Background correction procedures

In the following section we describe two different procedures for background correction: one based on the (m/z 44 intensity)/(m/z 47 background) relationship determined by scanning, and one based on the (m/z 49 intensity)/(m/z 47 background) relationships. We then apply these correction schemes to existing data from both instruments, demonstrating that they can eliminate or at least significantly reduce the apparent dependence of the Δ47 on the bulk composition of the gas (expressed as δ⁴⁷). The first method produces more accurate results on the ETH instrument, while the second one yields better results on the GU instrument.

1) Correction with directly determined m/z 47 backgrounds

To carry out the background corrections, the raw data of each measurement have to be extracted from each measurement to be corrected individually. The beam intensity data exported from the raw data file of each measurement are corrected by removing first the background determined by the Isodat^® software without gas flow into the source, and then by subtracting the background determined with the relationship between m/z 44 intensity and the corresponding m/z 47 background determined with the scanning procedure described above (Figs. 2.4 and 2.5). The corrected beam intensity data are then reprocessed to calculate δ¹³C, δ¹⁸O, δ⁴⁷ and Δ47 values with a spreadsheet. The difference in the calculated δ¹³C and δ¹⁸O values with the corrected background are less than 0.02%, thus well below the analytical error.

However, the changes in the calculated δ⁴⁷ and Δ47 values are very large, and are strongly dependent on the bulk isotope composition of the gas. An example calculation is available as Supplementary Table S2.1 and the data are reported in Supplementary Table S2.2 (see Supporting Information).

At ETH, all the analyses to determine the heated gas line and the equilibrated gas line for exchange with water at 25 °C were carried out at 20 V on the m/z 44 signal. The gas compositions presented in Figure 2.7 were measured between November 2011 and March 2012, whereas the background determination was carried out at the beginning of June 2012 after we observed the existence of the negative backgrounds in our instruments and characterized its dependence with the signal intensity of m/z 44. Since March 2012, internal carbonate standards of various bulk and clumped isotope compositions were used to verify that the heated gas line remained stable as described in Schmid and Bernasconi (2010). On this instrument, we observe that the heated gas line is stable over long periods of time, if no changes are made to the source tuning parameters, as was also observed in other laboratories

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(Huntington et al., 2009; Dennis et al., 2011). At GU, heated gas analyses were performed according to the methods outlined above. Background scans were carried out in early June 2012. We noticed no change in the m/z 49 beam intensity during reference gas measurements (set to 16 V at m/z 44) for the period end of April to mid June 2012. For the same period, a constant m/z 49 beam intensity was accompanied by an invariant heated gas regression line. In Figure 2.7 we show the effect of applying the new background correction to the analyses on the

‘Heated gas line’ and ’25 °C equilibrated gas line’ for the ETH instrument, and in Figure 2.8 we show the same effect on the May 2012 ‘Heated gas line’ for the GU instrument.

Figure 2.7 The current ‘heated gas line’ (circles) and the ‘25 °C equilibrated gas line’

(squares) of the ETH instrument calculated with the background determined by the software (open symbols) and calculated with the backgrounds determined with gas flowing in the mass spectrometer (filled symbols). The new background correction removes the dependence of Δ47

on d47 values.

Figure 2.8 The current heated gas line of the GU instrument calculated with the background determined by the software (circles) and recalculated with the backgrounds measured with gas flow into the source (triangles).

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For the ETH instrument, the uncorrected raw heated gas (HG) data reveal a slope of 0.00548 ± 0.00026 and an intercept of −0.8222 ± 0.0114. Applying the relationship between the m/z 47 background and the m/z 44 intensity (Fig. 4(D)) to correct raw data (after removing the corresponding, false m/z 44 background determined by the Isodat^® software), the HG line is characterized by a slope of 0.00018 ± 0.00025 and an intercept of −0.8222 ± 0.0012. The most striking feature of Figure 2.7 is that the dependence of the Δ47 value on the δ⁴⁷ value essentially disappears (slope 0.00018 ± 0.00025). The intercepts displayed by raw and corrected data are – within error – indistinguishable from each other, demonstrating that this correction procedure does not affect the measured Δ47 value of the heated reference gas. As for the heated gases, the dependence of Δ47 on δ⁴⁷ values for the gases equilibrated at 25 °C with waters of different oxygen isotope composition for the ETH instrument is also removed by the correction procedure. Uncorrected 25 °C gases reveal a slope of 0.00463 ± 0.00012 and an intercept of −0.0122 ± 0.0040, which, after correction, become −0.00041 ± 0.00013 and

−0.0003 ± 0.0042, respectively. As observed in other studies (Dennis et al., 2011), the uncorrected (and corrected) slopes are slightly different from those for the heated gases (Fig. 2.7). In contrast to the heated gases that have an absolute Δ47 of 0.0266, these gases have a theoretically determined absolute Δ47 of 0.9252 (Dennis et al., 2011). For a given temperature, the absolute Δ47 values differ from the measured ones (as represented by corresponding intercepts), because the measured Δ47 values are not expressed on the absolute scale, but relative to the Δ47 values of the reference gas used. In addition, the measured Δ47

values are affected by a process called ‘scale compression’ (Huntington et al., 2009; Dennis et al., 2011), which, in contrast to the dependence of uncorrected Δ47 on δ⁴⁷ values, might be related to isotope scrambling processes occurring in the ion source.

In this respect, the positive slope reflected by uncorrected gas data can be understood as follows: the relative contribution of the background to the measured intensity of m/z 44 is the same for both reference and sample gas only if the analyte gas has the same bulk isotopic composition as the reference gas. However, as the isotope abundances are simply ratios of the beam intensities, the contribution of a constant negative m/z 47 background will increase with increasing difference in the m/z 47 beam intensity between reference gas and sample gas. As a consequence, the original erroneous background correction results in a calculated Δ47 value which is 0.334% lower than the actual one for a sample gas with a much more negative bulk isotope composition (e.g. for the lightest CO2 with a δ⁴⁷ of −63.8%) than the reference gas.

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It should be noted that the background correction procedure described above not only removes the dependence of the Δ47 value on the bulk isotopic composition, but also that it is not accompanied by any loss in precision. This is shown by the similar errors for the uncorrected and corrected slopes and intercepts.

The same correction procedure was applied to data generated by the GU instrument, and results are presented in Figure 2.8. A first striking feature of the GU instrument is the much steeper slope of the heated gas line, which is clearly related to the much larger negative backgrounds.

The uncorrected May 2012 HG line has a slope of 0.02254 ± 0.00028. The same correction applied to the heated gas data results in an improvement of the steepness of the slope, but the correction based on the direct measurement of the m/z 47 peak leads to a slight overcorrection of the data, as expressed by the negative slope of −0.00225 ± 0.00025 (Fig. 2.8; triangles).

Both datasets demonstrate that the ‘non-linearity’ is simply related to an improper determination of the background of the collectors. The correction procedure based on the (m/z 44 intensity)/(m/z 47 background) relationship removes the observed non-linearity completely on the ETH instrument for which the m/z 44 slit widths is nearly double the m/z 47 slit widths. The correction is demonstrated to be valid for gases with a wide range of bulk compositions (δ⁴⁷ values ranging from −64 to +20%) and also in degree of isotopic ordering from close to stochastic distribution to equilibrium with water. However, the same procedure leads to a slight overcorrection at the GU instrument, for which the m/z 44 and 47 cups have identical slit widths. We propose that this is a result of the different cup design. When determining the m/z 47 background on the side of the peak on the GU machine using the HV scan method, the m/z 44 beam is not collected in the m/z 44 cup. This is different to conditions prevailing during measurement and during HV scans using the ETH instrument with the wide m/z 44 collector (Fig. 2.1). The fact that the m/z 47 background at GU is determined under

conditions not representative of the real measurements might explain why the (m/z 44 intensity)/(m/z 47) background relationship obtained by the scan method leads to a

slight overcorrection of data. This indicates that the background during measurement, when the m/z 44 beam falls into the Faraday cup, is probably slightly smaller than the one determined by peak scanning.

2 Background effects on Faraday collectors 2) Corrections with m/z 49 intensity

As discussed above, the signal observed on m/z 49 is a close representation of the background on cup 49. Therefore, if a good correlation between the m/z 49 signal and the background 47 exists, this signal might be used to directly and continuously monitor the background on m/z 47 during the measurement, and potentially improve even further the quality of the data. In Figure 2.9 we depict the relationships between the m/z 49 signal and the m/z 47 backgrounds for the ETH (Fig. 2.9(A)) and GU (Fig. 2.9(B)) instruments. These relationships were used to correct the raw data, as with the direct determination. The results are shown in Figures 2.10(A) and 2.10(B) for ETH and GU, respectively. It is now observed for both instruments that the heated gas data is overcorrected, i.e. both corrected HG lines exhibit negative slopes significantly different from zero: −0.00172 ± 0.00025 at GU (Correction 1) and

−0.0019 ± 0.0003 at ETH. However, for the GU instrument if we do not consider the absolute minimum of m/z 47, but instead the minimum to the right of m/z 47 (Fig. 2.1) and correlate it with the mass 49 intensity, a relationship is obtained that – if used for the correction of May

2012 Heated gas data – removes the heated gas slope nearly quantitatively to

−0.00069 ± 0.00025 (Fig. 2.10(B), Correction 2). Such a heated gas slope is already sufficient to correct samples that have a δ⁴⁷ value within 15% of the reference gas, introducing a maximum error of ±0.01% for the samples close to the extreme compositions.

Figure 2.9 Correlation of the m/z 49 signal with the background determined on the left side of the m/z 47 peak for the ETH (A) and GU (B) instruments. Note the excellent linear relationship.

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Figure 2.10 Correction of the heated gas line (open symbols) with the m/z 49 vs. background m/z 47: ETH (A) and GU (B). Correction 1 of GU is calculated with the minimum background, correction 2 with the background on the right of the peak (for details, see text).

At present it is not clear what the reason for the overcorrection is, but at GU it might be again related to the different slit widths of cups 44, 47 and 49. Applying the HV scan method at GU, the m/z 49 intensity vs. m/z 47 background relationship used for data correction is monitored under conditions that do not represent real measurement conditions, with the m/z 44 beam being collected by the mass 44 cup. Additional factors such as small contributions from non-CO2 isotopologues might also be of importance, as is suggested by the observation that heated gas data is also slightly overcorrected at ETH, although the m/z 44 peak widths completely covers that of m/z 49 (Fig. 2.1). Additional studies are ongoing to better understand the behavior of the m/z 49 signal.

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