Researchers have introduced a new technique for the rapid detection and identification of extremely low levels of gases, which could pave the way for advanced real-time sensors. These sensors could be utilized in various fields, such as environmental monitoring, breath analysis, and the regulation of chemical processes.
A novel approach has been developed by researchers to swiftly detect and identify very low amounts of gases. This innovative method, known as coherently controlled quartz-enhanced photoacoustic spectroscopy, holds promise for highly sensitive real-time sensors that can be applied in areas like environmental monitoring, breath analysis, and chemical process oversight.
According to Simon Angstenberger, the lead researcher from the University of Stuttgart in Germany, “Many gases are found in trace amounts, making it crucial to detect low concentrations in various industries and applications. Our method differs from other trace gas detection techniques that utilize photoacoustics; it isn’t restricted to particular gases and doesn’t need prior knowledge of which gas might be present.”
The researchers have reported in Optica, a journal published by Optica Publishing Group focusing on significant research, that they successfully captured a full methane spectrum from 3050 to 3450 nanometers in just three seconds. Typically, this would take about 30 minutes.
Angstenberger noted, “This innovative technology can aid in climate monitoring by detecting greenhouse gases like methane, which significantly contributes to climate change. Moreover, it may have potential for early cancer diagnosis via breath analysis and in chemical manufacturing plants for identifying leaks of toxic or flammable gases, as well as for process management.”
Introduction of coherent control
Spectroscopy helps identify various chemicals, including gases, by examining their individual light absorption properties, similar to a “fingerprint” for each gas. However, swiftly detecting low gas concentrations necessitates not only a rapidly tunable laser but also a highly sensitive detection system and accurate electronic timing control of the laser.
For their study, the researchers utilized a laser with rapid tuning capabilities developed by Stuttgart Instruments GmbH, a university spinoff. They employed quartz-enhanced photoacoustic spectroscopy (QEPAS) as the sensitive detection system. This technique uses a quartz tuning fork to detect gas absorption by measuring its vibrations at a resonant frequency of 12,420 Hz, which is stimulated by a laser modulated to the same frequency. The laser emits quick pulses of heat to the gas between the fork’s prongs, causing movement and generating a measurable piezoelectric voltage.
Angstenberger explained, “The tuning fork’s high quality factor allows it to resonate for an extended period, enabling detection of low concentrations through a process known as resonant enhancement. However, this quality limits the speed of data acquisition. When we switch wavelengths to capture the molecular fingerprint, the fork continues to vibrate. Hence, we need to halt its movement before recording the next feature.”
To tackle this challenge, the researchers devised a method called coherent control. This technique involves adjusting the timing of the laser pulses by exactly half of the fork’s oscillation cycle while maintaining the same frequency of the laser output. This adjustment allows the laser pulse to reach the gas when the fork’s prongs are contracting. This method helps to reduce the fork’s oscillation because the gas’s heat and expansion counteract the prongs’ motion. After a few laser flashes over a brief period, the fork ceases to vibrate, enabling the next measurement.
Rapid gas identification
Angstenberger stated, “Integrating coherent control with QEPAS allows for extremely quick identification of gases using their vibrational and rotational signatures. Unlike traditional setups that are limited to certain gases or singular absorption peaks, our method enables real-time monitoring with a wide tuning range of 1.3 to 18 µm, which means it can detect almost any trace gas.”
The researchers evaluated their new method using the laser from Stuttgart Instruments alongside a commercially available QEPAS gas cell to examine a pre-calibrated methane sample containing 100 parts per million of methane. They discovered that while regular QEPAS scanning too quickly would distort the spectral fingerprint, the coherent control method maintained clarity and accuracy.
As their next step, the researchers intend to investigate the limitations of this new technology to ascertain its maximum speed and the lowest concentration it can detect. They are also keen to explore its capability to sense multiple gases, ideally simultaneously.
