Dear all, This is my virginal posting so excuse any failures in protocol. In response to Steve Nelson's posting I thought this might be of use. I use triple distilled water (is this ultra pure?) with other media added to culture bacteria on certain target contaminants. I have added known concentrations of CO2 to the headspace of a closed system and studied CO2 eqilibration by analysing the fractionation of the Delta 13C value left in the headspace as a function of time. Accounting for temperatures from 4C to 35C and headspace concentrations of 1% to 10% equilibration is complete within 60 minutes. I hope this helps, James Hall ---------------------- James Hall [log in to unmask]