Particle size measurement with sophisticated laser technology ensures several advantages. They include: simple operation, short analysis time, repeatability and reliability, comparable results, cleverly designed dispersion units and a fully automatic analysis. All these features are now available in one single instrument which is capable of analysing particles whose size can vary between 10 nm to the millimetre range. The instrument is therefore ideal for production and quality control purposes as well as for research and development applications.
“Static Light Scattering”, “Laser Scattering”, “Laser Diffraction” and “Laser-Granulometry” are interchangeably used to refer to the same particle size determination technology. The sample material is irradiated with a light beam and the scattered light intensity is measured in as many directions as possible. Based on this anisotropic intensity distribution and with the aid of a suitable scattering theory, the particle size can be determined.
Limitations of the Technology
As large particles result in small diffraction angles, it is possible to measure the smallest diffraction angles consistently because of the upper measurement limit. The stability and adjustability of the optical setup depend on the capability to separate the diffracted light of these small angles from the undiffracted laser beam. For a large number of instruments, the upper measurement range has been set at 2 mm. The lower obtainable measuring range of the static light scattering is defined on the basis of the scattering processes. If the scattering particles become smaller, a point will be reached, where the intensity of the scattered light is the same in all directions.
The Instrument Design
In most cases, a laser is utilized as a light source, but several manufacturers use LED’s or conventional light sources. The central advantage of lasers is the high light intensity and the excellent beam quality, which is very important for the accurate measurement of the scattered light. The Conventional Design and the Inverse Fourier Design are explained below. The Inverse Fourier Design is the design used in the FRITSCH Laser Particle Sizers ANALYSETTE 22.
The conventional design is such that the measuring cell is moved in a wide, parallel laser beam and the scattered light is directly depicted behind the measuring cell, with a lens on an angle-resolving semiconductor detector. One of the benefits of this setup is the fact that even thick measuring layers can be used, which is advantageous especially with aerosols. However, the main drawbacks regard the limited capability of measuring large scattering angles and very small particles. The design allows covering a wide measurement range.
Figure 1. Conventional Design.
The difference between the Conventional Design and the Inverse Fourier Design is that in the Inverse Fourier Design the laser beam is moved through a focusing lens (the so-called “Fourier-Lens”) and the convergent laser beam moves through the measuring cell. Using the ANALYSETTE 22, it is possible to alter the distance of the measuring cell from the detector and therefore the detected angle range can be adapted to suit specific requirements. Since the measurement cell can be moved to detect backward scattering light, it is possible to measure very small particles. Since the measurement cell is positioned just in front of the detector, an additional laser beam can irradiate the sample from the opposite direction. The backward scattered light is then very efficiently captured from the detector, which is also used for the normal forward scattering. Since the distance between the measurement cell and the detector is small, a high sensitivity is achievable.
Figure 2. FRITSCH-Technology: Inverse Fourier-Design.
Figure 3. Measurement design for the nano particle size range.
The Dispersion Unit
The quality of the instrument is strongly influenced by its components such as the laser, the optical setup and the detector. The main challenge for the user is the sample treatment. In order to guarantee a reliable measurement, the sample material must be fragmented to its single primary particles. For instance, potential agglomerates have to be fragmented and then transported, in an optimum concentration, through the laser beam. This role is performed by dispersion units, classified as wet and dry dispersion units.
The wet dispersion unit is a closed circulatory system where mainly water is continuously recirculated and dispersed. In the dispersion process, an integrated ultrasonic generator is used. Its intensity can be adjusted through the operation software. Standard samples are added directly with an applicator into the dispersion unit. The system offers continuous feedback of the amount of sample added and signals when a sufficient amount of material for a dependable measurement is available. After a brief dispersion, a first measurement begins, generally followed by a second measurement in order to monitor potential changes of the dispersion condition.
Figure 4. FRITSCH Laser Particle Sizers ANALYSETTE 22 NanoTec plus – practical modular-system: Measuring Unit with Wet Dispersion Unit.
The benefits of the wet dispersion unit are its flexibility and the easy handling. The adjustability of the ultrasound, duration of variable dispersion and of the dispersion addition allow measuring depandably and reliably a wide range of samples. After a completed measurement the entire reservoir can be automatically emptied, rinsed and filled with new liquid.
When compared to the wet dispersion, the dry dispersion is not a closed circulatory system. Here, each sample portion is accelerated only once with compressed air through an anular gap Venturi nozzle system and broken up into primary particles. The dispersion effect is based on multiple, consecutively occurring strong pressure fluctuations, which leads to highly turbulent flow ratios. A strong shearing force is generated, which breaks the agglomerates apart. Compared with the wet dispersion, less energy is introduced into the sample material. The dispersion efficiency does not attain the level of the wet measurement.
Figure 5. FRITSCH Laser Particle Sizers ANALYSETTE 22 NanoTec plus with Dry Dispersion Unit.
It is possible to increase the efficiency of the dispersion process of the dry dispersion by accelerating the sample material on an impact plate positioned right in front of the measurement cell. By using relatively soft materials, the agglomerates are broken apart and a first reduction in size of the primary particles takes place.
Evaluation and Software
The operation and evaluation software of the ANALYSETTE 22, stores all measurements in a SQL database and at the same time meets the requirements of 21 CFR part 11. In order to guarantee an optimal reproducibility of the measurement results, the operation of the measuring process is performed by the SOP (Standard Operating Procedures), which can be flexibly programmed to suit the requirements of each sample.
Examples from Practical Experience
To conclude, two examples analysed with the Laser Particle Sizer ANALYSETTE 22 are considered.
In the first example, Al2O3 was ground for four hours in the Planetary Micro Mill PULVERISETTE 7 premium line that is shown in figure 6 as a black graph in the left area of the distribution. The blue graph on the right shows instead the distribution of the original material. The particle size distribution in this example span from approximately 30 – 40 nm to approximately 200 nm. Above this, in the range between approximately 200 and 500 nm a second peak occurs, which is caused by the abrasion of the ZrO2 used during the comminution.
Figure 6. Al2O3 comminuted with the Planetary Micro Mill PULVERISETTE 7 premium line - measured with the ANALYSETTE 22 NanoTec plus.
The advantage of the Static Light Scattering compared to the Dynamic Light Scattering for example used in the FRITSCH Nano Particle Sizer ANALYSETTE 12 becomes immediately clear: in one measurement, particle distributions from large (~ mm) down to below 100 nm size ranges can be continuously observed. For example, grindings starting with the original material up to the final fineness can be reliably analyzed.
The second example regards motor oil with different specifically added aggregates. It confirms once again the advantage of being able to measure a wide range of particle sizes in a single analysis. First, pure oil was used to perform the background measurement. This is performed prior to each measurement to separate the possible contamination of the measuring cell from the actual measurement data. Subsequently, the motor oil with the aggregates was added into the circulatory circuit and the actual measurement was performed. A multiple modal distribution where each mode could be allocated to a material was obtained.
Figure 7. Motor oil with different distinct added aggregates-measured with the ANALYSETTE 22 NanoTec plus.
This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.
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