Environmental pollutants detected at parts-per-quadrillion. It is just one of the uses of a new photoacoustic measurement technique that researchers have refined to measure sound generated in the hundreds of kilohertz range, a level far more sensitive than previously detectable.

Published by the Proceedings of the National Academy of Sciences, the study conducted by chemistry professor Gerald Diebold of Brown University, in cooperation with the lab of Fapeng Yu at China’s Shandong University, demonstrates the “most practical method” for detecting pollutants in the atmosphere. It is also the most precise method available.

The process of measurement was first discovered in the 1880s by Alexander Graham Bell. A beam of light was set to interact with specific molecules. The molecule – a liquid, solid, or gas -would expand with the absorption of light, thus creating movement measurable in the form of a sound wave. The sound wave could be calculated precisely at the point of oscillatory motion using devices set to record the acoustic signal.

Previously, photoacoustic signals had to be fairly significant in order to be detected, which put a limit on attainable data. As Gerald Diebold explained,

“…But when the concentration of the molecules you’re trying to detect gets down to the parts-per-trillion level, the signal become too weak to detect. We’ve developed a new photoacoustic technique that boosts the signal and enables us to get down to the parts-per-quadrillion level, which to our knowledge is a record.”

The scientific breakthrough, a device combining two laser beams, is able to measure three resonances, allowing the signal to increase with each resonance. The occurrence can be compared to an object in a swinging motion; if the object receives a repeated, gentle push it will continue to amplify in movement. In similar concept, the gyrations, or patterns of interference occurring between the laser beams can be measured at a very precise frequency.

A large piezoelectric crystal capable of measuring this delicate frequency is at the core of the photoacoustic science. The crystal vibrates with the compressed forces in the sound waves. The resonance in the crystal is sent to amplifiers and sensitive recording devices, allowing the acoustic signal to be identified. The technique can identify low pollutant gas concentrations and molecules with weak absorptions. Before now, these types of measurements were not achievable.

A notable convenience to working with frequencies at this extreme high level is the reduction in background interference. Electrical and wind sources are virtually eliminated, so blocking external noises is less of a concern. The potential of using this science to detect methane or other outdoor gases at a continuous measurement, without disruption, is a foreseen benefit to photoacoustic science.

The concepts and techniques of this photoacoustic detection continue to undergo research.


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Source: Science Daily
Photo: Gerald Diebold




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