Dear list members

We need to determine carbon-13 and oxygen-18 in mixtures of carbonates,
particularly calcite and dolomite.
We used the method of separation that makes use of the variable reactions
rate of these minerals with phosphoric acid.
To test the method we used different mix ratios of calcite and dolomite of
known isotopic composition, that were reacted at 25ºC. We collected the CO2
produced in the first hour as representative of the calcite, we allowed the
reaction to proceed for three more hours (pumping away the CO2) and we made
a new CO2 collection after 72 hs, that we assumed to represent the dolomitic
fraction.  However, we observed cross-contamination in the isotopic signals
of both minerals.

Can anybody help us in telling what can be wrong or if you suggest a
different technique for this purpose?

Thank you in advance,


Héctor O. Panarello
Pabellón INGEIS - Ciudad Universitaria
1428 Ciudad de Buenos Aires
Tel: +54-11-4784 7798
Fax  +54-11-47833024