How Laser Diffraction Particle Size Analysis Works

Laser diffraction is a widely used particle size measurement technique for determining particle sizes in the micron range. The reasons for this are as follows.

POINT1

The measurement range is wide, spanning from submicrons to millimeters

POINT2

Fast measurement… Results displayed in less than a minute

POINT3

Reproducibility… Since a large number of particles can be sampled in each measurement, reproducibility is high.

POINT4

Instant feedback… Enables monitoring and control of the particle dispersion process

POINT5

High sample throughput… Capable of performing hundreds of measurements per day

POINT6

No calibration required… Simple verification using standard samples

POINT7

Proven technology… Compliant with ISO 13320 (2009)

Measurement Principle

In laser diffraction, the particle size distribution is determined by measuring the angular dependence of the intensity of light scattered as a laser beam passes through a sample of dispersed particles.

As shown in the figure below, for larger particles, light is scattered at small angles relative to the laser beam, whereas for smaller particles, light is scattered at nearly the same intensity up to large angles. The data for each angle is then analyzed, and the particle size responsible for the scattering pattern is calculated using Mie theory of light scattering. The particle size is reported as the diameter of the equivalent volume sphere.

Optical Properties

A typical laser diffraction system consists of three main components.

Optical bench

The dispersed sample passes through the measurement area of the optical bench, where the laser beam illuminates the particles. A series of detectors then accurately measures the intensity of the light scattered at various angles by the particles in the sample.

Sample Dispersion Unit

To suspend the sample in the liquid and ensure homogeneity, a sample dispersion unit equipped with a circulation pump is used. This ensures that the particles are delivered to the measurement area of the optical bench at an appropriate concentration and in a stable, dispersed state. Abrasive grains that tend to agglomerate during standard processing are first treated with a surfactant and pre-dispersed using a homogenizer before being fed into the dispersion unit.

Measurement Instrument Software

The measurement software controls the system during measurement, analyzes the scattered data, and calculates the particle size distribution.

Considerations for Laser Diffraction Measurements

Since the particle size distribution is determined by comparing the scattered light intensity with theoretical values based on a spherical model, the results may not correlate well with other measurement methods (such as particle size measurement via image analysis) or with laser diffraction analyzers from other manufacturers.
The accuracy of peak separation is not as high as that achieved by microscopic methods.
The data may vary depending on the choice of refractive index used in the calculations.

Furthermore, when agglomerated abrasive particles are present, the results can vary significantly depending on the dispersion state of the sample, which is influenced by the conditions of the dispersion system (pump type, flow velocity, flow rate, type of ultrasonic irradiation device, and output). Therefore, it is important to always perform measurements using the same equipment and under the same conditions, as this improves reproducibility.
For this reason, it is essential to compare values under strictly defined measurement conditions.

資料提供：Source: Spectris Corporation, Malvern Panalytical DivisionOpens in a new window

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