Mobile, outdoor continuous-flow isotope-ratio mass spectrometer system for automated high-frequency C-13- and O-18-CO2 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 1.5m 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 CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07%. for delta(13)C and 0.34-0.14%o for delta(18)O of CO2 in air with mixing ratios ranging between 200-800 mumol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r(2)=1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta(13)C and delta(18)O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. delta(18)O exhibited a clear diurnal cycle (4parts per thousand range), but short-term (1-h interval) variability was small (average SD 0.38parts per thousand). Yet, the correlation between delta(18)O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta(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 -27.40parts per thousand ( 0.07parts per thousand 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 C-13 content of ecosystem respiration was approx. constant near -27.6parts per thousand. 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 CO2 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 (C) 2004 John Wiley Sons, Ltd.