Researchers have developed a new way to rapidly detect and identify very low concentrations of gases. The new method, called coherent-controlled quartz-enhanced photoacoustic spectroscopy, could be the basis for highly sensitive real-time sensors for applications such as environmental monitoring, breath analysis, and chemical process control.
“Most gases are present in trace amounts, so detecting low concentrations of gases is important in a variety of industries and applications,” said lead researcher Simon Angstenberger of the University of Stuttgart in Germany. “Unlike other trace-gas detection methods that rely on photoacoustics, our method is not limited to a specific gas, nor does it require prior knowledge of which gases may be present.”
Writing in Optica , a high-impact research journal from Optica Publishing Group , the researchers report that they obtained the entire methane spectrum, spanning from 3050 nanometers to 3450 nanometers, in just three seconds, a feat that typically takes about 30 minutes.
“This new technology could potentially be used to monitor the climate by detecting greenhouse gases such as methane, which are a major contributor to climate change,” Anstenberger said. “It could also have applications in early cancer detection through breath analysis, and in detecting toxic or flammable gas leaks and process control in chemical manufacturing plants.”
Added coherent control
Spectroscopy identifies chemicals, including gases, by analyzing the unique light absorption characteristics of each gas, which resemble a “fingerprint.” However, rapid detection of low gas concentrations requires not only rapidly tunable lasers, but also extremely sensitive detection mechanisms and precise electronic control of the laser timing.
In the new study, the researchers used an ultrafast tunable laser recently developed by their collaborators at Stuttgart Instruments, a university spin-out company. They 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 its resonant frequency of 12,420 Hz produced by a laser modulated at the same frequency. The laser heats the gas between the prongs of the tuning fork with rapid pulses, moving the fork and generating a detectable piezoelectric voltage.
“The high quality factor of the tuning fork causes it to ring for a long time, enabling the detection of low concentrations through a phenomenon scientists call resonance enhancement, but it limits the collection speed,” Angstenberger explains .“This is because the tuning fork is still moving as we change the wavelength to obtain a molecular fingerprint. We need to stop it moving somehow to measure the next feature.”
To overcome this problem, the researchers developed a technique called coherent control. In this trick, the laser output stays at the same frequency, but shifts the timing of the pulses by exactly half the period of the fork’s vibration. This ensures that the laser pulses reach the gas between the forks as the spokes move inwards. This trick reduces the fork’s vibration because as the gas heats up and expands, it inhibits the movement of the spokes. After a few flashes of the laser (a few hundred microseconds), the fork stops vibrating, allowing the next measurement to be made.
Rapid Gas Identification
“The addition of coherent control to QEPAS enables ultrafast gas identification through vibrational and rotational fingerprints,” said Angstenberger. “Unlike traditional setups that are limited to specific gases or single absorption peaks, we can achieve real-time monitoring over a wide laser tuning range from 1.3 to 18 µm, allowing the device to detect virtually any trace gas.”
The researchers tested their new method using a laser developed by Stuttgart Instruments and a commercially available QEPAS gas chamber to analyze a methane mixture that was pre-calibrated with 100 ppm methane in the gas chamber. They showed that with conventional QEPAS, the spectral fingerprint becomes blurred when scanned too quickly, but with the coherent control method, the fingerprint remains sharp and unchanged.
Next, the researchers plan to explore the limits of their new technology to determine its maximum speed and minimum sensor concentration. They would also ideally like to use it to sense multiple gases simultaneously.
