Dear Steve,

You can use these materials (NBS18 and NBS19) as O-isotope reference
(secondary standard) materials when analysing carbonates. However, the
O-isotope scale is the VSMOW-SLAP scale (defined as 0 and -55.5 permil,
respectively), and any O-isotope value must be related to this scale (if
reporting to VSMOW). This means, unfortunately, for carbonates an extra
traceability step, introducing an additional uncertainty.
Certainly it is not a good idea to have more than one scale - we can have
sub-scales (but introducing more uncertainty!), as I have explained on the
list not too long time ago (look into the archives).

NBS18 is given as: -23.00 plusminus 0.07 permil and NBS19 as: -2.20 permil
(here also should be a plusminus value, since it is not the defined primary
international standard!!!; Can you comment on that Manfred Groening? Might
be introduced this way because for carbon isotopes the material has a
defined value as primary standard...).
Values taken from: M. Groening (2004) International stable isotope reference
materials, In: Handbook of stable isotope analytical techniques, Vol. 1,
P.A. De groot (ed.), Elsevier: 874-906.

I still believe we have to drop the VPDB-'scale' for oxygen (a sub-scale
where conversion to VSMOW introduces uncertainty, although not included in
the commonly used equations - conversion cannot be absolute, however...) -
it is only a historical (and probably sentimental and conservative) artefact
and more confusing than adding anything to proper analysis, and not needed
in any sense.

Hopefully this helps.
Best wishes,
Pier.


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> From: "Crowley, Stephen" <[log in to unmask]>
> Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
> Date: Wed, 8 Nov 2006 09:17:16 -0000
> To: <[log in to unmask]>
> Conversation: Normalisation of oxygen isotope scale for carbonates
> Subject: [ISOGEOCHEM] Normalisation of oxygen isotope scale for carbonates
> 
> I'd like to ask if it is now generally accepted practice to normalise
> the oxygen isotope scale for carbonate analysis by using values for
> NBS18 and NBS19 of -23.00 and -1.95 per mille (VPDB)?
> 
> Regards,
> 
> Steve Crowley
> 
> Department of Earth & Ocean Sciences
> University of Liverpool
> 4 Brownlow Street
> Liverpool
> L69 3GP
> UK
> 
> 0151 794 5163/5164
>