Evolution of particle sizing technique
Written by: Paul Kippax
In the previous blog of this series, ‘Why has laser diffraction endured?’ I waxed lyrical about what Leonardo da Vinci may have thought of laser diffraction. Now that I find myself on the brink of a discussion covering the evolution of the technique, at the risk of being twee, I simply cannot resist the temptation to bring in the late, great Charles Darwin!
“It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change.” Charles Darwin.1809-1882
Survival of the adaptable
Laser diffraction relies on the accumulation of scattered laser light data collected at various angles. The wider the range of angles measured, the greater the system resolution, especially in relation to the particle size range which can be measured. Poor resolution compromises the generation of a reliable particle size distribution and makes quantifiable data at extreme size ranges unreliable.
Early laser diffraction systems used several different lenses to capture and focus light scattered at different angles onto the laser diffraction detector array. However, this required routine re-alignment of the instrument, which limited both productivity, and also impacted the accuracy of sample analysis, especially for samples with broad particle size distributions.
Some of these multi-lens systems remain in use, but most have now been replaced by instruments fitted with improved optics that address both measurement range and resolution.
While international standards (ISO13320) state that laser diffraction may be usefully applied over a range from 0.1 to 3000 microns, achieving this optimal capability has required significant evolution in optical hardware.
ISO13320 provides a useful summary analysis of the relative merits of the two optical set-ups that now dominate commercial laser diffraction system design. The traditional Fourier optical set-up – used extensively in instruments built during the 1980s – places the data collection lens after the sample measurement zone. This allows measurements to be made over a wide path length (which remains useful for spray measurement), but limits the maximum scattering angle that can be measured. The Reverse Fourier set-up, now recognised as a standard alternative, places the lens before the measurement zone. While this does restrict the measurement path length, it has the advantage of allowing detectors to be positioned both in front of and behind the cell, enabling measurements over a wider range of angles. This gives access to a broader dynamic range, without requiring lens changes, and consequently improves resolution for the presence of out-of-specification particles
Extending into the sub-micron range
The sub-micron range is an area of increasing interest in many fields as companies exploit the unique properties of nano-materials in product design. Hardware features that aim to extend laser diffraction measurement down below 0.1 microns include:
- Use of an extra light sources which have a small wavelength (e.g. blue light rather than red)
- One or more off-axis light sources, enabling collection of scattering data at higher angles using a standard focal-plane detector
- Scattered light detectors set at less than 90o but larger than the conventional range sampled by the Fourier set-up (forward scattering)
- Scattered light detectors at angles greater than 90o (backscattering)
Improvements in computing power and software capabilities have also driven significant developments in the evolution of laser diffraction measurements. In my next blog in the ‘Why has laser diffraction endured?’ series, I aim to review the evolution of automated laser diffraction measurements.