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[June 2002]
Back Scatter Optics Allow Particle Size Measurement at High Concentration
The back scatter optics in the recently introduced Malvern HPPS (high performance particle sizer) are said to be making possible the accurate measurement of a range of high concentration samples, such as suspensions and particularly emulsions, without the need for dilution.Malvern says that this is a significant advance since many high concentration systems are sensitive to the disruptive effects of dilution, which produce erroneous particle sizing results. Equally, it may be difficult or inconvenient to build in a dilution step.
An example of a highly concentrated system is that of silica particles used in polishing slurries for the semiconductor industry. Here, particle size distribution affects both the substrate removal rate and the surface finish. Typical slurries can contain silica at 20% volume concentration and are opaque.
Classical light scattering systems, explains Malvern, fail to give accurate particle sizing data due mainly to the presence of multiple scattering, whereby light scattered by one particle is rescattered by others before reaching the detector. HPPS back scatter optics minimise multiple scattering by automatically optimising the scattering path length.
Non-invasive back scatter technology is a dynamic light scattering technique that incorporates a unique optical configuration to maximise detection of scattered light while maintaining signal quality. The use of back scattering rather than the typical 90° detection angle is said to improve the sensitivity of the measurement and ensure the smallest possible interference from multiple scattering.
Earlier back scattering techniques suffered from drawbacks that included the need for contact between samples and detector optics, necessitating careful cleaning of the measurement optics between samples to avoid cross-contamination. The HPPS uses a standard disposable cell and, because there is no contact between sample and detector, cleaning is unnecessary.
Malvern says that the ability to move the sample cell and automatically locate the optimum measurement position is a feature unique to the HPPS. This automated positioning ensures that the setting is tailored to the characteristics of the specific sample. The optimum position for measurement is close to the centre of the cell for dilute samples to maximise the observation volume, and close to the cell wall for more concentrated samples to minimise the distance that light must travel through the sample.
The net result is high measurement sensitivity for samples as dilute 0.1ppm and the ability to measure at a concentration up to 20vol%.
ENDS