The effect of photosynthetic gas exchange on Δ17O of atmospheric CO2

Abstract

Rationale: Determination of 17 O directly from CO2 with traditional gas source isotope ratio mass spectrometry is not possible due to isobaric interference of 13 C 16 O 16 O on 12 C 17 O 16 O. The methods developed so far use either chemical conversion or isotope equilibration to determine 17 O of CO2. In addition, 13 C measurements require correction for the interference from 12 C 17 O 16 O on 13 C 16 O 16 O since it is not possible to resolve the two isotopologues. Methods: We present a technique to determine 17 O, 18 O and 13 C 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 17 O-correction algorithm for 17 O and 13 C respectively. Results: Scale contractions for 13 C and 18 O are slightly larger for fragment ion measurements compared to molecular ion measurements. 17 O and 17 O of CO2 can be measured on the 17 O + 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 17 O + 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 17 O measurements with the O-fragment method agree with the CO2-O2 exchange method over a range of 17 O values of-0.3 to + 0.7 ‰. Conclusions: Isotope measurements on atom fragment ions of CO2 can be used as alternative method to determine the carbon and oxygen isotopic composition of CO2 without chemical processing or corrections for mass interferences.

Exploring the potential of Δ17O in CO2 for determining mesophyll conductance

“…The fractionation associated with the initial fixation by the enzyme RubiscO (ribulose-1,5-bisphosphate carboxylase-oxygenase) or PEP (phosphoenolpyruvate) has no/negligible effect on the δ 18 O and Δ 17 O value of CO 2 ( Farquhar and Lloyd 1993 ; Adnew et al 2021 ). As a result, Δ A 18 O and Δ A Δ 17 O can be positive, 0, or negative depending on the oxygen isotope composition of the CO 2 entering the leaf and the isotope composition of leaf water ( Adnew 2020 ). Δ A 18 O is proportional to c m /( c a − c m ) × (δ 18 O m − δ 18 O a ) ( Farquhar and Lloyd 1993 ; Hofmann et al 2017 ; Koren et al 2019 ), and Δ A Δ 17 O is proportional to c m /( c a − c m ) × (Δ 17 O m − Δ 17 O a ) ( Hofmann et al 2017 ; Koren et al 2019 ; Adnew et al 2020 ) whereas Δ A 13 C is proportional to ( b − a 13 ) c c / c a , where a 13 is the 13 C discrimination due to diffusion and b is the discrimination due to Rubisco ( Farquhar et al 1982 , 1989 ; Farquhar and Lloyd 1993 ).…”

“…Details of the model setup are provided in the Supplemental data . Similarly, the photosynthetic discrimination in Δ 17 O (Δ A Δ 17 O) is calculated as shown in Equation ( 14 ) ( Adnew 2020 ; Adnew et al 2020 ).…”