Methane, Ethylene, and Ethane Detection by Differential Helmholtz Resonance Spectroscopy Using a 3345 nm Mid-Infrared Tunable Diode Laser Source

Round 1

Reviewer 1 Report

In the presented manuscript, the authors introduce a Photoacoustic spectroscopic set-up based on Differential Helmholtz Resonance spectroscopy, using an ICL around 3.345 micron. Results are presented on detection of methane, ethylene and ethane, as well as mixtures of these compounds. The results are presented in a clear way, but I would recommend the following suggestions before this work can be published.

In the introduction, ICLs and PAS are properly introduced, but the introduction of Helmholtz resonance photoacoustic spectroscopy specifically is lacking. Please consider adding a small paragraph explaining the principle of Helmholtz resonance photoacoustic cells.

Figure 1: The data in this figure is not presented in a clear way. For example, it does not make sense that the points in the figure are connected with a line. As it presents line strength data, the data points are individual points, that should not be connected. Secondly, please consider changing the y-axis into a log scale, so that the extra magnification is not needed. This is a common practice in line strength data, as the line strength usually can vary over orders of magnitudes.

Line 83-85: “Methane has sharp absorption peaks between 3345.56 nm and 3345.83 nm, which are one order of magnitude stronger than those of ethylene and ethane.”. I am missing some reflection on this in a discussion section of the paper. Shouldn’t such a significant difference in line strength be reflected in the results? Yielding a significantly better sensitivity for methane compared to the other molecules? And if not, please explain why.

Line 136:  “… but this noise can be significantly suppressed by …”. Would you be able to show this, or can this be quantified? You must have measured the differentiated signal as well as a single signal and give a measure for the reduction in noise. Consider adding a quantified result to this sentence.

Figure 4: Why is the profile of the signal not following the Lorentzian fit, but showing this asymmetric response? Please explain this in the text.

Line 152: “… at the resonance frequency …”. I am assuming this must be the aforementioned 710 Hz, but please add this in brackets to this sentence to avoid confusion.

Figures 7, 8, and 9: Consider adding error bars to subplots (b) to give an indication of the noise or imprecision of the detected signal. (d): Please add the correct unit to the y-axis: this probably should be ppm, but it is not stated here.

The detection limits from the Allan-Werle deviation are 98.8 ppb for methane, 252 ppb for ethylene and 33 ppb for ethane (in 2s) (stated in lines 211, 225, and 238). However, I am missing any kind of discussion or reflection here. Are these results as expected? What is causing the differences in detection limits between these species, etc. Maybe also reflecting on the point I raised about lines 83-85. This definitely needs to be added in a paragraph.

Table 1:Please refer in the header of the table to sensitivity instead of slope, if you are referring to “sensitivity” in the rest of the text. Furthermore, I do not see how to interpret this sensitivity in a meaningful way. Yes, for this sensitivity, in mV/ppm, it is a useful measure to compare the response per molecule at a given wavelength, but it does not say anything about the sensitivity for a certain molecule in an absolute way. Please consider to either change this to a meaningful figure (such as sensitivity in ppm) or to not call this current value a measure of sensitivity.

The multi-species analysis is the most relevant result of this work, as cross-interferences are a known issue to techniques revolving around ICLs or QCLs. However, in general, the article should provide more quantitative evidence that the multiwavelength fitting routine is a clear improvement over single wavelength approaches. Concretely, in table 2, I am missing values on precision. How much do retrieved concentrations vary per scan? What is the error margin on the gas mixtures?

Maybe this information can be provided, and be shown much more intuitive in a bar plot, as the table is already rather extensive and a plot would be easier to interpret.

Line 285: “The detection error…”. Apart from the errors being mentioned here in the text, it is not intuitive not see this from the data shown in the table. Do we have a lot of inprecision? Systematic errors? Again, a kind of (bar) plot would make this easier to see.

Figure 10 is not being referred to in the text, and its added value is unclear for the story. It is of course relevant measurement data. Therefore, I would recommend moving this figure to Supplmentary materials, or encorporate this figure in a better way in the text. Furthermore, I would recommend the authors to explain why figure (a) is showing this clear blob in the fitted spectrum around 77 mA. What is causing this deviation from the measured spectrum?

Instead of Figure 10, I would recommend showing only one measurement as an example for the multi-species fitting routine, in which instead of the combined fitted spectrum, the contribution to the fitted spectrum of the three different compounds is shown. Adding a residual plot to this would also strengthen the evidence that the fitting routine is working fine.

Some minor points:

Title, abstract: Please consider changing Middle IR to Mid IR, as this is a more commonly used name for this part of the IR spectrum.

Figure 3: Please consider explaining abbreviations used in the figure in the caption. E.g., MFC: mass flow controller.

Chapter 5: Please change Allen to Allan, as this is regularly spelled incorrectly, both in the text as well as in the caption.

Author Response

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Reviewer 2 Report

The article “A Methane, Ethylene, and Ethane Detection by Differential Helmholtz Resonance Spectroscopy Using a 3345 nm Middle IR Tunable Diode Laser Source ” of Zhe Wu, Yunxing Shi and Yuwang Han is a research paper regarding a photoacoustic setup for gas detection.

 

 

The core of the paper is the Helmholtz resonance photoacoustic cell used for photoacoustic spectroscopy. Two capacitive microphones are located into two parallel channels. Taking advantage of cell geometry, both channels produce acoustic signals of the same amplitude but with opposite phases. This enable accurate measurements with this setup.

The paper is clearly written and well organized. The authors pointed out the focal points of the topic in a clear and concise way. The introductory part is complete and didactically presented. Reference list should be extended in order to enhance the quality of this article.

In some sections, topics are exposed like a thesis. The measurements presented in the experimental part are consistent and adequate to the journal.

 

 

I have only few comments/suggestions in order to improve this manuscript.

 

-There are few refuses in the manuscript. I suggest to carefully check the manuscript in this sense.

 

-I suggest to check the english style. For example in this field the spectral region is known as mid-infrared (“middle IR” in the title and in the text appears unusual).

 

-Regarding Figure 1. In the present form could generate confusion. I suggest to merge top and bottom panel using a log scale.

 

-In the section 4, the authors should add the pressure as parameter to be optimized.

 

 

Author Response

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Reviewer 3 Report

The authors have developed a sensor for the detection of the multi gases based on the photoacoustic spectroscopic technique. Although the contents of the manuscript are reasonable still lack reasonable novelty which will lead to a strong will for publication. However, some of my comments will enhance the weightage of the manuscript for its publication.

Abstract: line 14-15

The claim of the authors that “the detection limit reached 98.8 ppb, 252 ppb, and   14, 33 ppb for methane, ethylene, and ethane, respectively” seemed too low.

What is the amplitude of the heat produced by the nonradiative transition due to the absorption of the molecules of the sample of ppb level?  What is the amount of change in pressure produced by changes in temperature and what is the sensitivity of the microphone i.e., the minimum amount of the change in pressure that can detect by the microphone used in the present experiment?   Thus it required the validation of the present results with other techniques.

Introduction

Line 36-38

The authors write “Popa et al. used a CO2 laser PAS system to measure five components, i.e., carbon dioxide, ammonia, ethanol, methanol, and ethylene [18]”

then how the present manuscript is superior.

Line 55-58

-------- low-frequency noise [23] and work at a lower pressure,-------.

How much low pressure can be detected?

Optimization of measurement parameters

Line 130-134

The authors claim that “For the given cell size, the intrinsic frequency of the Helmholtz resonance is calculated to be approximately 800 Hz”

how the authors have calculated it.

In Figure 4

What is the DHR signal

Line141-144

How the amplitude of the laser is optimized.

Line 155-158

Why at 2f, the signal is low? Give a proper explanation. What is the physics behind it?

Line 209-210

-------- a linearity coefficient of 4.953 mV/ppm can be derived from the linear fit.

This statement is not clear.

Lin1 201-212

--------- which is comparable to the QEPAS detection limit obtained under 200 Torr for a 1 s integration time [22].

then how the present manuscript is superior to the previous techniques.

Line 223-225

At the selected ethylene detection laser current, I2 = 73.88 mA, the signal values of different concentrations are taken to draw the working curve. The data in the figure show that the signal response values of ethylene under this laser current have a good linear relationship, and the linear coefficient of fitting the photoacoustic signal to the gas concentration is −1.778 mV/ppm. The Allen bias analysis shows (Fig. 8c) that the limit of ethylene detection is 252 ppb for an integration time of 2 s.

The above statement is not clear, because the sensitivity/detection limit depends on the slope of the curve and the background in the spectrum.

Line 251

What do you mean by four were diluted?

Line 267

How the deviation is calculated.

Line 268-274

In the presence of the multi gas, the absorption peaks of the individual gas may be blue or red-shifted. how authors have detected/minimized this.

Line 285-287

How the detection error is calculated.

 

 

Author Response

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Reviewer 4 Report

In this work, the authors present a conventional method based on the differential Helmholtz resonance spectroscopy to simultaneously detect methane, ethylene and ethane. My comments are as follows:

a) Line 94, what is the full name of ca.?

b) Please add an experimental diagram of modulation depth (wavelength or wavenumber) and signal amplitude.

c) Photoacoustic spectroscopy detects photoacoustic signals. Why does the first harmonic have background signal while the second harmonic dose not have the background signal?

d) Which wavelength is selected for ethylene detection?

e) In Figure 1, the absorption line intensity of methane is one order of magnitude higher than that of ethane. In the experimental results, why is the detection limit of ethane higher than that of methane ?

f) Formula 1 may be wrong, B(I) on the right of the equation is missing.

Author Response

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Round 2

Reviewer 1 Report

The authors have adequately addressed the previous comments and suggestions, and have substantially improved the quality of the manuscript.

Author Response

Thank you very much for reviewing our paper and your positive feedback is gratefully appreciated.

Reviewer 3 Report

Accept in present form

Author Response

Thank you very much for reviewing our paper and your positive feedback is gratefully appreciated.

Reviewer 4 Report

It is meaningless to use mV as the unit of modulation depth because different laser controllers have different conversion (current to wavelength) efficiency. Please use universal units, such as wavelength or wavenumber.

Author Response

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