David Denhaan
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                    Sent by: Stable        Subject:     VPDB vs PDB
                    Isotope
                    Geochemistry
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                    01/31/2000
                    09:20 PM
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                    to Stable
                    Isotope
                    Geochemistry









What is the difference in values between VPDB and PDB?  My understanding is
VPDB is set to 1.95 and PDB is set to zero.  Is this correct?

Thanks,

David DenHaan

National Soil Tilth Laboratory
Ames, Iowa

Not at all.  It depends upon the laboratory.  In 1984, Gonfiantini
(Advisory group meeting on stable isotope reference samples for geochemical
and hydrological investigations, Sept. 19-21, 1983, Rep. to the Director
General, Int. At. Energy Agency, Vienna, 1984) pointed out that NBS 19
calcite as measured by 31 stable isotope laboratories from 19 countries
ranged from +1.75 to +2.33 per mill.
This is due to the fact that the supply of PDB is exhausted, and many of
these laboratories had tied their scales to degraded isotopic reference
materials.  To correct the problem, IAEA and IUPAC recommend that 13C/12C
ratios be reported relative to VPDB by assigning a value of +1.95 per mill
to NBS 19 calcite.

The advantage of stating your results relative to VPDB is that you make
others aware that you know the difference between PDB and VPDB, and others
then know that analysis of NBS 19 in your laboratory would give +1.95 per
mill.  Alternatively, if you state carbon isotope ratios relative to PDB,
just state what value you would get had you analyzed NBS 19 in your
laboratory.  Others can then "adjust" your data as necessary.

Tyler Coplen
USGS