Particle characterization is the process of identifying various particles by particle shape, size, surface properties, charge properties, mechanical properties, and microstructure. There is a broad range of commercially available particle characterization techniques that can be used to measure particulate samples. Each has its strengths and limitations and there is no universally applicable technique for all samples and all situations.
Many criteria need to be considered while deciding which particle characterization techniques are required, and can include the following:
- Which of the particle properties are really important?
- What is the particle size range?
- Are the samples polydisperse or is there a need for a wide dynamic range?
- How quickly must measurements be done?
- Is there a need for measurement at high resolution?
- Is there need for good statistical sampling for robust QC measurement?
- Is there a need to disperse the sample wet or dry?
All particle characterization techniques involve a degree of sub-sampling in order to make a measurement. Even in the case of particle counting applications where the entire contents of a syringe are measured, only a small fraction of all the syringes on a production line are examined.
The root cause of issues surrounding unreliable measurements is very often related in some way to sampling. It is hence required that the sub sample measured by the instrument is as representative as possible of the whole.
While instruments such as laser diffraction require presentation of the sample as a stable dispersion, the effects of any sampling issues are reduced by homogenizing, stirring and recirculation of the material.
This does not deal with the challenge of taking a representative 10g aliquot from a 10,000kg batch. A spinning rifler is a common method widely used for increasing the robustness of powder sampling. Figure 1 shows an illustration of a spinning riffler device.
Figure 1. Illustration of a spinning riffler device.
There are two basic approaches to sample dispersion:
- Wet dispersion - particles dispersed in a liquid
- Dry dispersion - particles dispersed in a gas (usually air)
In wet dispersion, individual particles are suspended in a liquid dispersant. The wetting of the particle surfaces by the dispersant molecules lowers their surface energy, reducing the forces of attraction between touching particles.
In microscopy-based techniques, wet sample preparation methods can be used to initially disperse the sample onto a microscope slide.
In a dry powder dispersion, the dispersant is normally a flowing gas stream, most typically clean dry air. The dry dispersion process is normally a higher energy process than wet dispersion.
As shown in Figure 2, three different types of dispersion mechanism act upon the sample.
Figure 2. Illustration of the three dry powder dispersion mechanisms with increasing energy/ aggressivity.
The most dominant dispersion mechanism will depend on the dispenser design with particle-wall impaction providing more aggressive high energy dispersion than particle-particle collisions or shear stresses.
Techniques: Laser Diffraction Particle Sizing
Laser diffraction is a widely-used particle sizing technique, for materials ranging from hundreds of nanometers up to several millimeters in size. The key reasons for its success are:
- Wide dynamic range - from submicron to the millimetre size range
- Rapid measurements - results generated in less than a minute
- Repeatability - large numbers of particles are sampled in each measurement
- Instant feedback - monitor and control the particle dispersion
- High sample throughput - hundreds of measurements per day
- Calibration not necessary - easily verified using standard reference materials
- Well-established technique - covered by ISO13320 (2009).
Measurement of particle size distribution by laser diffraction is done by determining the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample.
Light is scattered by large particles at small angles relative to the laser beam and small particles scatter light at large angles as shown in Figure 3.
Figure 3. Scattering of light from small and large particles.
The Mie theory of light is used in laser diffraction to determine the particle size distribution, assuming a volume equivalent sphere model.
In cases where optical properties are not known, the user can either measure them or make an educated guess and use an iterative approach based upon the fit of the modelled data.
Sample dispersion units
Sample handling and dispersion are controlled by sample dispersion units that are designed to measure the sample either wet or dry. These ensure that the particles are delivered to the measurement area of the optical bench at the correct concentration and in a suitable, stable state of dispersion.
A typical laser diffraction system is made up of three main elements:
- Optical bench – The optical bench is shown in Figure 4
- Sample Dispersion Units
- Instrument software
Figure 4. Optical layout of a state-of the-art laser diffraction instrument.
Techniques : Dynamic Light Scattering
Dynamic light scattering (DLS), also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS), is a non-invasive, well-established technique for measuring the size of particles and macromolecules typically in the submicron region down to below 1nm. It can be used for sample measurement consisting of particles suspended in a liquid e.g. proteins, polymers, micelles, carbohydrates, nanoparticles, colloidal dispersions, and emulsions
The main benefits are:
- Particle size range ideal for nano and biomaterials
- Small quantity of sample required
- Fast analysis and high throughput
- Non-invasive allowing complete sample recovery
Particles in suspension undergo Brownian motion caused by thermally induced collisions between the suspended particles and solvent molecules.
If the particles are illuminated with a laser, the intensity of the scattered light fluctuates over very short timescales at a rate that is dependent upon the size of the particles; smaller particles are displaced further by the solvent molecules and move more rapidly.
A conventional dynamic light scattering instrument includes a laser light source, which converges to a focus in the sample using a lens. Light is scattered by the particles at all angles and a single detector, traditionally placed at 90° to the laser beam, collects the scattered light intensity.
In state-of-the art instruments, NIBS (Non-Invasive Back-scatter) technology extends the range of sizes and concentrations of samples that can be measured.
Techniques: Automated Imaging
Automated imaging is a direct high resolution technique for the characterization of particles from around 1 micron up to several millimeters in size. Individual particle images are captured from dispersed samples and studied to determine their particle size, particle shape and other physical properties.
Static imaging systems require a stationary dispersed sample whereas in dynamic imaging systems the sample flows past the image capture optics.
Typical applications include:
- Measurement of shape differences where particle size alone does not give differentiation
- Detection and/or enumeration of agglomerates, oversized particles or contaminant particles
- Size measurement of non-spherical particles such as needle shaped crystals
- Validation of ensemble based particle size measurements such as laser diffraction.
A typical automated imaging system is composed of three main elements:
- Sample presentation and dispersion
- Image capture optics
- Data analysis software
Techniques: Electrophoretic Light Scattering (ELS)
Electrophoretic Light Scattering (ELS) is a technique used to measure the electrophoretic mobility of particles in dispersion, or molecules in solution. This mobility is often converted to zeta potential to enable comparison of materials under different experimental conditions.
Particle related properties: Rheology
The physical properties of particulate based materials also have an influence on the bulk or macro properties of products and materials. Therefore when working with particulate based formulations it is also important to consider macro properties, such as rheology, in order to better understand the behavior of formulated materials.
About Malvern Panalytical
Malvern Panalytical provides the materials and biophysical characterization technology and expertise that enables scientists and engineers to investigate, understand and control the properties of dispersed systems.
These systems range from proteins and polymers in solution, particle and nanoparticle suspensions and emulsions, through to sprays and aerosols, industrial bulk powders and high concentration slurries.
Used at all stages of research, development and manufacturing, Malvern Panalytical’s instruments provide critical information that helps accelerate research and product development, enhance and maintain product quality and optimize process efficiency.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.