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

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.

These abundances can be changed by kinetic and equilibrium fractionation processes and other physicochemical effects. Variations in isotopic abundance are reported as deviations of a heavy-to-light isotope ratio in a sample relative to a reference material. In the case of oxygen isotopes, the two isotope ratios are 18  ln The factor λ i:e: ranges from 0.5 to 0.53 for such mass-dependent fractionation processes. [1][2][3] Ozone photochemistry is a well-known exception to this rule, and O 3 and related gases have a large oxygen isotope anomaly, expressed as Δ 17 O and referred to as mass-independent fractionation. We use the logarithmic definition to calculate Δ 17 O of CO 2 (Equation 4). 2,4,5 Note that the choice of λ is arbitrary since a variety of sources contribute to the isotopic composition of tropospheric CO 2 with different fractionations and different three-isotope slopes. In this study we used a λ value of 0.528 to calculate the Δ 17 O of CO 2 following Barkan and co-workers 6,7 and the 17 O-correction algorithm by Brand et al. 8 Since the discovery of mass-independent fractionation, 9 the Δ 17 O value has been used to study sources/sinks of atmospheric trace gases and chemical reaction pathways. Several studies have shown that CO 2 acquires Δ 17 O from O 3 via photochemical isotope exchange in the stratosphere. [10][11][12][13][14][15][16][17] When this CO 2 re-enters the troposphere 18-20 the Δ 17 O is successively reduced by oxygen isotope exchange with leaf, soil and ocean water. Isotopic exchange of CO 2 with leaf water is more efficient than with ocean water due to the presence of carbonic anhydrase in the leaves, and as a result the main sink for the Δ 17 O of CO 2 is exchange with leaf water. Precise measurements of the Δ 17 O of CO 2 may therefore help to better constrain the exchange of CO 2 between the atmosphere and the biosphere/hydrosphere. For several processes it has been shown that Δ 17 O is a more suitable tracer than the δ 18 O value alone. [21][22][23][24] Determination of Δ 17 O in CO 2 with traditional isotope ratio mass spectrometry techniques remains challenging due to the isobaric interference of 13 44.9932) and 12 [34][35][36] Very small variations in the δ 13 C value are used to quantify fluxes between atmosphere and hydrosphere and/or ocean [37][38][39][40][41] . Due to the mass interference of 12

| The 253 Ultra instrument
The 253 Ultra is the commercial version of a high mass resolution gas source multi-collector mass spectrometer, which was pioneered with the MAT 253 Ultra prototype in 2012. 48,57 The high mass resolution of the 253 Ultra enables the investigation of the abundance of isotopologues that suffer from isobaric interferences. The mass resolving power of the instrument can be tuned to m/Δm >35,000 and the peak stability over time is <5 ppm in mass; m/Δm is the width of a peak flank between 5% and 95% of the maximum peak signal. The instrument is controlled by the Qtegra™ software package (Thermo Fisher Scientific).
The ion source of the 253 Ultra is connected to a sample   Ion optical layout of the Thermo Scientific 253 Ultra high-resolution isotope ratio mass spectrometer. In the ion source, the ions are accelerated to 5 keV onto the source slit. After the electrostatic analyzer the ions are accelerated to 10 keV just before passing the crossover. The switchable intermediate aperture behind the magnetic sector is used for extra high mass resolution settings and the zoom lens allows for fine adjustments of peak overlap. The variable multicollector assembly is mounted on the focal detector plane of the mass spectrometer system. The RPQ filter lens discriminates for scattered ions and reduces abundance sensitivity. It is located behind the focal plane right in front of the ion counting detector [Color figure can be viewed at wileyonlinelibrary.com] the ions generated in the ion source. Double focusing can overcome this limitation. In a properly designed double-focusing system the electrostatic sector optics match the chromatic aberrations of the magnetic sector optics such that the combined system eliminates both, the angular and the chromatic aberrations up to the second order. 58 In the 253 Ultra the ions are generated at a potential of 10 kV.

| Characterization of the 253 Ultra for CO 2 measurement
We investigated the effect of equilibration time, emission current, source conductance and signal intensity on the ionization of CO 2 as suggested by Verkouteren et al 58,60 and Meijer et al. 61 We characterized the scale contraction effect of the ion source of the 253 Ultra at Utrecht University using two CO 2 gases (G1 and SCOTT, see To study the effect of equilibration time and source conductance,  Similarly, the scale contraction due to the emission current is calculated with respect to the results obtained at an emission current of 1 mA. The cross contamination (η) is calculated as: where y is 13 (for δ 13 C) or 18 (for δ 18 Figure 3C shows that the 16 Figure 3D shows the calculated effect of 16  The ion signals are registered in two Faraday collectors (L4 and Center) that are read out with resistors of 1.0 × 10 11 Ω and 1.0 × 10 13 Ω for 12 C + and 13 C + , respectively. The mass spectra covering the range for 12 C + and 13 C + are shown in Figure 4. The main interference for 13 C + (mass 13.0034 u) is 12 CH + (mass 13.0078 u), which requires a mass resolving power of 2900. This is well resolved with the medium-resolution slit of the 253 Ultra (m/Δm >7500).
To establish the scale contraction correction for fragment ion measurements, isotopically well-characterized pure CO 2 gases (see section 3.2) were analyzed both with the molecular ion method and with the fragment ion method. The CO 2 and O 2 working reference gases used in this study are summarized in Table 1. The two CO 2 samples, G3 and G4, are prepared from G2 by adding isotopically anomalous CO 2 generated by UV-induced isotope exchange between The reported internal precision of the fragment technique is compared with the expected error (precision) based on counting statistics (EECS), which is calculated as: where N is the average count rate (cps), t int is the integration time in seconds, n is the number of measurement cycles and the factor ffiffiffi 2 p accounts for the fact that the reference and the sample both introduce the same error to the δ value. Throughout the manuscript the error of a single measurement series is reported as the standard error of the mean. When we quantify errors for more than one measurement (series), we report the standard error times the Student's t-factor to cover the 95% confidence interval.

| O 2 -CO 2 exchange method
A schematic diagram of the O 2 -CO 2 exchange experimental setup at Utrecht University is shown in Figure S2 (supporting information).
where β is the molar ratio of CO 2  is wrapped in a sheet of platinum foil and platinum wire and placed inside a quartz reactor as shown in Figure S3 (supporting information

| Instrument characterization and scale contraction
Scale contraction decreases with equilibration time and source pressure (signal intensity), when the variable conductance window is fully opened and when the emission current is decreased. A detailed investigation of these parameters is presented in the supporting information ( Figures S5, S6, and S7, and Tables S1 and S2,   As shown in Figure 5A and Table 3, from these six replicates the Δ 17 O  Figure 5B and   As mentioned above, a requirement is that the mass scale remains very stable over the entire measurement period. At Utrecht University we monitor the stability of the mass scale by recording a medium-resolution mass spectrum at regular intervals during the measurement. Figures 7A and 7B show an example of a long-term fragment measurement during which the mass scale was very stable.

| Fragment measurement
However, the mass scale is not always as stable, and mass instabilities are one limitation for measurements that require long measurement times. Instabilities in the mass scale are more likely to contribute to the larger errors than counting statistics, factor Γ in  Figure 8A and when measured with the fragment method ( Figure 8A and Table 3).
To Furthermore, the agreement in the triple isotopic composition of oxygen between O 2 and CO 2 (produced by combustion) suggests that our isotope scales for CO 2 and O 2 are very compatible.  artificially enriched in 17 O as described in section 3. 5.2. As shown in Figure 9 and Student's t-factor for 95% confidence). The excellent agreement between the two totally independent methods provides an independent validation of the fragment ion technique.

D. C-fragment
The δ 13 C values of the two CO 2 gases G1 and SCOTT were measured against G2 with the C-fragment method and with the traditional measurement on the CO 2 molecule (evaluated with the Brand et al 8 procedure). As shown in Table 4, the δ 13 C values obtained from the C-fragment method and molecular measurement are the same within the error (at the ≈ 0.01‰ reproducibility level). A possible challenge for measuring δ 13 C values with the fragment method is the interference from the 12 CH + adduct due to ion source chemistry (e.g. in the presence of water). The 12 CH + adduct is only 0.004 u separated from 13 C + as shown in the mass spectra ( Figure 4). However, the figure also shows that this interference can be resolved at medium resolution.

| Scale contraction
We observe a higher scale contraction when measuring on the fragment ions than with the measurements on the molecular ions ( Table 2). The difference might be because fragment ions are more reactive than the molecular ions. High energy collisions between ions and the source material cause sputtering and implantation,

| Possible interferences
Oxygen isotope measurements on O fragment ions with low-resolution mass spectrometers are mainly limited by the interference from water and its OH fragment ions. The background level of water in mass spectrometers is always significant, and it also generally varies when switching between bellows in dual-inlet measurements. With the 253 Ultra, these interferences can be separated from the O + fragments ( Figure 2;  ) can be estimated using Equation 8. The magnitude of the interference depends on the isotopic composition, the fragmentation pattern (efficiency of producing O fragment ions relative to CO 2 ), ionization efficiency and the abundance of the impurity relative to the CO 2 (Equation 8).

| Future developments and applications
In where the sample-reference switching is not performed at all would enable longer observation times of the sample. 69 LIDI measurements were attempted with the 253 Ultra but not continued because of instability issues. An increase in stability may also enable measurements at the counting statistics limit, which would improve precision by a factor of 1. 5.
Compared with traditional δ 13 C measurements that require a 17 O-correction, the C-fragment is not subject to the following uncertainties related to the 17 O-correction: and CO can be measured directly on the C + fragment of these gases, without chemical conversion steps that are known to cause artifacts in traditional isotope techniques. [70][71][72][73] Furthermore, isotope measurements on atomic fragment ions may be combined with measurements of larger fragments of hydrocarbons to determine the position-specific carbon isotope composition of hydrocarbons. 55 The position-specific 15  In addition to these environmental applications, the analysis of atomic fragment ions of different compounds may be a useful tool to study fractionation processes in the ion source of an isotope ratio mass spectrometer. As discussed earlier, the scale contractions for isotopic measurements are different for the fragment ions and molecular ions of CO 2 . Examining these effects further may help to understand the chemistry and surface effects in the ion source of isotope ratio mass spectrometers by studying different fragments.
In addition, analysis of fragment ions facilitates measuring the isotopic composition of two different chemical compounds versus each other (e.g. δ 13 C value in CH 4 versus in CO 2 ). This can on the one hand provide information on ion source effects associated with fragmentation, but on the other it may also help to directly compare isotope scales between different compounds.