Hi folks,
                  A student here is undertaking an elevated CO2
experiment which involves adding extra CO2 to the ambient air in a
growth chamber. The 'tank' CO2 is at about -40 per/mil (13C) but after
dilution by the ambient CO2 in the chamber works out to be about -12
per mil, we are concerned that this is not different enough from
ambient to be able to trace the path of the carbon through the
experiment. We would like to know if there is any bottled CO2 out
there with a more extreme signature than -40. We are reluctant to
start using 13-C enriched CO2 for reasons of cost as the chamber is
not sealed and gas addition is constant.  Any help would be greatly
appreciated.

                                                             Cheers,
Darren

     Stable Isotope Facility
      ITE Merlewood
       Grange-over-Sands
        Cumbria,
          UK