>
> I suspect that there are as many different fractionation factors as there
> are labs with different methods of preparing carbonates. This was brought
> home to me when considering the results of an inter-lab comparison of the
> carbon and oxygen isotope composition of a series of CO2 gas samples. Labs
> were asked to calibrate using NBS-19 and an acid reaction fractionation
> factor of 1.01025. The range in measured delta 18-O compositions of any one
> gas was ca. 0.3 per mille. Gases in the test ranged over a 25 per mille
> delta 18-O compositional range. Over this range there were consistent
> differences between labs. i.e. irrespective of the gas composition one lab
> would consistently report data 0.1 per mille heavier than another etc. This
> I think can only be due to varying acid fractionation factors that apply to
> each lab.

With regard to your last statement, I think there are other *potential*
sources of these differences among test samples. One possibility
is the use of reference gases of different isotopic compositions in the
labs which made determinations of the delta 13C and delta 18O values of
those samples.

Lois Roe
Department of Geosciences
University of Arizona