Welcoming a new generation of particle size analyzers

Phil Black - PII Editor 25/11/2011

103 4 minutes read

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By Carl Levoguer – Product Marketing Manager, Malvern Instruments

Laser diffraction is now the particle sizing technique of choice across almost all of the processing industries. We look at how the newest systems extend its application and deliver the full benefits of laser diffraction technology to the widest possible user base.

For processors of particulate products, particle size measurement is an almost universal requirement. Particle size influences the rate of reaction achieved with a solid reactant or catalyst, and similarly dissolution rate, of a drug in vivo, for example. Size and size distribution also impact how particles pack together: in a suspension; or when filling a mould, as in the production of ceramic and metal components. Inhaled product formulators control particle size to ensure effective delivery to the lungs, while paint developers control it to achieve the right hue and tint. Food manufacturers manipulate particle size to influence our enjoyment of a product or our perception of its quality.

In many industries particle size data are therefore essential at every stage of the production cycle – from research through to QC. Laser diffraction meets this need in very many instances and has some intrinsic benefits which include:

o    Flexibility (wet and dry measurement)
o    Broad measurement range
o    Speed of data acquisition
o    Ease of automation

These advantages have been exploited to produce flexible, highly automated instruments for use in the laboratory, and robust systems for real-time process monitoring. However greater application has meant increased demand for smarter technology. This article examines how the very latest systems, as exemplified by the Mastersizer 3000 (Malvern Instruments) answer these industrial requirements.

Introducing laser diffraction
The essential elements of a modern laser diffraction analyzer are shown in Figure 1. A collimated beam of light, from one or more sources, passes through a sample, which scatters the light according to the size of any particles present. Large particles scatter strongly at relatively narrow angles to the incident beam while smaller particles scatter more weakly at wider angles. During measurement, a multi-element detector records the light scattered by the particles, the application of Mie theory delivering the particle size distribution responsible for the detected pattern.

Particle size analysis using laser diffraction is based on the fundamental laws of light behaviour and so calibration requirements are minimal. Furthermore, the technique is rapid, non-destructive, and suitable for a very wide range of industrial applications, encompassing both wet and dry measurement across a broad measurement range.

New hardware
Laser diffraction technology has advanced considerably over the last decade both in terms of the core optical hardware used to deliver performance and, crucially, the enabling software that makes that performance accessible to the user.

Bench space is now an issue in many laboratories so packing performance into a smaller footprint is becoming increasingly important. On the other hand pressures on the measurement range of the technique are equally intense. In many industries there is a trend towards working with finer particles and, of course, there is widespread interest in nanoscale materials.

The Mastersizer 3000 uses an innovative folded optical design to deliver the broadest of measurement ranges – from 10nm to 3500µm – with a single lens, in an optical bench just 690mm long. It incorporates a 10 mW blue solid state light source for precise resolution in the sub-micron range and applies the highest of data acquisition rates (10 kHz). Faster data acquisition rates mean more frequent sampling of the detected light scattering so in practical terms they equate to equivalent analytical detail in a shorter measurement time, or more detail in an equivalent measurement time. The bottom line is short measurement times, and better accuracy and system-to-system reproducibility, even for polydisperse samples with small particle populations at either end of the measurement range.

There have also been substantial developments to the hardware used to prepare the sample for measurement. In wet measurement, for example, new accessories that more efficiently apply ultrasound and agitation can cut dispersion times by as much as 50% (see figure 2). With dry measurement, on the other hand, application of the latest theory has enabled the development of a dry dispersion engine that extends the advantages of dry measurement – shorter measurement times and lower environmental impact – to the widest range of materials.

Lightening the analytical load
While expert interest may be stimulated by performance developments, others simply want to allow all potential users in an organisation to reliably use the technology and generate secure data. Laser diffraction systems automated via standard operating procedures (SOPs) are simple to use, once a method has been locked into the system, but reaching this point requires the application of appropriate method development expertise: the systematic consideration of sampling, dispersion and measurement conditions. Software developments are increasingly focused on providing valuable integrated support in this area.

With the Mastersizer 3000, for example, the interface allows users to change both wet or dry dispersion parameters in real time and instantly see the effect. This simplifies the identification of appropriate measurement conditions accelerating progress towards a robust method. The immediate feedback of relative standard deviations rapidly identifies the point at which repeatability targets (ISO13320 or USP) are met.

Once a method is locked down and stored, routine use is simply a matter of selecting and running the required protocol. At this point requirements turn to monitoring the success of individual measurements, a process that is helped considerably by being able to see the instrument working in real time. Of even more help though is software that provides feedback as to the success or otherwise of an analysis, as exemplified by the Data Quality Tool illustrated below. This software tool provides a critical assessment of the measurement data and results, and delivers expert interpretation at the point of measurement, telling the user, for example, if concentration levels are outside the range for accurate analysis. Its application enhances the security of the information flow by assuring the integrity of every analysis.

In conclusion
Laser diffraction is now a core materials characterization technique for numerous industries. Widespread application of the technique makes ease of use a defining issue, but other aspects of performance are also crucial, most notably precise resolution across the full measurement range which now extends well into the nano-region.

The introduction of a new laser diffraction instrument is a timely reminder of just how effectively the attributes of laser diffraction have been harnessed. With its unique optical design, small footprint and efficient dispersion accessories this latest system offers performance improvements that support the drive for excellence in data generation, flexible efficiency in working practices and optimal use of the technology by every user.

Malvern Instruments Limited
Malvern
Worcestershire

Can be contacted on

Tel: 01684 892456
Fax:01684 892789
E-mail: salesinfo@malvern.com
Web: www.malvern.com