>Hi everyone,
>
>I've got a small but annoying (and probably very simple) problem with the
>O18 ratios of SMOW and PDB and the relationship between gases calculated
>upon these.  If I use absolute ratios to convert from PDB to SMOW, nothing
>seems to work...
>
>The values I have are SMOW: 2005.2 and PDB: 2079.
>
>The standard relationship we use to convert from gases with respect to PDB
>and SMOW is Delta(SMOW)=1.03091 * Delta(PDB) + 30.91, implying that PDB is
>30.91 permil heavier than SMOW.

You will find a discussion of the parameters that go into this equation
in "Comparison of stable isotope reference samples" by Coplen, Kendall,
and Hopple, Nature, v. 302, p. 236-238, 1983.  This equation is
applicable for a value of -2.19 per mill for NBS 19.  Because the VPDB
scale is defined by adopting a value of -2.2 per mill (exactly) for
NBS 19, the relationship between VSMOW and VPDB that we use is

     Delta(VSMOW) =  1.03092 * Delta(VPDB) + 30.92

Precise 16-O, 17-O, and 18-O abundances in VSMOW, SLAP, and NBS 19
are needed.

You may want to discontinue reporting delta values relative to PDB
to indicate that you know (i) that PDB carbonate no longer exists and
(ii) that the VPDB scale is the recommended replacement for the PDB
scale.

For a recent discussion of isotope scales and isotopic reference
materials, see "Standards and intercomparison materials distributed
by the International Atomic Energy Agency for stable isotope
measurements" by R. Gonfiantini, W. Stichler, and K. Rozanski,
In  Reference and intercomparison materials for stable isotopes of
light elements, p. 13-29, IAEA-TECDOC-825, International Atomic
Energy Agency, Vienna, 1995.

>
>My problem is that I can't get any calculation with 2005.2 and 2079 to come
>up with a value of 1.03091, even allowing for the PDB ratio to be that of
>the gas extracted from PDB dissolved in Phosphoric at 25C.
>
>It's got to be easy, but I can't suss it out myself, so any help would be
>hugely appreciated.
>
>Thanks in advance,
>
>Andrew
>--
>Andrew Tait
>SURRC
>Rankine Avenue
>Scottish Enterprise Technology Park
>East Kilbride
>GLASGOW  G75 0QF
>
>email:  [log in to unmask]
>Tel:  (01355) 223332
>Fax:  (01355) 229898
>