Particle Size Analysis Basics

11 Apr, 2011

In many industries, the ability to determine the size of particles is not only useful but can be very important. A Laser diffraction particle size analyser is commonly used to provide this sort of information but the processes used to obtain particle size can be quite complex. Sampling, dispersion processes and the shape of materials all contribute to the complexity of particle size analysis.

Here we provide key information for understanding particle size analysis.

What does it mean to describe a particle?

When dealing with a three dimensional shape, one unique number cannot accurately be given to describe its size. While this remains true in so many situations and circumstances, it is particularly relevant when seeking to describe complex shapes such as a grain of sand or even a particle within a can of paint.

While many of us may question why this information is so important, it is imperative and influential for people such as Quality Assurance Managers within particular industries and organisations. For example, a Quality Assurance Manager may need to know whether the average size of particles has increased or decreased since the last production run.

The equivalent sphere

So often we seek to describe a shape by only one number. As has been mentioned, this is problematic as there is only one shape – a sphere – that can be accurately described by one number.

In order to arrive at a particular number to explain the size of a shape, equivalent sphere theory is frequently used. Using the equivalent sphere theory, some property of the particle is measured and it is then assumed that this refers to the diameter of a sphere to describe the particle. Essentially, this means that three or more numbers do not have to be used to describe a three dimensional particle. Although it is more accurate to describe three dimensional particles with three or more numbers, it is also inconvenient and can become unmanageable.

Using different techniques

When a particle is examined under a microscope, a two dimensional image is seen and the information that is deduced relates to the two dimensional shape. Consequently, there are a number of different diameters that can be reported. If the maximum length of the particle is used, then the result relates to a sphere with diameter of the maximum dimension. Conversely, if the minimum length is used for the calculation, then the result pertains to a sphere with diameter of the minimum dimension.

It is therefore important to note that each technique used to measure a particle will provide a unique result because it measures a different property of a particle.

In light of this, there is no absolute right or wrong answer. All answers are correct for the techniques that are used and the dimension of the particle that they are measuring.

What does this all mean?

For those to whom particle size analysis is important, different techniques for measuring particle size mean that there can be no standard size for particles such as grains of sand. If meaningful comparisons are to be made between different techniques, it must be done with standards containing spherical particles. Further to this, characterisation of particle size standards is only possible when the same technique has been used and this allows comparison between instruments that use the same technique.