Common methods for particle size analysis rely on the scattering of light after a light beam hits the particles in a solid state or in a solution, but acoustic spectroscopy uses another important principle to determine particle size distribution. When ultrasonic pulses are sent through a solution or emulsion of particles, the sound waves are attenuated as a direct function of particle volume or weight. This attenuation can be measured by detectors and the obtained signal can be used to calculate particle size without any additional approximations.
This technology adds some new features to particle size analysis. Unlike light scattering techniques, it can be used to measure particle size even in a mixture of different particles, given that they differ in density. This way, smaller particles will not be clouded by the presence of larger particles. Concentration of particles in the fluid has no effect on the measurement, so this method is appropriate in systems where sample dilution could change the result. Additionally, in acoustic spectroscopy we can exclude structure contributions to the size, since structural contributions to the signal attenuation can be calculated from theory and subtracted.
In modern acoustic spectrometers measurements can be made over a wide span of frequencies and they can give results of particle sizes in a range from 10 nm to over 10 µm. Another advantage of acoustic spectroscopy is that it can measure particle size distribution in any fluid system. This method requires no initial sample preparation or sample dilution and it can be used to monitor particle size over a certain time span, which is very useful for some industrial applications, for example during milling and pharmaceutical processes.
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