Dynamic light scattering (DLS) is a widely used technique for determining the size distribution of nanoparticles in suspension. Standardization of DLS measurements ensures consistency, accuracy, and comparability of results across different laboratories and instruments. International standards organizations such as ISO and ASTM have developed guidelines to regulate DLS measurements, while reference materials from institutions like NIST and JRC play a crucial role in instrument calibration and validation. Interlaboratory comparison studies further enhance measurement reliability, and uncertainty calculations provide a quantitative assessment of result variability.
ISO and ASTM standards provide a framework for DLS measurements, covering instrument performance, sample preparation, and data analysis. ISO 22412:2017 specifies requirements for DLS instrumentation and outlines procedures for measuring particle size distributions in the submicrometer and nanometer range. It defines key parameters such as measurement angle, temperature control, and laser wavelength, ensuring that instruments operate under consistent conditions. The standard also provides guidance on sample handling, including dispersion protocols to prevent aggregation and ensure representative measurements.
ASTM E2490-09 complements ISO 22412 by detailing best practices for DLS measurements, particularly in polydisperse systems. It emphasizes the importance of instrument alignment, baseline verification, and signal-to-noise ratio assessment. The standard also discusses the limitations of DLS, such as its reduced accuracy for highly polydisperse samples or particles outside the typical size range of 1 nm to 1 µm. Both ISO and ASTM standards stress the need for regular instrument calibration using traceable reference materials to maintain measurement accuracy.
Reference materials are essential for DLS calibration and performance verification. NIST provides several certified nanoparticle standards, including NIST RM 8011 (gold nanoparticles, nominal 10 nm), NIST RM 8012 (gold nanoparticles, nominal 30 nm), and NIST RM 8013 (gold nanoparticles, nominal 60 nm). These materials have well-characterized size distributions and are used to validate DLS instrument performance. The Joint Research Centre (JRC) of the European Commission also offers reference materials such as ERM-FD100 (silica nanoparticles, nominal 20 nm) and ERM-FD101 (silica nanoparticles, nominal 80 nm), which serve as benchmarks for interlaboratory comparisons.
The role of reference materials extends beyond initial calibration. They are used in routine quality control to detect instrument drift or misalignment. For example, a laboratory may measure a NIST gold nanoparticle standard weekly to confirm that the instrument continues to report the expected size within acceptable tolerances. Deviations from the certified values indicate the need for maintenance or recalibration. Reference materials also enable comparability between different DLS instruments, as all measurements can be traced back to a common standard.
Interlaboratory comparison studies are critical for assessing the reproducibility of DLS measurements across different facilities. These studies involve multiple laboratories analyzing the same nanoparticle sample using their respective DLS instruments. The results are compiled and statistically analyzed to identify systematic biases or inconsistencies. For instance, a study might reveal that certain instruments consistently overestimate particle size due to differences in detection optics or data processing algorithms. Such findings drive improvements in measurement protocols and instrument design.
The National Nanotechnology Initiative (NNI) and other collaborative programs have conducted interlaboratory studies to evaluate DLS performance. In one such study, participants measured monodisperse and bimodal polystyrene latex samples. The results showed good agreement for monodisperse samples but higher variability for bimodal distributions, underscoring the challenges of analyzing complex systems. These studies highlight the importance of standardized operating procedures and the need for ongoing training to minimize user-dependent errors.
Measurement uncertainty is a key consideration in DLS data interpretation. Uncertainty arises from multiple sources, including instrumental noise, sample preparation variability, and data fitting algorithms. ISO/IEC Guide 98-3 provides a framework for quantifying uncertainty in particle size measurements. The combined standard uncertainty (uc) is calculated by considering all significant contributors, such as temperature fluctuations, laser stability, and dust contamination.
A typical uncertainty budget for DLS might include the following components:
- Instrument repeatability (Type A uncertainty)
- Reference material certification (Type B uncertainty)
- Sample dispersion variability (Type A or B, depending on experimental design)
- Data analysis algorithm limitations (Type B)
For example, if a DLS measurement reports a particle size of 50 nm with an expanded uncertainty (U) of ±5 nm (k=2), this means there is a 95% confidence that the true size lies between 45 nm and 55 nm. The uncertainty value helps users assess the reliability of the data and make informed decisions based on the results.
In summary, DLS standardization relies on international standards, certified reference materials, interlaboratory studies, and rigorous uncertainty analysis. ISO 22412 and ASTM E2490 provide the foundational guidelines for instrument operation and data reporting. NIST and JRC reference materials ensure traceability and instrument performance validation. Interlaboratory comparisons identify measurement discrepancies and promote best practices, while uncertainty calculations quantify the reliability of reported results. Together, these elements establish a robust framework for accurate and reproducible DLS measurements in nanoparticle characterization.
The continued development of DLS standards and reference materials will further enhance measurement consistency as nanotechnology applications expand into fields such as medicine, energy, and environmental science. By adhering to standardized protocols and leveraging certified materials, researchers can ensure that their DLS data is both reliable and comparable across global laboratories.