Laser Diffraction in the Nano-Range for Automatic Particle Size Analysis

Fraunhofer diffraction, static light scattering, laser diffraction and granulometry are all terminology for the same method. The method is the light of a coherent light source scattered at particles to create a characteristic angle-dependent intensity distribution, the exact course of which is essentially established by the particle size.

To get the particle sizing instrument ready, take a laser beam, which is coherent light, place the particles at a defined position in the beam, and put a suitable detector in the right place.

Laser Particle Sizer ANALYSETT 22 NeXT

Figure 1. Laser Particle Sizer ANALYSETT 22 NeXT.

Some points to consider are:

  • How exactly should the laser beam be shaped?
  • Where exactly should the detectors be positioned?
  • What geometry should the detector elements have?
  • How and where do I place the particles in the laser beam?

And very important is: The particles must be prepared so that they are available in the required concentration and desired mode – singly or bound in agglomerates. This is the dispersion process, and some questions to consider are outlined below.

Each manufacturer of particle sizing instruments answers all these questions somewhat differently. The ISO standard 13320 provides a basic orientation. This standard discusses not only general requirements for appropriate instruments and the quality of measuring results with standardized sample systems but also the basic optical design of the measuring systems.

Invented by FRITSCH GmbH in the 1980s, the Reverse Fourier design, which was developed for the first time to market maturity, is a variant here. The newest generation of the ANALYSETTE 22 from FRITSCH is also based on this design, which has now been adopted by multiple suppliers.

Keeping it Simple

This current instrument is characterized by a central approach based on the experience of over 35 years: Keep it simple, starting with the light source. FRITSCH uses only one single light source with only one single wavelength, in contrast to many other suppliers.

Theoretically, numerous wavelengths, particularly for very fine materials, supply the advantage of gathering more information from the scattering processes for the Mie theory to be utilized. Anyone who has ever dealt with the practical implementation of these theoretical advantages has realized quickly that this is frequently associated with immense practical difficulties and that the theoretical benefits are negligible in comparison. In the case of the Mie theory, the refraction index and the absorption coefficient of the sample material must be known for the respective wavelength of the light utilized. This often presents a non-trivial challenge even for a single wavelength. The matter can quickly become confusing with numerous different wavelengths.

FRITSCH applies only one wavelength. A fiber-coupled green laser is utilized, which supplies the ideal wavelength for the reliable detection of both: finest particles well below one-tenth of a micrometer and large particles down to the millimeter range.

Furthermore, the system does not utilize several beams that could come from different directions at different points in the measuring cell, but only a single beam that is simple to control.

The Exact Figures

In its full expansion stage, the ANALYSETTE 22 NeXT Nano supplies a possible measurement range from 10 nm to 3800 µm. A slightly simpler version (NeXT Micro) enables particle size measurements between 0.5 and 1500 µm and because of this is already equipped for a variety of applications.

Reliable Thoroughness

Wet dispersion unit ANALYSETT 22 NeXT

Figure 2. Wet dispersion unit ANALYSETT 22 NeXT.

The dispersion unit for measurement in suspensions, in addition to the optical design, follows the idea of maximum simplicity and is also based on this principle. This part of the ANALYSETTE 22 NeXT, which is extremely crucial for daily work and the success of measurements, works totally without valves in the sample circulation system, a real innovation from FRITSCH.

All wet dispersion units presently available on the market have either relied on rotating multi-port valves or on clamping elements, in which, for instance, a hose branching in the circuit is squeezed either in one or the other direction and, therefore, plugging. Both methods have their specific weaknesses.

Rotating systems are slow and can be damaged by particles, which can jam and get stuck in the sealing surfaces. This can lead to sluggishness or even leakage. On the other hand, clamping systems typically have dead spaces in which particles can already settle during a measurement. Furthermore, particles may also get stuck in sealing surfaces.

FRITSCH elegantly avoids these problems by completely dispensing without valves in the sample circulation system, and so gathers a fast, reliable, highly flexible and compact dispersion system.

Ultrasonic Flexibly Applicable

Laser Particle Sizer ANALYSETT 22 NeXT with ultrasonic box.

Figure 3. Laser Particle Sizer ANALYSETT 22 NeXT with ultrasonic box.

Flexibility is the keyword: The utilization of ultrasonic is usually the technique of choice when agglomerates from the sample to be measured need to be degraded. However, there are always instances where ultrasonic is not necessary or even harmful.

Sometimes, it is worth preceding an ultrasonic treatment before the actual measurement and then to dispense completely in the main measuring circuit. FRITSCH consistently follows a new path here too.

The powerful and variably programmable ultrasonic box is provided as an independent module that can be easily inserted into the sample circuit system. Therefore, it is also viable to configure a system completely without an ultrasonic chamber. Just as the respective application needs.

Conclusion

The ANALYSETTE 22 NeXT represents a powerful and flexible system for determining particle size, which in its entry-level version as the NeXT Micro model can also provide a very attractive price.

Furthermore, a later expansion with respect to dispersion (for instance, through the optional ultrasonic box) or an upgrade of the Micro-version to the NeXT Nano is possible.

References and Further Reading

First published Int. Labmate (2005) Vol. XXX Is. VI

Acknowledgments

Produced from materials originally authored by Dr. Günther Crolly from Fritsch GmbH.

This information has been sourced, reviewed and adapted from materials provided by FRITSCH GmbH - Milling and Sizing.

For more information on this source, please visit FRITSCH GmbH - Milling and Sizing.

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