CONTINUOUS-FLOW ISOTOPE RATIO MASS-SPECTROMETRY OF CARBON-DIOXIDE TRAPPED AS STRONTIUM CARBONATE

The isotopic signal provided by differential discrimination against at mospheric carbon dioxide ((CO2)-C-13) by C-3 and C-4 plant photosynthe tic pathways is being widely used to study the processes of carbon (C) fixation, soil organic matter formation, and mineralization in nature . These studies have been facilitated by the availability of automated C and nitrogen (N) combustion analyzers (ANCA) combined with continuo us flow isotope ratio mass spectrometers (CFIRMS). Analysis of (CO2)-C -13, in these instruments requires consistent sample mass for best pre cision, a requirement that is easily satisfied for soil and tissue sam ples by adjusting sample weight. Consistent CO2 sample size is much mo re difficult to achieve using gas handling systems for samples of head space gases when CO2 concentrations vary widely. Long storage of gaseo us samples also is difficult. Extended respiration studies are most ea sily conducted by trapping CO2 in alkali and conversion to an insolubl e carbonate. Thermal decomposition of the carbonate in an on-line ANCA allows consistent and optimal CO2 sample mass to be obtained. The use of precipitated carbonates also facilitates storage of samples and en ables full automation of sample analysis using an ANCA interfaced to a CFIRMS. Calcium (Ca), strontium (Sr), and barium (Ba) carbonates were tested. Strontium carbonate (SrCO3) with the addition of vanadium pen toxide (V2O5) as a combustion catalyst was found most suitable.