Mobile, outdoor continuous‐flow isotope‐ratio mass spectrometer system for automated high‐frequency 13 C‐ and 18 O‐CO 2 analysis for Keeling plot applications

A continuous‐flow isotope‐ratio mass spectrometer (CF‐IRMS, custom‐made GasBenchII and Delta plus Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed‐path infrared gas analyser (IRGA). The CF‐IRMS and IRGA were housed in an air‐conditioned travel van. Air was sampled at 1.5 m above the 0.07‐m tall grassland canopy, drawn through a 17‐m long PTFE tube at a rate of 0.25 L s −1 , and fed to the IRGA and CF‐IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas‐tight pump, and a sample loop attached to the eight‐port Valco valve of the continuous‐flow interface. Air was pumped through the 0.25‐mL sample loop at 10 mL s −1 (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO 2 and CO 2 ‐free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21–0.07‰ for δ 13 C and 0.34–0.14‰ for δ 18 O of CO 2 in air with mixing ratios ranging between 200–800 μmol mol −1 . The CO 2 peak area measured by the IRMS was proportional to the CO 2 mixing ratio (r 2  = 1.00), allowing estimation of sample air CO 2 mixing ratio from IRMS data. A 1‐day long measurement cycle of CO 2 , δ 13 C and δ 18 O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. δ 18 O exhibited a clear diurnal cycle (4‰ range), but short‐term (1‐h interval) variability was small (average SD 0.38‰). Yet, the correlation between δ 18 O and CO 2 mixing ratio was relatively weak, and this was true for both the whole data set and 1‐h subsets. Conversely, the δ 13 C of all 541 samples measured during the 25.2‐h interval fitted well the Keeling regression (r 2  = 0.99), yielding an intercept of −27.40‰ (±0.07‰ SE). Useful Keeling regressions (r 2  > 0.9, average r 2  = 0.96) also resulted from data collected over 1‐h intervals of the 12‐h long twilight and dark period. These indicated that 13 C content of ecosystem respiration was approx. constant near −27.6‰. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory‐based CF‐IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO 2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance. Copyright © 2004 John Wiley & Sons, Ltd.