Dear Pier and Ty, 

I completely agree with Ty. 
The combined uncertainty of CO2 analyses, including the calibration to the primary CO2 standard in use or secondary standards (NIST CO2 gases - RM 8562-8563-8564) is much smaller than the combined uncertainty, when d18O(CO2) on the VPDB_CO2 scale converted to the VSMOW scale.
In the latter case any inter-comparison of d18O(CO2) would be limited not by analytical capabilities but mostly by the scale conversion factor. 

As long as many important measurements are made on CO2 gas (carbonates, monitoring of air CO2) and these practically do not relate to waters, I see that as a reason to report data on the VPDB_CO2 scale. 
In that case anyone can easily rescale values, with a nicely propagated uncertainty! 

Kind regards, 

Sergey


............................................ 
Dr. Sergey Assonov 
Joint Research Centre (JRC) of European Commission (EC) 
Institute for Reference Materials and Measurements, Isotopic Measurements Unit 
Retieseweg 111, 2440 Geel, Belgium 
phone: +32-14-571623 
fax:     +32-14-571863 
............................................ 

 


________________________________

	From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Pier de Groot
	Sent: Monday, December 11, 2006 6:28 PM
	To: [log in to unmask]
	Subject: Re: [ISOGEOCHEM] On VSMOW - VPDB again
	
	
	Hi Ty,
	
	If the conversion is so uncertain, why this is not shown by a large uncertainty in the conversion equations than???
	There are even more serious implications. If we keep the obsolete VPDB scale alive, then there are arguments for those working with other materials, such as organic components or atmospheric gases, also to have their own scales (proposals were already seen into that direction). If we do that, we completely loose any possibility ever to compare and will drift away from each other seriously in the end.
	
	I very well realize the analytical problems of analyzing different materials with different methods (even same materials with different methods already can be tricky enough; new methods are introduced over time, as also will be in future, and actually make comparisons not easier), and the eventual uncertainty related to the calibration procedures. But the problem will be even bigger if we do not try to make a uniform scale for each isotopic ratio (how, for instance, 17O would fit into the 18O/16O scale is another problem, added to the already existing ones).
	
	The decision in 1985 was probably a compromise one, while in metrology compromises never are a solution (to my humble opinion)....
	But considering the difficulty of the variety of sample materials and analytical methods, I can understand that (compromise) decision, but not necessarily I do need to agree with it, do I? I still think we should have a single scale, and if that introduces more uncertainty to measurements of some materials, we have to live with that fact. Now we have to live with another incorrect situation, that not cures any of the problems... Resuming, I can conclude that we agree on the fact that at this stage of our capability of analyzing we cannot reach an ideal situation or anything that comes close to that.
	
	Thanks a lot for your answer to my posting as I asked to the community, I really appreciate that Ty. 
	Nothing seems to be so complicated in analytical procedures as is metrology!
	And put two analysts together and you will get two opinions and two different outcomes of measurement (but hopefully not very far appart... i.e within the range of uncertainties for both).
	
	Best wishes,
	Pier
	
	
	
	
________________________________

	From: Tyler B Coplen <[log in to unmask]>
	Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
	Date: Mon, 11 Dec 2006 10:57:03 -0500
	To: <[log in to unmask]>
	Subject: Re: [ISOGEOCHEM] On VSMOW - VPDB again
	
	
	Pier, 
	
	The fact that we do not know accurately the conversion relationship between the VSMOW and VPDB delta O-18 scales is exactly the reason that the VPDB delta O-18 scale should be retaining.  It can be used in oceanography, for example, to easily express delta O-18 values of carbonates with the highest accuracy and precision.  This was recognized by IAEA consultants, meeting in Vienna in September of 1985, who anchored the VPDB delta O-18 scale by assigning a consensus value of -2.2 per mill exactly to NBS 19 calcium carbonate. 
	
	You are quite correct that it is important to normalize delta O-18 data.  VPDB delta O-18 data should be normalized so that if carbonates differing in delta O-18 values by -55.5 per mill were analyzed, their reported values would differ by -55.5 per mill.  This can be accomplished by interspersing NBS 18 carbonate among unknowns and normalizing delta O-18 data by assigning a delta O-18 value on the VPDB scale to NBS 18 of -23.01 per mill (Coplen, 1996, More uncertainty than necessary, Paleoceanography, vol. 11, p. 369-370). 
	
	For any scale, authors are strongly encouraged to provide in every published report the delta values for internationally distributed isotopic reference materials had they been analyzed with their unknowns.  In this manner, their valuable data can be used most effectively by researchers in the future. 
	
	If analytical techniques improve sufficiently with new equipment and techniques, then it might be possible to go to one scale.  I look forward to that day. 
	
	Sincerely, 
	
	Ty 
	
	
	Pier de Groot <[log in to unmask]> 
	Sent by: Stable Isotope Geochemistry <[log in to unmask]> 12/11/2006 06:52 AM 

	Please respond to
	Stable Isotope Geochemistry <[log in to unmask]> 

	To 

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	Subject 

	On VSMOW - VPDB again 
	
	
	
	
	Dear all,
	
	I received questions about the conversion of VSMOW into VPDB (or visa versa) scale. It was noted that a while ago there was discussion already on this matter on the list, and Ty Coplen presented a corrected equation (caused by a better measurement of 'absolute' isotopic rations).
	
	What still surprises me in this matter, where we do our best to get this conversion as good we can, that all equations are given as absolute conversions. This, unfortunately, gives the impressiuon that we have two defined absolute scales for O-isotope ratios - a situation that physically cannot exist. We only can have one defined scale (what to my knowledge is the VSMOW-SLAP scale), and all others must be related to that one, meaning introduction of uncertainty if converting to that 'secondary' scale (simple rule of traceability). Now, in all the equations published up to now this uncertainty is completely left out. There should be a plus/minus added with equations.
	
	Even better would be to stop using completely this obsolete VPDB sub-scale for O-isotopes - it is just historical and only is continued out of sentiments and conservatism, and does not add anything else than confusion.
	It only introduces serious mistakes, as clearly shown by the conversion equations, missing the added uncertainty of this conversion....
	Is there any conclusion/decision on this important matter by the IUPAC commission? They should be aware of this wrong representation!
	
	I really hope to get some reactions on this posting. It is an important subject that should be solved.
	I bring this up again because I think it still is not solved - it would be better if an organisation, like for instance IUPAC, is giving the correct directions in this matter. If these already would exist, I like to hear where to find it - then I must have overlooked one or two things!.
	
	Best wishes,
	Pier.
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