Dynamic Light Scattering (DLS) is a commonly used term to describe a technique which measures the particle size and estimated distribution of submicron particulate systems. In addition, the terms Photon Correlation Spectroscopy (PCS) and Quasi-Elastic Light Scattering (QELS) have also been used historically to refer to the same analytical principle. By any name, the technique is widely recognized throughout the pharmaceutical and industrial world reflected in the existence of several standards describing the technique (i.e. ISO 22412, ISO 13321, and ASTM E2490-09).
Dynamic Light Scattering requires particles approximately less than a micron in size to be homogenously suspended in a fluid (aqueous or organic). A few examples of suitable systems could include aggregated proteins, pigment components, micelles or emulsion droplets suspended in a continuous phase. The nano-dispersed system is placed into the optical path of a LASER. The LASER is then scattered upon interacting with the particles in the suspension which are moving by Brownian Motion. The scattered light is captured by a detector over the course of the analysis to determine the rate of diffusion (i.e. how fast the particles move within a system due to Brownian Motion) and the average Hydrodynamic particle size (referred to as the Z-Average) is calculated on an Intensity weighted basis using the Stokes-Einstein equation. In simple terms, small particles move/diffuse more rapidly than larger particles. While other reporting formats are available, the most widely accepted and recommended way to report results from DLS is on an Intensity basis using the Z-Average along with the Polydispersity Index (PDI). The PDI is an indicator of the “broadness” of the particle size distribution.
While the technique is straight forward in principle, achieving representative results relies on knowledge of the dispersed system (such as optical parameters of the particle/liquid and viscosity of the dispersed system). In addition, optimal results require an understanding of the strengths and limitations of the DLS instrument being used. Different instrument manufacturers vary certain aspects of the data acquisition process such as the angle of detectors or the data processing algorithms. These variations can generate different results between instruments or require a need to alter sample preparation and concentration.
Sample preparation is of the utmost importance in DLS analyses as the analytical principle assumes an “infinite dilution” in an attempt to limit the effect of particle/particle interactions. As a result, depending on the instrument and sample properties, dilution of the sample is sometimes necessary for analysis. However, this requires careful thought and consideration in proper method development as to not destabilize the dispersed system. PTL’s years of experience in sample preparation and variety of in-house DLS equipment allows us to assist with all aspects of our client’s projects and provide solutions. We have the expertise necessary to develop and validate an accurate, precise and robust method for your submicron sized samples.
Below is a list of the DLS instruments available at PTL. Interested clients should contact us to discuss if your sample and project is appropriate for Dynamic Light Scattering analysis and how this analytical technique can provide additional answers to your project.
Malvern Zetasizer Nano - Click here for typical sample report.
Nicomp 380/ZLS - Click here for typical sample report.