Dear Darren,

First, retroconvertion of PDB and SMOW data to VPDB and VSMOW may not be 
possible if isotopic results of reference materials are not available.

Coplen, Normalization of oxygen and hydrogen isotope data, Chemical 
Geology (Isotope Geoscience Section), v. 72, p. 293-297, 1988, discusses 
the mechanics of converting isotopic data to VPDB and VSMOW and should 
be helpful.  If you have additional questions, please contact me directly.

An example showing the conversion of measured delta O-18 values to VSMOW 
is given by Coplen, Kendall, and Hopple, Comparison of stable isotope 
reference samples, Nature, v. 302, p. 236-238, 1983.  Note that the values 
in column 7 are expressed on a scale such that SLAP is -55.5 per mill 
exactly relative to VSMOW and that the value of NBS 19 in this column is 
-2.19 per mill.  To convert data in this column from PDB to VPDB, one would 
adjust the coefficients used to generate column 7 data so that NBS 19 
becomes -2.2 per mill exactly instead of -2.19 per mill.

For C-13 data it is only necessary to adjust the isotopic results so that 
the delta C-13 of NBS 19 becomes +1.95 per mill.  If the value of NBS 19 
is not given by authors of an article, you may need to contact the authors 
to determine what value they would have reported for their measurement of 
NBS 19.

Tyler B. Coplen
U.S. Geological Survey
431 National Center
Reston, VA  20192