|
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| | Geochemistry |
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| To: [log in to unmask] |
| cc: |
| Subject: Re: Further Discussion on Carbonate Corrections |
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Shouldn't we proceed as follows:
1. Generate CO2 from NBS 19 using phosphoric acid and following an
appropriate and established procedure.
2. Calibrate a working reference CO2 using the NBS 19 CO2 (+1.95 and -2.2
per mill exactly; apply an appropriate fractionation correction for 18O,
usually +10.25 per mill) using the mass spectrometer, preferrably a dual
inlet system. Make sure that the inlet conditions are identical for both
gases.
3. Use the working gas from here on to measure sample CO2.
4. Occasionally check the working gas against NBS 19 and assign a new value
if a significant change is observed.
So, I do not understand the business of measuring NBS 19 as a sample,
except for point 4.
Regarding offsets and scaling: I think those must be treated in a strictly
separated manner.
If the mass spectrometer suffers from scaling compression (and most do),
this may be independently checked observing the dependence of measured
isotope ratios upon switching time and / or the pump down behaviour upon
closing the changeover valve. These procedures are well described in the
literature (including our own publications, see our web page below).
Offsets of the working gas should be established using NBS 19 (or other,
well known secondary reference materials). The anchor to the VPDB scale
should be mentioned in all publications. When recalculating isotope values
on a different reference scale (e.g. working reference scale versus VPDB
scale), please be aware of the non-additive nature of relative units like %
or per mill. The general formula for relating deltas to a different
reference ratio is del(ac) = del(ab) + del(bc) + (del(ab) del(bc))/1000;
for further info you may look into Werner and Brand, RCM; 15 (2001) 501.
Regards Willi
tcp121 wrote:
Greetings...
I have been following the recent discussions on the list concerning
correcting inorganic O and C isotope data for carbonate samples. We
have a MAT 253 and a Kiel III device in our lab. We recently had the
same discussion...why is the correction we are using additive rather
than multiplicative (is that a word?).
When we run NBS-19, values come out as follows: d13C = 1.82 ‰ and
d18O
= -2.37 ‰. I've always corrected my data by adding the necessary
correction factor that brings NBS-19 in line, to all of my samples as
well. Ideally, the reference gas would be calibrated so that this
step
is unnecessary, so I performed an experiment:
Our reference gas values: d13C = -13. 36 ‰ and d18O -10.95 ‰. I
pulled
up an old run of NBS-19 and changed the reference gas values by the
additive correction factor, so d13C = -13.36 + 0.13 = -13.23 ‰ and
d18O
= -10.95 + 0.17 = -10.78 ‰. Reevaluating this NBS-19 run with these
values gave d13C = 1.95 ‰ and d18O = -2.20 ‰.
I then took a few sample runs to see what would happen. One sample:
d13C = 0.00 ‰ and d18O = 0.41‰. Changing the reference gas values for
this sample and reevaluating gave d13C = 0.13 ‰ and d18O = 0.58 ‰,
which is additive (this was a great test sample...0.00‰ should not
have
changed at all if the correction is multiplicative). I also tested a
more depleted sample: original d13C = -5.60 ‰ and d18O = -8.01 ‰.
With
the new ref gas values this changed to d13C = -5.47 ‰ and d18O =
-7.84
‰, again which is additive.
Unless there is some flaw in the way the Isodat 2.0 software is
calculating delta values, this seems to me as proof that addition to
correct works...at least in this range of sample data. I know of one
motherland engineer who has stated that delta values are not additive
in this way. Though the correction seems to hold within the data
range
above, as we move further away from NBS-19, such as around NBS-18
(d18O
= -23 ‰), there will be an error that appears (maybe around 0.1‰ at
this point). Would he care to comment with some formula examples to
show the exact way of correcting data when your reference gas is not
perfectly calibrated?
Any other thoughts?
Thanks...
Tim
-----------------------------------------------
Tim Prokopiuk
B. Sc. Geology/Technician
Saskatchewan Isotope Laboratory
Room 241
Department of Geological Sciences
University of Saskatchewan
114 Science Place
Saskatoon, Saskatchewan, Canada
S7N 5E2
Phone: (306) 966-5712
Fax: (306) 966-8593
Email: [log in to unmask]
--
.....................................................................
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/
GASIR 2005 in Jena, October 10-12:
http://www.bgc-jena.mpg.de/service/iso_gas_lab/gasir2005/index.shtml
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