Optical Methods of Methane Detection

Multimode interference methane sensor based on a ZIF-8/PDMS composite film

A multimode interference methane sensor based on a ZIF-8/PDMS composite film is proposed. The sensing principle is that the refractive index of the ZIF-8/PDMS composite film changes when it adsorbs methane, leading to a measurable optical path difference during the coupling of the cladding higher-order modes and the fundamental mode in the multimode interference fiber (MMI). The environmental methane concentration is then detectable by detecting the wavelength shifts of the interference peaks in the resulted spectrum. Through simulations and experiments aimed at enhancing sensor sensitivity, we optimized three parameters within the sensor structure: the length of the Tapered Single-Mode Fiber (TSMF), the composite film thickness, and the TSMF taper diameter. The experimental results indicate that the sensor's sensitivity reaches a maximum of 0.231 nm%-1. Additionally, the sensor exhibits excellent structural stability and measurement repeatability. The response time is as short as 40 s, and the recovery time ranges between 3 and 5 min. The proposed multimode interferometric methane sensor based on the ZIF-8/PDMS composite film has great potential to support highly sensitive methane concentration detection in many applications.

A cavity ringdown spectrometer for methane isotope analysis using a 1.65 µm distributed feedback diode laser with fiber optical feedback loop

The development of a 1.65 µm cavity ringdown methane spectrometer for methane isotope analysis is reported. In order to reduce the laser linewidth, simple optical feedback with an 11 m external fiber cavity using a retroreflector was implemented and it improved the sensitivity. The detection limit at the ppt level for both 12CH4 and 13CH4 concentrations at 100 Torr gas pressure was evaluated from the Allan-Werle plot calculated from the dataset obtained at the fixed laser frequency. In contrast, the detection limit estimated from the baseline noise on the absorption spectrum was a few ppb for both methane isotopologues due to the periodic background oscillations that remained even after baseline correction. The system demonstrated the direct measurement of ambient methane in atmospheric room air, and the estimated 13CH4 ratio as well as the methane concentration were in good agreement with the reference values of ambient air.

Real-Time Measurement of CH4 in Human Breath Using a Compact CH4/CO2 Sensor

The presence of an elevated amount of methane (CH4) in exhaled breath can be used as a non-invasive tool to monitor certain health conditions. A compact, inexpensive and transportable CH4 sensor is thus very interesting for this purpose. In addition, if the sensor is also able to simultaneously measure carbon dioxide (CO2), one can extract the end-tidal concentration of exhaled CH4. Here, we report on such a sensor based on a commercial detection module using tunable diode laser absorption spectroscopy. It was found that the measured CH4/CO2 values exhibit a strong interference with water vapor. Therefore, correction functions were experimentally identified and validated for both CO2 and CH4. A custom-built breath sampler was developed and tested with the sensor for real-time measurements of CH4 and CO2 in exhaled breath. As a result, the breath sensor demonstrated the capability of accurately measuring the exhaled CH4 and CO2 profiles in real-time. We obtained minimum detection limits of ~80 ppbv for CH4 and ~700 ppmv for CO2 in 1.5 s measurement time.

Highly Sensitive Capacitive MEMS for Photoacoustic Gas Trace Detection

An enhanced MEMS capacitive sensor is developed for photoacoustic gas detection. This work attempts to address the lack of the literature regarding integrated and compact silicon-based photoacoustic gas sensors. The proposed mechanical resonator combines the advantages of silicon technology used in MEMS microphones and the high-quality factor, characteristic of quartz tuning fork (QTF). The suggested design focuses on a functional partitioning of the structure to simultaneously enhance the collection of the photoacoustic energy, overcome viscous damping, and provide high nominal capacitance. The sensor is modeled and fabricated using silicon-on-insulator (SOI) wafers. First, an electrical characterization is performed to evaluate the resonator frequency response and nominal capacitance. Then, under photoacoustic excitation and without using an acoustic cavity, the viability and the linearity of the sensor are demonstrated by performing measurements on calibrated concentrations of methane in dry nitrogen. In the first harmonic detection, the limit of detection (LOD) is 104 ppmv (for 1 s integration time), leading to a normalized noise equivalent absorption coefficient (NNEA) of 8.6 ⋅ 10^-8 Wcm^-1 Hz^-1/2, which is better than that of bare Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS), a state-of-the-art reference to compact and selective gas sensors.