Dynamic Light Scattering (DLS)

Dynamic Light Scattering (DLS) is a rapid particle size method used to screen for aggregation and assess overall size homogeneity. It is widely used as a low-resolution, high-throughput technique, but its regulatory role depends strongly on the modality.

At a glance:

AAVs / proteins: Characterization and stability screening only (not a standalone release method)

mRNA-LNPs: Accepted release and stability test for particle size and polydispersity (PDI)

Best used when: You need fast detection of aggregates or gross size shifts

Not appropriate when: You need species-level resolution (e.g., empty/full capsids, monomer/dimer separation)

DLS reports an intensity-weighted hydrodynamic diameter (Z-average) and polydispersity index (PDI). Because scattering intensity scales strongly with particle size, even trace amounts of aggregation can dominate the signal—making DLS an excellent early-warning tool, but a poor method for detailed quantitation of complex mixtures.

• Legacy Techniques: Classical light scattering methods that measured only static scattering intensity. Early DLS instruments (also known as Photon Correlation Spectroscopy, PCS) used less powerful lasers and slower correlators, limiting their sensitivity and speed.
• Established Standard: Modern cuvette-based DLS systems. These are the standard platforms for routine, rapid assessment of particle size and polydispersity, though higher-resolution methods are required for definitive characterization.
• Emerging Alternatives: Multi-Angle DLS (MADLS), which measures scattering at multiple angles to produce a higher-resolution, angle-independent size distribution. Another evolution is its integration as a detector for SEC (SEC-DLS) to measure the size of species as they elute from the column.

DLS measures the hydrodynamic size of particles by analyzing the time-dependent fluctuations in scattered light caused by Brownian motion.

1. Brownian Motion: Particles in a liquid move randomly due to collisions with solvent molecules. Small particles move quickly, while larger particles move more slowly.
2. Light Scattering: A laser illuminates the sample, and the moving particles scatter the light. This causes the intensity of the scattered light at a fixed detector to fluctuate over time.
3. Correlation: A digital correlator measures the rate of these intensity fluctuations. Small, fast-moving particles cause rapid fluctuations; large, slow-moving particles cause slower fluctuations.
4. Size Calculation: The software uses the Stokes-Einstein equation to convert this rate of fluctuation into an intensity-weighted average hydrodynamic diameter (Z-average). A polydispersity index (PDI) is then calculated from the cumulant analysis of the correlation function to describe the distribution width.

• Output: The primary outputs are the Z-average (intensity-weighted mean hydrodynamic diameter) and the Polydispersity Index (PDI) (a measure of the distribution width, from 0 to 1). A size distribution graph is also generated.
• Analysis: The Z-average provides the average size, while the PDI indicates the homogeneity of the sample.
• Interpretation: For a monodisperse sample like AAV, a "good" result is a Z-average close to the expected diameter (~25 nm) and a low PDI (e.g., < 0.2). An increased PDI or an apparent larger-size component (>100 nm) suggests aggregation, which should be confirmed by higher-resolution methods.

• Fast & Non-Invasive: Measurements are very rapid (minutes) and do not alter the sample.
• High Sensitivity to Large Aggregates: Exceptionally sensitive to the initial formation of large aggregates, making it an excellent screening tool for instability.
• Low Sample Requirement: Requires only microliter volumes of sample.
• Easy to Use: Modern instruments are highly automated, making routine screening straightforward.

• Low Resolution: Cannot resolve species with small size differences. It is not suitable for distinguishing individual AAV particles from small aggregates or an empty capsid from a full one.
• Intensity-Weighted Bias: The results are heavily weighted towards larger particles. A very small amount of aggregation can significantly skew the average size (Z-average), making it a qualitative screening tool rather than a precise quantitative method for purity.
• Dust Interference: Very sensitive to extraneous contaminants like dust. Meticulous sample handling and filtration are required.
• Not a Standalone Release Method (for most protein/AAV products): Due to its low resolution and semi-quantitative nature, DLS is generally used as a characterization or screening tool for protein and AAV products. However, for modalities where particle size itself is a primary mechanism of action, such as Lipid Nanoparticles (LNPs), DLS is a critical, primary method used for lot release and stability testing.

DLS in Context: Comparison to Other Particle Characterization Methods

DLS is one of several complementary techniques for particle size and aggregation
analysis in biologics. The following table compares DLS to SEC-MALS and AUC - the
two other primary methods used for aggregate characterization and capsid analysis
in CMC programs. Each method offers different trade-offs between resolution,
throughput, and cost.

• System Suitability: System Suitability: Verified using traceable size standards (e.g., NIST polystyrene latex beads) to confirm instrument performance and reproducibility of the Z-average measurement.
• Precision: Demonstrating low variability in Z-average and PDI measurements upon repeated analyses of the same sample.
• Range: Defining the concentration range over which the instrument can provide reliable measurements.
• Robustness: Assessing the impact of minor variations in measurement parameters (e.g., temperature).

Standardization of DLS methods is critical, since results can vary with instrument settings, sample handling, and analysis models. Consistent parameters such as angle of detection, temperature, buffer conditions, and analysis algorithm (cumulants vs. distribution) should be defined and controlled. Reference materials such as NIST-traceable polystyrene latex bead standards—are routinely used to verify instrument performance and ensure reproducibility of Z-average and PDI values. Lifecycle management of these standards and routine system suitability checks are necessary to maintain data comparability across studies, sites, and time.

• Gene Therapy (AAVs): A standard characterization test used to confirm the average particle size is correct (~20-25 nm) and, most importantly, to rapidly screen for the presence of aggregates, often before analysis by AUC or SEC.
• mRNA-LNP Products: A routine release and stability test. DLS is the industry-standard method for measuring the CQA of Particle Size & Polydispersity, which regulators expect to be reported for LNP products.
• mAbs & Proteins: Used during formulation development and characterization as a rapid, orthogonal method to detect the early onset of aggregation.