Stable Isotope Geochemistry


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Debajyoti Paul <[log in to unmask]>
Fri, 1 Jul 2005 15:24:41 -0500
text/plain (88 lines)
The first method of simple addition and subtraction has no logical
derivation.  If you have to do it, then the second method of
normalization (multiplying by the ratio of true/measured) is
appropriate.  Hope this helps.

Department of Earth and Environmental Science
6900 N. Loop 1604 W.
San Antonio, Texas 78249

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Penny Higgins
Sent: Friday, July 01, 2005 2:57 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Correcting to standards - procedure

Greetings all,

I hope this isn't a totally dumb question. What seemed pretty
straight-forward to me has become a large discussion in our lab. Maybe
been doing this wrong all along - I just want to be sure.

We're running carbonate samples for d13C and d18O on a gasbench
to a DeltaPlus XP. NBS-19 is the standard we're using.

So, I run a bunch of samples with a few NBS-19s tossed in for good
The values for NBS-19 don't come out exactly as specified by Coplen 1994
d13C = 1.95 and d18O = -2.20. Instead, I get d13C = 1.98 and d18O =

Ordinarily, I'd just do simple adding or subtracting to all the samples
the run to make the NBS-19s "right." I subtract 0.03 from ALL the d13C
values and subtract 0.02 from all the d18O values. My understanding is
then I can report my results as VPDB.

I was just told that a more correct way to do it is to calculate the
between the real value and the measured value of NBS-19 and multiply
with all of my samples. That is, the ratio of "true" and measured d13C
NBS-19 is 0.986, which I multiply all my d13C values with. The ratio of
"true" and measured d18O for NBS-19 is 0.990, and I multiply all my d18O
values with that.

This second method just seems wrong, because any measured sample near
would not change. But it makes sense in the sense that everything we are
measuring is done in terms of ratios.

So, at the risk of wasting bandwidth, which is the correct method? Is
some instances where both methods are required? Am I delusional (it



                       Dr. Pennilyn Higgins
                        Research Associate

   Stable Isotope Ratios in the Environment Analytical Laboratory

         Department of Earth and Environmental Sciences
         University of Rochester
         227 Hutchison Hall
         Rochester, NY 14627

                    [log in to unmask]

Office: 209b Hutchison Hall               Lab: 209 Hutchison Hall
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