How to choose an instrument for particle analysis

There are multiple technologies to choose from, including:

• Laser Diffraction (LD)
• Dynamic Light Scattering (DLS)
• Several types of Electron Microscopy
• Separation technologies such as HPLC or GPC
• Small-angle X-ray Scattering (SAXS).

The following sections will focus on laser diffraction, dynamic light scattering, scanning electron microscope (SEM), transmission electron microscopy (TEM). See SAXS nanostructure analysis for more information on the basic principles of small-angle X-ray scattering (SAXS).

Besides the particle size, there are a number of different particle parameters that can be measured by analysis instruments. These include:

• particle shape
• charge
• concentration
• density
• molecular mass
• turbidity
• count
• porosity
• and stability

Often, several of these parameters are combined in a single instrument. For example, a number of instruments provide a combination of nanoparticle size, zeta potential and molecular mass. However, other instruments may be required for particle counting, assessing particle shape, or to separate out particle mixtures.

Some techniques, such as laser diffraction, are flexible and can analyze particles in dry powder form, in liquid suspension, or even in aerosol. Others, such as DLS (dynamic light scattering), require the samples to be suspended in either an aqueous or organic solvent. If in suspension, the concentration of the particles is important because if the concentration is high (i.e. 40 %w/v to 50 %w/v) some techniques such as DLS may struggle to provide good data whereas acoustic techniques may be more useful.

Another question related to formulation is the amount of sample available. In many cases, the sample is measured in liters and the sample volume is not an issue for most types of instrumentation. However, some samples, such as recombinant proteins or antibodies, are extremely expensive to produce and are therefore only made in quite small amounts and volumes. It is important to know the available sample volume to work out which technique can be used. Many instruments for particle analysis such as DLS instruments require only a few microliters of solvent and often have special low-volume cuvettes available to measure this low-volume sample.

This is particularly relevant for zeta potential analysis of protein samples. Zeta potential works by applying a voltage to a sample of interest, measuring its rate of motion, and then correlating that to its zeta potential or effective charge in solution. This voltage, if applied too powerfully or for too long, can easily damage or degrade protein or nucleic acid samples. New developments in zeta potential technologies help eliminate this issue by enabling much faster measurements and measurement at lower potentials. Additionally, new cuvette designs with an Omega-shaped sample loop also help limit protein loss when compared to the more traditional U-shaped cuvettes.

Stability is also important in terms of particle or protein aggregation. If particles are aggregating over time, they may settle out of suspension and falsify the analysis or they may simply yield different results at different times and under different conditions.

There are different choices when it comes to data accessibility: Some systems provide access to the raw data while others limit users to existing report templates.

For work in a GMP- or FDA-regulated environment, the instrument software needs to be compliant with 21 CFR Part 11 and have the necessary compliance paperwork and IQ/OQ protocols.

Different technologies may give slightly different results, even with the same sample. For example, TEM results are based on a number analysis of a dry particle and the values tend to be smaller than DLS results, which are often based on intensity analysis of a hydrated particle.