Towards a Miniaturized Photoacoustic Detector for the Infrared Spectroscopic Analysis of SO₂F₂ and Refrigerants

Abstract

Sulfuryl fluoride (SO₂F₂) is a toxic and potent greenhouse gas that is currently widely used as a fumigant insecticide in houses, food, and shipping containers. Though it poses a major hazard to humans, its detection is still carried out manually and only on a random basis. In this paper, we present a two-chamber photoacoustic approach for continuous SO₂F₂ sensing. Because of the high toxicity of SO₂F₂, the concept is to use a non-toxic substituent gas with similar absorption characteristics in the photoacoustic detector chamber, i.e., to measure SO₂F₂ indirectly. The refrigerants R227ea, R125, R134a, and propene were identified as possible substituents using a Fourier-transform infrared (FTIR) spectroscopic analysis. The resulting infrared spectra were used to simulate the sensitivity of the substituents of a photoacoustic sensor to SO₂F₂ in different concentration ranges and at different optical path lengths. The simulations showed that R227ea has the highest sensitivity to SO₂F₂ among the substituents and is therefore a promising substituent detector gas. Simulations concerning the possible cross-sensitivity of the photoacoustic detectors to H₂O and CO₂ were also performed. These results are the first step towards the development of a miniaturized, sensitive, and cost-effective photoacoustic sensor system for SO₂F₂.

1. Introduction

The detection of sulfuryl fluoride (SO₂F₂) has gained increasing interest in recent years since it is widely used as a fumigant insecticide in houses, food, and shipping containers [1]. SO₂F₂ is neurotoxic and therefore poses a major hazard to humans, e.g., to inhabitants of treated houses or workers in the logistics industry, with an occupational exposure limit (OEL) of 5 ppm and an immediate danger for life and health starting from 200 ppm [2]. Since the adoption of the Montreal Protocol in 1987, in which the use of methyl bromide as a fumigant was phased out because of its harm to the ozone layer, SO₂F₂ has largely replaced methyl bromide, and the amount used drastically increased to more than 2000 metric tons per year [3,4]. Apart from its toxicity, SO₂F₂ is a potent greenhouse gas with a global warming potential of 4780 and an atmospheric lifetime of 36 years [5,6]. The main degradation pathway in the environment is hydrolysis, e.g., in oceans and wetlands [6,7,8].

Due to the high necessity of SO₂F₂ measurement to minimize its use and therefore release into the environment, a wealth of different sensors and sensor principles have recently been developed and have been reported in the literature [9,10,11,12,13,14,15,16]. Apart from fumigation, SO₂F₂ detection is often employed in the failure monitoring of fully enclosed insulated gas switching equipment (gas-insulated switchgear, GIS) because SO₂F₂ is a decomposition product of SF₆-filled GIS. Such sensors must be able to monitor SO₂F₂ at very low concentrations (low ppm to ppb range) to safely warn below the OEL of 5 ppm. Most of the proposed sensors involve transition metals [9,13] or semiconductors [10,11], which detect a chemical reaction of SO₂F₂ with the respective material. Such sensors have the advantage of being very sensitive and compact, yet they often suffer from cross-sensitivities to other gases and ambient parameter variations, such as temperature and humidity. Several studies on metal/semiconductor-based sensors relied on density functional theory (DFT) simulations of chemical reactions with SO₂F₂ and took changes in the ambient conditions into account [12,13,14,15,16].

Alternatively, gases can often also be measured very well optically via their characteristic absorption in the infrared, as they have distinct absorption bands in the spectral range between 3 µm and 16 µm. Simple non-dispersive IR (NDIR) gas sensors work with a few fixed spectral channels and pyroelectric detectors. To minimize influences from interfering gases or drift effects from the light source, an additional reference channel is usually used for compensation, which measures in an absorption-free wavelength range. The wavelengths and spectral bandwidths are selected using thin-film interference filters, which are each optimized for the wavelength of the absorption band of the gas to be measured [17,18]. Naik et al. [19] reported an NDIR-based sensor for SO₂F₂ using a spectral absorption around 6.64 µm for the concentration range of 500–30,000 ppm. For higher sensitivity and resolution, SO₂F₂ can also be detected using tunable laser absorption spectroscopy (TDLAS) with suitable light sources and optical long-path cells, as proposed in [20,21]. Yao et al. [20] employed on interband cascade laser (ICL) at a 3619 nm central wavelength, combined with a 5.33 long-path cell to reach a detection limit of ~4 ppm. Zhang et al. [21] conducted a simulation study on the comparison of an ICL at 3619 nm and a quantum cascade laser (QCL) at 6653 nm, either using TDLAS or photoacoustic detection to reach a sub-ppm detection limit: 0.34 ppm for the ICL and 0.66 ppm for the QCL. Photoacoustics is another highly selective infrared spectroscopic measurement technique and is widely used to measure IR-active gases at low concentrations, from high-precision industrial process monitoring to trace gas monitoring [22,23,24]. The operating modes of photoacoustic systems can be divided into resonant and non-resonant. Resonant cells are mainly used in trace gas monitoring, which requires a very high level of sensitivity. With lasers as light sources, resonant photoacoustic systems can achieve sensitivities in the ppb range [25,26]. However, resonant systems are comparatively complex and expensive. Recently, different resonant photoacoustic systems have been introduced for SO₂F₂ sensing [27,28,29,30], achieving such low detection limits. However, such systems are laboratory-based due to their size and complexity. In contrast, non-resonant systems can be much smaller and less complex, making them suitable for use outside the laboratory. Broadband infrared sources such as filament emitters or planar thermal emitters are often employed. An advantage of broadband emitters is that all absorption lines of the target gas contribute to the signal, which allows the optical path length to be reduced. A prominent application example is room climate monitoring, in which the CO₂ concentration is evaluated as a parameter for air quality [31].

Therefore, we propose a non-resonant, two-chamber photoacoustic approach to meet the requirements of size, cost, and complexity reduction to obtain a sensor for continuous SO₂F₂ sensing in field applications, e.g., freight container monitoring. The scheme of a two-chamber photoacoustic sensor is shown in Figure 1. Our core concept is to use a non-toxic substituent gas with similar absorption characteristics in the photoacoustic detector chamber for indirect, but still selective, SO₂F₂ detection. Fourier-transform infrared (FTIR) spectroscopy was used to obtain high-resolution spectra of the possible substituents 1,1,1,2,3,3,3-heptafluoropropane (R227ea), 1,1,1,2,2-pentafluoroethane (R125), 1,1,1,2-tetrafluoroethane (R134a), and propene. The resulting infrared spectra were used to simulate the sensitivity of the substituents of a photoacoustic sensor to SO₂F₂ in different concentration ranges and at different optical path lengths.

2. Materials and Methods

2.1. Measurement Setup

To estimate the sensitivity of the hermetically sealed photoacoustic detector cells to SO₂F₂, simulations were performed, requiring high-resolution spectra of the target gas, SO₂F₂, and the possible substituents. For this purpose, the transmission measurements of SO₂F₂ and the substituent gases were determined using an FTIR spectrometer (Vertex v80, Bruker, Billerica, MA, USA). The decadic absorption coefficients were calculated from the resulting spectra.

The measurement setup consisted of the FTIR spectrometer combined with a 10 m long-path gas cell (Pike Technologies, Madison, WI, USA), mass flow controllers (Bronkhorst, The Netherlands), and a lid (Figure 2a). The lid was needed to evacuate the sample compartment of the spectrometer to avoid absorption from atmospheric gases. However, the long-path gas cell was higher than the sample compartment, so the sample compartment could not be evacuated with the standard lid of the spectrometer. A lid with corresponding dimensions, as seen in Figure 2a, was constructed and designed. An aluminum cylinder (250 mm × 15 mm × 360 mm) with a polycarbonate plate (250 mm × 15 mm) was fixed through screws to a rectangular aluminum plate (282 mm × 293 mm × 10 mm). Four holes for straight bulkhead fittings were inserted in the bottom plate. The gas inlet was connected to one of the bulkhead fittings on the outside of the lid, and a second bulkhead fitting was connected to the gas exhaust (see Figure 2b). The inlet and the outlet of the long-path gas cell were connected to the respective bulkhead fittings on the inside of the lid. In case the sample compartment was intended to be flushed with nitrogen gas (N₂), the third bulkhead fitting was connected to the N₂ source and the fourth one was connected to the gas exhaust. For evacuating, these two were closed so that the entire measurement setup remained vacuum tight.

Before adjusting different concentrations of each gas and recording the resulting spectra with the FTIR spectrometer, the measurement parameters, such as the mirror velocity, resolution, etc., were set. For all measurements, consistent parameters were defined. A mercury–cadmium–telluride (MCT) photo detector cooled with liquid N₂ was used to achieve high sensitivity.

Table 1. Parameters used for the transmission measurements with the FTIR spectrometer (Vertex 80v, Bruker).

Detector Type	LN-MCT Photoconductor
Resolution	0.08 cm−1
Mirror velocity	80 kHz
Acquisition mode	Single-sided, forward–backward
Phase correction method	Mertz
Apodization function	Three-term Blackman–Harris window

summarizes all parameters.

The transmission spectra were recorded after setting different gas concentrations, starting from the gas cylinder concentration down to 5–15 ppm. Certified gas bottles were obtained from Westfalen, Germany (1000 ppm SO₂F₂ in N₂), and TEGA, Germany (refrigerants), and used as received. At high gas concentrations, the strong absorption bands were already saturated, while weak absorption bands were not saturated and were clearly measured. On the other hand, at low gas concentrations, the strong absorption bands were clearly measured, while the weak ones were not. The decadic absorption coefficients were calculated from the transmission spectrum at each concentration, and the calculated spectra were merged. The resulting spectrum then contained both the weak and strong absorption bands with the desired resolution (Figure 3).

2.2. Simulations

The sensitivity of the photoacoustic detectors filled with the substituent pure gas to the target gas, SO₂F₂, was simulated. This was performed using a script written in JavaScript, similar to that in [32]. The script calculated the integral absorbed power in the detector cell filled with a definite pure gas (100 Vol.-%). By varying the concentration of the target gas in the absorption path at a defined optical path length, the integral absorbed power in the detector decreased. This change was defined as the sensitivity. Furthermore, a suitable window material was selected for the hermetic sealing of the detectors. Silicon has an average transmission of about ~50% in the infrared region [33]. It is easy to obtain and can easily be sawed into desired small detector windows. For this purpose, the transmission measurements of a single-side-polished (SSP) Si wafer and a double-side-polished (DSP) Si wafer were determined using the FTIR spectrometer (Figure 4).

The numerical calculations were performed for each substituent gas using the decadic absorption coefficients calculated from the FTIR measurements and by considering the IR transmission of the DSP Si wafer. Both the detector gas and the optical path length were varied.

$$I_{Emitter}\left(\lambda \right)=\frac{d{P}_{Emitter}}{d\lambda }=\frac{2\pi \cdot h\cdot c^2}{{\lambda }^5}\cdot \frac{A}{\exp \left(\frac{h\cdot c}{k_B\cdot \lambda \cdot T}\right)-1}$$

(1)

$$A\left(\lambda \right)=1-T\left(\lambda \right)=1-{10}^{-\alpha \cdot \left(\lambda \right)\cdot l\cdot c},$$

(2)

$$P_{Abs}={{\displaystyle \int }}_{{\lambda }_1}^{{\lambda }_2}{I}_{Emitter}\left(\lambda \right)\cdot A_{Detector}\left(\lambda \right)\cdot T_{Window}\left(\lambda \right)\cdot T_{Abs.cell}\left(\lambda \right)d\lambda ,$$

(3)

where I is the emitted spectral power of the emitter, P_Emitter is the optical power of the emitter, λ is the wavelength, h is Planck’s constant, c is the speed of light, A is the active emitter area of the emitter, k_b is Stefan–Boltzmann’s constant, T is the temperature, α is the decadic absorption coefficient of the gas, T_window and T_Abs.cell are the transmission through the window and the absorption cell, respectively, A_Detector is the absorption in the detector, and P_Abs is the integral absorbed power in the detector cell. Using these Equations, the emitted spectral power of the infrared emitter, with an active emitter area 2.2 mm × 2.2 mm, and the integral absorbed power in the detector were calculated [35,36]. The length of the detector chamber was fixed to 1.5 mm. The sensitivity of the detectors filled with SO₂F₂ and the refrigerants (100 Vol.-%) to 0–10,000 ppm SO₂F₂ at an optical path length of 50 mm was simulated. Similarly, simulations for the measurement range of 0–50 ppm SO₂F₂ with an optical path length of 1.6 m were conducted. The cross-sensitivity to atmospheric gases such as H₂O (0–4 Vol.-%) and CO₂ (0–2000 ppm) were also investigated for both optical path lengths.

3. Results and Discussion

3.1. Infrared Spectroscopic Characteristics of SO₂F₂ and Refrigerants

The resulting infrared spectra of SO₂F₂ and the possible substituents are plotted in Figure 3. SO₂F₂ had three main strong absorption bands in the wavelength range between 6500 and 6800 nm, 7700 and 8100 nm, and 10,500 and 12,200 nm (Figure 3a).

As seen in Figure 3b and

Table 2. List of the substituent gases with the wavelength ranges of absorption that overlapped with SO₂F₂.

Substituent	Wavelength Range of Absorption Overlapping with SO2F2 (nm)
R227ea	7700–8100
	10,900–11,250
	11,400–11,800
R134a [32]	6550–6800
	7700–8050 11,000–12,200
R125	7700–8100
	10,900–12,000
Propene	6500–6800
	7700–7800 7900–8100 10,950–12,000

, R227ea had absorption bands that overlapped with SO₂F₂ between 7700 and 8100 nm, 10,900 and 11,250 nm, and 11,400 and 11,800 nm. R134a had bands that overlapped with the target gas between 6550 and 6800 nm, 7700 and 8050 nm, and 11,000 and 12,200 nm [32]. R125 had bands that overlapped with the target gas between 7700 and 8100 nm and 10,900 and 12,000 nm. Propene had weaker absorption compared with the other refrigerants but also had bands that overlapped with SO₂F₂ between 6500 and 6800 nm, 7700 and 7800 nm, 7900 and 8100 nm, and 10,950 and 12,000 nm.

Figure 4 shows the decadic absorption spectra of the possibly interfering atmospheric gases H₂O, CO₂, and CH₄, which were calculated and plotted using the HITRAN database [34].

3.2. Simulation of the Sensitivity of the Photoacoustic Detectors

The infrared transmission through 500 µm of SSP Si and 500 µm of DSP Si and the spectrum of SO₂F₂ in the wavelength range between 2500 and 15,000 nm are shown in Figure 5. The infrared transmission through DSP Si was higher than the transmission though SSP Si. The average infrared transmission through DSP Si was about 52 % in the wavelength ranges where SO₂F₂ absorbs.

Figure 6 shows the emitted spectral power of the infrared source simulated using Equation 1 with an emitter temperature of 650 °C as well as the transmitted spectral power through DSP Si. According to these simulations, the minimum spectral power transmitted into the detector was about 0.006 mW/nm at wavelengths between 7800 and 8100 nm, and it dropped to about one third at wavelengths between 10,500 and 12,200 nm. Nevertheless, the transmitted power was sufficient for the planned application.

The simulated sensitivity of each photoacoustic detector to 0–10,000 ppm SO₂F₂ in the absorption cell (50 mm length) is plotted in Figure 7a. As expected, the highest sensitivity was obtained with the SO₂F₂ detector. Among the substituents, R227ea showed the highest sensitivity to SO₂F₂. Although R125 absorbed stronger than R227ea, R134a, and propene between 10,500 and 12,200 nm, the absorbance of R227ea between 7700 and 8100 nm was higher than those of R134a, R125, and propene, and the spectral power transmitted through the detector between 7700 and 8100 nm was three times higher than that between 10,500 and 12,200 nm. For this reason, the R227ea detector was more sensitive to SO₂F₂ than the other detectors. The propene detector showed the lowest sensitivity to SO₂F₂, which can be explained by its weak infrared absorption compared to the other substituents in the considered wavelength range. Moreover, the relative signal change of the SO₂F₂ detector versus the SO₂F₂ concentration change in the absorption cell was not linear, while that of the refrigerant detectors was.

Figure 7b–d show the cross-sensitivity of the photoacoustic detectors to 0–4 Vol.-% H₂O, 0–2000 ppm CO₂, and 0–5 ppm CH₄, respectively. The highest cross-sensitivity was obtained with SO₂F₂ as the detector gas. The relative signal change of a SO₂F₂ detector to ambient 0–4 Vol.-% H₂O in the absorption cell was 10× lower than that to 0–10,000 ppm SO₂F₂. This is because the absorption bands of SO₂F₂ overlap with those of H₂O, especially at 6500–6800 nm. The cross-sensitivity of the SO₂F₂ detector to ambient CO₂ (0–2000 ppm) was 4× lower than that to ambient H₂O. The lowest cross-sensitivity to ambient H₂O was obtained with the R125 detector, while that to CO₂ was obtained with propene. The R227ea detector showed a maximum relative signal change of 0.5 % to CO₂ and H₂O. The cross-sensitivity of the photoacoustic detectors to 0–5 ppm CH₄, which is in the range of the relevant background concentration in the atmosphere, was negligible. However, the problem of the cross-sensitivity of the photoacoustic detectors to CO₂ and H₂O must be taken into account when calibrating the sensors and can be solved by additionally integrating humidity and CO₂ sensors into a final sensor system or by integrating a humidity sensor and an IR-blocking filter.

Figure 8a shows the simulated sensitivity of each photoacoustic detector to 0–50 ppm SO₂F₂ in a long-path absorption cell (1.6 m optical path length). In these simulations, only the concentrations of SO₂F₂ and the optical path length were changed. Everything else was kept as in the previous simulations. Therefore, R227ea still showed the highest sensitivity to SO₂F₂ among the substituents. Moreover, the relative signal change of the SO₂F₂ detector as a function of the SO₂F₂ concentration change in the long-path absorption cell became linear, like that of the refrigerant detectors. The cross-sensitivity of the photoacoustic detectors to 0–4 Vol.-% H₂O and 0–2000 ppm CO₂ also became more critical in this case (Figure 8b,c), while the cross-sensitivity to 0–5 ppm CH₄ was still low and negligible (Figure 8d).

4. Conclusions

In this article, an investigation of a new photoacoustic detector approach for the measurement of SO₂F₂ is presented. It is based on the concept of filling a non-toxic substituent gas that has absorption bands that overlap with the target gas into a photoacoustic detector chamber to measure SO₂F₂ indirectly. The refrigerants R227ea, R125, R134a, and propene were selected as possible substituents. An infrared spectroscopic analysis of SO₂F₂ and the refrigerants was performed using an FTIR spectrometer. The resulting infrared spectra were used as a basis for simulating the sensitivity of the detector filled with the substituents to SO₂F₂ in different concentrations and at different optical path lengths. In addition, the cross-sensitivity of the detectors filled with the substituents to ambient CO₂ and H₂O was simulated. The simulations revealed that R227ea had the highest sensitivity to SO₂F₂ among the substituents and could be used as a detector gas. Detection limits below 50 ppm and 1.5 ppm could be reached with a 50 mm absorption cell and a 1.6 m long-path absorption cell, respectively, with a thermal broadband emitter and a compact setup. The simulation results also showed the cross-sensitivity of the photoacoustic detectors to ambient gases such as H₂O and CO₂, which have to be taken into account in experimental studies. Based on these results, photoacoustic sensors for the measurement of SO₂F₂ will be designed, built, and experimentally characterized in the future.