| 30 Dec 2025
Technical Note: Design, construction, automation, and calibration of a low-volume He measurement line optimized for laser-ablation analyses
Abstract. A noble-gas analysis line capable of accurate and precise measurements of small absolute amounts of 4He released from crystals is a key analytical step in the production of (U-Th)/He chronologic data. He analysis lines that are custom-built in-house can be optimized for specific lab needs and facilitate continued maintenance, repair, and upgrades. However, there is little information in the published literature about the methods and approaches for building a He line. Here, we describe the design, construction, automation, and metrological calibration of a custom 4He extraction and analysis line as part of establishing laser-ablation (U-Th)/He methods in the University of Colorado Thermochronology Research and Instrumentation Lab (CU TRaIL). The line, called the Jimbochron, is designed to precisely measure very small (~fmol) amounts of 4He while being fully automated and easily modifiable in the future. These goals are achieved by minimizing the line volume, adopting a unique double-hexagonal manifold configuration, installing a high-sensitivity quadrupole mass spectrometer, and developing LabView code for instrument communication and automation that is open and straightforward to update. We also explain the steps used to calibrate the Jimbochron metrologically from first principles with a new in-house calibration volume to ensure high-accuracy He measurements. The Jimbochron is about two orders of magnitude more sensitive than our existing 4He line and now routinely generates accurate and precise He data for laser-ablation (U-Th)/He applications.
This manuscript describes the established procedure for design and construction of a helium measurement system for (U-Th)/He analysis using a 3He spike and a QMS originally intended for use as an RGA. Surprisingly this has not been written up anywhere as far as I know, so this is a valuable contribution. The link to the intended use for laser ablation analysis is incidental to the content of the manuscript, so I do not address it further in the review. I limit my comments mostly to places where I think the utility of the manuscript would be improved by additional discussion, consideration of alternatives or complications, and presentation of measurements or data associated with the topic under discussion. I think there are several places where the manuscript needs such changes in order to truly serve as a blueprint for someone setting up a noble gas mass spectrometry lab without access to existing facilities, which is the goal stated in the introduction. I encourage rapid publication once these minor revisions have been considered.
Section 2.2 and other places pressures are mentioned
The only way to calculate the pressure in one of these vacuum systems is using the mass spectrometer. I think it would be instructive to include such a calculation to show how deep into UHV one really must be to make good noble gas measurements.
Pumps, getters, and gauges (section 3.5)
Why these pumps? For a vacuum system dedicated to helium measurements, why choose a turbo pump with such a poor He compression ratio? And more important, why these getters? Why stray from the conventional choices, and what considerations or cautions should be given to other people building vacuum lines for noble gas analysis? Which ones are run hot and which at room temperature? How hot, and why?
What problems have been observed (line 214) with having a turbo pump directly connected to the vacuum line, or is this just speculative? What about the problems that arise from having the high surface area of a flexible tubing in the UHV system?
Bakeout is briefly mentioned in the tank filing procedure, and bakeability is mentioned in section 2.2, but bakeout procedures and the limits imposed on hardware choices by baking probably deserve further discussion somewhere.
Quadrupole choices and performance claims (sections 3.6 and 6)
I agree that the Hiden 3F is a good choice for this application, but I did not learn much about it from the manuscript. The brief section 3.6 asserts that the Hiden has a higher sensitivity than "other QMSs often used in (U-Th)/He labs," but it does not show any comparison data or even state the sensitivity of the Hiden 3F or how it was determined. The summary (section 6) mentions sensitivity numbers in V/fmol that are not previously shown elsewhere in the manuscript. Sensitivity, abundance sensitivity, and detection limits are frequently conflated in QMS advertising literature. The authors should show measurement data, discuss important considerations like source configuration, measurement conditions, and duty cycle, and deconflate volume and mass spectrometer sensitivity in the discussion of instrument choice.
The specifics of the Hiden 3F and the choice of two QMS instruments also deserves additional consideration. Was this really just made because the lab already had the SRS instrument on hand? What customization was required to measure up to mass 20 in a way that affects the resolution of the He peaks, and why? Can we see said peaks? Why would these instruments have different sensitivities? Why plan to use the less sensitive one for CRH measurements?
UPS system (section 3.7)
Has the UPS system been tested? Section 3.7 (lines 244-246) provides a cursory mention of the backup capabilities of the UPS, but there is no test data provided. The UPS model listed in Table 1 is a consumer-grade line-interactive UPS. This type is meant for things like home computers and is typically avoided in instrument labs because they experience a delay in switching to battery power during a failure, and because the conditioning capabilities are limited compared to a double-conversion UPS. The authors should discuss this decision and show test data demonstrating that this UPS actually works to keep all of these sensitive electronics operating when stress tested.
Section 3.7 also mentions that the backup compressor is on a UPS, but it doesn't list the model of either the UPS or the compressor. I wonder also if this setup has been tested. It would require a massive UPS unit to handle the inrush current of a starting air compressor motor. Lab-grade UPS systems are expensive to purchase and maintain, and even when properly maintained represent an additional point of failure. Most labs that experience long power failures frequent enough to merit this investment will also have backup generators that would make it unnecessary to back up things like air compressors and backing vacuum pumps that do not need to run constantly and that are challenging for UPS systems. If power failures are so rare that no backup generators are necessary, it's frequently better to just ensure that everything fails in the least damaging way possible when power is lost. Backing up the compressed air better than the electronics is actually detrimental in this case because you can end up having sensitive items like turbo pumps trip and fail with all of the valves still open. None of this is discussed and it really ought to be for the benefit of people operating in different situations regarding electricity reliability and building services. I think some of the decisions (consumer-grade UPS, air compressor backup) need to be reconsidered or at least defended with data.
Communications section about valves (section 4.1)
Considering and ordering new hardware from Festo is a pretty annoying experience even for experienced lab operators because of the impenetrable product codes and massive catalog, so I think this is once instance where more detail would actually be helpful to some readers. The setup of the connections and the manifold model are not included, nor are the considerations that went into those choices described. I think a "USB to ribbon cable" probably leaves out a step or omits that this is some proprietary USB to serial converter that then terminates in a ribbon cable.
Also, "PCB Board" is redundant.
Labview code (section 4.2)
The Labview section states that Labview is "affordable" due to the University of Colorado's site license, and the code is described as "open." I think it is important to disclose that Labview requires very expensive licenses that are not available as part of large site licenses to all potential users of publicly funded research, and I would argue that Labview (or "G") code that requires a proprietary IDE cannot really be considered open. That aside, the manuscript states that the code is provided through Github but this repository isn't actually linked. And I think Geochronology either requires or strongly encourages putting code in a repository that provides a doi rather than just providing a link to a dynamic resource like Github.
Sections 5.1-5.5
I don't think the "3T" and "4T" tanks are defined anywhere before the manuscript just starts using them.
The manuscript cites a couple examples of isotope dilution in very different contexts but not any of the examples of this exact procedure being used for decades in other labs, which oddly implies that this technique is new. It is a fair point that someone ought to have published it a long time, so maybe there is not much to cite. I think it would be reasonable to cite some previous work using the procedure even if they don't describe it in detail, and/or mention how well established this is.
Why not show some data? It would probably be easier for an unfamiliar reader to understand the discussion if it referred to some measurement plots.
Choose one of cc or mL to use for volumes throughout.
Quantitative discussion of the uncertainties in the volume calibrations would be valuable. The assertions in lines 375-377 in particular deserve some data behind them.
It is difficult to assess Table 3 without seeing the measurement data, but it seems like some optimistic assumptions might have been made about the relative accuracy of the manometric pressure measurements, especially at the low end of the scale. These decisions, and the measurements themselves, ought to be shown and discussed.
Are the tanks not cylindrical? Surely the volume of large cylindrical tanks could be measured more accurately with a ruler than with the procedure described here. Alternatively, just fill them with water and weigh them.
Summary (section 6)
Why would more vacuum pumps improve the pumpdown time? It's hard to imagine this is limited by pumping speed.
As mentioned above the sensitivity calculations need a lot more data and context and should not appear for the first time in the summary section. Same goes for the background measurement. And what about comparison to sector mass specs and other setups than just the Alphachron?