Fabry–Perot interferometers (FPIs), comprising foundry-compatible dielectric thin films on sapphire wafer substrates, were investigated for possible use in chemical sensing. Specifically, structures comprising two vertically stacked distributed Bragg reflectors (DBRs), with the lower DBR between a sapphire substrate and a silicon-oxide (SiO₂) resonator layer and the other DBR on top of this resonator layer, were investigated for operation in the near-ultraviolet (near-UV) range. The DBRs are composed of a stack of nitride-rich silicon-nitride (SiN_x) layers for the higher index and SiO₂ layers for the lower index. An exemplary application would be formaldehyde detection at sub-ppm concentrations in air, using UV absorption spectroscopy in the 300–360 nm band, while providing spectral selectivity against the main interfering gases, notably NO₂ and O₃. Although SiN_x thin films are conventionally used only for visible and near-infrared optical wavelengths (above 450 nm) because of high absorbance at lower wavelengths, this work shows that nitride-rich SiN_x is suitable for near-UV wavelengths. The interplay between spectral absorbance, transmittance and reflectance in a FPI is presented in a comparative study between one FPI design using stoichiometric material (Si₃N₄) and two designs based on N-rich compositions, SiN_1.39 and SiN_1.49. Spectral measurements confirm that if the design accounts for phase penetration depth, sufficient performance can be achieved with the SiN_1.49-based FPI design for gas absorption spectroscopy in near-UV, with peak transmission at 330 nm of 64%, a free spectral range (FSR) of 20 nm and a full-width half-magnitude spectral resolution (FWHM) of 2 nm.

Many environmental, industrial, and security applications demand in-field analysis of chemical vapors. Whereas microscale gas chromatographs (µGCs) are promising candidates, reliable in-field chemical analysis particularly demands repeatability, humidity tolerance, and in-field reference. Using a µGC with substantial monolithic integration (of preconcentrators, separation columns, and capacitive and photoionization detectors), this paper reports chip-level and system-level advancements towards reliable chemical analysis. Thermal management is advanced using tailored heater designs to compensate for boundary conditions and cooling. Fence electrodes are incorporated into on-chip photoionization detectors, reducing responses due to humidity by >98%. The repeatability of retention time is advanced by introducing closed-loop flow control, reducing the relative standard deviation of retention time to only 0.29–0.43%, which represents a 4–5× improvement over open-loop flow control. A miniature reservoir for a chemical reference standard is also incorporated on board, providing the ability to correct for drifts in retention time and the ability to directly measure retention times relative to the reference chemical. A set of blind false alarm tests was performed for fixed target analytes in the presence of partially coeluting interferent species. A separate set of blind chemical recognition tests was also performed for various analytes of concealed identities. Overall, the results were largely successful and showed the promise of the reported µGC instrument and modules for broad chemical screening and long-term in-field deployment.

This paper reports the first microscale gas chromatography (μGC) system in which all fluidic components are monolithically integrated into a single 15 × 15 mm ² chip, including three Knudsen pumps, a preconcentrator, a separation column, and a capacitive detector. Knudsen pumps utilize thermal transpiration along narrow channels to induce gas flow, providing a compact, motionless solution that enables monolithic integration with high reliability and a long operational lifetime. The flow switching within the system is provided by a unique arrangement of multiple Knudsen pumps that eliminates the need for valves. To realize monolithic integration, the preconcentrator, separation column, and detector are arranged in a planar layout and are designed to be microfabricated using a common fabrication process. Methods to provide heat dissipation and thermal isolation are incorporated. In the experimental evaluation, the fabricated μGC system was operated to analyze the headspace of chemical mixtures, including alkenes, glycol ethers, aromatics, and mercaptans. The results showed a quantification accuracy of ±8.5% for the individual species within the mixtures, which is suitable for specific process monitoring applications in the chemical industry where concentrations of select target species and byproducts must be assessed continuously. The monolithic integration of gas pumps into a μGC system has not been previously reported and is an important step toward further miniaturization of μGC systems.