Breakthrough in Gas Sensing: Real-Time Detection of Trace Concentrations Unveiled!

“Most gases exist in minimal quantities, so identifying gases at low levels is crucial across numerous industries and applications,” stated research team leader Simon Angstenberger from the University of Stuttgart in Germany. “Unlike other trace gas detection techniques that depend on photoacoustics, our method is not constrained to certain gases and does not necessitate prior knowledge of the gas that may be present.”

In Optica, the journal published by Optica Publishing Group focused on high-impact research, the researchers report the acquisition of a complete methane spectrum ranging from 3050 to 3450 nanometers in just three seconds, a remarkable achievement that would generally take approximately 30 minutes.

“This advanced technology could be applied for climate surveillance by detecting greenhouse gases such as methane, which is a significant factor contributing to climate change,” noted Angstenberger. “It also has potential utility in early cancer diagnosis via breath analysis and in chemical manufacturing facilities for identifying toxic or flammable gas leaks and for process management.”

Incorporating coherent control

Spectroscopy identifies substances, including gases, by examining their distinct light absorption properties, similar to a “fingerprint” for each gas. To rapidly detect low gas levels, however, entails not only a laser that can be adjusted quickly but also an exceedingly sensitive detection system and precise electronic regulation of the laser timing.

In this recent study, the researchers employed a laser with an exceptionally fast tunable wavelength that was developed by collaborators at Stuttgart Instruments GmbH, a spin-off from the university. They also utilized quartz-enhanced photoacoustic spectroscopy (QEPAS) as the sensitive detection mechanism. This spectroscopy method employs a quartz tuning fork to detect gas absorption by electronically measuring its vibrations at a resonant frequency of 12,420 Hz, induced by a laser modulated at the same frequency. The laser heats the gas between the fork’s prongs in rapid pulses, causing them to oscillate and generating a measurable piezoelectric voltage.

“While the high quality factor of the tuning fork, which enables it to resonate for an extended period, allows us to detect low concentrations through what scientists refer to as resonant enhancement, it also restricts acquisition speed,” clarified Angstenberger. “This is due to the fact that when we adjust wavelengths to obtain the molecule’s fingerprint, the fork is still oscillating. To capture the next feature, we must find a way to halt the movement.”

To address this challenge, the researchers developed a technique known as coherent control. This involved adjusting the timing of the pulses by exactly half an oscillation cycle of the fork while the laser output power remained consistent. This ensures that the laser pulse arrives at the gas between the fork when its prongs are moving inward. This technique dampens the fork’s oscillation since the gas heats up and expands, counteracting the prongs’ movement. After several bursts of laser light — within a few hundred microseconds — the fork ceases to vibrate, allowing the next measurement to be taken.

Rapid gas identification

“Incorporating coherent control into QEPAS facilitates ultra-fast identification of gases utilizing their vibrational and rotational fingerprints,” stated Angstenberger. “Unlike conventional setups limited to specific gases or individual absorption peaks, we can achieve real-time monitoring with a broad laser tuning range of 1.3 to 18 µm, making it capable of detecting virtually any trace gas.”

The researchers evaluated the new technique using the laser developed by Stuttgart Instruments and a commercially available QEPAS gas cell to analyze a pre-calibrated methane mixture with 100 parts per million of methane in the gas cell. They demonstrated that with standard QEPAS, scanning too quickly blurs the spectral fingerprint, whereas with the coherent control method, it remains clear and unchanged.

As a subsequent step, the researchers intend to investigate the constraints of the new technology to ascertain its maximum speed and lowest detection concentration. They also aspire to use it for sensing multiple gases, ideally simultaneously.