>Dear ALL,
>
>Can anyone refer me to an article which details how we can convert old PDB
>and SMOW data to VPDB and VSMOW?
>
>
>Tschüs
>

It is my understanding that PDB and VPDB are essentially the same provided
that the reported values on the "OLD" PDB scale were calibrated to that
scale using NBS-19 values of -2.20 and 1.95 per mil for oxygen and carbon
respectively. This may or may not be the case as NBS-19 values for carbon
have been reported as 1.93 per mil by Coplen at al., 1983 and1.92 per mil
by the National Bureau of Standards (NIST) as recently as 1990.
Interestingly, many laboratories also calibrated thier ref gas to NBS-20 as
well as NBS -19 in the past and often when using values of -4.14 and -1.06
per mil (for oxygen and carbon respectively) for this standard (NBS-20)
actually determined that NBS- 19 gave values that were several hundreths of
a per mil different than those earlier reported values (hence the new value
reported exactly as 1.95 per mil). NBS-20 has problems of it's own in terms
of potential oxygen exchange with atmospheric water vapour because it is a
finely ground powder. It is no longer distributed  and this is unfortunate
as a good dessicator certainly alleviates that problem!. I think the thing
that Ty is trying to do is just to make sure that everyone calibrates the
same way to the same values because in the past as the real PDB (or the
Chicago PDB-1 standard) expired many laboratories used internal lab
standards that they believed they knew very well relative to PDB. All these
different labs would use these different standards and believe they were on
the PDB scale (which they were as far as they were concerned). The problem
was they were all at slightly different sliding scales so that when a
believed to be homogenous "toilet seat standard" was distributed to 31
laboratories the values ranged from -2.42 to -1.53 per mil for oxygen and
1.75 to 2.33 for carbon. Now this "toilet seat" standard is uniquely
defined as NBS-19 and a general consensus for its values relative to PDB
have been defined as well. If you calibrate to NBS-19 with these values
then you report your data relative to VPDB. Implicit in data reported to
VPDB is that everyone will know what standard with what values you used to
calibrate your mass spec. For older data conversions at different labs you
have to know the nuts and bolts of how everything was done in the lab. Best
of Luck, Cap

Douglas "Cap" Introne
Stable Isotope Laboratory
Sawyer Environmental Research Center
University of Maine
Orono, ME 04469

Tel. (207) 581-2192
Fax (207) 581-3490
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URL: http://iceage.umeqs.maine.edu/geology/sil/home.htm