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 fully automatic analysis. All these features are now available in one single instrument which is capable of analyzing particles whose size can vary between 10 nm to the millimeter 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 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 3.8 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 excellent beam quality, which is very important for the accurate measurement of the scattered light. The Conventional Design and the Reverse Fourier Design are explained below. The Reverse 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 Reverse Fourier Design is that in the Reverse 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. 35 years ago, FRITSCH was the first company in the industry to bring a revolutionary alternative to the conventional design onto the market in the form of laser diffraction in a convergent laser beam: By positioning the Fourier lens in front of the measuring cell, a convergent laser beam passes through the measuring cell. The scattered light is focused directly on the detector without additional optical elements. This design is now in widespread use and can be designed so that the main detector can be used to capture small scattering angles for measuring larger particles.
For detecting particles with a diameter of less than 100 nm, it is necessary to measure the backward scattered light (scattering angle greater than 90°). For this purpose, the detectors were specially positioned close to the measuring cell in the ANALYSETTE 22 NeXT Nano. A laser with green light that is also used simultaneously for measuring forward and sideward scattering is used as a light source. Particular attention was placed in the design of the backward detectors on the suppression of undesirable signal components, caused, for example, by reflection on the measuring cell glasses.
Figure 2. FRITSCH-Technology: Reverse Fourier-Design.
Figure 3. Measurement design for the nanoparticle 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 in most cases by a wet dispersion unit.
The wet dispersion unit is a closed circulatory system where mainly water or other organic solvents are continuously recirculated and dispersed. In the dispersion process, a separate ultrasonic box can be integrated, if you measure frequently sample material that tends to agglomerate. 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 on 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 in the dispersion condition.
Figure 4. FRITSCH Laser Particle Sizer ANALYSETTE 22 NeXT Nano
The benefits of the wet dispersion unit are its cleverly reduced design and robust engineering. It is particularly durable and practically maintenance-free. Doing completely without valves and moveable seals in the sample circulation system ensure, for example, that no dead spaces occur and no sample material can accumulate and settle. The use and the adjustability of the ultrasound, duration of variable dispersion, and of the dispersion addition allow measuring dependably and reliably a wide range of samples. After a completed measurement the entire reservoir can be automatically emptied, rinsed, and filled with new liquid. The fill level measurement is done without contact using an ultrasonic sensor. Without soiling. Without wear.
Evaluation and Software
For the control, recording, and perfect evaluation of your measuring result your ANALYSETTE 22 NeXT is delivered with the FRITSCH MaS control software in which all user entries, parameters, and results are saved automatically and revision-proof in an SQL database. And by integration into a local computer network, all measuring data can also be conveniently analyzed on other computers.
The ANALYSETTE 22 NeXT software also contains completely predefined Standard Operating Procedures – SOPs for short – for nearly all typical measurement tasks, making operation especially easy. Via a well-arranged input mask, you are completely free and flexible in modifying these SOPs to perfectly suit your measurement requirements.
Examples from Practical Experience
To conclude, two examples analyzed 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 of 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 NeXT Nano.
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 distributions where each mode could be allocated to material was obtained.
Figure 7. Motor oil with different distinct added aggregates-measured with the 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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