Basic Guide to Particle Size Analysis -3

Particle Characterization Techniques

Particle characterization techniques available for measuring particle samples are commercially available to a wide extent. Each technique has its unique relative strengths and limitations and is not generally applicable to all samples and situations.

Which Particle Characterization Technique Do I Need?

When determining which particle characterization technique you need there are many conditions to consider.

•   Which particle characteristics are important to me?
•   What is the range of particle sizes I’m measuring?
•   Is the sample polydisperse, requiring a wide dynamic range?
•   How quickly can measurements be conducted?
•   Is high resolution measurement required?
•   Is good statistical sampling needed for robust QC measurement?
•   Should the sample be dispersed wet or dry?
•   What budget do I have prepared?

  The table below provides some basic guidance for judging which of the commonly used techniques is most suitable for specific applications. The indicated size ranges are guidelines only and exact specifications will vary depending on the equipment.

Sampling

  All particle characterization techniques assume some degree of subsampling to conduct measurements. For instance, a particle counter measuring the entire contents of a syringe will inspect only a fraction of all syringes on a product line. 

  The root cause of issues with unreliable measurements often lies in the way samples are taken. Therefore, it is essential that the subsample measured by equipment represents the entire sample as closely as possible.

  Equipment (e.g., laser diffraction) requires samples to be stably dispersed by homogenizing materials, stirring and recirculating, thus minimizing effects caused by arbitrary sampling issues.

  However, this does not address the challenge of representing 10g aliquot from a 10,000kg batch.

  A widely used method to enhance the robustness of powder sample sampling is using a device known as a spinning riffler.

In a spinning riffler, sub-samples are extracted from the flowing powder through a hopper into the rotation axis of containers at regular intervals. In case any of the sample portions enter the hopper, each container will contain a representative sub-sample.

Sample Dispersion

  Many particle characterization techniques require the sample to be analyzed in a dispersed form, with individual particles spatially separated. For this, there are two basic approaches.

• Wet dispersion – Particles are dispersed in a liquid

• Dry dispersion – Particles are dispersed in a gas, usually air

Wet Dispersion

  In wet dispersion, individual particles become suspended within a liquid dispersant. Wetting the particle’s surface with molecules of the dispersant reduces attraction between contacting particles, lowering the surface energy and allowing particles to be separated and suspended.

  Adding such a wetting behavior and subsequent improvement in particle dispersion behavior can significantly improve. It is usual to apply a small amount of energy to the sample to disperse the individual particles. This typically involves stirring or shaking the sample in bulk, while for very fine materials or tightly bound agglomerates, ultrasonic radiation is sometimes used.

In microscope observation-based techniques, the initial sample can be dispersed onto a microscope slide via wet sample preparation methods. After the dispersant has evaporated, the dispersed particles can be analyzed in their dry state.

Dry Dispersion

  In dry powder dispersion, the dispersant is typically a flowing gas stream, most commonly clean dry air. The nature of dry dispersion processes generally involves higher energy processes than wet dispersion.

  As depicted below, three different types of dispersion mechanisms apply to the sample. These three different types of dispersion mechanisms for increasing energy input include:

  The most widely used dispersion mechanism will depend on the design of the disperser. Impact between particles and a wall causes more aggressive, high energy dispersion compared to particle-to-particle collision or shear forces.

Using expensive or potentially hazardous solvents can make dry dispersion an attractive option. However, due to high inter-particle forces within the material, it is very challenging to fully disperse particles less than 1 micron using dry dispersion.

  For fragile particles, applying sufficient energy for dispersion without breaking particles during the process requires particular care. In such cases, wet dispersion methods should be validated as a reference.

Technique 1: Laser Diffraction Particle Size Analysis

  Laser diffraction is a widely used particle size analysis technique for materials ranging from hundreds of nanometers to millimeters. Main reasons for its success are:

•   Wide Dynamic Range – from less than 1 micron to millimeter size
•   Rapid Measurements – results produced in less than a minute
•   Reproducibility – large number of particles measured at each sampling
•   Immediate Feedback – for monitoring and controlling particle dispersion
•   High Throughput – capability for hundreds of measurements per day
•   No Calibration Required – easy verification using standard reference materials
•   Certified by ISO 13320 (2009)

Principle

  Laser diffraction measures particle size distribution by measuring changes in angular variation of light scattered by a dispersed particle sample as it passes through a laser beam.

  As shown below, large particles scatter light at small angles relative to the laser beam while small particles scatter light at large angles. Subsequently, measurement of scattering intensity data at different angles allows calculation of particle size using Mie scattering theory to generate the scattering pattern. Particle size is recorded as the volume-equivalent spherical diameter.

Optical Properties

  Laser diffraction assumes a volume equivalent spherical model and utilizes Mie scattering theory to calculate particle size distribution.

  Mie theory requires both the optical properties (refractive index and absorptive index) of the dispersant and the sample being measured to be known. Dispersant optical properties are usually found in open literature and many modern instruments come equipped with a database to include common dispersants. 

  When optical properties are unknown for the sample, users may either measure the sample or make conjectures and use an iterative approach by comparing results with modeled data to actual data.

  A simplified approach uses the Fraunhofer approximation which doesn’t require sample optical properties but extraordinary care must be taken when assessing samples with particles below 50μm or relatively transparent samples.

Device Equipment

  A typical laser diffraction system consists of three main elements.

1. Optical Bench

  The dispersed sample passes through the measurement zone of the optical bench where the laser beam interrogates the particles. A series of detectors then precisely measure light intensity scattered by particles at a wide range of angles.

2. Sample Dispersion Unit

  Sample handling and dispersion are managed by a sample dispersion unit designed for either wet or dry measurements. These units ensure particles are delivered at correct concentration and in an appropriately stable dispersed state to the measurement zone at the optical bench.

  Wet sample dispersion units use aqueous or solvent-based liquid dispersants. To maintain the sample’s state of suspension and homogenization, the sample continuously recirculates through the measurement zone.

  Dry powder sample dispersion units suspend samples in a flowing gas stream, typically dry air. Usually, the entire sample only passes through the measurement zone once, so capturing data at high speed, commonly up to 10kHz, is advisable for representative sample measurement.

3. Instrument Software

   The instrument software controls the system during the measurement process and analyzes scattering data to calculate the particle size distribution. In more advanced measurements, immediate feedback and expert advice on the quality of the results are provided as the method unfolds.

Laser diffraction applications are certified by the international standard ISO 13320: 2009 and are strongly recommended for anyone using this technology routinely.

Next Introduction Content

 Technique 2. Dynamic Light Scattering

 Technique 3. Automated Imaging Technology

 Technique 4. Electrophoretic Light Scattering