the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Jul 2026
Technical Note: Stable Chloride Measurements at CAMS
Abstract. We present a new method for preparing silicate samples for stable chloride analysis using the low-energy injection components of an accelerator mass spectrometry (AMS) system. This method uses Nb over AgBr as a bulking agent during sample preparation which drastically reduces background Cl signals (ion source memory) by a factor of ~100 and allows for faster, cheaper, and more precise analyses on low-Cl samples (<10 μg Cl). We demonstrate this approach with measurements of stable chlorine isotope ratios (35Cl/37Cl) performed on silicates, prepared in both AgBr and Nb matrices, that characterize chlorine concentrations of samples containing ~1–500 ppm Cl. While this method was developed primarily to interrogate silicates, it performs equally well on other commonly analyzed sample matrices including carbonates and water. This Nb-based method is now the standard procedure at the Center for Accelerator Mass Spectrometry (CAMS) for preparing all stable Cl targets, including those accompanying 36Cl analyses.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4091', Anonymous Referee #1, 09 Sep 2026
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RC2: 'Comment on egusphere-2026-4091', Anonymous Referee #2, 10 Sep 2026
general comments
The authors of the "Technical Note: Stable Chloride Measurements at CAMS" present a systematic study to perform stable isotope measurements with an AMS ion source. Their progress to reduce the ion source memory for a highly volatile element such as Cl by very simple means is remarkable and highly useful to the community. Ion source memory is an important parameter in addition to general efficiency and output as well as potential suppression of the isobaric interference, which typically are the main criteria to optimize the composition of an AMS target. The study expanded the use of the Nb technique from silicates to diverse original sample matrices, which is highly relevant for labs that are engaged in analysis of other samples, e.g. ice or water.
specific commentschapter 1.1. Explicitly state the materials used for the cathode and the wheel.
Line 83-85 The term "Nb wheel" may give the wrong perception of a wheel facibrated of Nb. Rephrase to clarify that you ran wheels containing targets with different matrices.
Fig. 2: What is the time scale on the x-axis? The term acquisition sequence is not defined anywhere else. How long is one acquisition sequence? Are they continuous or followed by target changes?
Line 153-155: Specify Nb powder properties (mesh size, purity) and clarify the sequence of addition for AgNO3 relative to the Nb powder. Is the AgNO3 added quasi simultaneously to the solution as the Nb powder?
chapter 2.2.: Please give more information on the stability of ratios over time when sputtering a sample, e.g. in a plot. Are there any effects from cratering? The measurements seem to take only few minutes. How much of the material is used up in this time? How many "passes" are performed per measurement? Please clarify the temporal measurement process, how the "pass" relates to the acquisition sequence.
Please also include some graphics to document the temporal evolution of the output for typical samples and blank cathodes and the magnitude of the blank correction. This could be combined with the data in Fig. 2.
line 233: State the precision of F_norm~0.997. Is the deviation from 1 significant and what is the reason? Could it be related to different focusing during the fast bouncing or a different sputtering yield?
line 335: Define ion source recovery time. How does this compare to values published for other ion sources (e.g. in Finkel et al. NIMB 294, 121–125 and Pavetich et al. NIMB 329, 22–29? )
line 337: What is the criterion for a "high-Cl sample"?
line 339/340: What is the typical amplitude and period of the current fluctuations?
line 373: Please comment in more detail, maybe somewhere else in the paper, on the difference in Sulfur output for Nb targets compared to AgBr targets.
technical correctionsLine 50: It should read "switching from AgBr to Nb"?
Citation: https://doi.org/10.5194/egusphere-2026-4091-RC2
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- 1
The manuscript “Technical Note: Stable Chloride Measurements at CAMS” describes new sample preparation method and mass spectrometric approach to measure stable Cl concentration. The measurement of stable Cl concentration via isotope dilution is somewhat finicky task and advances in the field are warmly welcomed. Introduction of Nb matrix instead of commonly used AgBr seem to have merit, alas it comes with a cost. I’m supportive of the manuscript but have few concerns and recommendations for improvement.
First, and this is mostly a matter for the editor to decide, the benefits of the manuscript lie in the improvement of the low concentration Cl measurements. This in turn has very limited impact on exposure age dating with 36Cl given improving the measurement precision and/or accuracy of low Cl concentrations have very little impact on 36Cl derived ages. That is, if the Cl concentration is low there is very small contribution of 36Cl being produced via neutron capture on 35Cl, and uncertainties in the number of target nuclides have no bearing on final 36Cl ages. As such relevance of the manuscript to Geochronology is peripheral at best. Having said this I’m supportive of the manuscript but think that it would be better suited to more mass spectrometry focused journal, where more detailed discussions of the background corrections and uncertainty analysis would be at home also.
My second overarching criticism is in the style of the manuscript, and again this might be a matter for the editor. That is, currently I feel the manuscript reads more like laboratory notebook rather than self-contained journal article. For example, sections called ‘Experiment 1’ and ‘Experiment 2’ might be unfortunate hindsight, but they also imply that ‘Experiment 3’ and ‘Experiment 4’ will be reported next month. Whilst titled as ‘Technical note:’ I still feel the authors could have been more complete in their presentation, for example they could have used high-energy measurement of 35/37 ratios that are free of molecular interferences, and/or mass scans to identify molecular fragments, and thus show sufficient removal of fluorides that justifies use of low energy measurement in the future. I understand that high-energy measurement might not be desired as end goal as it might be less stable than low-energy measurement, but it can still be used for diagnostic purposes.
My last general comment refers to laboratory jargon. The descriptions of the measurement sequence/process are currently difficult to follow. I understand this is common feature as each laboratory develops its own language and terms that might be hard for others to follow. For example, my understanding of the measurement process is that each sample is measured once, and this measurement is artificially divided into 15 internal subdivisions that are then used to calculate the internal spread and standard deviation for the that measurement. The final error is then elevated if multiple measurements on standards have higher standard deviation to this value. Either way consider simplifying the terms and being clearer on the process.
I believe figures would benefit from few improvements:
Figure 1 and the associated discussions. Figure shows increasing 37Cl- current as a function of the cathode voltage from 50ug sample without specifying if this is AgBr or Nb target, presumably latter but it is pertinent to mention this at someplace. Operational limits are not demonstrated by this data, I don’t doubt them, just wondering the value of this figure. It is well known that the current output typically increases as a function of cathode voltage in Cs sputter sources, see e.g. early work by Middleton et al. Is the response similar/different to AgBr targets, what if one increases Cs delivery, or ioniser power? Presumably what is important here is the signal to noise or more specifically signal to source memory, which is discussed later. Does AgBr and Nb need to be run at the same ion source settings? Can the figure be expanded to contain more general conclusion, e.g. 5 times increase in Cl- output with Nb vs AgBr?
Figure 2: I have real trouble understanding this figure. Please elaborate and/or device alternative method of demonstrating the advantage of the reduced memory. Currently both x and y axis are ambiguous: integrated current – over what time/samples, acquisition sequence – what does this mean in terms of time and samples measured? What are the spikes in the graphs? Why does the 37Cl- with Nb integrated charge never go below 1 nC? Note: my understanding of this figure improved after I finished the entire manuscript but presume other readers would have similar challenges, so I left this comment as is. I would recommend just to make a simple plot of source memory 35 or 37 current as a function of time after running Cl targets with both AgBr and Nb and then repeat with reverse order to make sure build up from Nb is not reason for elevated memory from AgBr.
Figure 3, 6, 7: all the mass scans. I’d recommend using log scale so that the achieved interference suppression becomes visible.
Figure 5: not sure this is needed. It is basic and can easily be referenced.
Figure 9: introduce small shift between multiple samples on x-axis to make errors visible.
Specific comments:
-Paragraph lines 26-31: gives the impression that 10-100 mg of AgCl is still needed for measurement. Mention that samples with Cl masses around 1mg are routinely measured for 36Cl.
-Paragraph between 32-46 currently oversells. Measuring stable Cl from separate vs combined with 36Cl targets has a long history and each have their benefits; there doesn’t have to be ultimate answer here either. Also, ANU AMS laboratory has been measuring stable Cl form separate targets for a long time, I suggest a reference would be appropriate here. Similarly note such as “significantly reduces the amount of expensive isotopically enriched Cl carrier” is little misleading in the context. That is, I agree with the statement, but feel that currently the reader is misled by implying from the above paragraph that close to 100mg of AgCl is needed, or that pure 37Cl carrier is needed, when ~1mg of 35Cl enriched carrier suffices. Then the sentence “This method provides additional flexibility because the carrier can be enriched in either 35Cl or 37Cl, …” is simply not true. I routinely measure combined stable Cl and 36Cl samples that are enriched with 35Cl without significant 36Cl background from the 35Cl enriched carrier, there is no difference here.
-Line 87: “Collectively, these early findings demonstrated the potential benefits from using Nb as a matrix for stable Cl isotope analyses of silicates.” So far there have been no evidence or mention of why the method would be beneficial for silicates, all the presented evidence is simply about Cl currents and source memory and applies to any Cl measurement, not just silicates.
-Line 140: “centrifuging us referred” typo, should be is.
-Around lines 200-215 there is mention of ‘pass’ few times. Consider if this is needed. The magnet bounce cycle is given, and one could simply just state typical measurement time of X min instead of introducing the term ‘pass’.
-Line 233: “Measured standard ratios are typically ~0.3% higher than the natural ratio, resulting in Fnorm of ~0.997.” Please state what the normalisation factor is for pure enriched carrier samples to validate that the instrumental fractionation is constant between endmembers.
-Line 235: “Method one calculates the intra-target standard deviation of the 15 individual analyses that make up one measurement ‘pass’” This sounds like the sample is measured once and arbitrarily divided into 15 units that are then used to calculate standard deviation. The measurement could be equally divided into any number of units. This approach is fine for estimating the stability of a single measurement, but the 15 units shouldn’t be considered as independent analyses.
In summary, despite my lengthy and perhaps somewhat critical comments I’m highly supportive of the manuscript and would like to see the work published as it offers a new approach to stable Cl measurements with increased sensitivity, that would have a benefit for the community. This work has potential to enable new studies into stable Cl concentration and/or fractionation. Many of my comments, I believe, are rather easy to address, e.g. improving the figure clarity, and will improve readability of the manuscript. My recommendation would be to consider more mass spectrometry specific journal but leave this up to the editor and authors to decide.
Best regards.