Determination of the triple oxygen and carbon isotopic composition of CO₂ from atomic ion fragments formed in the ion source of the 253 Ultra high‐resolution isotope ratio mass spectrometer

Abstract

Rationale Determination of δ17O values directly from CO2 with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of 13C16O16O on 12C17O16O. The methods developed so far use either chemical conversion or isotope equilibration to determine the δ17O value of CO2. In addition, δ13C measurements require correction for the interference from 12C17O16O on 13C16O16O since it is not possible to resolve the two isotopologues. Methods We present a technique to determine the δ17O, δ18O and δ13C values of CO2 from the fragment ions that are formed upon electron ionization in the ion source of the Thermo Scientific 253 Ultra high‐resolution isotope ratio mass spectrometer (hereafter 253 Ultra). The new technique is compared with the CO2‐O2 exchange method and the 17O‐correction algorithm for δ17O and δ13C values, respectively. Results The scale contractions for δ13C and δ18O values are slightly larger for fragment ion measurements than for molecular ion measurements. The δ17O and Δ17O values of CO2 can be measured on the 17O+ fragment with an internal error that is a factor 1–2 above the counting statistics limit. The ultimate precision depends on the signal intensity and on the total time that the 17O+ beam is monitored; a precision of 14 ppm (parts per million) (standard error of the mean) was achieved in 20 hours at the University of Göttingen. The Δ17O measurements with the O‐fragment method agree with the CO2‐O2 exchange method over a range of Δ17O values of −0.3 to +0.7‰. Conclusions Isotope measurements on atom fragment ions of CO2 can be used as an alternative method to determine the carbon and oxygen isotopic composition of CO2 without chemical processing or corrections for mass interferences.

Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>

“…Pure CO2 (either normal CO2 or isotopically enriched CO2) was mixed with the incoming air to produce a CO2 mole fraction of 500 ppm. The isotopically enriched CO2 was prepared by photochemical isotope exchange between CO2 and O2 under UV irradiation, as described in detail in (Adnew et al, 2019).…”

“…After the isotope measurement, the remaining gas in the bellow of the IRMS was frozen back into the break seal tube for the measurement of Δ 17 O. The Δ 17 O of CO2 was determined using the CO2-O2 exchange method (Mahata et al, 2013;Barkan et al, 2015;Adnew et al, 2019). A detailed description of the CO2-O2 exchange system at Utrecht University is given in (Adnew et al, 2019) and the method is only described here briefly.…”

“…The Δ 17 O of CO2 was determined using the CO2-O2 exchange method (Mahata et al, 2013;Barkan et al, 2015;Adnew et al, 2019). A detailed description of the CO2-O2 exchange system at Utrecht University is given in (Adnew et al, 2019) and the method is only described here briefly. Equal amounts of CO2 and O2 were mixed in a quartz reactor containing a platinum sponge catalyst at the bottom and heated at 750 o C for 2hrs.…”

Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>

“…17 O of CO 2 has been suggested as an additional independent tracer for constraining global GPP (Hoag et al, 2005;Thiemens et al, 2013;Hofmann et al, 2017;Liang et al, 2017b;Koren et al, 2019) because the processes involved in plant-atmosphere gas exchange are all mass dependent. Therefore, 17 O at the CO 2 −H 2 O exchange site in the leaf will vary much less than δ 18 O.…”

“…1 in their box model. Koren et al (2019) incorporated all the physicochemical processes affecting 17 O of CO 2 in a 3D atmospheric model and investigated the spatiotemporal variability of 17 O and its use as tracer for GPP. Using these and other similar models, numerous measurements of 17 O in atmospheric CO 2 from different locations have been performed and used to estimate GPP (Liang et al, 2006;Barkan and Luz, 2012;Thiemens et al, 2014;Liang and Mahata, 2015;Laskar et al, 2016;Hofmann et al, 2017).…”