I have recently reviewed measurements of isotope ratio calibrations for
carbon and oxygen, see

Kaiser, J. (2008) Reformulated 17O correction of mass spectrometric stable
isotope measurements in carbon dioxide and a critical appraisal of historic
`absolute' carbon and oxygen isotope ratios. Geochim. Cosmochim. Acta 72,
1312-1334.

http://dx.doi.org/doi:10.1016/j.gca.2007.12.011

In the paper, I also discuss the relationship between different oxygen
isotope scales and associated uncertainties. I argue, for example, that the
difference between the 18O/16O ratios of atmospheric oxygen and VSMOW could
be as high as (24.36±0.06) ‰ if the scale was normalised to VSMOW/SLAP
difference of –56.18 ‰. This difference (to within 0.01 ‰) was reported by
three laboratories as a result of a lab intercomparison exercise, see

Verkouteren, R. M. and Klinedinst, D. B., 2004. Value assignment and
uncertainty estimation of selected light stable isotope reference materials:
RMs 8543-8545, RMs 8562-8564, and RM 8566 (2004 Edition). In: NIST
(Ed.),NIST Spec. Publ. NIST, Gaithersburg, Maryland, USA.

As Willi Brand pointed out, all these values are subject to re-measurement,
which is why we now have three different oxygen isotope scales (VSMOW, VPDB
and Air-O2). It could also be argued that we actually have four scales, the
fourth being VPDB-CO2, because the relation between the VPDB and VPDB-CO2
scales is just an assigned difference based on uncertain measurements.

Regards
Jan Kaiser

****************************************************************
Dr Jan Kaiser
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ
United Kingdom
Tel. +44 (1603) 59-3393 (Office: 01.36)
Fax  +44 (1603) 59-1327