Particle Size Analysers and How They Work

Particle size analysis employs a number of different methods to measure the size distribution of particles in the nano, micron and millimetre range. The best type of analysis will depend on the type of particle you need to measure. Often, several kinds of analysis are used to gather a more complete picture. Read on to find out which types of analysis would best suit your needs.

Laser diffraction analysis

Laser diffraction is a popular method for measuring particle size distribution, particularly between the 0.01 to 3500 micron range.

How laser diffraction works

A beam of light of fixed wavelength, projected at a group of dispersed particles will scatter light at different angles depending on their size. Larger particles will scatter light at smaller angles while smaller particles will scatter light at larger angles. A series of detectors measure the intensity of light scattered by the particles and their size is determined using an optical model of light behaviour.

Mie theory

In laser diffraction analysis Mie theory is used to calculate the distribution of particle size. Mie theory requires knowledge of both:

• The refractive index (RI) in relation to both the dispersant and the sample being measured, and,
• The imaginary refractive index – the absorption or transparency of the sample (which accounts for chemical properties and surface roughness).

Most laser diffraction instruments will have built-in databases where the dispersant measurement can be obtained. For samples where the optical properties are not known and cannot be measured, an iterative approach can be taken on an educated guess. Alternatively, the Fraunhofer model provides a simpler approach by assuming the RI difference between the particle and and the medium is infinite. However this approximation breaks down as particle size decreases below 50 micrometres and as particles become more transparent. Mie theory therefore remains the prefered method in the ISO standard ISO13320 as it covers a wider measurement range.

Benefits of laser diffraction

Benefits of laser diffraction include:

• Can measure a wide dynamic range – Laser diffraction can measure from submicron to millimetre lengths.
• Rapid results – Can produce results in less than a minute.
• Easily repeatable – Laser diffraction technique produces a large sample with each measurement, and requires very little configuration to repeat sampling.
• High throughput – Hundreds of measurements can be taken per day.
• Instant feedback – Results are produced quickly.
• Easy to calibrate – Standard reference materials provide ease of calibration covered by ISO13320:2009.

Laser diffraction instruments

The typical laser diffraction particle size analyser usually comprises of three key elements:

• The Optical Bench – Once the sample is dispersed, the Optical Bench contains the measurement area, where a beam of laser light illuminates the particles, and a series of detectors to measure the scattered light pattern.
• Sample Dispersion Units – Ensure the correct concentration of sample is dispersed to the measurement area. They come in two varieties, wet and dry.
• Laser Diffraction Software – Software is required to control the instrument, analyse the data and calculate the size distribution of the particles in each sample.

Dynamic Light Scattering (DLS)

DLS is an established technique for the analysis of particles in the submicron region down to below 1 nanometre. It is commonly used to measure particles suspended in liquids such as:

• proteins
• polymers
• micelles
• carbohydrates
• nanoparticles
• emulsions
• colloidal dispersions

Dynamic Light Scattering encapsulates two similar techniques of particle size analysis:

• Photon Correlation Spectroscopy (PCS)
• Quasi-Elastic Light Scattering (QELS)

How DLS works

Thermally induced collisions between suspended particles and the molecules of the solvent cause the particles in suspension to undergo random movement known as Brownian motion.

DLS measures the speed at which particles are diffusing due to Brownian motion and relates this to the size of particles. When a sample is illuminated by a laser, the intensity of scattered light fluctuates at a rate dependant on the particle size. Smaller particles within the solvent are displaced further and at a more rapid pace. By analysing the intensity of these fluctuations, the velocity of Brownian motion or the translational diffusion coefficient can be used to determine particle size using the Stokes-Einstein equation.

Advantages of DLS

Due to it’s capacity for measuring submicron sized particles the advantages of DLS over other forms of particle size analysis include:

• non-invasive (the complete sample can be recovered)
• quick analysis times with speedy throughput
• analysis can be completed at high or low concentrations using a small sample size
• ideal for measuring a broad range of biomaterial and nano particles

Dynamic Light Scattering instruments

DLS instrumentation requires a laser light and a lense to converge the light. The process typically works as follows:

1. The laser is focused on the sample through the lense.
2. Light is scattered by the particles.
3. A detector, typically placed at 90 degrees (or 173 degrees, depending on the particular Zetasizer Advance model) to the light source, collects the scattered light.
4. Intensity fluctuations of the scattered light are converted into electrical pulses.
5. These pulses feed into a digital correlator which generates a particle size based on an autocorrelation function.

The Zetasizer Advance series of instruments detect the scattering information at 173 degrees. This is known as backscatter detection and has significantly better performance than systems using 90 degree scattering optics. Some advantages include:

• Reduced multiple scattering effects enabling higher concentrations to be measured
• Contaminants such as dust effects are reduced
• Variable measurement positioning can reduce flaring effects and allow small particles and dilute or highly concentrated samples to be measured.

Automated imaging

Used predominantly for particles between 1 micron and several millimetres in size, automated imaging captures high resolution images of up to thousands of particles to create statistically representative distributions of particles. Automated imaging is often used in conjunction with other particle size analysis techniques like laser diffraction for a deeper understanding of the distribution or to validate previous findings.

Benefits of automated imaging

Benefits of automated imaging include:

• the ability to detect agglomerates, oversized particles or contaminants
• measurement of shape difference
• measurement of size of non-spherical particles

Automated imaging techniques

There are three key techniques in automated imaging to measure particle size distribution.

Scanning Electron Microscopy (SEM)

SEM imaging takes place in a vacuum chamber therefore samples need to be fixed and dried thoroughly to prevent unwanted interactions between the electron beam and any atmospheric molecules. Using an SEM, a focused beam of electrons scans the surface to produce images of a sample. As the electrons in the beam interact with the atoms in the sample, the microscope gathers information about the sample’s surface topography and composition. Once an SEM image is acquired, automated analysis and classification software enables the particles to be further processed and characterised.

The Phenom SEM range offers superfast, easy-to-use, high-resolution imaging including elemental composition analysis of large samples.

Optical microscopy

Optical microscopy illuminates a sample using visible light to provide a magnified image. Sample dispersion is key for correctly measuring particle size through imaging. The goal is spacial separation of particles in the field of view and will depend on whether the imaging technique is dynamic or static. Dynamic imaging uses a flow cell through which the sample is passed during a measurement. In static imaging the dispersed sample is placed on a flat surface like a microscope slide. In both cases, the use of automated dispersion prevents inconsistencies and improves procedural efficiency.

ATA Scientific offers a range of high quality image analysers including the Malvern Morphologi 4 ID particle image analyser, which has been designed for automated high-resolution particle size analysis between 0.5 and 1000 microns. It automatically targets individual particles and provides the size, shape, and chemical identification of each.

The right analytical tools for you

Need help finding the right particle size analytical equipment for your application? Contact ATA Scientific today to for a free consultation and discover the right instruments for your project.