Carbon isotope determination for separate components of heterogeneous materials using coupled thermogravimetric analysis/isotope ratio mass spectrometry.

A gas-tight thermal analysis system (Netzsch STA 449C Jupiter) has been connected to an isotope ratio mass spectrometer (PDZ Europa 20-20) via an interface containing an oxidizing furnace, water trap, and gas-sampling valve. Using this system, delta(13)C has been measured for CO(2) derived from the thermal decomposition of carbonate and oxalate minerals and organic materials at temperatures that correspond to different decomposition events. There is close agreement between measured and published delta(13)C values for carbonate and oxalate minerals, which have simple decarbonation reactions on heating. Cellulose and lignin-rich materials show much more complex thermal decomposition, reflecting differences in their purity and structure, and measured delta(13)C values vary with the temperature of gas sampling. Provided that measurements are made at temperatures that correspond to the decomposition of cellulose and lignin (indicated by maximum weight loss), internally consistent data can be obtained. However, measurements for cellulose and lignin are systematically enriched in delta(13)C (by up to 1.4 per thousand) with respect to those reported for reference materials, possibly due to the slower combustion kinetics (compared with EA-IRMS). Thermogravimetric analysis/isotope ratio mass spectrometry (TG-IRMS) is ideal for materials and samples for which it is not possible to use other isotopic measurement techniques, for example because of sample heterogeneity.