Jena Reference Air Set (JRAS)
Carbon and oxygen isotopic signatures of atmospheric CO2 are used to elucidate source and sink information pertaining to this very important greenhouse gas. Intra-annual variability of δ13C and δ18O values of carbon dioxide are around ~0.3 and ~ 0.6 ‰, respectively, and the decadal trend towards 13C depleted CO2 is currently ~ 0.2 ‰/decade. In order to resolve these very small variations the analytical measurement precision needs to be extremely high, and to compare results from different laboratories their results need to be comparable. The World Meteorological Organisation-Global Atmospheric Watch (WMO-GAW) has formulated compatibility goals for δ13C and δ18O measurements of atmospheric CO2 of 0.01 and 0.05 ‰, respectively. This means that the results of different laboratories for the same sample should not deviate by more than the stipulated compatibility goals. To improve inter-laboratory measurement compatibility the WMO-GAW has designated the stable isotope laboratory of the Max Planck Institute for Biogeochemistry (BGC-IsoLab) to be the central calibration laboratory (CCL) for isotopic measurements of atmospheric CO2.
Following the principle of identical treatment (IT-principle) isotopic analyses of atmospheric CO2 need to be standardised with air. Thus air standards need to be produced in which the isotopic composition of CO2 is directly linked to the primary isotopic standards provided by the IAEA. These standards are the NBS 19 (now exhausted) and IAEA-603 calcium carbonates. The BGC-IsoLab has developed instrumentation and techniques to produce reliable and long-lasting stable isotope ratio standards for CO2 in samples of clean air (Ghosh et al., 2005, Brand et al. 2009, Wendeberg et al. 2013). The specially designed Acid Reation and Air Mixing System (ARAMIS) generates CO2 from international primary, and in-house calcium carbonate standards:
CaCO3 + H3PO4 --> Ca(H2PO4)2 + CO2 + H2O
The evolved CO2 gas is mixed and equilibrated into synthetic CO2-free-air so that the CO2 concentration is close to ambient levels. These produced CO2-in-air mixtures serve as standards that directly link isotopic measurements of atmospheric CO2 to the VPDB-CO2 scale. A set of standards is called “Jena Reference Air Set” (JRAS), and these standards define the Jena Reference Air Set scale (JRAS-06) that has been in operation since 2006 to standardise isotopic measurements of atmospheric CO2. JRAS-06 links atmospheric CO2 measurements to the Vienna Pee-Dee Belemnite scale (VPDB) through two carbonates, MAR-J1 and OMC-J1 (Wendeberg et al., (2013). MAR-J1 is a marble purchased from a local vendor. The ground fraction < 250 µm is used and its characteristics are very close to those of the primary standard NBS 19. OMC-J1 is an Otavi-Meieberg calcite. Details on the composition of the calcites can be found in Ghosh et al. (2005) and below. MAR-J1 acts as quality control and are routinely analysed at the BGC-IsoLab. The δ13C and δ18O signatures of MAR-J1 are 1.96 ± 0.01 ‰ and -2.58 ± 0.03 ‰, respectively. The δ13C and δ18O signatures of OMC-J1 (final batch, not 1st drill) are -4.373 ± 0.01 ‰ and -8.93 ± 0.03 ‰, respectively. MAR-J1 along with the working standard (high pressure Jena air cylinder "Westfalen 10-2000) were calibrated against 10 NBS19 syntheses in 2004/05, and these NBS19 syntheses are the link to the VPDB-CO2 scale. The JRAS-06 scale is propagated through time using high pressure air as working standards. These standards are routinely checked against MAR-J1 calcium carbonate derived CO2-in-air syntheses to verify scale stability.
The BGC-IsoLab distributes the JRAS standards to laboratories that are part of the WMO-GAW frame work. The standards come in the form of two 5L glass flasks filled to ~ 1.5 bar with calcite derived CO2 mixed into CO2-free-air. These flasks are designed to be used with a dual inlet isotope ratio mass spectrometer setup (DI-IRMS). With the advent of laser spectroscopy based techniques that require more analyte, the BGC-IsoLab has also started to offer the analysis of high pressure air tanks on its local DI-IRMS systems (see “instruments and techniques”). The results of air analysed on these systems is automatically given on the JRAS-06 scale.
MAR-J1: The <250 µm size fractions weighing about 900 g was labeled ‘MAR-J1’. Texture and appearance of the powder is similar to NBS 19 carbonate material. Other fractions, 250-315 µm (~500 g) and 315-400 µm (~300 g), were designated as ‘MAR-J2’ and ‘MAR-J3’ and stored for future use. Quantitative analysis using ICP-MS and ICP-OES indicated an average CaCO3 content of 98.0 % and 2.0 % MgCO3. Al, Fe, Cu, Mn, Na, K together were less than 0.1 %. NBS 19 (TS limestone) is very similar: in line with literature XRF data we obtained 98.1 % CaCO3 and 1.8 % MgCO3. Other elements were 0.08 % in total. The similarity of the two materials is further confirmed by observing the carbonate reaction yield with NBS 19 and MAR-J1 resulting in the same amounts of CO2 gas.
OMC-J1: The composition analysis of the Otavi-Meieberg calcite using ICP-AES and ICP-OES has given 98.7 % CaCO3 and 0.9 % MgCO3 with non-carbonate cationic impurities summing up to 0.4 %. The crushing, milling and sieving left us with 1270 g powder with a grain size <100 µm (‘OMC-J0’), 700 g between 100 and 200 µm (‘OMC-J1’) and 1800g between 200 and 400 µm (‘OMC-J2/3’). In order to avoid oxygen exchange with ambient moisture or CO2 all fractions are kept in glass or PE jars topped with Ar.