ISO 26824:2022 — Particle Characterization of Particulate Systems — Vocabulary

1. Standardised Terminology for Particle Characterisation

ISO 26824:2022 (second edition, prepared by ISO/TC 24/SC 4) defines a comprehensive vocabulary for the characterisation of particles and particulate systems, covering size scales from nanometres to millimetres. This standard consolidates terms from approximately 24 published ISO standards and 15 active projects, providing a uniform nomenclature essential for interdisciplinary applications ranging from mining and construction to pharmaceuticals, food technology, medicine, microelectronics, and nanotechnology. The 2022 edition significantly reorganized the first edition (2013) into 17 numbered subclauses within Clause 3, improving navigability and cross-referencing between related measurement techniques.

The vocabulary is organised into 17 clause groups covering different measurement principles and particle properties: representation of size data (3.1), particle shape (3.2), pore size and surface area (3.3), sedimentation methods (3.4), electrical sensing zone (3.5), laser diffraction (3.6), dynamic light scattering (3.7), image analysis (3.8), single particle light interaction (3.9), small angle X-ray scattering (3.10), sample preparation (3.11), electrical mobility analysis (3.12), electrical charge conditioning (3.13), acoustic methods (3.14), focused beam method (3.15), particle dispersion in liquids (3.16), and zeta potential determination (3.17). Each clause contains terms with definitions, notes, and source references to the originating ISO standard.

2. Key Distinctions: Agglomerates vs. Aggregates vs. Primary Particles

One of the most important contributions of this vocabulary is the clear distinction between agglomerates and aggregates — terms frequently confused across industries. An agglomerate is a collection of weakly or medium-strongly bound particles where the external surface area approximates the sum of individual component surface areas; bonding forces are weak (van der Waals, physical entanglement). An aggregate comprises strongly bonded or fused particles with external surface area significantly smaller than the sum of component surface areas; forces include covalent bonds or sintering. This distinction, illustrated with schematic diagrams in the standard, is fundamental for quality control in powder processing and for regulatory classification of nanomaterials.

The standard also defines primary particles (original source particles), constituent particles (identifiable integral components of a larger particle), and secondary particles (agglomerates or aggregates as larger entities). These hierarchical definitions are essential for nanotechnology applications, where the distinction between primary nanoparticles and their agglomerated or aggregated forms has significant implications for toxicity assessment, regulatory classification, and product performance characterisation. The definitions are harmonised with ISO/TC 229 (Nanotechnologies) to ensure cross-domain consistency. For example, a nanomaterial supplied as aggregates of 100 nm primary particles may be classified differently depending on whether reported as 100 nm or >1 um entities.

3. Engineering Design Insights for Particle Analysis Laboratories

The structured vocabulary in ISO 26824:2022 directly supports the selection and specification of particle characterisation instruments. For laser diffraction, terms such as volume-weighted mean diameter D[4,3] and percentile diameters D10, D50, D90 are unambiguously defined, preventing misinterpretation when specifying equipment or reporting results. For pharmaceutical dynamic light scattering (DLS) used in protein aggregation studies, the vocabulary clarifies the critical distinction between intensity-weighted, volume-weighted, and number-weighted size distributions — a common source of confusion that can lead to orders-of-magnitude differences in reported mean sizes for polydisperse samples. For zeta potential measurement (Clause 3.17), terms related to electrophoretic mobility, streaming potential, and electroacoustic methods are all included.

The standard also addresses data uncertainty and representation of classification analysis results, which are crucial for inter-laboratory comparability. Terms related to sample preparation (3.11) highlight methods for obtaining representative subsamples from bulk powders, suspensions, or aerosols, and preventing measurement artefacts from improper dispersion, agglomeration, or sedimentation during analysis. Many measurement discrepancies between laboratories trace to differences in sample conditioning rather than instrument performance.

Understanding the standardised vocabulary in ISO 26824:2022 is essential for any laboratory performing particle characterisation measurements. Proper application of these definitions ensures that particle size data are correctly interpreted and compared across different measurement techniques and laboratories. The standard serves as a crucial reference for quality control documentation, regulatory submissions, and technical communication in industries ranging from pharmaceuticals to advanced materials manufacturing.

Proper application of particle characterisation terminology is essential for quality control documentation, regulatory submissions, and technical communication. The distinction between different weighted distributions and the correct use of percentile diameters are fundamental skills for any particle analysis laboratory seeking ISO 17025 accreditation for particle size measurements.

ISO 26824 serves as an indispensable reference document for particle characterisation laboratories worldwide.

4. Frequently Asked Questions