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Subject:
From:
Kinga M Revesz <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Date:
Wed, 6 Jul 2005 09:40:01 -0400
Content-Type:
text/plain
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text/plain (172 lines)
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


|---------+---------------------------->
|         |           "Willi A. Brand" |
|         |           <[log in to unmask]
|         |           MPG.DE>          |
|         |           Sent by: Stable  |
|         |           Isotope          |
|         |           Geochemistry     |
|         |           <[log in to unmask]
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|         |           07/06/2005 04:21 |
|         |           AM               |
|         |           Please respond to|
|         |           Stable Isotope   |
|         |           Geochemistry     |
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  |       To:       [log in to unmask]                                                                       |
  |       cc:                                                                                                     |
  |       Subject:  Re: Further Discussion on Carbonate Corrections                                               |
  >---------------------------------------------------------------------------------------------------------------|




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]



--
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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

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        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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