Ex) Article Title, Author, Keywords
Current Optics
and Photonics
Ex) Article Title, Author, Keywords
Curr. Opt. Photon. 2023; 7(4): 457-462
Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.457
Copyright © Optical Society of Korea.
Yonghee Kim , Byung Jae Chun, Lim Lee, Kwang-Hoon Ko, Seung-Kyu Park, Taek-Soo Kim, Hyunmin Park
Corresponding author: *yonghee922@kaeri.re.kr, ORCID 0009-0005-8232-6278
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Quantitative measurement of trace ethane is important in environmental science and biomedical applications. For these applications, we typically require a few tens of part-per-trillion level measurement sensitivity. To measure trace-level ethane, we constructed a cavity ring-down spectroscopy setup in the 3.37 μm mid-infrared wavelength range, which is applicable to multi-species chemical analysis. We demonstrated that the detection limit of ethane is approximately 300 parts per trillion, and the measured concentration is in agreement with the amounts of the injected sample. We expect that these results can be applied to the chemical analysis of ethane and applications such as breath test equipment.
Keywords: Cavity ring-down spectroscopy, Ethane, Mid-infrared, Quantitative measurement
OCIS codes: (280.1545) Chemical analysis; (300.6340) Spectroscopy, infrared; (300.6390) Spectroscopy, molecular
Ethane (C2H6) is one of the most abundant hydrocarbon volatile organic compounds (VOCs) in the atmosphere, with average concentrations of approximately 1,000 parts per trillion (ppt) for the Northern Hemisphere [1]. Over the last few decades, many quantitative studies of trace ethane have been conducted in various fields such as environmental science [1–4] and biomedical applications [5–12].
Atmospheric ethane is one of the major greenhouse gases, and its origin lies in human activities such as natural gas leakage during production and transportation, biofuel use, and burning of biomass [2]. There is now growing interest in the concentration of ethane and its relation to climate change. Information on atmospheric ethane concentrations, such as regional distribution, seasonal oscillation, and annual changes, offers opportunities to better understand the earth system and its reflection of human activities [1–3]. Also, ethane is related to various chemical reactions in the atmosphere such as the formation of tropospheric ozone [4].
From a clinical point of view, exhaled human breath contains thousands of VOCs, and many studies have shown a correlation between concentrations of VOCs and various diseases [13–17]. A breath test is a non-invasive diagnostic method of diseases by measuring specific molecules in the exhaled breath. It is known that ethane in human breath is produced by oxidation damage of cellular membranes, in a process known as oxidation stress [5]; This is related to a wide range of diseases such as lung cancer [6, 7], chronic obstructive pulmonary disease (COPD) [8], cystic fibrosis [9], asthma [10] and vitamin E deficiency [11]. Intensive studies have been conducted on ethane as a potential biomarker for diagnosing diseases and monitoring oxidation stress [5–12].
To measure the quantity of ethane and its delicate changes in air or human breath, measurement techniques with precision below several tens of ppt is required. Currently, gas chromatography coupled to mass spectrometry (GC-MS) is a standard method for analyzing the composition and concentration of trace-level samples. However, the GC-MS method has some disadvantages such as a complex sampling procedure, long measurement time and high cost. Therefore, in specific applications for which the chemical species and measurement range are limited, other measurement technologies such as electrochemical sensors [13, 15] and optical methods [16, 17] have been suggested. Among these, optical methods using molecular spectra have distinct advantages compared to GC-MS, such as high sensitivity, real-time measurement, simple sampling procedures, and relatively low cost. Thus, these methods are expected to be widely used in various fields such as clinical applications based on exhaled human breath analysis [16, 17].
In the case of ethane measurement, the mid-infrared wavelength region is generally used because an ethane molecule has strong absorption lines near the 3.3 μm wavelength range. To measure the trace levels of ethane, various spectroscopic techniques such as Fourier transform infrared (FTIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) using a multi-pass cell [5, 6, 12], off-axis integrated cavity output spectroscopy (OA-ICOS) [18, 19] and cavity ring-down spectroscopy (CRDS) [7, 20, 21] have been reported. These methods have shown detection limits of 70 to 500 ppt with 1 second measurement time and further demonstrate the potential of optical methods for clinical applications.
In this paper, we focus on the CRDS method to achieve several tens of ppt level sensitivity for the quantitative measurement of ethane molecules. We demonstrate ethane measurement using the 3.37 μm mid-infrared wavelength region (corresponding to 2,967.0 to 2,969.0 cm−1), which can be applicable to multi-species chemical analysis.
In section II, we briefly introduce the CRDS method, the wavelength selection for this study, and details of the experimental setup. In section III, we present experimental results on ethane measurement and discuss system sensitivity. Finally, we summarize results and discuss future work in section IV.
CRDS is a highly sensitive quantitative measurement technique using the decay time (or ring-down time) of light in an optical cavity. This technique, which was first demonstrated at 1988 [22], has been widely used to analyze small amounts of optical absorption.
When a laser is spatially mode matched to a cavity mode and injected into a high finesse optical cavity with length
For an empty cavity, cavity loss depends on only cavity mirror reflectivity
Cavity loss increases in the presence of target molecules inside the cavity due to the light absorption of target molecules. Then, ring-down time
where
Notably, CRDS is very sensitive to injected laser beam alignment; However, the method is free from laser power fluctuation, in principle, because the ring-down time is independent from the laser power. This offers CRDS the unique capability of conducting trace-level sample measurements, which can lead to the development of robust commercial equipment.
Ethane has a strong absorption band in the mid-infrared wavelength region of 3.31–3.38 μm (approximately 2,960.0 to 3,020.0 cm−1) as shown in Fig. 1(a) [24, 25].
The largest two absorption lines of ethane are 2,983.28 cm−1 and 2,996.87 cm−1 (corresponding to 3.3520 μm and 3.3368 μm, respectively). Most previous studies [5, 12, 18, 19, 21] have used the wavelength region near these lines, depending on the light source and interference by methane or water molecules.
In this study, we selected the 3.37 μm (2,967.0 to 2,969.0 cm−1) wavelength region, as shown in Fig. 1(b), which is different from the typical region used for ethane measurement. The first candidate absorption lines were at approximately 2,967.5 cm−1, of which the
However, at this time, we can use only a 503.21 parts-per-million (ppm) ethane sample, which is a too high concentration to measure using the suggested line. Thus, we used two different ethane absorption lines at
We set up the CRDS experimental apparatus for ethane measurement as shown in Fig. 2, with a light source, acousto-optic modulator (AOM), optical fiber for spatial mode filtering, mode matching optics, CRDS cell, photo-detector, and data acquisition system.
The light source was required to be in the 3.37 μm range and the wavelength can be tunable. Nowadays, the desired conditions can be easily achieved using a quantum cascade laser (QCL) or inter-band cascade laser (ICL). We used a 7-mW distributed feedback ICL laser diode (DFB ICL; Nanoplus Nanosystems and Technologies GmbH, Meiningen, Germany).
To measure the ring-down time, we needed to switch off the laser beam after the build-up of cavity mode. We used an AOM (AGM-402B9M; IntraAction Corp., IL, USA) for fast optical switching. The optical rise time of AOM is 116 ns, sufficient to measure ring-down time on the order of μs.
When we directly coupled the laser beam to the cavity after the AOM, the coupling efficiency was not sufficient to reach the proper sensitivity because the laser beam profile was deformed. To improve the laser spatial mode, we coupled the laser beam to a single-mode zirconium fluoride fiber (P3-23Z-FC-1; ThorLabs, NJ, USA). After passing through the fiber, a TEM00 mode laser beam was obtained with power of 1.5 mW, which was adequate for CRDS experiments.
Mode matching optics, consisting of three plano-convex lenses, was used to match the laser spatial beam mode and cavity fundamental mode. We designed the system such that the first and second lenses controlled the beam size and the third lens controlled the beam divergence.
The CRDS cell was 50 cm long, and the optical cavity was constructed with two identical concave mirrors with reflectivity higher than 99.95% and curvatures of 1,000 mm (Layertec, Mellingen, Germany) at each end of the cell. The FSR of the cavity was calculated at 299.79 MHz. To scan the cavity modes, we needed to change the length of the cavity. This was achieved by controlling the modulation amplitude and phase of three piezoelectric actuators (PE-4; ThorLabs) independently at one of the cavity mirrors. This was necessary to measure the ring-down time in all experimental conditions while sweeping the laser wavelength.
The photodetector required proper detection sensitivity and bandwidth. We used an HgCdTe photovoltaic detector with a preamplifier (PIP-US-LS; Vigo Photonics, Ozarow Mazowiecki, Poland) that had high sensitivity in the mid-infrared range.
To obtain the cavity ring-down signal, we switched off the laser beam when the Fabry-Perot signal of the cavity was above a certain triggering value and then recorded the decay signal. Next we obtained the ring-down time by exponential fitting of the measured signal. Optical switching control was performed using a delay/pulse generator (DG535; Stanford Research System, CA, USA). A data acquisition system containing a data fitting process, number of averaging, and time interval between data points, was programmed by using Labview.
We measured the ring-down time for the empty cavity of the experimental setup described in section II. Typical ring-down time data is shown in Fig. 3(a). Each measurement took 5 seconds for 16 averaged cavity decay signals in this experiment.
As a result of repeating the measurements 1,000 times, the ring-down time for the empty cavity was measured and found to have an average value of 18.239 μs with a standard deviation of 0.198 μs [Fig. 3(b)].
The value of ring-down time calculated using Eq. (1), with cavity mirror reflectivity of 0.9995, is 3.336 μs. The measured ring-down time corresponds to a cavity mirror reflectivity of 0.9999. This was reproducible in several experimental conditions.
From Eqs. (1)–(2), we can estimate measurement sensitivity using the minimum detectable values for reciprocals of ring-down time. We obtained an average absorption coefficient
If we consider our first candidate line for ethane measurements at approximately 2,967.5 cm−1, with
To improve system performance, including the measurement sensitivity and acquisition rate, we can consider the following in future work.
First, if we consider only ethane measurement, we can easily achieve higher sensitivity by using absorption lines with large
Second, we can decrease the noise level of the cavity decay signal by increasing the average number of acquisitions [26, 27]. In this case, since a longer measurement time will be required, we will have to improve both the system stability and the acquisition rate. For large average numbers of acquisitions in particular, every decay signal will need to be as identical as possible. This can be achieved by improving cavity alignment, laser frequency stabilization, cavity temperature stabilization, and vibration isolation. Furthermore, in this experiment, measurement time was limited to a few seconds by the intrinsic parameters of the oscilloscope (DS6104; Rigol, Oregon, USA) such as communication time and acquisition time. However, in principle, the acquisition rate can reach the cavity mirror position scan rate, which was 200 Hz in this experiment. If we optimize the data acquisition system, we can decrease the measurement time to below one second.
We measured ring-down times under identical conditions for an empty cavity and for a cavity in which an ethane sample was injected. We used a certificated ethane sample with a 503.21 ppm concentration (AirKorea Co., Yeoju, Korea). After sample injection, the pressure of the cavity was measured at 0.27 torr. This means that the number of ethane molecules inside the cavity was the same as that of a 178.77 parts-per-billion (ppb) ethane sample at 1 atm. If we ignore pressure broadening effects, the measurement condition can be considered as effectively an ethane concentration of 178.77 ppb.
The measurement results are presented in Fig. 4. The measured spectrum of ethane shows good agreement with the reference data [25]. As mentioned in section 2.2, we focused on the two ethane absorption lines at
From the ring-down time data for the two absorption lines, we calculated the concentration of ethane and compared it to the effective concentration from pressure. The measured ethane concentrations showed consistency, with two different absorption lines (212.9 ppb for the
For quantitative measurement, accurate absorption cross-section data of ethane is required. In this study, however, we used the absorption cross-section data taken from [25], which shows broadening effects due to different measurement conditions. In addition, to improve measurement accuracy, we will need to consider measurement using a sample with a lower concentration and the pressure uncertainty inside the cavity introduced by the sample injection and pumping process.
We measured ethane in the 3.37 μm mid-infrared region using the CRDS method. The configured CRDS setup showed a minimum detectable absorption coefficient of 2.249 × 10−8/cm, corresponding to a 322 ppt detection limit, with an absorption line at 2,967.5 cm−1. We demonstrated the ethane measurements using a certificated sample. Quantitative calculations from the measured data were consistent and showed agreement with the amount of injected sample. These results showed the possibility of using CRDS for trace gas analysis, including for atmospheric ethane measurement and breath test applications.
In addition, we expect that the CRDS system can be used as a multi-species VOCs analyzing system applicable to breath test equipment in the future.
The authors declare no conflicts of interest.
This work was supported by the KAERI Institutional Program (Project No. 524430-23).
KAERI Institutional Program (Project No. 524430-23).
Curr. Opt. Photon. 2023; 7(4): 457-462
Published online August 25, 2023 https://doi.org/10.3807/COPP.2023.7.4.457
Copyright © Optical Society of Korea.
Yonghee Kim , Byung Jae Chun, Lim Lee, Kwang-Hoon Ko, Seung-Kyu Park, Taek-Soo Kim, Hyunmin Park
Quantum Optics Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea
Correspondence to:*yonghee922@kaeri.re.kr, ORCID 0009-0005-8232-6278
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Quantitative measurement of trace ethane is important in environmental science and biomedical applications. For these applications, we typically require a few tens of part-per-trillion level measurement sensitivity. To measure trace-level ethane, we constructed a cavity ring-down spectroscopy setup in the 3.37 μm mid-infrared wavelength range, which is applicable to multi-species chemical analysis. We demonstrated that the detection limit of ethane is approximately 300 parts per trillion, and the measured concentration is in agreement with the amounts of the injected sample. We expect that these results can be applied to the chemical analysis of ethane and applications such as breath test equipment.
Keywords: Cavity ring-down spectroscopy, Ethane, Mid-infrared, Quantitative measurement
Ethane (C2H6) is one of the most abundant hydrocarbon volatile organic compounds (VOCs) in the atmosphere, with average concentrations of approximately 1,000 parts per trillion (ppt) for the Northern Hemisphere [1]. Over the last few decades, many quantitative studies of trace ethane have been conducted in various fields such as environmental science [1–4] and biomedical applications [5–12].
Atmospheric ethane is one of the major greenhouse gases, and its origin lies in human activities such as natural gas leakage during production and transportation, biofuel use, and burning of biomass [2]. There is now growing interest in the concentration of ethane and its relation to climate change. Information on atmospheric ethane concentrations, such as regional distribution, seasonal oscillation, and annual changes, offers opportunities to better understand the earth system and its reflection of human activities [1–3]. Also, ethane is related to various chemical reactions in the atmosphere such as the formation of tropospheric ozone [4].
From a clinical point of view, exhaled human breath contains thousands of VOCs, and many studies have shown a correlation between concentrations of VOCs and various diseases [13–17]. A breath test is a non-invasive diagnostic method of diseases by measuring specific molecules in the exhaled breath. It is known that ethane in human breath is produced by oxidation damage of cellular membranes, in a process known as oxidation stress [5]; This is related to a wide range of diseases such as lung cancer [6, 7], chronic obstructive pulmonary disease (COPD) [8], cystic fibrosis [9], asthma [10] and vitamin E deficiency [11]. Intensive studies have been conducted on ethane as a potential biomarker for diagnosing diseases and monitoring oxidation stress [5–12].
To measure the quantity of ethane and its delicate changes in air or human breath, measurement techniques with precision below several tens of ppt is required. Currently, gas chromatography coupled to mass spectrometry (GC-MS) is a standard method for analyzing the composition and concentration of trace-level samples. However, the GC-MS method has some disadvantages such as a complex sampling procedure, long measurement time and high cost. Therefore, in specific applications for which the chemical species and measurement range are limited, other measurement technologies such as electrochemical sensors [13, 15] and optical methods [16, 17] have been suggested. Among these, optical methods using molecular spectra have distinct advantages compared to GC-MS, such as high sensitivity, real-time measurement, simple sampling procedures, and relatively low cost. Thus, these methods are expected to be widely used in various fields such as clinical applications based on exhaled human breath analysis [16, 17].
In the case of ethane measurement, the mid-infrared wavelength region is generally used because an ethane molecule has strong absorption lines near the 3.3 μm wavelength range. To measure the trace levels of ethane, various spectroscopic techniques such as Fourier transform infrared (FTIR) spectroscopy [2], tunable diode laser absorption spectroscopy (TDLAS) using a multi-pass cell [5, 6, 12], off-axis integrated cavity output spectroscopy (OA-ICOS) [18, 19] and cavity ring-down spectroscopy (CRDS) [7, 20, 21] have been reported. These methods have shown detection limits of 70 to 500 ppt with 1 second measurement time and further demonstrate the potential of optical methods for clinical applications.
In this paper, we focus on the CRDS method to achieve several tens of ppt level sensitivity for the quantitative measurement of ethane molecules. We demonstrate ethane measurement using the 3.37 μm mid-infrared wavelength region (corresponding to 2,967.0 to 2,969.0 cm−1), which can be applicable to multi-species chemical analysis.
In section II, we briefly introduce the CRDS method, the wavelength selection for this study, and details of the experimental setup. In section III, we present experimental results on ethane measurement and discuss system sensitivity. Finally, we summarize results and discuss future work in section IV.
CRDS is a highly sensitive quantitative measurement technique using the decay time (or ring-down time) of light in an optical cavity. This technique, which was first demonstrated at 1988 [22], has been widely used to analyze small amounts of optical absorption.
When a laser is spatially mode matched to a cavity mode and injected into a high finesse optical cavity with length
For an empty cavity, cavity loss depends on only cavity mirror reflectivity
Cavity loss increases in the presence of target molecules inside the cavity due to the light absorption of target molecules. Then, ring-down time
where
Notably, CRDS is very sensitive to injected laser beam alignment; However, the method is free from laser power fluctuation, in principle, because the ring-down time is independent from the laser power. This offers CRDS the unique capability of conducting trace-level sample measurements, which can lead to the development of robust commercial equipment.
Ethane has a strong absorption band in the mid-infrared wavelength region of 3.31–3.38 μm (approximately 2,960.0 to 3,020.0 cm−1) as shown in Fig. 1(a) [24, 25].
The largest two absorption lines of ethane are 2,983.28 cm−1 and 2,996.87 cm−1 (corresponding to 3.3520 μm and 3.3368 μm, respectively). Most previous studies [5, 12, 18, 19, 21] have used the wavelength region near these lines, depending on the light source and interference by methane or water molecules.
In this study, we selected the 3.37 μm (2,967.0 to 2,969.0 cm−1) wavelength region, as shown in Fig. 1(b), which is different from the typical region used for ethane measurement. The first candidate absorption lines were at approximately 2,967.5 cm−1, of which the
However, at this time, we can use only a 503.21 parts-per-million (ppm) ethane sample, which is a too high concentration to measure using the suggested line. Thus, we used two different ethane absorption lines at
We set up the CRDS experimental apparatus for ethane measurement as shown in Fig. 2, with a light source, acousto-optic modulator (AOM), optical fiber for spatial mode filtering, mode matching optics, CRDS cell, photo-detector, and data acquisition system.
The light source was required to be in the 3.37 μm range and the wavelength can be tunable. Nowadays, the desired conditions can be easily achieved using a quantum cascade laser (QCL) or inter-band cascade laser (ICL). We used a 7-mW distributed feedback ICL laser diode (DFB ICL; Nanoplus Nanosystems and Technologies GmbH, Meiningen, Germany).
To measure the ring-down time, we needed to switch off the laser beam after the build-up of cavity mode. We used an AOM (AGM-402B9M; IntraAction Corp., IL, USA) for fast optical switching. The optical rise time of AOM is 116 ns, sufficient to measure ring-down time on the order of μs.
When we directly coupled the laser beam to the cavity after the AOM, the coupling efficiency was not sufficient to reach the proper sensitivity because the laser beam profile was deformed. To improve the laser spatial mode, we coupled the laser beam to a single-mode zirconium fluoride fiber (P3-23Z-FC-1; ThorLabs, NJ, USA). After passing through the fiber, a TEM00 mode laser beam was obtained with power of 1.5 mW, which was adequate for CRDS experiments.
Mode matching optics, consisting of three plano-convex lenses, was used to match the laser spatial beam mode and cavity fundamental mode. We designed the system such that the first and second lenses controlled the beam size and the third lens controlled the beam divergence.
The CRDS cell was 50 cm long, and the optical cavity was constructed with two identical concave mirrors with reflectivity higher than 99.95% and curvatures of 1,000 mm (Layertec, Mellingen, Germany) at each end of the cell. The FSR of the cavity was calculated at 299.79 MHz. To scan the cavity modes, we needed to change the length of the cavity. This was achieved by controlling the modulation amplitude and phase of three piezoelectric actuators (PE-4; ThorLabs) independently at one of the cavity mirrors. This was necessary to measure the ring-down time in all experimental conditions while sweeping the laser wavelength.
The photodetector required proper detection sensitivity and bandwidth. We used an HgCdTe photovoltaic detector with a preamplifier (PIP-US-LS; Vigo Photonics, Ozarow Mazowiecki, Poland) that had high sensitivity in the mid-infrared range.
To obtain the cavity ring-down signal, we switched off the laser beam when the Fabry-Perot signal of the cavity was above a certain triggering value and then recorded the decay signal. Next we obtained the ring-down time by exponential fitting of the measured signal. Optical switching control was performed using a delay/pulse generator (DG535; Stanford Research System, CA, USA). A data acquisition system containing a data fitting process, number of averaging, and time interval between data points, was programmed by using Labview.
We measured the ring-down time for the empty cavity of the experimental setup described in section II. Typical ring-down time data is shown in Fig. 3(a). Each measurement took 5 seconds for 16 averaged cavity decay signals in this experiment.
As a result of repeating the measurements 1,000 times, the ring-down time for the empty cavity was measured and found to have an average value of 18.239 μs with a standard deviation of 0.198 μs [Fig. 3(b)].
The value of ring-down time calculated using Eq. (1), with cavity mirror reflectivity of 0.9995, is 3.336 μs. The measured ring-down time corresponds to a cavity mirror reflectivity of 0.9999. This was reproducible in several experimental conditions.
From Eqs. (1)–(2), we can estimate measurement sensitivity using the minimum detectable values for reciprocals of ring-down time. We obtained an average absorption coefficient
If we consider our first candidate line for ethane measurements at approximately 2,967.5 cm−1, with
To improve system performance, including the measurement sensitivity and acquisition rate, we can consider the following in future work.
First, if we consider only ethane measurement, we can easily achieve higher sensitivity by using absorption lines with large
Second, we can decrease the noise level of the cavity decay signal by increasing the average number of acquisitions [26, 27]. In this case, since a longer measurement time will be required, we will have to improve both the system stability and the acquisition rate. For large average numbers of acquisitions in particular, every decay signal will need to be as identical as possible. This can be achieved by improving cavity alignment, laser frequency stabilization, cavity temperature stabilization, and vibration isolation. Furthermore, in this experiment, measurement time was limited to a few seconds by the intrinsic parameters of the oscilloscope (DS6104; Rigol, Oregon, USA) such as communication time and acquisition time. However, in principle, the acquisition rate can reach the cavity mirror position scan rate, which was 200 Hz in this experiment. If we optimize the data acquisition system, we can decrease the measurement time to below one second.
We measured ring-down times under identical conditions for an empty cavity and for a cavity in which an ethane sample was injected. We used a certificated ethane sample with a 503.21 ppm concentration (AirKorea Co., Yeoju, Korea). After sample injection, the pressure of the cavity was measured at 0.27 torr. This means that the number of ethane molecules inside the cavity was the same as that of a 178.77 parts-per-billion (ppb) ethane sample at 1 atm. If we ignore pressure broadening effects, the measurement condition can be considered as effectively an ethane concentration of 178.77 ppb.
The measurement results are presented in Fig. 4. The measured spectrum of ethane shows good agreement with the reference data [25]. As mentioned in section 2.2, we focused on the two ethane absorption lines at
From the ring-down time data for the two absorption lines, we calculated the concentration of ethane and compared it to the effective concentration from pressure. The measured ethane concentrations showed consistency, with two different absorption lines (212.9 ppb for the
For quantitative measurement, accurate absorption cross-section data of ethane is required. In this study, however, we used the absorption cross-section data taken from [25], which shows broadening effects due to different measurement conditions. In addition, to improve measurement accuracy, we will need to consider measurement using a sample with a lower concentration and the pressure uncertainty inside the cavity introduced by the sample injection and pumping process.
We measured ethane in the 3.37 μm mid-infrared region using the CRDS method. The configured CRDS setup showed a minimum detectable absorption coefficient of 2.249 × 10−8/cm, corresponding to a 322 ppt detection limit, with an absorption line at 2,967.5 cm−1. We demonstrated the ethane measurements using a certificated sample. Quantitative calculations from the measured data were consistent and showed agreement with the amount of injected sample. These results showed the possibility of using CRDS for trace gas analysis, including for atmospheric ethane measurement and breath test applications.
In addition, we expect that the CRDS system can be used as a multi-species VOCs analyzing system applicable to breath test equipment in the future.
The authors declare no conflicts of interest.
This work was supported by the KAERI Institutional Program (Project No. 524430-23).
KAERI Institutional Program (Project No. 524430-23).