Without a Particle of Doubt

Published on: 
In the Lab eNewsletter, Pharmaceutical Technology's In the Lab eNewsletter, January 2022, Volume 17, Issue 1

Particle analysis provides assurances of the quality and performance of the final dosage form in pharmaceutical development.

Whether characterizing particle size or structure, or investigating any possible contamination, particle analysis plays a critical part in bio/pharmaceutical development and manufacture. “Particle analysis is a critical tool to define, help manage, and deliver product quality,” confirms Michael Lindsay, SGS Global scientific director.

“Desired product performance greatly influences how the API needs to perform with regards to solubility, rate of solubility, and permeability to access the target site or be available for systemic circulation,” Lindsay continues. “Typically, the API is blended with other non-active ingredients, each with their own contributing property attributes, to help deliver the API in the desired media to a patient as a solid, semi solid, liquid or gas/powder.”

Essentially, achieving a sufficiently homogenous mixture and product is a key manufacturing goal, but there are multiple characteristics that can impact this, such as physical aspects (size, shape, density), particle charge, smoothness, and the ability to blend and flow, Lindsay notes. “These features can be critical quality attributes in process steps from: initial synthesis through crystallization, if needed; processing and blending in production into the final package to contribute final product performance,” he says. “Collectively, these factors influence which particle analysis methods are used to evaluate material behavior to determine criticality; and, with defined specifications become necessary to ensure consistent product quality and performance.”

During manufacture, drug products come into contact with other surfaces and can be exposed to the air, which could lead to potential contamination from particulates, Lindsay asserts. “Therefore, particle analysis also serves as an investigative tool to ensure there are no unacceptable contaminants,” he says. “Where contaminants are present, particle analysis helps identify root causes for such contaminants, so that actions are taken to eliminate them to further ensure product safety.”

Current techniques

There are numerous techniques that are currently available for particle analysis, which can be categorized around both chemical and physical properties, Lindsay explains. Additionally, it is possible to use single or multiple methods to reveal the expected materials performance so that an acceptable product can be yielded, he adds.

Particle size methods. “Material solubility is a significant contributor to product performance where particle size reduction corelates with increased surface area to influence rate of dissolution. Thus, multiple methods are used to measure particle size,” Lindsay says. “A desirable decrease in particle size can facilitate higher dissolution with increased surface area and can also increase the rate of degradation and impact product stability too. Therefore, particle size analysis is one of the most important techniques.”

A relatively easy method that requires minimal expertise and no sample preparation is sieving, which includes air jet sieving, specifies Lindsay. However, with this method between 10 g and 100 g of material is consumed, there is a lower particle size limit (>20–50 µm), the range of sieves reduces data points, and it is only possible to be used with dry materials, he states.

Through the use of light microscopy with high resolution-image analysis, it is possible to fully visualize and inspect particles, Lindsay confirms. This method provides such advantages as a 1–1000 µm dynamic range, visualization of solubility (if it occurs), capability of confirming aggregation and particle fragmentation, and the ability of confirming crystallinity, degree of crystallization, crystal quality, and shape. Considerations to take into account when using this approach are solvent evaporation; multiple fields of view; and non-random particle orientation if particle size or shape is also being measured.

Laser diffraction only requires a small amount of sample (>10 mg to a few grams), can be used with wet or dry particles, and offers a dynamic size range of 0.01–2500 µm (dependent on instrument, technique, and configuration of instrument), Lindsay continues. For wet analysis, however, the solubility of the material should be considered, he adds. Additionally, expertise for method development is required, and the particle shape needs to be contemplated when the technique assumes a sphere.

A small quantity of sample (±1 mg or less as a minimum) is also required when using dynamic light scattering (DLS) as a technique for particle size analysis. The dynamic range offered is between 0.3 nm–10 µm and the estimated average molecular weight range catered for is 1 x 103 to 1 x 107 Daltons (both dependent on instrument, technique, and configuration of instrument), Lindsay states. Similar considerations are required for DLS as with laser diffraction: solubility, expertise, and particle shape.

Dynamic image analysis or Micro-Flow Imaging (ProteinSimple, Calif., USA) is another particle size technique that only requires a small quantity of sample (<1 mL). This approach offers a dynamic range of 1–300 µm (dependent on instrument, technique, and configuration of instrument), provides the ability to see particles, allows visualization and measurement of particle shape and morphology, tests most of the sample, can be used with high viscosity samples (up to 20 centipoise), and can enable visualization of contaminant particles, Lindsay asserts. Solubility and expertise are considerations to take into account with this method, along with the need to view sufficient images for confirmation of particle distribution because particles are visualized in a random orientation.

“A consideration for all methods discussed is that the sample tested must be representative of the bulk; particle size is measured through a variety of techniques, where in development orthogonal approaches may be taken to evaluate which provides the more meaningful data to control quality,” Lindsay stresses.

Particle property evaluation methods. “Other techniques can be employed to visualize particles, isolated or not, to better describe the particle and aid in identification of potential contaminants,” states Lindsay.

When evaluating surface area of particles, it is possible to use the Nitrogen Brunauer-Emmett-Teller (BET) method. “Surface area most influences solubility of a material in solution and can be additionally measured as an orthogonal technique to particle size,” Lindsay says. “As particle size decreases surface area increases per unit mass and reported as m2/g to positively increase potential dissolution rate. Thus, particle size/surface area is used to help control dissolution rate in some products.”


Measuring the surface area of particles becomes particularly important for chemically unstable compounds, Lindsay explains. For these compounds, if there is an increased surface area there will be an increased degradation rate, which influences the formulation protection or coating that has been implemented to prevent degradation.

Surface charge method can be used to evaluate the Zeta potential—the charge on a particle at the shear plane. This property is important for the understanding and prediction of how particles will interact in and successfully stay in suspensions with other particles, Lindsay confirms.

Flowability of particles, which is critical for powder blends, can be measured with a variety of methods, such as funnel, angle of repose, powder viscometry/rheology, and so on. “The ability to flow evenly is a requisite for gravity assisted processes, where flow is uneven or there are vibrations differences in flow, density, particle size can lead to size or material separation to de-mix a once uniform blend,” Lindsay says.

Pycnometry/a densitometer can be used to measure the material density at a material level and to measure that property at a bulk scale, it is possible to use tap and bulk density testing. The latter method can also provide details that will allow for an understanding of how particles will settle together, Lindsay notes.

Particle identification methods. All products to be administered via injection need to undergo a visual inspection to check for particulates as per regulatory guidance, such as the United States Pharmacopeia Chapter <790> and European Pharmacopoeia Chapter 2.9.20 (1,2). The visual checks involve an inspection of the sample against a black and against a white background, with the naked eye and without magnification, under standardized light conditions, Lindsay states.

Light microscopy can be used to visualize the particles and their surface properties to a size of 1 µm. Additionally, this technique allows for particle geometry to be evaluated and measured, and can provide confirmation of crystal properties, Lindsay reveals. A simple count of particles in liquid samples, at set sizes between 10 µm and 25 µm, can be done using light obscuration, he adds.

Microscopy coupled with Fourier-transform infrared or Raman spectroscopy may be used to identify particles greater than 10–20 µm, although particles must be cleaned of other materials to improve spectral quality, Lindsay stresses. Scanning electron microscopy and energy dispersive X-ray analysis are useful for the visualization of the material and for elemental identification, particularly for metal/alloy contaminants.

Great advances

“Particle analysis techniques have advanced greatly from the simple sieve to the use of laser and high-speed image and spectroscopic systems to gather more meaningful sample data on the particle to better describe the particle shape, size, density, and identity faster,” says Lindsay. “Also, providing the sample is representative, results are obtained with much less material thus enabling accurate measurements on very expensive materials at lower cost.”

Thanks to advancements in optical and spectroscopic techniques, it is now possible to capture images of the particle flow in real-time, allowing for particle identification, Lindsay reveals. Additionally, smaller particles (>0.3 nm) can be optically detected and measured using light scattering techniques, he adds.

“Data processors, detectors, and processing speed have contributed significantly to the advancement of new instrumentation that are both smaller and faster than previous instruments,” Lindsay says. “Bench instrument components are now being built into flowing or blending processes to also enable real-time in-situ non-destructive analysis and include final in vial measurements so that no additional material is consumed. Spectroscopic techniques, with complex libraries and processing for spectral analysis, enable real-time monitoring of blends to confirm material concentration and uniformity.”

In the future, continued advancement in analytical measurement abilities for all aspects of particle analysis to the limits of what physics will allow is expected, Lindsay emphasizes. Further miniaturization of instruments to enable measurements of lower volumes and greater speed in flowing systems and an increased use of process analytical technology (PAT) solutions that separate offline analysis will transpire, he states.

“In time industry should expect to see more analytical instrument techniques be part of the PAT solution to reduce some laboratory testing outside of the particle analysis areas too,” Lindsay adds. “The regulations already support alternate techniques to better manage risk and assure quality. Data analytics with smarter artificial intelligence will help identify and correct processes, within defined and approved design and quality-by-design spaces, seamlessly to enable more effective production.”


1. USP, USP General Chapter <790>, “Visible Particulates in Injections,” USP 37–NF 32 (Rockville, MD, 2014).
2. EDQM, Ph. Eur. Chapter 2.9.20, “Particulate Contamination: Visible Particles” (Strasbourg, France, 2008).

About the Author

Felicity Thomas is the European editor for Pharmaceutical Technology Group.