Laser diffraction is an excellent technique for determining particle size as it allows the user to measure particle size on both a liquid and dry basis. When working with dry materials choosing the most appropriate way to disperse the material will depend on the characteristics of the sample as well as the goal of the analysis. But knowing this information, how do you decide which dispersion technique may be right for you?
How Laser Diffraction Works
Before discussing which factors can help you decide whether a liquid or dry dispersion is more appropriate for your material, it is essential to discuss the basics of laser diffraction. For those unfamiliar with laser diffraction, it is an ensemble technique that evaluates the light scatter from a cloud of suspended particles in a medium as they pass through a laser. The scattering angle is then correlated to an equivalent spherical diameter based on additional input by the user. For laser diffraction analyses at Particle Technology Labs, there are two types of dispersion techniques – liquid and dry.
A liquid analysis typically involves an appropriate carrier fluid which allows the particles to remain suspended and separate during the analysis and that does not cause adverse effects to the material (such as dissolution or reaction). With liquid analyses, the sample remains suspended in the carrier and the particles are analyzed as a cloud as they pass repeatedly through the path of a laser. This allows the user to evaluate the stability of the analysis over the course of multiple measurements.
With dry analyses, the dry material is pulled with compressed air through the instrument and the path of the laser directly into a collection vacuum. This means that the particles are not recirculated as they are with liquid analyses, due to this there is little understanding on the stability of dry analyses during measurement compared to liquid.
Now that we’ve discussed what laser diffraction entails, it is also important to talk about what type of dispersion techniques may be necessary for each type of analysis. With liquid analyses samples may be manipulated if needed using different dispersion techniques, with dry analyses this is not as simple. As such, certain dry materials may not be appropriate for dry analyses and vice versa. With dry analyses, the sample is fed through a compressed air stream. The type of sample disperser, feed rate of the sample and applied dispersion energy (i.e. pressure), are the only variables that may be altered to disperse the particles without causing particle attrition.
Some materials are more agglomerated than others so different dispersion techniques may be necessary to disperse the particles. Dry materials are able to be manipulated more with liquid analyses compared to dry. Some examples of these techniques include hand mixing, vortexing and sonicating. Vortexing involves applying shear stress to the sample in order to separate the material. This technique is useful when working with fragile but agglomerated material since the shearing can separate the particles from one another while causing minimal damage to whole particles. Sonication is a technique that disperses sample material by sending longitudinal waves through the sample preparation, ideally dispersing soft agglomerates while not affecting primary particles. These dispersion techniques involve careful selection. Understanding the sample’s characteristics as well as the goals of the analysis can help determine which dispersion techniques will be right to use.
When deciding between liquid and dry dispersions, there are several factors that should be considered such as safety, sample quantity, the natural state and end use of the sample, sample characteristics, and the compatibility the sample will have with a fluid or dispersant.
Sample types that work well for dry analyses include:
- Materials that are designed to be dispersed into air or will be used on a dry basis (e.g. aerosols and powdered drugs)
- Substances with high solubility in liquid mediums
- Samples with magnetic properties (tend to agglomerate in liquid mediums)
- Materials that are free flowing or have high densities causing them to readily settle in a liquid medium
Liquid analyses may be more applicable when:
- Analyzing fragile materials
- Working with toxic substances (easily controlled in liquid state which reduces risk of inhalation)
- Handling hygroscopic products (tend to absorb moisture from the air)
- Looking at fine particles (below 10 microns) or substances that don’t flow well
- Only small sample quantities are available
How Can PTL Help?
Particle Technology Labs is a fully independent, cGMP compliant and ISO accredited laboratory. Our lab has 30 years of experience in the particle characterization field working with a multitude of sample types and techniques. As an independent laboratory, we offer a variety of instruments manufactured by multiple vendors. Our experts can help evaluate if your sample is appropriate for a liquid or dry laser diffraction analysis. If you have any questions about which technique may be appropriate for your sample, please do not hesitate to contact us. The experts at Particle Technology Labs can guide you through the entire process from selecting the best technique for your needs, conducting the analysis, and assisting with data interpretation. Here at Particle Technology Labs we also provide consultations for testing inquiries free of charge, and we are always happy to discuss your testing needs.
By Jackie Balog, Particle Characterization Chemist II.