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
*******************************************************************
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
[log in to unmask]
Office: 209b Hutchison
Hall Lab: 209 Hutchison Hall
Voice : (585)
275-0601 Outer lab: (585) 273-1405
FAX : (585)
244-5689 Inner lab: (585) 273-1397
http://www.earth.rochester.edu/SIREAL/index.html
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