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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