Hi Penny
The first method of simple addition or subtraction of a small amount
will generally be fine the d13C values of your samples and references
are all fairly similar.
My preference is the second method you refer to, to keep a consist form
of correction in the lab (it principle).
When are quite right in saying that the second method doesn't seem quite
right at first, especially when the sample values are close to 0 per
mil.
However, to avoid this apparent anomaly, instead of directly using the
true and measured value of NBS-19 to determine the scale
expansion/contraction correction factor, use the difference between your
working reference and the NBS-19.
For example, in our lab we routinely determine the d13C of plant
material from marine environments, which can be anywhere between -7 to
-20 per mil. So, I will use a wheat flour working ref (-25.34 per mil)
and USGS41 (+37.76) to determine the scale expansion/contraction
correction i.e. all sample values must fall between the two standards
and the calculation is applied after blank and O17 corrections.
For argument sake
d13C (USGS41 meas) = +47.58 per mil
Correction factor = d13C(flour) - d13C(USGS41 real)
--------------------------------
d13C(flour) - d13C(USGS41 meas)
= -25.34 - 37.76)/(-25.34 - 47.58)
= 0.86
Now apply this to your sample measurement
d13C(sample corr) = ((d13C(flour)-d13C(sample meas)) x 0.86) +
d13C(flour)
To check to see if this is working I also apply the correction to a
Sucrose ANU aka IAEA-CH6 sample that is run with each batch.
I find this approach works for me every if I ensure the amount carbon in
each sample and ref is very similar, otherwise you then need to worry
about linearity effects.
Some references you may wish to look at are;
Bohlke, J.K. & Coplen, T.B. (1993) "Interlaboratory Comparison of
Reference Materials for Nitrogen-Isotopes for Nitrogen-Isotope-Ratio
Measurements" 51-66, in References and Intercomparison Materials for
Stable Isotopes of Light Elements (IAEA-TECDOC-825).
Coplen, T.B. (1988). Normalization of Oxygen and Hydrogen Isotope Data",
Chemical Geology, 72, 293-297
Verkouteren, R.M & Lee, J.N (2001) Web-based Interactive Data
Processing: Applications to Stable Isotope Metrology. Fresenius J Anal
Chem, 370, 803-810
Hope this helps
Jason
Jason Tranter
Scientific Officer
School of Natural Sciences
Edith Cowan University
100 Joondalup Drive
Joondalup
Western Australia
Australia
6027
Ph +61 8 6304 5769
Fax +61 8 6304 5842
Email [log in to unmask]
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Penny Higgins
Sent: Saturday, 2 July 2005 3:57 AM
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
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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