Determining Particle Size Distributions Using Laser Particle Size Measuring Instruments

Laser diffraction is a fast and effective method of measuring particle size, making it extremely useful in quality control, with the technique being widely used across industry.

Laser diffraction is used to analyze feedstocks, before delivery and upon reception, for applications in which particle size impacts processability or the quality of the final product. Laser particle sizing is also used to manage processes where the size distribution of intermediates is monitored between stages.

Measuring Principle

Laser diffraction is based on a well understood scientific principle – diffraction. When light is passed through a particulate sample it is diffracted and behind each particle an angle-dependent intensity distribution is formed (comparable to a complex shadow), that appears as dark and light alternating rings.

The size of the particle determines the intervals between the dark and light concentric rings with large particles resulting in closely bordering rings, and small particles resulting in large intervals between the different rings. Using Mie-theory it is possible to calculate the distances of the rings, whereas with large enough particle diameters the Fraunhofer- approximation can be utilized.

Practical Realization

Using this principle for the practice of particle measurement involves firing a laser beam through the sample, with the sample moving through a measuring cell. The transmitted laser light is detected by a light sensor sat behind the measuring cell, in the beams optical path, and the angular resolution of the transmitted, scattered light is recorded.

The instrument has a unique optical design which collects all of the scattering patterns from particles of the same size and collates them to produce one ring system. This means that when there is a high population of a specific particle size the ring system associated with that size will be of a higher intensity.

The majority of samples in the real world do not have collections of particles with discrete sizes but instead show a wide distribution of different particle sizes, which results in overlapping of the different ring systems. This complex data mix can be resolved using mathematical processing, which results in quasi unfolding of the overlaps resulting in a calculated size distribution for the particles.

The sensor used to measure the angle-dependent scattering intensity does not measure continuous changes, but instead measures in discrete amounts that cover a range of angle areas. This means that the calculated particle size distribution is not continuous and is instead given in discrete size intervals. The range of these size intervals (also known as size widths) has a positive logarithmic relationship with the particle size, meaning if a logarithmic size axis is used the size ranges are shown as being the same size.

Significance of Detector Elements

The width of each size range is dependent on several factors, with the most significant being the detector’s angular resolution. The angular resolution of the detector depends on how many detector elements there are and the ratio of the covered particle size area.

For example, if a size range of 0.1 – 2000 µm has 50 detector elements then it will have larger individual size ranges than a size range of 0.1 – 100 µm with 50 detector elements. It is important to note that it is also possible to calculate more individual size ranges than there are detector elements, however this makes little physical sense in terms of resolution and no additional information can be collected.

For the reasons given above a significant factor when choosing a laser particle size analyzer is the number of effective detector elements that it possesses.

Dispersion Parameters

As discussed in the Principle of Measurement section the sample being measured is moved through a measuring cell. During measurement it is crucial that the sample is present as individual particles and not as agglomerates. For this reason, prior to measurement the sample must be fully dispersed. This dispersion can either be dry or wet.

In a dry dispersion the sample is fired through a jet system into circulating air. The rapid pressure changes in the circulating air system results in the tumbling of the sample and this breaks apart any large agglomerates that are present. Smaller agglomerates may not be broken down, with how effective the method is at breaking small agglomerates depends on the binding forces in the agglomerates, which vary significantly depending on the nature of the sample.

For low-binding agglomerates dry dispersion methods can break up agglomerates of sizes just below 1 µm, however dry systems will not work in the nano-range.

Wet dispersion is a more effective method of breaking down agglomerates making it useful for nanoanalysis or for the processing of difficult samples. In wet dispersion the sample is added to a liquid in a closed loop at concentrations between 1 – 10% volume, depending on the sample.

The liquid is then pumped around the loop and often ultrasound is applied, the mechanical forces from the pumping and ultrasound then result in the breakup of agglomerates more effectively than the dry dispersion method.

For particle size analysis of many different types of sample choosing the correct dispersion parameters (e.g. dry or wet, tensides, ultrasound etc.) is the most important and difficult part of the process.

Variable Dispersion

As the choice of parameters is key to carrying out the best possible analysis it is important that modern laser particle analysis systems are flexible. The ANALYSETTE 22 NanoTec’s wet dispersion unit allows both the ultrasonic power and the pumping speed to be adjusted, with three different liquid options for the closed loop.

The materials used to build the system (PTFE, stainless steel, glass and viton) allow most organic solvents to be used.

The ANALYSETTE 22 NanoTec has a modular design of a measuring unit with a fitted dispersion unit and a folded optical path. Two lasers are used, IR and green, to provide a wide measuring range (between 0.08 – 2000 µm) at tight dimensions. The system’s control software is also highly flexible, meaning even the most difficult dispersions can be carried out effectively.

Measuring Example

The cumulative curve for three different dairy products is shown in illustration 1. To obtain these results a small volume of sample was added to the liquid closed loop and exposed to ultrasound for a defined time period.

It can be seen that the homogenized milk contained the smallest particles, whereas cream contained larger particles – most likely droplets of fat.

Particle size distribution of homogenized milk (red) and cream (blue) measured with the ANALYSETTE 22 NanoTec.

Illustration 1: Particle size distribution of homogenized milk (red) and cream (blue) measured with the ANALYSETTE 22 NanoTec.

The measurements of four different types of chocolate (two premium dark chocolates and two milk chocolates) are shown in illustration 2. The milk chocolates were found to have particles of a greater size whereas the premium dark chocolate (black) was found to have particles of a finer size – corresponding with the chocolate’s softer mouthfeel.

It can also be observed that chocolate with a higher cocoa content (red, 99% cocoa) has finer particles than chocolate with a lower cocoa content (black, 70%).

Measured cumulative curves of four different kinds of chocolate with an ANALYSETTE 22 NanoTec. Important when measuring chocolate is the utilization of a suitable solvent, otherwise the measuring cells quickly become soiled and the reproducibility of the measurement clearly suffers.

Illustration 2: Measured cumulative curves of four different kinds of chocolate with an ANALYSETTE 22 NanoTec. Important when measuring chocolate is the utilization of a suitable solvent, otherwise the measuring cells quickly become soiled and the reproducibility of the measurement clearly suffers.

This information has been sourced, reviewed and adapted from materials provided by Fritsch GmbH.

For more information on this source, please visit Fritsch GmbH.

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