JRC scientists, in collaboration with scientists from two National Metrology Institutions, have succeeded for the first time to link nanoparticle size results, measured by disc centrifugal liquid sedimentation, to the International System of Units (SI) of length, the metre. This makes the measurements of nanoparticles more accurate.
Nanomaterials have attracted enormous scientific and technological interest due to their unique physicochemical properties. The smart use of nanomaterials has already shown their exciting potential to revolutionise a wide range of every day applications and technologies.
With the increased use of nanomaterials in consumer products, there is also a growing concern about their potential risks. In protecting EU citizens, different sectoral legislation with specific provisions for nanomaterial identification and characterisation have been adopted. Although the regulations may apply a different scope of the term ‘nanomaterial’, they all require accurate particle size measurements.
A variety of established techniques such as electron microscopy, dynamic light scattering, particle tracking analysis and disc centrifugal liquid sedimentation (disc-CLS) is available for particle size analyses. While each of these techniques has specific advantages and disadvantages, the method based on disc-centrifugal liquid sedimentation (CLS) is increasingly used because of its relative low cost and its power to discriminate individual particles from particle agglomerates.
The disc-CLS method is based on the principles of Stokes' law. In theory, the method can be used without calibration but this requires a complex and careful assessment of each method parameter. To avoid this cumbersome approach, an alternative method which depends on calibrating the sedimentation time scale with reference particles has become common practice, instead.
The main drawback of the calibration-based (routine) method is that many of the reference particles, used for calibration, are not well-characterised in terms of particle size and particle density, thereby undermining the traceability to the SI and potentially introducing systematic errors in the final measurement result.
During this collaborative study of scientists from the European Commission's JRC, the UK National Physical Laboratory (NPL) and the Australian National Measurement Institute (NMI), it was demonstrated for the first time that particle size results obtained by disc-CLS can be traceable to the ultimate metrological reference, i.e. the SI unit of length, the metre.
To achieve this, particle size results were calculated using the fundamental Stokes equation (calibration-free) and compared these results with particle size results measured with the routine disc-CLS (calibrated with reference particles).
The described methodology resulted in a simplification of the complex traceability network, allowing laboratories to realise SI-traceability of their own results.
Read more in:
V. Kestens et al.: "Establishing SI-traceability of nanoparticle size values measured with line-start incremental centrifugal liquid sedimentation", Separations 6 (2019) 15