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The Magic of Photoacoustic Spectroscopy: How Alexander Graham Bell Used Sunlight to Uncover the Secrets of Sound
In 1880, Alexander Graham Bell conducted a groundbreaking experiment in scientific history, discovering that when a beam of sunlight was rapidly interrupted by a rotating slotted disk, the thin disk produced a sound. This experiment revealed an incredible connection between light and sound, which over time evolved into today's photoacoustic spectroscopy technology. The core of this technology is to measure the effect of absorbed electromagnetic energy (especially light) on matter, and this is achieved through sound detection.
The basic principle of the photoacoustic effect is that when light is absorbed by a substance, local heating causes thermal expansion, which in turn generates pressure waves or sound.
Bell's discoveries were not limited to visible light; he also found that sound could be produced when materials were exposed to the non-visible parts of the solar spectrum, such as infrared and ultraviolet light. By measuring the sound under different wavelengths of light, the photoacoustic spectrum of the sample can be recorded, which is crucial for identifying the absorbing components of the sample. The technique can be used to study solids, liquids and gases.
Application and Technology
Modern photoacoustic spectroscopy has become an important means of studying gas concentrations and is capable of detecting trace amounts of gas down to the part-per-billion or even part-per-hundred-billion level. Although modern photoacoustic detectors still rely on Bell's basic principle, several improvements have been made to increase sensitivity. Instead of using sunlight, powerful lasers are now commonly used to illuminate the sample because the intensity of the sound produced is proportional to the intensity of the light. This technique is called laser photoacoustic spectroscopy (LPAS).
The role of the ear is replaced by a highly sensitive microphone, which is further amplified and detected by a lock-in amplifier to increase sensitivity.
In addition, the sound signal can be further amplified by enclosing the gas sample in a cylindrical cavity and adjusting the modulation frequency to the acoustic resonance of the sample cavity. The use of cantilever-enhanced photoacoustic spectroscopy technology can further improve the sensitivity and achieve reliable monitoring of gases.
Example
An example demonstrating the potential of photoacoustic technology occurred in the 1970s, when researchers used a balloon-borne photoacoustic detector to measure the temporal changes in nitric oxide concentrations at an altitude of 28 kilometers. These measurements provide key data for understanding the problem of ozone depletion caused by human-caused nitrous oxide emissions. This early work relied on the development of the RG theory by Rosencwaig and Gersho.
Applications
One of the main capabilities of using FT-IR photoacoustic spectroscopy is the ability to evaluate samples in situ, which can be used to detect and quantify chemical functional groups and chemicals, particularly for biological samples, without the need for pulverization or chemical analysis. deal with. Samples of shells, bones, etc. have been studied. The application of photoacoustic spectroscopy has also helped to evaluate molecular interactions in bone that are relevant to OI.
While most academic research has focused on high-resolution instrumentation, very low-cost instrumentation has been developed and commercialized over the past two decades for applications such as gas leak detection and CO2 concentration control. Typically uses low-cost heat sources and is operated by electronic modulation. The use of semipermeable membranes rather than valves for gas exchange, low-cost microphones, and proprietary signal processing using digital signal processors have significantly reduced the cost of these systems.
The future of low-cost photoacoustic spectroscopy may be achieved with fully integrated micromechanical photoacoustic instruments. Photoacoustic methods have also been used to quantitatively measure macromolecules such as proteins by using nanoparticles that emit strong acoustic signals to label and detect target proteins. Photoacoustic-based protein analysis is also applied in point-of-care testing.
Additionally, photoacoustic spectroscopy has many military applications, such as detecting toxic chemical agents. The sensitivity of photoacoustic spectroscopy makes it an ideal analytical technique for detecting trace amounts of chemicals associated with chemical attacks. LPAS sensors can be widely used in industry, security (nerve agent and explosive detection) and medicine (breath analysis).
Photoacoustic spectroscopy has continued to evolve since Bell, combining optics and acoustics to open new doors for scientific exploration. As technology continues to advance, how will scientists use this technology to explore unknown areas?
