Photoacoustic spectroscopy approach enables real-time detection of low gas concentrations

Researchers have developed a new method to quickly detect and identify gases at very low concentrations. This new approach, called coherently controlled quartz-enhanced photoacoustic spectroscopy, could form the basis of highly sensitive real-time sensors for applications such as environmental monitoring, breath analysis, and chemical process control.

“Most gases are present in small amounts, so detecting low concentrations of gases is important in a variety of industries and applications,” said research team leader Simon Angstenberger from the University of Stuttgart in Germany. said. “Unlike other trace gas detection methods that rely on photoacoustics, our method is not limited to specific gases and does not require prior knowledge of gases that may be present.”

In Optica, researchers report that they obtained a complete methane spectrum spanning 3,050 to 3,450 nanometers in just 3 seconds. This typically takes about 30 minutes to accomplish.

“This new technology could be used for climate monitoring by detecting greenhouse gases such as methane, which are potent contributors to climate change,” Angstenberger said. “It also has applications in the early detection of cancer through breath analysis, and in chemical production plants for leak detection and process control of toxic or flammable gases.”

Adding consistent controls

Spectroscopy identifies chemicals, including gases, by analyzing the chemicals’ unique light absorption properties, which are similar to the “fingerprint” of each gas. However, rapid detection of low gas concentrations requires not only a rapidly tunable laser but also a very sensitive detection mechanism and precise electronic control of the laser timing.

In the new study, the researchers used a very fast tunable laser recently developed by collaborators at the university-independent company Stuttgart Instruments. We also utilized quartz-enhanced photoacoustic spectroscopy (QEPAS) as a highly sensitive detection mechanism.

This spectroscopy uses a quartz tuning fork to detect gas absorption by electronically measuring vibrations at a resonant frequency of 12,420 Hz caused by a laser modulated at the same frequency. The laser heats the gas between the fork lobes in rapid pulses, moving the fork lobes and creating a detectable piezoelectric voltage.

“The tuning fork’s high quality factor allows it to be struck for long periods of time, allowing us to detect low concentrations through what scientists call resonance enhancement, but limiting the speed of acquisition,” Angstenberger said. explained. “Because even if you change the wavelength to get a fingerprint of the molecule, the fork is still moving. To measure the next feature, it has to stop moving somehow.”

To overcome this problem, researchers developed a trick called coherent control. This involved shifting the timing of the pulse by exactly half the fork’s oscillation cycle while keeping the laser output power at the same frequency.

This allows the laser pulse to reach the gas between the forks as the fork protrusions move inward. As the gas heats up and expands, it resists the movement of the prongs, so this trick reduces fork vibration. After several flashes of the laser light (more than a few hundred microseconds), the fork stops vibrating and the next measurement can be performed.

rapid gas identification

“Adding coherent control to QEPAS allows us to use vibrational and rotational fingerprints to identify gases very quickly,” Angstenberger said. “Unlike traditional setups that are limited to specific gases or single absorption peaks, real-time monitoring can be achieved over a wide laser tuning range from 1.3 to 18 µm, allowing detection of virtually any trace gas.”

Researchers tested a new method using a laser developed by Stuttgart Instruments and a commercially available QEPAS gas cell to analyze a pre-calibrated methane mixture containing 100 ppm methane in a gas cell. They showed that with regular QEPAS, the spectral fingerprint becomes blurred when the scan is too fast, but with the coherent control method, the spectral fingerprint remains sharp and unchanged.

As a next step, the researchers plan to investigate the limits of the new technology and determine its maximum speed and minimum sensing concentration. I would also like to use this to sense multiple gases, ideally at the same time.