A rapid and precise technique for measuring δ 13 C‐CO 2 and δ 18 O‐CO 2 ratios at ambient CO 2 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 ( 13 C and 18 O) of CO 2 at ambient CO 2 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 δ 13 C and δ 18 O of CO 2 at ambient concentration were ±0.164 and ±0.247‰, respectively. This modified GC/IRMS was used to test the effects of storage on the 18 O and 13 C isotopic ratios of CO 2 at ambient concentrations in four container types. On average, the change in the 13 C‐CO 2 and 18 O‐CO 2 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 0.20‰, respectively. The 13 C ratios in aluminum canisters (Scotty II and IV, Scott Specialty Gasses) after one month of storage were depleted, on average, by 0.73 and 2.04‰, respectively, while the 18 O ratios were depleted by 0.38 and 1.20‰ for the Scotty II and IV, respectively. After a month of storage in electropolished containers (Summa canisters, Biospheric Research Corporation), the 13 C‐CO 2 and 18 O‐CO 2 ratios were depleted, on average, by 0.26 and enriched by 0.30‰, respectively, close to the precision of measurements. Samples were collected at a mature hardwood forest for CO 2 concentration determination and isotopic analysis. A comparison of CO 2 concentrations determined with an infrared gas analyzer and from sample voltages, determined on the GC/IRMS concurrent with the isotopic analysis, indicated that CO 2 concentrations can be determined reliably with the GC/IRMS technique. The 13 C and 18 O ratios of nighttime ecosystem‐respired CO 2 , determined from the intercept of Keeling plots, were −26.11‰ (V‐PDB) and −8.81‰ (V‐PDB‐CO 2 ), respectively. Copyright © 2002 John Wiley & Sons, Ltd.