Laser diffraction is a well-established method for the analysis and determination of particle size distributions. In this approach, a laser diffraction particle size analyzer measures a particle ensemble dispersed in a liquid or gaseous medium by deflecting the light waves of a laser beam and capturing the resulting diffraction pattern of the laser-scattering angles. In this article we clarify what laser diffraction is, how it operates, what it measures and the benefits and limitations of this method.
There are a number of measurement technologies for the determination of particle size distributions. Over the past few years laser diffraction has become established as one of the leading methods in laboratory environments for analyzing suspensions, emulsions, granulates and fine powders. It makes use of the principle that light, when hitting a particle, scatters more intensively and in smaller angles the bigger the particle is. Thus a laser diffraction analyzer does not measure the particle size but the angle and the intensity of the scattered light in the measured sample. The samples may be dispersed in different media. It is even possible to use laser diffraction for the analysis of particles in an air stream. The particle sizing range of the laser diffraction method is impressive, as it can measure particles between 10 nm and 3 mm, although ideal measuring results for particle size starts at around 100 nm. For smaller particle sizes the laser diffraction method has some problems because of the low intensity of the scattered light.
Basically you can divide the workflow of a laser diffraction analyzer into two steps: Firstly, it measures the angles and the intensity of the light scattered by the particles in the sample, afterwards the gathered data is converted into particle size distributions. In order to achieve this, a laser diffraction instrument consist of various components. The starting point of a laser diffraction analysis is typically formed by two light sources (although someties there is just one or even three). These light sources have different wavelengths. The laser beams are directed through a dispersed sample so that the particle stream is transported diagonally through the first laser beam. The velocity of this transport is not an issue. Each laser beam measures different sized particles. By using lenses or optical fibers the laser beams are additionally broadened to maximize the measurement area. When the beams collide with the particles they are reflected, scattered, refracted or absorbed. In the last step, detectors measure the scattered light on a wide angle range through a Fourier lens (a system of lenses) and determine the particle size distribution provided by the measurement data of the angles and the intensity of the scattered light.
Whenever laser light encounters a particle or another barrier diffraction phenomena occur. These phenomena can be monitored by laser diffraction, as light spreads in ring-shaped waves when it reaches a spherical particle. The diffraction angles are determined by the wavelength of the light and by the size of the particle: big particles show a small diffraction angle and a strong intensity, while small particles have a big diffraction angle and a weak intensity. When we analyze particles with laser diffraction, the individual scattering patterns overlap and are presented in one total pattern, so they need to be distinguished. To do this job, an algorithm compares the measured values with the expected theoretical values of different size categories to estimate the relation between the size categories in the entire sample volume. Two theories are used to analyze these diffraction phenomena: The Fraunhofer theory (also called Fraunhofer diffraction) describes the intensity distribution of the diffraction angles and is well-suited to measuring large or opaque particles. For the use of the Fraunhofer approximation no knowledge of the refractive index is needed, but transparent or very small (smaller than 50 µm) particles negatively influence the measurement results. Using the Mie theory for laser diffraction analysis instead, diffraction phenomena like the refractive index or the absorption index are also taken into account. Scattered or refracted light can be measured very well, while reflected or absorbed light might influence the measurement results negatively, so that their influence must be considered for the measurement and calculation. Thus Mie theory is well-suited for the analysis of small particles.
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The laser diffraction method certainly impresses with its huge dynamic particle size range. It is possible to analyze particles of just 10 nm in size and also particles up to 3 mm with the same particle measurement instrument. This gives laser diffraction analysis an unmatched versatility but also compromises its specification. For this reason it is mostly used in a working range between 100 nm and 1000 µm, which leads us to another benefit of the laser diffraction method: its versatility ensures that a laser diffraction analyzer can be used for daily and for challenging tasks in a variety of industries. The method is able to characterize liquid particle samples as well as fine powders, suspension or emulsions. Furthermore, the measurements can be done quickly – it only takes about two minutes. In addition measurements are repeatable as a relatively large number of particles are analyzed and the sample throughput for laser diffraction instruments is high.
As for most established methods for particle characterization, laser diffraction also has clear limits and many disadvantages. For example, laser diffraction is not designed for analyzing ultra-low or ultra-high concentrated samples because of the interferences caused by multiple scattering. Furthermore, laser diffraction is not able to determine the exact size of very fine particles. Another problem with LD is the low resolution when analyzing big particles. Also, laser diffraction is not capable of determining the shape of particles. When measuring sub-micrometer particles the refractive index is needed (this describes the optical characteristics of the particles). Particularly with polymineralic mixtures measuring the refractive index takes time, is very difficult to determine and highly cost-intensive. In addition, non-spherical particles cause more strongly scattered patterns and a laser diffraction analyzer cannot determine such particles accurately. With laser diffraction there is also the possibility to detect particles that are not actually there because of large laser angles. Polydisperse samples can negatively influence measurement results and it is not possible to continuously monitor particle populations over a defined time period.
What is laser diffraction used for? Over the years, the laser diffraction method has established itself in manifold industries for research as well as for quality control because of its great versatility. In research, laser diffraction is used to develop new materials and to analyze their characteristics, while in quality control it is used to ensure that produced goods comply with all requirements. In the field of soil research the laser diffraction method is utilized to examine structures and sediments in the earth, like slurry or clay, by measuring the particle size distribution. Laser diffraction is also used to analyze particles in estuaries, as the method delivers the size, the stability and the density of soil particles and thus grants insights into the movement patterns of natural as well as polluting particles. Furthermore, a laser diffraction analyzer can investigate how particle sizes affect the taste of food, for example. In the case of pigments, laser diffraction can determine how particle sizes influence the coloring of pigments. Moreover, laser diffraction is used for the measurement of ceramics, fertilizers, emulsions and pharmaceutical powders.
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