"It also must be notified that there is a clear difference between
'off-line'
(CO2, O2) and 'on-line' (CO) methods of roughly 2 - 3 per mil., whatever
the
reason. Worrying enough...."
There is an explanation for that difference:
The off line methods are using high temperature (520 or 800 C), sealed
tube, carbon combustion to produce CO2 out of KNO3. At that temperature,
there is an isotope exchange between the oxygen of the CO2 and the oxygen
of the combustion tube (quartz, Vycor, Pyrex). Therefore, the isotope
ratio scale factors of the "off line" methods are compressed to 0.3 - 0.7
that of the scale factor of the on line method. The oxygen isotopic value
of the quartz is about 17 per mill, consequently if the oxygen isotope of
the nitrate around +23 permill (such as it is of the IAEA N-3), the
difference between on and off line method would be about -(2-3) per mill.
However, if the O-18 in nitrate is about +54 per mill, the difference would
be about -10 per mill, and if the oxygen of the nitrate is -30 per mill,
the difference would be around +15 per mill.
This is demonstrated in our paper "Comparison of delta O-18 measurements in
nitrate by different combustion techniques. by Revesz and Boelke, Anal.
Chem., 2002, 74, 5410-5413.
Cheers:
Kinga
Kinga Revesz
Chemist, Stable Isotope Laboratory
U.S.Geological Survey
MS 431
Reston, VA 20192
T:703-648-5865
FAX: 703-648-5274
http://www.isotopes.usgs.gov
Pier de Groot
<pier.de.groot@PA To: [log in to unmask]
NDORA.BE> cc:
Sent by: Stable Subject: Re: d 18O value for IAEA-NO-3
Isotope
Geochemistry
<[log in to unmask]
UVM.EDU>
12/09/2003 09:26
AM
Please respond to
Stable Isotope
Geochemistry
Moritz and Eddy,
The value I actually gave was the value as given in one of my book chapters
on standards by Manfred Groening, referring to the Boehlke et al. (2003)
paper. I looked again in the Boehkle et al (2003) paper, and indeed, a
number of values is given.
I am not certain why Manfred selected this particular value (can you react
yourself on this, Manfred?), but I guess since Boehlke and coworkers took
into consideration the 'capital delta'17O signature (quoted is the work by
Greg Michailski and coworkers: Michailski et al. (2002) Anal. Chem., 74:
4989-4993) of IAEA-NO-3 (= IAEA-N3) in their determination of the d18O
value, it was considered the most reliable determination of the d18O for
that reason!
It also must be notified that there is a clear difference between
'off-line'
(CO2, O2) and 'on-line' (CO) methods of roughly 2 - 3 per mil., whatever
the
reason. Worrying enough....
In any case, we really need to have consensus about the values of standards
and reference materials. Having a list of values for the same standard or
reference material is not serving anybody.
If the given d18O value is the best one (I followed Manfred Groening in
this) might raise dispute by some. But we have to decide which value we do
accept! We need to have an isotopic value solidly calibrated and traceable
to the N-isotope scale - that should be the choice. Otherwise we cannot
compare with each other and may bias our interpretations.
Pier.
> Eddy,
>
> The Boehlke paper is indeed the latest report on d18O of IAEA-N3. But be
> aware that there are quite a number of other (lower) values reported (in
> parts compiled in the Boehlke paper).
>
> Moritz
>
>
> On Tuesday, December 9, 2003, at 05:16 AM, eminet wrote:
>
>> Dear list members,
>> Has anyone of you a d18O value for the reference IAEA-NO-3 (for which
>> d15N=4.7)?
>> Thanks,
>> Eddy
>>
>> Eddy Minet
>> Centre for the Environment
>> Trinity College Dublin
>> Dublin 2 (Ireland)
>> e-mail: [log in to unmask]
>> tel: +353(0)1 608 2403
>> mobile: +353(0)86 827 4921
>>
>>
> Moritz F. Lehmann
> Department of Geosciences
> Princeton University
> Guyot Hall
> Princeton, NJ 08544
> phone: (609) 258-7544
> fax: (609) 258-0796 e-mail: [log in to unmask]
>
>
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