Laser diffraction particle size analysis
The laser diffraction particle size analysis or laser granulometric measurement refers to the measurement of the distribution of the size of solid or liquid particles in a liquid or gaseous medium with the help of deflection ( diffraction ) of the light waves of a laser beam . Devices called laser diffraction measurement systems, laser diffraction sensors or laser diffraction particle size analyzers are used for this purpose.
construction
The particle flow consists of the particles to be measured in a liquid or a gas. The particles are transported by the medium across the laser light. It doesn't matter how quickly that happens. At this point in the device, the laser light is a plane wave , it consists of similar light waves that go in the same direction. It is formed by a laser beam made wider. This is done with lenses, sometimes also with fiber optic cables . Under certain circumstances, light sources of the same or different wavelengths are used, which also increases the measuring range. The radiation is reflected, diffracted or passes through the particles unhindered. Behind the particle flow there is another lens (in the picture: 5) or actually a lens system. It is also summarized as a Fourier lens due to its property in mathematical models. The light that reaches this lens is measured and electronically evaluated at the optoelectronic sensor , which is mounted in its focal length (in the picture: 6). The parallel running laser light is simultaneously focused again on a point and does not interfere with the rest of the measurement. The resulting image on the touch screen is round and point-symmetrical . Accordingly, the sensor is also round, and it is sufficient if it is semicircular.
functionality
The laser diffraction system is basically based on Fraunhofer diffraction ; on closer inspection, the Mie theory is also used. According to the former, the diffraction pattern of a spherical particle is related to the particle size. Specifically, the density of the diffraction rings increases the larger the particle is. If the particles are not spherical, patterns arise that are no longer symmetrical like rings, but are nevertheless point-symmetrical. The position of rotation depends on the position of the particle. If, however, many particles are used and the sum of the different light intensities of the respective semicircular rings is formed on the sensor, the mean size (a sphere of equal volume) can be deduced regardless of the particle position.
The Fraunhofer diffraction will range micrometer use in particle sizing down to the. It describes the part of the light deflection that comes about exclusively through diffraction. If light falls on an obstacle, e.g. B. a particle, it comes among other things to diffraction. For sufficiently large particles, the deflection of light is dominated by diffraction. A great advantage of the Fraunhofer theory is that no knowledge of the optical properties of the material being examined is required. The Mie theory is used to determine the size of particles whose diameter is not significantly greater than the wavelength of the light used. It is based on the measurement of the scattering of electromagnetic waves on spherical particles. With the Mie theory, the refractive index and absorption index of the sample material must be known. The lower limit of the size range that can be measured using the Mie theory is around 10 nm.
application
Thanks to the plane wave, the system can bridge greater distances for the X-raying of samples, which is particularly important with gaseous media, e.g. B. Sprays, is practical. The devices can be adjusted, set up and automated electronically in a wide range.
For powder samples with larger particles (significantly larger than 1 µm), the evaluation can be carried out according to Fraunhofer theory. The light diffracted by large particles is detected by the ring detector, which is located on the optical axis light source - cuvette - ring detector. In this case it is not necessary to enter or submit the optical parameters of the material to be measured. This is why the Fraunhofer evaluation is used for powder mixtures of which the optical properties of refractive index and absorption are not known at the laser light wavelengths used.
The Mie theory can be used for calculations for finer particles. To do this, you have to enter the refractive index and the absorption of the material at the light wavelength used. By using several light wavelengths, more precise optical models can be created, which enable an improved reproduction of the particle size distributions in the range below a particle size of 1 µm.
Web links
Individual evidence
- ↑ AH de Boer, D. Gjaltema, P. Hagedoorn, HW Frijlink: Characterization of inhalation aerosols: a critical evaluation of cascade impactor analysis and laser diffraction technique. In: International Journal of Pharmaceutics. 249, No. 1–2, 2002, pp. 219–231, doi: 10.1016 / S0378-5173 (02) 00526-4 : " Size classes are independent of the flow rate."
- ↑ Wolfgang Witt, Thomas Stübinger: Particle size analysis with absolute accuracy . 2011, section Laser diffraction with high accuracy , p. 14.
- ^ Mie scatter