Different Particle Sizing Techniques Provide Different Results – But What Is the Truth?

A common point of view is that different particle sizing techniques provide different results, AZoM talks to Gerhard Raatz of RETSCH Technology to discover the truth.

Can you give a brief overview of RETSCH Technology and the work you do?

Retsch Technology is a company 100% focused on particle size analysis. Our main area of business is dynamic image analysis of bulk materials for quality and production control as well as for research and development. Our offering includes the CAMSIZER P4 which is suitable for dry, pourable solids in a size range from 20 µm to 30 mm, as well as the CAMSIZER X2 which characterizes powders, granulates and suspensions in a size range from 0.8 µm to 8 mm. We are part of the Verder Scientific Group, together with other renowned manufacturers of analytical equipment, including our sister company Retsch, the leading supplier of laboratory mills and sieve shakers. We command a comprehensive global distribution network and subsidiaries to support our customers around the globe. Our German headquarters are in Haan (near Duesseldorf).

Could you please explain the basic theory behind dynamic image analysis?

At first glance, the principle of dynamic image analysis is quite simple. A stream of particles passes a light source and a camera system captures the shadow projections on digital images. The software evaluates every image and provides size and shape of the recorded particles. The particles can pass the measurement zone either in free fall, in an air flow, or suspended in a liquid. The big challenge in dynamic image analysis is to process a vast amount of data in a very short time. The CAMSIZER X2, for example, acquires more than 300 image frames per second and one image may potentially contain 100 particles or more. Analysis of a huge dataset with many particles ensures statistically sound results and high reproducibility. This sets dynamic image analysis apart from microscopy which is called static image analysis because the particles are not moving relative to the camera during the measurement. In microscopy, the number of frames and particles that are acquired and evaluated is significantly lower. Hence, the result suffers from poor statistics and repeatability.

How does this technique differ from static laser light scattering?

In laser scattering, as described in ISO 13320, the size distribution is calculated from light intensity and scattering angle of a laser beam interacting with the sample. It is therefore an indirect measurement. Sophisticated software algorithms and various assumptions and approximations are necessary to calculate the size distribution. One basic assumption in any light scattering theory is that all particles are spherical. But we all know that real-world particles are hardly ever perfect spheres. In laser diffraction, for example, an elongated crystal can be interpreted as a large or small particle depending on the orientation in which it passes the laser beam. Consequently, no information on particle geometry is available.

Image analysis is a direct measurement of particle size and the basic principle is “what you see is what you get”. This means that width, length, area, perimeter as well as several parameters describing the particle shape can be obtained directly from each particle projection.

Another important difference is that a laser granulometer evaluates a signal that is generated simultaneously by a huge number of particles, whereas in image analysis, particles are evaluated individually one by one.

What advantages does dynamic image analysis have over static laser light scattering?

Image analysis has a clear and direct particle size and shape definition whereas laser diffraction only provides a size distribution based on an equivalent diameter. The accuracy is therefore much better in image analysis and the information content is higher. With image analysis length, width and various shape parameters like aspect ratio, roundness, circularity, symmetry, and many more are directly accessible. Laser diffraction evaluates a signal which is generated by an ensemble of particles, i. e. the particles are not examined individually. This leads to limited resolution and low sensitivity, especially for very small concentrations of oversize and undersize particles. Dynamic image analysis can detect oversize and undersize particles even if the concentration is as low as 0.01 % of the sample. Precise characterization of such outliers is a very important information for many applications.

Are these advantages similar to the advantages dynamic image analysis has over sieve analysis?

Sieve analysis is the most common method for particle sizing, especially in routine analysis for quality and production control. Most product specifications in the industry are based on sieving data. Sieve analysis, however, is a very time-consuming and error-prone technique. Including weighing, sieving, and cleaning, the whole process can add up to 20 or 30 minutes. The typical analysis time in dynamic image analysis lies between 2 and 5 minutes, with virtually no manual labour, so the time saving alone is a strong pro for dynamic image analysis. It is possible to achieve a much higher sample throughput, more thorough quality control and shorter reaction time. Unlike sieve analysis, dynamic image analysis is practically automatic and maintenance-free and is therefore much less susceptible to any type of error.

Beside the time disadvantage, sieve analysis is associated with some other drawbacks: sieves may be damaged without being noticed by the operator or they may gradually change their aperture size due to wear and abrasion. However, the most common mistake in sieving is overloading. If apertures get blocked by particles jammed into the openings, passage of the fine particles is inhibited.

Sieves have significant variations and tolerances in their real aperture size, although they are produced and inspected according to an ISO standard. The average aperture size of a 500 µm sieve for example can deviate by +/- 16.2µm, and single apertures up to 580.5 µm are still allowed.

The good news is that by choosing the appropriate evaluation options and size definition in the CAMSIZER software, the data obtained by sieving and image analysis can be brought into perfect agreement, provided that the sieve analysis is executed correctly. Hence, product specifications based on sieving may remain unaltered when changing to CAMSIZER.

What are the down sides of dynamic image analysis?

If the size of the sample is within the range of the instrument, dynamic image analysis is superior to any other technique. Unfortunately, for particles below 1 µm photo-optical methods are not applicable. Here, laser light scattering is the best choice.

Do you have an example of these advantages in practice?

Our customers come from many different industries and the spectrum of applications is extremely wide. On our website we present many application notes and technical reports which will give an overview of the range of use of the CAMSIZER P4 and CAMSIZER X2. I will name just a few examples here: Metal and plastic powders for additive manufacturing and 3-D printing are analysed for the size distribution, but also for particle shape. Non-round, elongated, fused and oversize particles may influence flowability for the worse and cause problems in the sintering process. With dynamic image analysis these unwanted particles can be recognized easily. Other application examples are pharmaceutical APIs and excipients, foodstuff, construction materials, minerals, activated carbon, sand and gravel, extrudates and of course any material that is produced by spray or fluidized bed granulation.

What other benefits would a user have when using the CAMSIZER?

Image analysis provides valuable data and offers new insights into the nature of a sample. Thus, CAMSIZER instruments are not only used in quality control but also in research and development. With the Particle X-Plorer software it is possible to conveniently identify, evaluate and display individual particles, for example the unwanted, non-spherical particles in additive manufacturing that I have previously mentioned.

In routine analysis, one big advantage of the CAMSIZER is that the instruments are virtually maintenance-free and can be deployed in a rough industrial environment as well as in a laboratory. Once the measurement routines are established, the handling is as easy as this: fill in sample and push a button. Hence, a CAMSIZER can be operated by anyone with minimum training and it runs 24/7 to provide uninterrupted quality and production control.

What sets the CAMSIZER apart from any other dynamic image analyzers on the market?

Retsch Technology offers a complete range of dynamic image analyzers for all types of samples: free flowing granulates, agglomerated powders, and suspensions. The CAMSIZER instruments are extremely versatile, the modules for wet and dry dispersion of the CAMSIZER X2 can be exchanged within a few seconds. For the CAMSIZER P4, an AutoSampler is available to save the operator even more time and work.

The most important advantage in my opinion is the unmatched dual camera technology which enables a measuring range of more than factor 1000 between smallest and largest particle. Here, two cameras with different magnifications analyze the particle flow simultaneously. A high-resolution ZOOM camera captures and evaluates small particles with great accuracy, a BASIC camera with a large field of view analyzes a vast number of particles and provides excellent measurement statistics. The results from both cameras are combined by a sophisticated algorithm to produce an accurate result over the entire size range. There is no need for changing lenses, cameras, or adjusting the optical system.

What’s next for RETSCH Technology?

We always strive for 100 % customer satisfaction. This means that our efforts do not end after purchase or installation. Application support, training and counselling continue throughout the lifetime of the instrument which can be 20 years or more. What is always important for us is to know our customers’ needs so we can react accordingly. Therefore, our team is constantly improving our instruments and developing both hardware and software to explore new fields of applications.

Finally, new product releases are scheduled for 2019, so I recommend signing up to our customer newsletter to stay informed.

About Gerhard Raatz

Gerhard Raatz graduated in Physics in 1986 at the Martin-Luther-University Halle/Saale.

He worked in the field of mechanical process engineering in a research institute until 1990. Since 1991 he has been active in the sales of particle measurement systems for some large international companies. Since 2004 he is Sales Director at RETSCH Technology.