Dear Anthony,
the problem of "non-linearity" is not resticted to dD measurements but
may apply also for d13C measurements using GC-C-IRMS. You can do
correction for this bias as suggested for d13C (see refs below).
Kind regards
Bruno
Glaser, B. and Amelung, W., 2002. Determination of 13C natural abundance
of amino acid enantiomers in soil: methodological considerations and
first results. Rapid Communications in Mass Spectrometry, 16: 891-898.
Glaser, B., 2005. Potential and constraints of compound-specific stable
isotope (d13C) analysis in environmental research. Journal of Plant
Nutrition and Soil Science, 168: 633-648.
Gross, S. and Glaser, B., 2004. Minimization of foreign carbon addition
during derivatization of organic molecules for compound-specific d13C
analysis of soil organic matter. Rapid Communications in Mass
Spectrometry, 18: 2753-2764.
Schmitt, J., Glaser, B. and Zech, W., 2003. Amount-dependent isotopic
fractionation during compound-specific isotope analysis. Rapid
Communications in Mass Spectrometry, 17: 970-977.
_____________________________________________
PD Dr. Bruno Glaser
Abteilung Bodenphysik
Universität Bayreuth
Universitätsstr. 30
95440 Bayreuth
Deutschland
Email: [log in to unmask]
Web: http://www.bayceer.uni-bayreuth.de/bophy
_____________________________________________
-----Ursprüngliche Nachricht-----
Von: Stable Isotope Geochemistry [mailto:[log in to unmask]] Im
Auftrag von Anthony L. Michaud
Gesendet: Montag, 16. April 2007 20:23
An: [log in to unmask]
Betreff: Re: [ISOGEOCHEM] The "start value" of the H3 factor
Jonathan,
I'm glad you pointed this out. I have been wondering for some time how
groups evaluate their data quality when doing GC/TC analyses on H2 peaks
that are below 5V. The H3 factor is indeed not correct at this level.
Similarly, I've been wondering to what extent the incorrect H3 factor
skews
the portion of the peak front and tail that is below 5V. I sent a note
to
Thermo a year and a half ago to ask about how isodat performs the H3
factor
determination and applies it (you can see it here:
http://kfleb.asu.edu/H3factordeterminationproblem.ppt). Unfortunately, I
was
told from Thermo that the info on how the correction is made is not
available and that there is no intention to make it available. It seems
to
me that if groups want to make measurements down at the 1-2V level then
they
really need to perform some sort of check with real standards that
demonstrates that the isotope ratio measured at that level behaves in a
consistent, correctable way. This is something that I would expect
reviewers
to request when seeing manuscripts that present d2H results for such low
level signal.
Although there are several good articles out on the origin of the H3
factor
issue and how best to apply it (peakwise vs pointwise), I don't know of
anything that discusses how to make a correct measurement on something
with
a continuous flow signal below 5V. For our part, we have a faculty
member
who is interested in some d2H measurements for what will amount to very
small peaks. The good thing is that she understands that the uncertainty
of
the measurement will likely be several 10's of permil at least. That's
ok
for her application though.
I'm probably way out of the loop concerning all of the literature out
there
with d2H measurements on low amplitude peaks so I would be very pleased
if
someone could point me to an article that gives a good demonstration of
an
evaluation of the uncertainty of a d2H measurement made for a 1V peak,
for
example. Given that the H3 factor is not linear, a simple standard
deviation
of replicate analyses is not enough to do this: the measured value
itself
will change at those lower levels so a measurement can be precise but
not at
all accurate.
Thanks in advance for any pointers.
Anthony
------------------------------------------------------
Anthony L. Michaud
KFLEB
School of Earth and Space Exploration
Arizona State University
Tempe, AZ 85287-1404
Tel: 480-727-8033
Fax: 480-965-8102
Web: http://kfleb.asu.edu
***Windsurf Everywhere***
------------------------------------------------------
-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf
Of Jonathan Karr
Sent: Monday, April 16, 2007 9:41 AM
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] The "start value" of the H3 factor
Not sure about the newer models, but for the Delta Plus XL the H3
determination is not linear under 5V according to the manual
Harlow, Benjamin wrote:
>
> Effie,
>
> In the method you used, under "Peak detection @H2", you will see the
> starting value of the H3 factor. Here you can select overwrite and put
> in a starting value of 5. You could try this using the reevaluate
> option for the CF document in question. But I think you should use a
> broader span of voltages to calculate the H3, like 1-7V for example.
> That way you are giving isodat more of slope to work with, thus
> reducing error in the estimate of the H3.
>
> Isodat will start with the last H3 calculated, so you need to force it
> to start at a more realistic starting point. I have one method I only
> use for calculating the H3 where I overwrite or define the H3 starting
> value to be 5.
>
> Hope this helps,
>
> Ben Harlow
>
> Washington State University
>
>
------------------------------------------------------------------------
>
> *From:* Stable Isotope Geochemistry [mailto:[log in to unmask]]
> *On Behalf Of *effie
> *Sent:* Monday, April 16, 2007 12:40 AM
> *To:* [log in to unmask]
> *Subject:* [ISOGEOCHEM] The "start value" of the H3 factor
>
> Hi all,
>
> I have started to look into dD analysis, and I have some problems
> concerning the H3 factor. I hope you can enlighten me with some
> explanations. The analysis is of water using the TC/EA and DeltaplusXP
> MS. Back ground levels are of 10 for mass 3, and 30 for mass 2. During
> the determination of the factor (that is preceded by the zero on off
> measurement while increasing H2 ref gas pressure. Increasing being in
> the intensity range of 1000mV to 2000mV), the software (Isodat 2.0)
> determines a value from which it starts to calculate the H3 factor. On
> screen it will show it as "start value". The start value is of 42, the
> H3 factor which is determined is around that figure (with a standard
> deviation 0.6).
>
> My questions:
>
> What is that "start value" and how it is determined?
>
> Assuming my factor should be less then 10, what may cause a larger
figure?
>
> Thanks in advance,
>
> Effie
>
> *Effie Eliani
>
> **Stable isotopes lab
>
> ***Geological Survey of Israel
>
> This mail was sent from mail.pineapp.net
>
>
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--
Jonathan D. Karr, Ph.D.
Technical Director, Duke EnVironmental stable Isotope Laboratory (DEVIL)
Duke University Dept.of Biology & Nicholas School of the Environment and
Earth Sciences
Phytotron Bldg, 71 Science Drive
Durham, NC 27708-0340
(919) 660-7418
Fax: (919) 660-7425
[log in to unmask]
laboratory website: http://www.biology.duke.edu/jackson/devil/
Adversity is an opportunity
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