the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Jun 2025
Xact625i vs PX-375: A Comparative Study of Online XRF Ambient Multi-Metal Monitors
Abstract. This study provides a comprehensive evaluation of the Xact625i and PX-375 online energy dispersive X-ray fluorescence (ED-XRF) instruments for real-time trace elemental monitoring at a rural background site over six months. This represents the first direct comparison between the two instruments, assessing their performance against inductively coupled plasma mass spectrometry (ICP-MS), the reference method for elemental analysis. Both instruments demonstrated strong measurement capabilities, with the Xact625i achieving higher sensitivity for trace metals and showing closer agreement with ICP-MS using 24-hour averaging of 2-hour data (r² = 0.89 vs 0.78 for the PX-375). The PX-375 reported higher overall concentrations, primarily due to overestimations of Si and S. Both instruments correlated well for Ca, Fe, Zn, and Pb, while detection limits affected Ni and Cd. The Xact625i exhibited superior performance in measuring elements such as S, V, and Mn. While correlations with ICP-MS were high, systematic over- and underestimations in absolute concentrations were found, particularly for the PX-375. When directly comparing the two online instruments using raw 2-hour data, a strong agreement was observed (mean r² = 0.95). However, systematic slope discrepancies persisted, in line with comparisons against ICP-MS. The findings confirm the reliability of these two ED-XRF instruments for high-time-resolution elemental monitoring as a complementary technique to traditional filter analysis, enabling detailed source apportionment studies, improved trend analysis, and more responsive air quality management strategies. Future work comparing ED-XRF to laboratory-based methods could refine harmonisation efforts and address systematic differences in absolute concentrations.
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Status: open (until 01 Aug 2025)
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RC1: 'Comment on egusphere-2025-2350', Anonymous Referee #1, 27 Jun 2025
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This paper benchmarks two ambient XRF instruments against each other and offline ICP-MS measurements. This is an important paper that falls within the remit of AMT because of the recent interest in metals measurements and the consequent proliferation of instruments such as these. The divergence in quantitative agreement is less than ideal, however it is important that these disagreements are documented.
Overall, I would say that this paper is publishable, however there are a few shortcomings. Some parts lack technical detail, which is important for a methodology-heavy paper such as this, while I do not think the analysis goes deep enough in places.
General comments:The analysis of the data seems quite superficial, in that while Deming regression is an appropriate test, the intercept is fixed at zero and nonlinearities are largely ignored. This to me is an omission, because many elements of interest such as Pb, Cr and As very clearly demonstrate nonlinear relationships in the scatter plots. Given the remit of the paper, these must be discussed both in terms of the possible causes and implications. It may also be prudent to perform and additional numerical fit to quantify this effect, such as using a different fit function or testing a nonzero intercept from the Deming regressions. A cursory inspection would indicate to me that if the intercept on arsenic in particular were to be allowed to vary, a much more favourable slope would be yielded (for whatever reason), so these effects cannot be ignored.
There needs to be more information regarding the offline sampling and analysis. While some of this may be considered obvious to those familiar with the reference methods, these must still be stated for the sake of those who are not. Specifically:
- More information on the Leckel samplers should be given (e.g. inlets, blank strategy)
- The manufacturer(s) and product codes of the filters used for offline analysis should be given, along with specification (in particular pore size) and any pre-treatment (e.g. washing, baking).
- The 'shaker' used for the aqueous extraction needs describing better and the product information on the syringe filter included.
- More information given on the extraction process used for ICP, in particular the use of a microwave digester (I think this is implied, but not stated) and the fact that the substrate is included in the digestion.
- While the digestion protocol is standard, the authors should still comment on the acid mixture used in the digestion and the likely recovery rate (particularly of silicates), noting that other mixtures based on aqua regia and hydrofluoric acid are used elsewhere.
The conclusions does not feature nearly enough quantitative information, with the comparison using descriptive terms instead. Given the slopes are crucially important, these should be referred to and summarised quantitatively with reference to a range that the authors would consider acceptable. Furthermore, quantitative comparisons of these compared to previous equivalent literature should also be included.
Specific comments:
Title: That the instruments are compared to ICP-MS as well is important because this represents an independent point of reference, so this should be incorporated into the title.
Line 51: The authors should refer to the most recent directive (2024/2881), even though the limits were maintained from previous.
Line 107: A map should be included, showing the location of the sampling site and context such as the distance from major pollution sources.
Sections 2.2.1/2.2.2: Because the manufacturers regularly modify instruments, more information on the specific physical units used should be given here (ideally hardware revision numbers). Furthermore, because some of the discrepancies found are later attributed to software, the version numbers of the inversion software should be given.
Figure 1: The means by which this data was calculated should be more explicit. 'Relative Accuracy' is not very specific.
Line 272: 'Gravimetric' is not an appropriate term because this refers to the weighing of filters. Beta attenuation is a 'gravimetric equivalent' measurement.
Figure 2 (and elsewhere): "Total Concentration" should not be used because it could easily be confused with PM. "Total Detectable Concentration" would be more appropriate.
Line 293: Worth stating that Deming regression takes the uncertainty of both variables into account. It should also be stated that the intercepts were fixed at 0 (may seem obvious, but it is important to state).
Figure 3 (and other equivalent plots): The use of grey makes the plots hard to read, in particular the whiskers. Suggest using different colours. It would also be a useful guide to the eye to include the 1:1 line on these plots as well.
Line 329: "Both departments" is a very strange turn of phrase. Revise.
Line 468: The term "offset" I would take to mean a systematic additive error, but the analysis performed here would be insensitive to this because the intercepts were fixed at zero. The authors should be more specific about what they mean here. If they are saying there is such an error, then the fits should be performed again without the constraint.
Line 488: Given that the measurement site was part of various networks including EMEP and ACTRIS, surely some of the data is in the public domain?
Citation: https://doi.org/10.5194/egusphere-2025-2350-RC1
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