Sensor technology: New method for real-time monitoring of gases

Most gases, such as methane, which is harmful to the environment, only occur in very small quantities. However, being able to detect and control gases even in low concentrations I s important for many industries and applications. Simon Angstenberger's research team at the University of Stuttgart's 4^th Physics Institute has developed and successfully tested a new measurement method. It enables the monitoring of a large number of gases in real time. The researchers explain how this works in the specialist journal “Optica” (DOI: 10.1364/OPTICA.544448).  

In contrast to conventional methods that detect trace gases, the new technology is not limited to specific gases and does not require any prior knowledge of a gas that may be present. It is intended to lay the foundation for the use of highly sensitive real-time sensors in various areas. Professor Harald Giessen, Head of the 4th Physics Institute, explains: "This technology could, for instance, be used to monitor emissions and detect greenhouse gases like methane, which are major contributors to climate change." “It could also potentially be used for the early detection of cancer through breath analysis. And it could be used in chemical production plants, for example to detect toxic or flammable gas leaks or for process control.”

Gases are detected using spectroscopy. This method measures the light that a gas “captures”.  Which wavelength of light is absorbed to what extent is unique for each molecule. This means that every gas leaves a “fingerprint”.  To detect low gas concentrations quickly, a laser with rapidly tunable wavelength is required. Such a laser works like a guitar string, which can be quickly tuned by shortening or lengthening it. However, this also requires an extremely sensitive detection mechanism and precise electronic control over the laser timing—ensuring that each laser pulse is deployed at exactly the right moment for measurements.

The researchers used a laser with an extremely fast tunable wavelength developed by Stuttgart Instruments GmbH, a spin-off of the University of Stuttgart. They combined it with the so-called QEPAS method. QEPAS stands for “quartz-enhanced photoacoustic spectroscopy” - a highly sensitive method used to detect the smallest amounts of gas. A quartz tuning fork is stimulated using rapid laser pulses.  The laser periodically heats the gas between the prongs of the tuning fork. The heat produced as the gas absorbs the light generates sound waves precisely between the tuning fork's prongs, causing them to move outward. This process is repeated and the tuning fork begins to vibrate. The laser and tuning fork operate at the same frequency. This increases precision, but limits the recording speed. This is because: “In order to capture the entire spectrum, we have to measure many different wavelengths one after the other,” explains Angstenberger. Because the fork vibrates, the residual signal from the previous measurement is detected in each new measurement. That means we have to stop the movement somehow.”

To solve this problem, the researchers developed a new method based on known measurement principles in atomic physics: coherent control. In doing so, they shift the timing of the laser pulses by exactly half an oscillation cycle. The frequency of the laser pulse remains unchanged. As a result, the laser pulse reaches the gas between the fork when the tines move inwards. This dampens the vibration of the fork. “When the gas gets hot and expands, it counteracts the movement of the tines. The fork stops vibrating and the next measurement can be carried out,” says Angstenberger.

“The addition of coherent control to QEPAS enables the ultra-fast identification of gases based on their specific fingerprints,” says the physicist. This enables the real-time detection of virtually any trace gas. Using a technique known as coherently controlled quartz-enhanced photoacoustic spectroscopy, researchers have successfully captured a complete methane spectrum from 3050 to 3450 nanometers in just three seconds—a process that typically takes around 30 minutes. The measured concentration was 100 methane molecules to one million air molecules. This means that methane can be detected with the new method even if it only escapes in diluted form.

Tests also revealed that with the conventional QEPAS method, tuning too quickly causes the spectral fingerprint of gas molecules to become blurred. In contrast, the coherent control method preserves a clear and unchanged spectral signature. In the next step, Angstenberger and his team want to explore the limits of the new technology. The aim is to determine the maximum speed and the lowest measurable gas concentration and to test several gases simultaneously. “We have laid the foundations for a technology that we now want to develop further for the market,” says Angstenberger.

Publication
S. Angstenberger, M. Floess, L. Schmid, P. Ruchka, T. Steinle, H. Giessen, "Coherent Control in Quartz-Enhanced Photoacoustics: Fingerprinting a Trace Gas at ppm-Level within Seconds", Optica 12, 1-4 (2024). DOI: 10.1364/OPTICA.544448.