Re: Correcting to standards - procedure
Actually, Penny is right to be skeptical about the advice she has received, at least in the way it is expressed here.  Consider this example:  When reported on the PDB scale , Julie's offset between measured and expected values of d18O is 0.02 per mil, or roughly 1 % of the absolute value (-2.2).   Julie's 'second method' would suggest this is the amount by which all data should be compressed to get them onto the PDB scale (i.e., a measured value of +100 should be turned into a reported value of ca. +99).   If, instead, Julie had discussed exactly the same data using the SMOW scale, her offset between expected and observed values would have been almost identical (~0.02 per mil), but divided by a much larger number (something like 30; I don't have my IAEA book on-hand!),  thus implying a much smaller proportional stretching factor (ca. 0.066 %; i.e, turning a measured value of +100 into +99.934 ”).  Nothing has changed about the actual isotope ratios measured, nor their proportional offset from expected ratios, but by arbitrarily changing the reference frame used to convert raw data to delta values, the slope of the 'stretching factor' has been changed by 15x.

Any method that is based on additions, subtractions, multiplications or divisions using data reported on an arbitrary delta scale is bound to encounter greater or lesser problems of this kind (though division and multiplication based on ratios of delta value is a particularly bad idea).

One way to deal with offsets of this kind, assuming they represent instrumental or analytical fractionation rather than some additive component (like a blank), is by applying a stretching factor based on the ratio of measured to expected isotope ratios, not delta values.  When delta values are near 0 and vary little (i.e., for typical O, C and N isotope data), this will result in a correction quite close to that obtained by just adding or subtracting delta values (Penny's first, and preferred method).  When delta values are far from 0 or vary greatly (as for most H isotope data), this method can differ considerably from simple additions or subtractions.  Jason's suggestion regarding stretching factors based on measured differences between two or more standards that differ greatly in delta value will yield a similar result (this is the norm for correction of instrument- or method-specific hydrogen isotope fractionations).

John

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.


Paul
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
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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