In September 2023, a new standard was published for Tunable Resistive Pulse Sensing (TRPS) analysis of particle size distribution. The standard is titled ISO 13319-2:2023, and was published by the International Organization for Standardization (ISO) – the nongovernmental organization and publisher of standards, reports, specifications and guides covering the major sectors.
In general, ISO standards are developed to provide customers, regulators and governments with higher confidence in product safety, reliability and quality. The TRPS ISO standard therefore aligns with this, aiming to improve reproducibility and accuracy by providing general considerations and background information on measuring particle size distribution and concentration.
What is in the TRPS ISO standard?
The TRPS ISO standard serves as a comprehensive guide for users to gain insight into the general operation of TRPS instruments, understand its foundational principles, and navigate practical considerations. Of note, the standard was developed to be company- and product agnostic, meaning that the information is very general as it should be relevant to any TRPS instrument.
Key aspects include information about the general operation and setup of TRPS, methodologies for accurate measurements (i.e. calibration), and instrument and sample considerations (including tips for sample preparation, nanopore wetting and coating, and calibrant and particle concentration). Practical tips are also included, like guidance on selecting the nanopore size, and how to avoid/alleviate bubble formation and nanopore blockages.
Theoretical information covers:
- equations and terminology for TRPS size and concentration measurements
- the role of various electrokinetic effects
- the impact of off-axis transport and the importance of nanopore size selection
- how polarisation affects size and concentration measurements and how applying pressure is important in negating this effect
- dynamic size range and the likelihood of coincidence events
Historical context of TRPS
TRPS belongs to the family of electrical sensing zone analytical techniques. Of note, the addition of the ISO TRPS standard, ISO 13319-2:2023, follows a 2019 ISO publication on the electrical sensing zone method used in Coulter Counters.1
The electrical sensing zone technique, discovered by W. H. Coulter around 1950, has long been utilised for size and count analysis of cells, bacteria and other fine particles. However, advancements in electronics and aperture fabrication have recently broadened its application to nanoparticles. Of note is the use of TRPS across the fields of nanomedicine and extracellular vesicles.
Unique attributes and advantages of TRPS
TRPS stands apart from many other measurement techniques. It is an electrical method – not optical – that measures the change of impedance as individual particles pass through a nanopore.2 It is distinctive as it is a single-particle analysis characterisation technique, not an averaging ensemble technique. TRPS reliably measures particle size distribution, concentration3-5, and zeta potential6,7 of synthetic and biological particles. Measurement capabilities have varied by instrument.8 The Exoid, our latest TRPS instrument, measures particles in the size range of 40 nm to 11 µm.
By employing a multi-pressure calibration procedure, TRPS achieves reproducible and highly accurate particle concentration measurements across specific particle size ranges.5,9 For zeta potential measurements, this calibration procedure is extended to various voltages.7 A main advantage of TRPS is its ability to analyse a broad size range at high resolution, due to the ability to use a range of nanopore sizes.8 Its accuracy has been highlighted in various studies comparing other analytical platforms.8,9
Publication of ISO standard highlights TRPS as a standardisable technique
The publication of the ISO standard reflects how TRPS is a measurement technique with many standardisable aspects. With the use of multi-pressure methodology to determine size distribution and concentration, calibration particles, and the variety of nanopores available to accommodate different particle sizes, TRPS offers users a standardisable approach to high-resolution nanoparticle analysis.