Key Advantages and Challenges in Laser Diffraction for Particle Analysis

Laser diffraction is a popular particle sizing technique because it offers a number of advantages. However, laser diffraction is not without its challenges. Sampling errors can vary the results in any particle sizing research — particularly when measuring larger particles.

In this article, we will cover the advantages of laser diffraction, as well as the inevitable challenges it faces. We will also discuss the Mastersizer 3000, the world’s most popular particle sizing instrument, as the best solution for particle size analysis and identify a best practice sampling method.

Advantages of laser diffraction

The spherical modelling theory remains the only accepted and logical choice used in a commercial device intended to analyse a wide range of samples, regardless of the actual particle shape and size.

Laser diffraction advantages include:

• An absolute method grounded in fundamental scientific principles. In this method, it’s not necessary for an instrument to be calibrated against a standard. However, validation of equipment is possible to prove that it’s performing to a standard that can be traced.
• Wide and dynamic range. In measuring particle size, a reliable analytical instrument will allow the user to measure particles between approximately 0.01 to 3500 microns in size.
• Flexibility. Laser diffraction offers new possibilities for measuring materials. It’s even possible to measure the paint that’s sprayed from a nozzle in a paint booth. The pharmaceutical and agricultural industries are two of the many industries that have benefited greatly from such advances.
• Dry powders. Even dry powders, from cohesive to fragile materials, can be measured through the technique of laser diffraction. Although this may result in a poorer level of dispersion than if a liquid dispersing medium was used, dry powders can be directly measured using a dry powder dispersion accessory like the Malvern Aero S. In combination with a suspension analysis, it can support the assessment of the amount of agglomerated material in a dry state.
• Liquid suspensions and emulsions. It’s possible to use a recirculating cell to measure liquid suspensions and emulsions. This technique promotes a high level of reproducibility and facilitates the use of dispersing agents and surfactants to determine the primary particle size. If it’s possible to do so, it’s preferable to take measurements in a liquid suspension.
• Sample measured. This technique allows for the whole of the sample to be measured. As the sample passes through the laser beam, diffraction is measured for all particles.
• Rapid. This technique is so rapid that results can be derived in one minute or less. Feedback can therefore quickly be provided and repeat analyses can also be made quickly.
• Repeatable. This technique is highly repeatable and knowing that the results can be relied upon ensures compliance to any requirements of the regulatory authorities (ISO 13320:2009 and USP 429)
• Scope for additional light sources. Laser diffraction particle size analysers don’t just measure simple diffraction effects.Light sources that don’t make use of lasers are sometimes used to enhance the primary laser source to reveal extra information about particle size and shape.

Challenges of laser diffraction

Like many other scientific undertakings, sampling and method of procedure are crucial in the final data interpretation. Despite the prevalence of laser techniques, there are circumstances which warrant the need to visually confirm the outcome using an orthogonal tool (i.e. microscopy).

One of the biggest challenges for laser diffraction methods is getting a representative sample out of a larger bulk product.

Sampling errors are the largest source of variation in any particle sizing experiment (including laser diffraction), especially when it involves measurement of larger particles.

It’s also essential that the sample preparation method is tailored to the material being measured. Particularly, one must choose between wet and dry dispersion, with aspects such as the natural state of the sample, its potential to be dispersed and the volume of the sample all coming into consideration.

Wet dispersion is the most commonly used method due to its suitability for a wider range of samples.

A good way to ensure sampling and dispersion are adequate is to look at repeatability and reproducibility of results:

• Repeatability is a measure of the stability of a single sample (measured multiple times) and will indicate if a sample is well dispersed.
• Reproducibility looks at several sub-samples of the same material and provides insight into the effectiveness of a sampling procedure.

Whilst modern equipment can give quite precise results, it can never be assumed that the size of particles (produced through laser diffraction or any other type of particle sizing measurement) won’t differ from their true dimension.

Choosing the right solution

The Mastersizer 3000 is a state-of-the-art particle size analyser that produces robust and reproducible data for particle size analysis.

The Mastersizer 3000 operates with the principle of laser diffraction, which states that particles will scatter light in different ways depending on their size. Analysis of the scattered light’s angular intensity leads to accurate results relating to particle size distribution.

Some of the Mastersizer 3000’s most important features include:

• Measurement range. It’s a particularly broad one, ranging from 10 nm to 3.5mm. Using a series of detectors the Mastersizer accurately measures the intensity of light scattered by particles within a sample for both red and blue light wavelengths and over a wide range of angles which enables high resolution especially at the ‘sub-micron’ level.
• Minimal footprint. The optical bench spans no more than 690mm.
• High performance in both wet and dry dispersions. The Aero S dry powder dispersion accessory aids in dry dispersion through the adjustable hopper, different feed tray designs and a choice of two different venturis — one that uses shear forces to disperse the sample and another that uses impaction — whilst the Hydro range accessories handle wet dispersion extremely effectively. Particles are delivered to the measurement area of the optical bench at the correct concentration and in a suitable, stable state of dispersion.

This allows the system to deal equally efficiently with a variety of materials ranging from pigments to milk powder and coffee.

• Intuitive software. Every measurement is delivered quickly and easily, and the software is extremely powerful and intuitive with user guided workflows.
• Rapid Analysis.The Mastersizer 3000 captures light scattering at a rate of 10,000 snapshots per sec, with typical measurement times of 10sec for even polydisperse samples.

Best practice sampling preparation method

In order to overcome the challenges outlined earlier, the following three-step best practice sampling method should be followed when operating the Mastersizer 3000:

1. Preparing the sample. There are two dispersion modes that can be utilised — wet dispersion, for aqueous or organic dispersants, and dry dispersion, for powder samples. This ensures that everything from coarse granulates to incredibly finely dispersed emulsions is covered.
2. Measurement. The accuracy of the laser diffraction technique is dependent on two factors: the stability and wavelength of the light source, and the sensitivity of the detector array. As we have previously touched on, the Mastersizer 3000 features a top-of-the-line detector array, with red and blue light sources capable of resolving materials as small as 10 nanometres and as large as 3.5mm in size. As a result, even highly polydisperse samples are able to be measured accurately.
3. Reporting. Utilising the Mie theory of light scattering, the Mastersizer 3000 measures angular intensity of light scattering in order to determine a particle size distribution. From there, results will be presented volumetrically, and measurement parameters can be tracked in real time. This allows for immediate analysis and data comparison alongside defined standards.

The Mastersizer 3000 is the latest particle size analyser that is capable of providing accurate and fast distributions in both wet and dry dispersions.

Making use of the Mastersizer 3000 system

The flexibility of the Mastersizer 3000 system allows the user to develop a robust method for every type of sample and, compared with other laser diffraction systems, the Mastersizer 3000 significantly broadens the range of materials and applications to which this measurement technique can be applied.

Contact the ATA Scientific team to receive a quote today.