Laser diffraction is a simple particle size determination technique that provides rapid and reliable particle size measurement. It has been used in almost all branches of industry from the chemical industry to the food industry and the building materials industry.
Principle of Laser Diffraction Technique
Scattering of light on a particle generates an angle-dependent intensity distribution behind the particle that comprises a ring system with light and dark areas. The large particles produce closely bordering rings with intensity distributions; whereas, the small particles produce rings with large intervals. It is possible to accurately calculate the distances of the rings using the Mie-theory. However, the Fraunhofer- approximation can be applied for particles with sufficiently large diameters.
Practical Realization of Laser Diffraction Principle
In the practical realization of laser diffraction principle, the sample material, that is being transported via a measuring cell continuously for the most part, is illuminated by a laser beam. The scattered laser light is detected by a sensor placed within the optical path behind the measuring cell through angular resolution by the particles diffracted.
The scattering patterns of all illuminated particles of the same size in the measuring cell are projected onto the same ring system by means of a special optical design of the equipment. Besides having individual particle sizes, the most realistic samples have particles of a wide band of varying diameters. This causes overlapping of many different ring systems. The particle size distribution can be obtained by quasi unfolding of these overlaps using an appropriate mathematical process. There is a logarithmic increase of width to individual size intervals with increasing particle size; the individual size ranges seem to be of the same width with a logarithmic application of the axis.
Importance of Detector Elements
Different factors influence the individual size ranges, of which the angular resolution of the detector is crucial as it depends directly on the count of the detector elements, respectively and more accurately, the ratio of the count of the covered particle size area. Although a large number of size range intervals can be calculated from a few detector elements, it is not possible to acquire any additional information. Hence, the effective amount of detector elements is a key parameter for assessing a laser particle size measuring instrument.
For most samples, wet dispersion represents the ideal method of preparation for particle size measurement. The process involves adding the sample material to a liquid closed circuit. The selection of the appropriate dispersion parameters poses the major challenge in the measurement of the particle size distribution.
The flexibility of the dispersion possibilities is the critical aspect of modern laser particle measuring instruments. For instance, the wet dispersion unit of ANALYSETTE 22 NeXT (Figure 1), the latest FRITSCH model of the ANALYSETTE 22 series, can adjust not only the pump speed but also can work with or without ultrasonic. In addition, a variable suspension volume between 150 ml and 500 ml of filling the liquid closed-circuit can be selected and the unit is made of materials that allow handling of the most prevalent organic solvents.
Figure 1. ANALYSETTE 22 NeXT.
With the completely revised ANALYSETTE 22 NeXT you choose according to your requirements: The ANALYSETTE 22 NeXT Micro with a measuring range of 0.5 – 1500 μm for all typical measurement tasks or the high-end instrument ANALYSETTE 22 NeXT Nano with an extra-wide measuring range of 0.01 – 3800 μm for maximum precision and sensitivity for smallest particles with an additional detector system.
Get all the decisive advantages with the model that meets your requirements: especially easy operation and cleaning, short analysis times, reliably reproducible results and the recording of additional parameters such as temperature and pH value during wet dispersion. State-of-the-art technology at an unbeatable price.
Figure 2 depicts the cumulative curve for three different dairy products. As can be seen, the finest particles of homogenized milk can be clearly differentiated from the substantially larger droplets of fat from the cream.
Figure 2. Particle size distribution of homogenized milk (red) and cream (blue) measured with an ANALYSETTE 22 NeXT Nano.
Figure 3 illustrates the measuring of four different types of chocolate samples, two milk chocolates and two brands with a very high cocoa content. The clearly large particle size diameters are of the milk chocolate; whereas, the costly product (black) exhibits a finer particle size distribution when compared to the less expensive milk chocolate. The measurement also clearly differentiates the bitter chocolate as the particle size of 99% cocoa content (red) chocolate is finer than 70% cocoa content chocolate. For measuring chocolate, the use of an appropriate solvent is essential because the measuring cells become soiled rapidly. This, in turn, affects the reproducibility of the measurement.
Figure 3. Measured cumulative curves of four different kinds of chocolate with an ANALYSETTE 22 NeXT Nano.
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
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