Language
Country/Area
No result found
The wonderful combination of laser and sound waves: How to improve the sensitivity of photoacoustic spectrum?
Photoacoustic spectroscopy is a measurement method that detects the effect of absorbed electromagnetic energy (especially light) on matter through sound waves. In 1870, Alexander Graham Bell first discovered the photoacoustic effect and demonstrated that thin sheets emit sounds when exposed to rapidly interrupted bursts of sunlight. This absorbed light energy locally heats the material, causing thermal expansion, which in turn causes pressure waves or sound. Bell later showed that invisible parts of the solar spectrum (such as infrared and ultraviolet) can also emit sound. Photoacoustic spectroscopy records the photoacoustic spectrum of a sample by measuring the sound produced by light of different wavelengths. This spectrum is used to identify components absorbed in the sample.
The photoacoustic effect can be used to study solids, liquids and gases.
Uses and Technology
Modern photoacoustic spectroscopy is a powerful technique that can study the concentration of gases at the level of parts per billion (ppb) or even parts per trillion (ppt). Although modern photoacoustic detectors still rely on Bell's principle, some improvements have been made to increase sensitivity. Unlike sunlight, powerful lasers are now used to illuminate samples. Because the intensity of the sound generated is proportional to the intensity of the light, this technique is called laser photoacoustic spectroscopy (LPAS). The traditional ear has been replaced by a sensitive microphone whose signal is enhanced and detected using a lock-in amplifier. The gas sample is enclosed in a cylindrical cavity, and the sound signal is further amplified by adjusting the modulation frequency to the acoustic resonance of the sample cavity. By using cantilever-enhanced photoacoustic spectroscopy, sensitivity can be further improved, enabling reliable monitoring of gases.
The potential of using photoacoustic spectroscopy lies in its ability to perform in situ evaluations without damaging the sample.
Example analysis
In the early 1970s, Bartel and his collaborators used static photoacoustic detectors to measure changes in the concentration of nitric oxide in the stratosphere at an altitude of 28 kilometers. These measurements provide important data on ozone depletion due to anthropogenic nitric oxide emissions. In some early studies, it was based on the development of Rosenkweig and Gilesho theory (RG theory).
Application Scope
An important capability of FTIR photoacoustic spectroscopy is the ability to evaluate samples in their in situ state, which can be used to detect and quantify chemical functional groups and thereby identify chemical species. This is particularly useful for biological samples, which can be evaluated without the need to break into powder or undergo chemical processing. Samples such as shells and bones have been studied. The application of photoacoustic spectroscopy has helped evaluate intraosseous molecular interactions associated with osteogenesis imperfecta.
While most academic research over the past two decades has focused on high-resolution instrumentation, there has also been development in the opposite direction, and very low-cost instruments have quietly entered the market.
In recent years, many low-cost heat sources have been electronically modulated, gas exchange through semipermeable membranes, low-cost microphones, and proprietary digital signal processing technology have significantly reduced the cost of such systems. Future applications of low-cost photoacoustic spectroscopy may enable fully integrated micromachined photoacoustic instruments.
Photoacoustic methods have been used to quantitatively measure large molecules such as proteins. Photoacoustic immunoassays label and detect target proteins by using nanoparticles that generate strong acoustic signals. Protein analysis based on photoacoustic technology is also used in point-of-care testing.
Photoacoustic spectroscopy also has many military applications, one of which is the detection of toxic chemical agents. The sensitivity of photoacoustic spectroscopy makes it an ideal analytical technique for detecting trace amounts of chemicals associated with chemical attack. LPAS sensors can be used in industrial, security (nerve agent and explosives detection) and medical (breath analysis) and other fields.
With the development of technology, the sensitivity and accuracy of photoacoustic spectroscopy continue to improve. Will we be able to discover the potential impact of human activities on environmental health in the future?
