The JRC produced two new certified reference materials (CRMs) to allow quality control of the size measurements of powders. These CRMs will support a wide range of industries that need to ensure the particle size of their products such as pigments, cement, metal powders.
- colours and dyes (e.g. pigment particles),
- chemicals (e.g. catalysts and ingredients),
- building and construction products (e.g. sand and cement particles),
- advanced materials (e.g. metal powders for 3D-printing and powder metallurgy)
- food (e.g. coffee powder)
- pharmaceutical and cosmetic industries (e.g. ingredients).
The particle size can be measured by a number of methods which probe different material properties.
Particularly popular are imaging methods (observing a particle in an optical or electron microscope) or laser diffraction (exploiting the size dependency of the angle of scattered light).
Although these methods either do not require calibration with particles of known size (laser diffraction) or require just calibration of the length scale (imaging), calibration alone cannot ensure reliable results.
Reliable and reproducible results are best achieved by measurements using certified reference materials (CRMs) mimicking as closely as possible real samples. This means, a perfect CRM should consist of irregular particles that span a rather wide size range.
In the past, the European Commission produced two quartz materials certified for particle size as determined by gravimetric sedimentation and by sieving. These methods are rarely used nowadays, so the JRC replaced these two materials with CRMs based on corundum, well characterised by laser diffraction and imaging methods.
Different grades of commercial corundum were mixed and subdivided to obtain one batch with particle size between 1 and 8 µm (ERM-FD066) and another batch with particle size between 14 and 80 µm (ERM-FD069), respectively.
The homogeneity of each batch was confirmed by measurements and it was demonstrated that vibrations during subsequent dispatch to customers do not lead to de-mixing. The certified values were obtained from a collaborative study amongst expert laboratories from Europe, the Americas, Asia and Australia. The laboratories used laser diffraction (both with dispersion in liquid and in gas) as well as optical and electron microscopes to characterise the volume and number based particle sizes. Based on these measurements, certified values were assigned for the different evaluation models in laser diffraction and for imaging methods.
The successful completion of the certification project resulted in two CRMs to allow laboratories to demonstrate their capabilities in the determination of particle size. The materials are available from the JRC's online catalogue or from the JRCs authorised distributors.