the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Jan 2025
Determination of pressure baseline corrections for clumped-isotope signals with complex peak shapes
Abstract. Pressure baseline corrections have been proposed to mitigate pressure-dependent background effects and reduce the apparent dependence of Δ47 on δ47 (non-linearity) observed in clumped-isotope studies of CO2. In this work, we describe the determination of pressure baseline corrections for signals whose peak tops vary considerably across their width. Our study focuses on peaks with very small signal-to-baseline ratios (1.005 to 1.025) generated by the clumped isotopes 17O18O (linearly increasing peak top) and 18O18O (negatively curved peak top). The measurements were all performed in pure-oxygen gas using the compact, low-mass-resolution Elementar isoprime precisION Isotope Ratio Mass Spectrometer. We demonstrate that our corrections significantly reduce the influence of secondary electrons and that the adjusted clumped-isotope signals correctly increase with signal intensity. Furthermore, we extensively discuss correction procedures of varying complexity and explain why the best results were obtained by predicting multiple background values from the corresponding on-peak signals. Through this approach, we typically achieved standard deviations around 1 · 10-9 (35/32), 0.2 ‰ (δ35), 0.5 ‰ (Δ35), 7 · 10-9 (36/32), 0.1 ‰ (δ36) and 0.1 ‰ (Δ36) for at least 120 intervals (20 s integration). For the capital delta values, this corresponds to standard errors of the mean of less than 0.05 ‰, achieved with a total integration and analysis time of approximately 40 min and 6 h, respectively. We also show that the uncertainties of certain measurement parameters can be further reduced by optimising the measurement position (acceleration voltage) and applying additional drift corrections. For instance, for 35/32- and 36/32-related parameters we observed improvements of up to 1 order of magnitude and a factor of 7, respectively. Based on Monte Carlo simulations, we also show that the main uncertainties in our capital delta values are related to the on-peak signals, predicted backgrounds and the peak top curvature (only for Δ36). Additionally, we present a brief study on the influence of pressure baseline corrections on major oxygen-isotope ratios and their delta values. While these corrections had an insignificant effect on their uncertainties, the absolute values of 33/32 and 34/32 changed markedly.
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RC1: 'Comment on egusphere-2024-3572', Stefano Bernasconi, 10 Feb 2025
This manuscript describes a methodology for baseline corrections in an Elementar isoprime mass spectrometer used for the analysis of clumped isotopes of O2. The authors discuss the challenges posed by complex peak shapes and low signal-to-noise ratios in this low resolution instrument. The presented PBL correction procedure significantly improves measurement precision. Accuracy is not assessed due to the lack of standards. The method involves predicting multiple background values from on-peak signals, achieving sub-permil uncertainties in key isotopic ratios.
With Monte Carlo simulations uncertainty contributions from different factors are determined, and the major sources of error are discussed.
These corrections procedures can be useful for other similar instruments with low resolution and to improve the quality of data when the signal to noise ratio is small due to the low abundance of an isotopologue.
The paper is generally well written just some sections in the introduction could be shortened (see below). I suggest it can be accepted with minor revisions
Line 78 “By definition” instead of “by construction”
Lines 80-100 this paragraph should be shortened and updated. It is now clear that the negative backgrounds are the cause of the observed nonlinearities and PBL corrections are a must in clumped isotope analysis. HG and EG corrections are a wourkaround that does not correct the direct cause of the nonlinearity and are unnecessarily complicated.
Many papers have shown that PBL corrections eliminates the need for HG an EG corrections. He 2012 and Bernasconi 2013 were the start, but other papers have confirmed it, and all the results in the interlaboratory calibration exercise presented in Bernasconi et al. 2021 only utilize PBL corrected data.
Lines117-123 not really necessary,could be removed.
Line 124: it is not necessary to hit the center of the cup, the flat portion of the signal represent the width of the cup where the entire beam is collected in the cup. The signal can also be measured correctly on the side of the peak, as long as it’s in the flat part of the peak.
Bottom of page 9 : beginning of the sentence is missing.
Line 230-235 you could mention that such large changes in background shapes were also reported by Meckler et al. 2014 (DOI: 10.1002/rcm.6949) for a MAT253 mas spectrometer. As you mention, it is indeed very important to monitor backgrounds with scans as the shape and magnitude can strongly change. Just peak jumping to the side of the peak is not a sufficient way to monitor the change in background.
Line 437 what do you mean that the procedure for correcting is “ more involved”?
Line 580 this is only a problem on curved peak tops, it the peak top is flat than the exact position is not so important.
625 But now it is I-CDES (Bernasconi et al. 2021), the tying to the theoretical values is the first step, but the interlaboratory comparability can only be ensured by using traceable standards that can be measured in different laboratories.
The code and data should be made available on a repository.
Citation: https://doi.org/10.5194/egusphere-2024-3572-RC1 -
RC2: 'Comment on egusphere-2024-3572', Amzad Laskar, 14 Feb 2025
The manuscript addresses an important issue related to pressure baseline changes for complex peaks upon the introduction of sample gases (O2 here) in a gas-source mass spectrometer (Elementar Isoprime). This is particularly relevant for the precise measurement of rare isotopologues, such as clumped isotopes. The authors proposed correction procedures and recommend scanning multiple background values corresponding to on-peak signals for improved baseline corrections. While the study is suitable for publication, some revisions are required.
The manuscript is too lengthy in its current form. It would benefit from improvements in language, as well as the removal of less important descriptions, elimination of repetitive content. Additionally, some figures (4-8), table (table 1) and a significant amount of texts could be moved to supplementary materials for better organization.
The manuscript mentions the mass spectrometer's ability to measure signal currents for masses 35 and 36 for O2, and associated error estimates are provided. Their statements at the beginning of introduction “we observed that our device is sensitive enough to measure the multiply-substituted oxygen isotopologues 17O18O and 18O18O in pure-oxygen gas, despite its low mass resolution and its use of 1011 Ω resistors on the corresponding cups” is a bit misleading. The isobaric intereference to 18O18O from 36Ar cannot be avoided but resolved in the mass spectrometer (for that a resolving power of ~1170 or better is required). This is because a sample is very unlikely be 100% Ar free (even after passing through GC) and the amount of Ar present in a sample from the background (even for pure O2) would also interfere 18O18O measurement. Although the primary focus is on monitoring background shifts rather than measuring 18O18O, 17O18O, the observed errors could stem from variable contributions from isobaric interferences such as 36Ar and 35Cl for 18O18O and 17O18O, respectively. In section 3, authors mentioned that they cannot distinguish 17O17O from 16O18O due to low resolving power of the mass spectrometer used. The mass resolving power is not enough to separate 36Ar from 18O18O, let’s leave separating 17O17O from 16O18O. For separation of 17O17O from 16O18O, mass resolving power required is 8117. I would say separation of these two isotopologues is not possible even with a mass resolution of >8117 because of the intensity imbalance of the two isotopologues (16O18O signal is >27,000 times more than 17O17O, 10 % valley definition is practically inapplicable). Therefore, I suggest authors to make the statements a bit carefully. I Also suggest authors to discuss these somewhere in their manuscript.
Please shorten the conclusion section.
Detailed comments on the manuscript are provided in the annotated PDF.
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RC3: 'Comment on egusphere-2024-3572', Anonymous Referee #3, 18 Mar 2025
The manuscript provides a comprehensive account of how a low-resolution mass spectrometer, specifically the Elementar Isoprime precisION, can be utilised to measure clumped isotopes of oxygen. The measurements are conducted on pure oxygen. Due to secondary electrons the signal of the clumped isotopes is reduced and needs correction. Additionally, several other corrections are required to ensure reliable results. The study concludes that this low-resolution instrument can achieve results comparable to those obtained with more costly high-resolution alternatives. However, an absolute calibration of the system is currently lacking.
The manuscript is well written and highly detailed but is overly long and repetitive. The principles discussed are valuable and may have broader applicability to other measurements and instruments. Nevertheless, the manuscript would benefit considerably from substantial condensation and a more results-focused revision.
Abstract: The abstract should, in my view, begin by stating that minor isotopes are significantly affected by secondary electrons, which are dependent on bulk mass flow. It should then outline that the manuscript addresses the methodology for correcting this effect.
Line 10: The term on-peak signals is introduced without prior explanation. Clarification is needed at this stage.
Line 13: The phrase at least 120 intervals is ambiguous. How many intervals have actually been used for the calculation of standard deviations?
The introduction can be shortened significantly.
3.1 The conclusion of this section is that secondary electron suppression is not able to remove the secondary electrons fully and background corrections are still necessary. Please write this.
Figure 9: Please adjust the Y-scales.
The conclusions could be more concise. I recommend summarising the key successes of the approach and providing the achieved standard errors, ensuring the focus remains on the core findings.
Citation: https://doi.org/10.5194/egusphere-2024-3572-RC3
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