Mobile, outdoor continuous‐flow isotope‐ratio mass spectrometer system for automated high‐frequency ¹³C‐ and ¹⁸O‐CO₂ analysis for Keeling plot applications

Abstract

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 CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 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.

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

“…2.2.2 below), CO 2 mole fraction was substituted by CO 2 peak area for the Keeling plots. Schnyder et al (2004) demonstrated a proportional relationship between CO 2 mole fraction and CO 2 peak area. Measurement uncertainty of the mass spectrometer (SD of replicate measurements) was 0.09 ‰ for δ 13 C, and corresponded to ∼2 µmol mol −1 for the CO 2 peak area.…”

“…For a description of the two respiration measurement approaches in the field see below. The CO 2 mole fraction and δ 13 C were analysed in the field with an infrared gas analyser (LI 7000; Li-Cor, Lincoln, NE, USA) and a continuous-flow isotope-ratio mass spectrometer (Delta Plus Advantage; Thermo Electron, Bremen, Germany) interfaced with a Gasbench II (providing sample gas separation via a built-in gas chromatograph, and sample and reference gas injection to the mass spectrometer; Thermo Electron, Bremen, Germany) (Schnyder et al, 2004). To ensure synchronous analysis of both quantities for the Keeling plots (see Sect.…”

“…Natural variability in atmospheric CO 2 would cause the same disequilibrium effect as a change of headspace CO 2 inside the chambers. Diurnal cy- cles of δ 13 C atm can show amplitudes of ∼10 ‰ (e.g., Schnyder et al, 2004). Using a diffusion-based model Nickerson and Risk (2009a) predicted a disequilibrium effect within 0.05 ‰ resulting from daytime-nighttime changes of both atmospheric CO 2 concentration and δ 13 C. However, inclusion of the dissolution mechanism would likely multiply this disequilibrium effect.…”

Diel variations in the carbon isotope composition of respired CO<sub>2</sub> and associated carbon sources: a review of dynamics and mechanisms

“…A highly variable range in soil δ 13 C R of 0.3-12.5 ‰ occurred in an experimental garden with deciduous trees (Moyes et al, 2010). The information on dynamics of ecosystem respiration (δ 13 C R assessed by Keeling-plot approaches) presents again a very heterogeneous picture: while Ogée et al (2003) and Schnyder et al (2004) found only minor nocturnal variation of δ 13 C R (<3 ‰), others report that nocturnal ecosystem δ 13 C R presented the largest variation among different respiratory components (Kodama et al, 2008;Unger et al, 2010a). Nocturnal variations in δ 13 C R were 6.4 ‰ in a grassland (Bowling et al, 2003), 4.2-8.1 ‰ in a Mediterranean woodland (Werner et al, 2006;Unger et al, 2010a), 6.1 ‰ in a Pinus sylvestris stand (Kodama et al, 2008), 2.6-3.6 ‰ in a subalpine forest (Bowling et al, 2005;Riveros-Iregui et al, 2011), and 3.8 ‰ a beech-dominated deciduous forest (24 h-cycle, Knohl et al, 2005).…”

A new instrument for stable isotope measurements of &lt;sup&gt;13&lt;/sup&gt;C and &lt;sup&gt;18&lt;/sup&gt;O in CO&lt;sub&gt;2&lt;/sub&gt; – instrument performance and ecological application of the Delta Ray IRIS analyzer

“…For measurements of the isotopic composition of CO 2 , IRMS has typical precisions of approximately 0.02 to 0.1 ‰ for 13 C and 0.05 to 0.2 ‰ for 18 O. IRMS has been widely used for isotope studies in environmental sciences, though it shows limited applicability for in situ measurements (Griffis, 2013), but see also the field-applicable continuous flow IRMS described by Schnyder et al (2004). Disadvantages of flask-sampling-based IRMS techniques include high sample preparation effort and costs (Griffis, 2013), low temporal resolution and discontinuous measurements.…”