Mid-IR laser-based sensor for hydrogen peroxide detection

Hydrogen peroxide (H₂O₂) is a strong oxidizing agent that is associated with the generation of hydroxyl radicals in the atmosphere. It is also involved in several environmental processes, including the degradation of pollutants in water by advanced oxidation processes.^{1, 2} In addition, H₂O₂ is relevant in the medical field as a reactive oxygen species used as a biomarker of lung and airway-related inflammation. Mid-IR laser-based sensor systems, with minimum detection limits (MDLs) at the parts-per-billion (ppb) level, have previously been reported for monitoring of H₂O₂.^3–5 Although these configurations have high sensitivity levels and continuous real-time detection capabilities, they are subject to interference from water lines in the selected spectral regions, which is a potentially significant concern for their field operation.³ For future development of mid-IR laser-based sensor systems, it is therefore necessary to explore alternative strong, interference-free H₂O₂ absorption lines.

The strong oxidation potential of H₂O₂, its relatively broad germicidal spectrum, and the innocuous character of its decomposition products (water and oxygen) have led to the extensive use of this species for decontamination and sterilization of clean production sites and healthcare facilities.^6–8 Vapor phase hydrogen peroxide (VPHP) units—in which aqueous H₂O₂ solutions are used to generate gas-phase H₂O₂ concentrations of 200–1200ppm—are normally used in H₂O₂-based sterilization/decontamination techniques. Other applications of H₂O₂ include its use as a bleaching agent in the production of pulp and paper, and for sterilization of packing materials and medical devices. Potentially high levels of H₂O₂ can be observed at production and decontamination sites, and therefore monitoring the concentration of this species is crucial for evaluating exposure risks (e.g., above the US Occupational Safety and Health Administration Agency's average permissible exposure levels).

In this work,⁹ we addressed the potential interference from water and other trace gas species during the detection of H₂O₂. To that end, we chose to study an interference-free absorption line at a wavenumber of 1234.055cm⁻¹ (see Figure 1). We have thus developed a sensor system for the specific and selective detection of H₂O₂, which is based on this interference-free absorption wavelength.

Figure 1. Transmittance of hydrogen peroxide (H₂O₂) at a pressure of 20Torr, temperature of 293.15K, and with a 76m optical path length. The potential interferences from other gas species—water (H₂O), methane (CH₄), nitrogen oxide (N₂O), and carbon dioxide (CO₂)—are illustrated. Data taken from the high-resolution transmission molecular absorption (HITRAN) database.

In our sensor system we use a continuous-wave external-cavity quantum cascade laser (CW EC-QCL) that has a mode-hop-free (MHF) tuning range of 1225–1285cm⁻¹. We couple this EC-QCL with a commercial multipass absorption cell that has an optical path length of 76m. We use wavelength modulation spectroscopy, with second harmonic detection, for data processing. The modulation amplitude and pressure levels in our sensor system have been optimized for sensitive detection of H₂O₂. The actual configuration of our developed sensor system is shown in Figure 2.

Figure 2. Photograph of the H₂O₂sensor system configuration. The sensor architecture consists of a Daylight Solutions 21080-MHF CW EC-QCL (mode-hop-free continuous-wave external-cavity quantum cascade laser), optical components for focusing and improving the laser beam shape (wedge beam splitter, 400μm pinhole, mirrors, and two lenses with focal lengths of 4 and 5cm), an Aerodyne Inc. AMAC-76 multipass absorption cell, and a PVMI-3TE-8 Vigo mid-IR detector.

We performed a calibration of our sensor system by generating different concentrations of gas-phase H₂O₂. To realize these different gas concentrations, we allowed air to flow over aqueous solutions (between 0.1 and 2% w/w) of H₂O₂. We used the standard high-resolution transmission molecular absorption (HITRAN) database to determine the corresponding concentration of H₂O₂ in the gas phase, based on the direct absorption signal at each mixing ratio level. The response of the sensor system at different H₂O₂ concentrations is illustrated in Figure 3. We find that there is a linear relationship between the system response and the concentration of H₂O₂. We also observe stability in the response at different mixing ratios.

Figure 3. Response of the sensor system to different concentrations of H₂O₂at 20Torr. R²: Coefficient of determination.

Our current configuration for the sensor system allows detection of H₂O₂ at the ppb level (MDL of about 25ppb for a 280s optimum integration time). This configuration has the potential for continuous monitoring of H₂O₂ at industrial sites, as well as at locations that are undergoing VPHP-based decontamination. The relatively compact layout of our sensor means that it can be deployed at a variety of locations for the determination of average permissible exposure levels of gas-phase H₂O₂. In addition, our system has no restrictions associated with the relative humidity of the measurement sites. The interference-free absorption line that we selected for our sensor system, together with the MDL that we can achieve, also means that our system is suitable for the direct analysis of exhaled breath (in which water concentrations close to the saturation level are expected).

In summary, we have developed a sensor system that is based on the integration of a CW EC-QCL (with an MHF operating range of 1225–1285cm⁻¹) and a commercial multipass absorption cell (with a 76m optical path length) for the detection of H₂O₂ at the ppb level. With an optimized configuration, we can achieve an MDL of about 25ppb with a 280s integration period. From the results of our calibration tests, we find that the sensor system exhibits a linear response at different H₂O₂ concentrations, when it is operated at optimum pressure and modulation amplitude. We have targeted a specific interference-free H₂O₂ absorption line at 1234.055cm⁻¹, which alleviates interference issues that have been reported in previous mid-IR-based H₂O₂ sensing systems. Our system is therefore suitable for the monitoring of H₂O₂ at industrial sites, in decontamination/sterilization locations using VPHP, and in medical applications. In our future work we will focus on further improving the sensitivity of the sensor system. We aim to reduce its MDL to a level that makes it suitable for use in additional circumstances (e.g., the monitoring of atmospheric concentration of H₂O₂).