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 I've
been doing this wrong all along - I just want to be sure.

We're running carbonate samples for d13C and d18O on a gasbench connected
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 measure.
The values for NBS-19 don't come out exactly as specified by Coplen 1994 as
d13C = 1.95 and d18O = -2.20. Instead, I get d13C = 1.98 and d18O = -2.22.

Ordinarily, I'd just do simple adding or subtracting to all the samples in
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 that
then I can report my results as VPDB.

I was just told that a more correct way to do it is to calculate the ratio
between the real value and the measured value of NBS-19 and multiply that
with all of my samples. That is, the ratio of "true" and measured d13C for
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 zero
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 there
some instances where both methods are required? Am I delusional (it happens)?

Stumped,

~Penny

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                       Dr. Pennilyn Higgins
                        Research Associate

                             "SIREAL"
   Stable Isotope Ratios in the Environment Analytical Laboratory

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

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