Digital microscopy provides a direct, visual approach to understanding particle morphology, making it an essential tool to complement many physical particle characterization analyses. It can reveal more information regarding a material’s particle shape, structure, and degree of dispersion or tendency toward agglomeration, details that some sizing methods often overlook due to geometric assumptions and limitations on resolution. These insights support more informed decisions in method selection, quality control, and regulatory documentation.
Orthogonal Method Support
Most particle sizing techniques generate distributions based on assumptions that simplify measurement models. Because of this, visual information from imaging becomes essential when utilizing techniques such as laser diffraction or sieving, ensuring the technique aligns with the material’s characteristics.
This is especially true for laser diffraction, which calculates particle size based on how light scatters through a suspended sample. The technique relies on a user-defined optical model, a mathematical calculation that incorporates the material’s refractive index and estimated absorption properties, to interpret scattering behavior and generate a volume-based distribution.
Since particle shape, texture, and transparency directly affect how the laser interacts with the sample, selecting the most appropriate optical model is critical for representative results. Digital microscopy supports this process by providing visual confirmation of the particles’ shape and inferred absorption characteristics—such as opacity or contrast (see examples in Figure 1), helping users select the most appropriate model for the material.
Similarly, sieving assumes that particles are free-flowing, non-agglomerated, and will orient themselves to pass through mesh openings based on their smallest dimension. However, real-world materials often include flat, elongated, or agglomerated particles that behave unpredictably. Digital microscopy helps identify these characteristics in advance, offering insight into why some materials may not pass through the mesh as expected, and provides a visual context that supports a more informed interpretation of sieve data.
While digital imaging provides valuable qualitative insight into particle morphology and supports the interpretation of particle size data, it is important to note that this technique is informational rather than quantitative. Although images can offer a general sense of particle size and shape, this is not a manual sizing method and should not be used as such. For clients seeking quantitative particle sizing through microscopy, PTL offers automated static image analysis, a technique that provides robust particle size and shape data. Please read our other blogs, “Using Image Analysis to Answer Your Most Challenging Particle Characterization Questions” and “PTL Now Offering USP 788, USP 789, and USP 787 Testing” for more information.
Figure 1. Digital Images of Various Morphologies.
Quality Control
Digital microscopy plays a valuable role in quality control by revealing anomalies and material features that traditional sizing methods may overlook. These images can uncover foreign particulates, contamination, agglomerates, or unexpected morphological changes that could affect product performance or processability.
Visual records also support long-term material monitoring, enabling comparisons between lots, formulations, or suppliers. This is particularly useful for identifying batch-to-batch variability or structural changes that may raise concern during manufacturing or formulation.
What about Regulatory Compliance?
Digital microscopy is referenced in multiple pharmacopeial standards, such as the United States Pharmacopeia (USP), and plays a critical role in determining whether materials meet established acceptance criteria. While its applications vary across monographs and general chapters, digital imaging is particularly important in cases where morphology or crystallinity directly impacts product quality or classification. Two examples where digital microscopy is specifically required to confirm key material attributes include crystallinity assessment under USP <695> and cornstarch identification as outlined in the USP monograph.
For crystallinity analysis, digital microscopy is referenced in USP <695>, which in turn refers to the “Crystallinity Characterization” subsection of USP <776> Optical Microscopy. The acceptance criteria involve observing particles under cross-polarized light to identify birefringence and extinction positions. A particle is in the birefringent position when it displays interference colors, as shown by the particle circled in red in Figure 2. When the microscope stage is rotated and the particle fades or disappears, like the circled particle in Figure 3, it is in the extinction position. The presence of both birefringence and extinction positions on the same particle confirms the material’s crystalline structure.
Figure 2. Circled Particle is in the Birefringent Position.
Figure 3. Circled Particle is in the Extinction Position.
In the case of the monograph for cornstarch identification, digital microscopy is used to verify compliance with the criteria outlined in the USP chapter, specifically within the “Identification A” subsection. This analysis, offered by PTL, involves observing the material under polarized light to detect a central cavity or “hilum” (Figure 4). Under cross-polarized light, compliant particles exhibit a distinct black cross intersecting at the hilum (Figure 5).
Figure 4. Particles under Polarized Light to detect a Hilum.
Figure 5. Particles under Cross-Polarized Light to detect a Black Cross at the Hilum.
Conclusion
Digital microscopy is more than just pretty pictures; it gives insight into how materials behave and their morphology, which helps fill the gaps left by traditional particle sizing techniques. It’s a useful tool that can provide long-term monitoring of materials and be used to show that a material meets the acceptance criteria for various USP chapters. It is overall a great technique to use in tandem with particle sizing techniques, or on its own for quality control and regulatory compliance needs. Contact us for a quote today for your digital imaging needs!
By Laura Stallter, Particle Characterization Chemist II.
