A rapid and precise technique for measuring δ13C-CO2 and δ18O-CO2 ratios at ambient CO2 concentrations for biological applications and the influence of container type and storage time on the sample isotope ratios

A simple modification to a commercially available gas chromatograph isotope ratio mass spectrometer (GC/IRMS) allows rapid and precise determination of the stable isotopes (C-13 and O-18) of CO2 at ambient CO2 concentrations. A sample loop was inserted downstream of the GC injection port and used to introduce small volumes of air samples into the GC/IRMS. This procedure does not require a cryofocusing step and significantly reduces the analysis time. The precisions for delta(13)C and delta(18)O of CO2 at ambient concentration were +/-0.164 and +/-0.247parts per thousand, respectively. This modified GC/IRMS was used to test the effects of storage on the O-18 and C-13 isotopic ratios of CO2 at ambient concentrations in four container types. On average, the change in the C-13-CO2 and O-18-CO2 ratios of samples after one week of storage in glass vials equipped with butyl rubber stoppers (Bellco Glass Inc.) were depleted by 0.12 and by 0.20parts per thousand, respectively. The C-13 ratios in aluminum canisters (Scotty II and IV, Scott Specialty Gasses) after one month of storage were depleted, on average, by 0.73 and 2.04parts per thousand, respectively, while the O-13 ratios were depleted by 0.38 and 1.20parts per thousand for the Scotty II and IV, respectively. After a month of storage in electropolished containers (Summa canisters, Biospheric Research Corporation), the C-13-CO2 and O-18-CO2 ratios were depleted, on average, by 0.26 and enriched by 0.30parts per thousand, respectively, close to the precision of measurements. Samples were collected at a mature hardwood forest for CO2 concentration determination and isotopic analysis. A comparison of CO2 concentrations determined with an infrared gas analyzer and from sample voltages, determined on the GC/IRMS concurrent with the isotopic analysis, indicated that CO2 concentrations can be determined reliably with the GC/IRMS technique. The C-13 and O-18 ratios of nighttime ecosystem-respired CO2, determined from the intercept of Keeling plots, were -26.11parts per thousand (V-PDB) and -8.81parts per thousand (V-PDB-CO2), respectively. Copyright (C) 2002 John Wiley Sons, Ltd.