Exploring the Benefits of Laser Diffraction in Modern Particle Size Analysis

Many materials in many industries, such as food, medicine, and catalysis, exist in powder form. The particle size distribution of powder particles has an important impact on the quality and performance of materials. Laser particle size analyzer is an important tool for detecting the particle size characterization of powder materials. Laser diffraction particle size analyzer utilizes the light source scattering principle for powder measurement and has the advantages of wide measurement range, fast result analysis and good repeatability.

Working principle of laser diffraction particle size analyzer

Laser particle sizer measurements utilize the principle of light scattering, which is based on optical scattering and particle dynamics. The principle is based on optical scattering and particle dynamics, i.e., light passing through an inhomogeneous medium interacts with the particles, absorbing, reflecting, refracting, transmitting, and diffracting the light away from the original light path. The laser diffraction particle size analyzer measures the intensity and angular distribution of the light signals scattered by the particles by irradiating the sample with laser light and then deduces the size distribution of the particles.

1. The laser generates a monochromatic, unidirectional laser beam directed at the sample to be measured.

2. The particles in the sample scatter the laser beam, forming scattered light. The intensity and angle of the scattered light from the particles are related to the size of the particles, with larger particles scattering a higher intensity of light and a smaller scattering angle, and vice versa.

3. The laser particle size analyzer collects and records the light signals scattered by the particles.

4. The laser particle size analyzer analyzes the intensity and angular distribution of the scattered light from the particles to deduce the size distribution of the particles in the sample.

The laser diffraction particle size analyzer utilizes this principle to quickly and accurately measure the size distribution of particles in a sample. The laser diffraction particle size analyzer is widely used in powder metallurgy, pharmaceutical, chemical, food, building materials and other fields of particle size analysis and quality control.

Schematic diagram of the measurement method of laser particle sizer

Laser Particle Size Analyzer Structure

The beam emitted by the laser passes through a spatial filter and a beam-expanding lens to become a parallel beam of light with a specific wavelength. The particulate powder is dispersed in an air stream or liquid medium. It is transported into the sample chamber by a dispersion system to ensure that the particles are uniformly dispersed. The parallel light is diffracted and scattered by the sample chamber, and the scattered light is imaged on the focal plane by a Fourier lens. A multi-ring photodetector is arranged on the focal plane. The scattered light information detected by the detectors is converted into electrical signals, which are measured by a computer to characterize the particle size distribution.

Imaging procedure

1. Laser: generally a He-Ne laser or a semiconductor laser, which produces a monochromatic, unidirectional laser beam that irradiates the sample to be measured;

2. Lens system: divided into a pre-incidence beam expanding lens and a spatial filter. The lens system collimates and filters the beam emitted by the laser to make it have a parallel monochromatic beam.

3. Dispersion system: adequate sample dispersion is a prerequisite for obtaining accurate analytical results, according to the different dispersants divided into wet dispersion and dry dispersion. Wet dispersion is usually achieved by ultrasound and stirring. The dispersion effect is related to the ultrasound time, frequency and stirring speed; dry dispersion is divided into free-fall and jet dispersion.

4. Sample chamber and feeding system: The particles to be measured are transferred into the sample chamber through a feeding system to ensure uniform dispersion of the particles;

5. Photodetectors: Photodetectors are distributed on the focal plane of the Fourier lens through which the scattered light passes and are numerous and arranged in a ring. Distributed in the center of the sensor to collect the end of the diffracted beam, used to determine the volume concentration of the sample. Peripheral detectors receive the scattered light's distribution angle and energy and convert it into electrical signals.

6. Computer and imaging system. The computer receives the electrical signals and then measures and extrapolates them to obtain the particle size distribution characteristics.

Classification of Laser Particle Sizer Testing Methods

The testing methods of laser particle sizer can be divided into 2 types: dry and wet. Due to the different dispersing mediums used for dry and wet test methods, test results will be different.

The dry method uses air as the dispersing medium. It utilizes the principle of turbulent dispersion, which enables the sample particles to be fully dispersed, and the dispersed sample is then introduced into the optical system for testing.

Advantages : It is fast, easy to operate, accurate, and reproducible. It can test samples dissolved in water and is suitable for detecting various coarse and fine particle size distributions.

Disadvantages: Large sample size and complex influencing factors.

The wet method adds the sample directly into the water or ethanol and other dispersing media for dispersion. Then, through the optical system, the particle size distribution is calculated.

Advantages: A small sample size is suitable for detecting fine particle size distribution.

Disadvantages: Poor repeatability of measurement results.

Precautions:

Dry feeding try to choose a smaller pressure to reduce the impact on the sample particles and avoid secondary fragmentation of the particles. For some difficult to disperse the sample, such as iron oxide, the density is larger, you need to choose a larger dispersion pressure.

Wet feed to be adjusted by changing the stirring rate, ultrasonic time. For some lighter samples that can float on the dispersing medium, the test time of the sample should be extended to facilitate the full dispersion of the sample. Also the wet test should use ultrasound to remove air bubbles, otherwise trailing peaks will be formed in the results.

Influence factors of test results:

1. Dispersant selection: One of the important conditions for a laser particle sizer test is sample dispersion. Choosing the right dispersant can lower the liquid surface tension, destroying the van der Waals force between the particles. The main role of the dispersant is to prevent the particles from settling and coagulating so that they are well dispersed. Theoretically, the dispersant should be chosen as a transparent, optically balanced liquid that does not react with the sample.

2. Detector distribution: The larger the laser diffraction ring radius, the larger the diffraction angle, the weaker the light intensity. Increasing the number of detectors can reduce signal leakage and improve the accuracy of the measurement results.

3. Shade selection: If the shade value is too high, there will be multiple scattering and can not be measured; if the shade value is too low, the signal will not be detected enough to affect the measurement accuracy.

4. Calibration standard material selection: To ensure the accuracy and traceability of the laser particle sizer calibration results, you need to calibrate the instrument based on certain industry instrument calibration specification processes.

5. Scanning speed: Fast scanning can improve data accuracy, repeatability and stability.

6. Instrument maintenance: The degree of maintenance of the laser particle sizer has a greater impact on the test results. The laser particle sizer needs to be calibrated and maintained regularly.

Precautions

(1) Laser Particle Size Analyzer should be warmed up for 15-30 min before use;

(2) Sample requirements: remove organic matter, carbonate, and fully decompose the sample before testing. For dynamic scattering measurement, the narrower the particle size distribution range, the better;

(3) Background normal standard: the decreasing trend of light energy should be less than 80 at most, and the light intensity should be about 80%;

(4) Ensure that the entire surface of the dispersive feed and cuvette channel is clean and that the cuvette is well-sealed and leakage-free.

(5) Sample dispersion must be good, so choose the appropriate dispersant, sufficient ultrasonic time and stirring treatment.

(6) Using stable shading can ensure the reproducibility of the results.

Conclusion

Laser particle sizers have the advantages of high precision, fast analysis speed, and good reproducibility of results. In the particle size distribution range of narrow particle testing, performance is a particularly outstanding advantage. Archaeology, pharmacy, food, materials science and other fields are widely used in the environment.