Lois,

the reference gases used in the various mass spectrometers would have been
calibrated using CO2 produced by phosphoric acid digestion of NBS-19. Thus
even though of different composition they should have been calibrated with a
gas of theoretically known composition. I still remain convinced that the
problem lies with variable apparent fractionation factors that are observed
with the different acid digestion methods.

Paul


-----Original Message-----
From: Lois J Roe <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Wednesday, February 25, 1998 4:40 PM
Subject: Re: acid fractionatiobn factors


>>
>> 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