Particle size analysis methods: Laser Diffraction vs. Dynamic Image Analysis

This review compares Laser Diffraction (LD) and Dynamic Image Analysis (DIA), two commonly used particle size analysis techniques. LD determines particle size by examining light scattering patterns, making it suitable for rapid, high-throughput analysis over a wide size range. However, it assumes particles are spherical, which can reduce accuracy when dealing with irregularly-shaped samples. In contrast, DIA captures high-resolution images of individual particles, offering detailed information about both size and morphology, including shape factors like elongation and circularity. Although DIA delivers more precise morphological details, it has a lower throughput compared to LD. The optimal particle size measurement technique depends on specific application requirements. LD excels in high-throughput analysis when particle shape is less critical, while DIA is preferred for meticulous shape characterization.

While DIA shares similarities with traditional light optical microscopy (LOM) and static image analysis (SIA) in its use of light and optics (see Figure 2), it differs in key ways: 

• particles are in motion and continuously exchanged during the measurement
• particles are randomly oriented towards the camera
• the detection and analysis of particles are automated. 

These features enable DIA to analyze a large number of particles in a single measurement, ensuring the statistical quality of the resulting distributions, making it one of the more robust particle size analysis techniques for detailed particle morphology characterization.

Laser diffraction: LD typically covers a broad size range, from sub-micron to several millimeters, making it suitable for applications where a wide particle size distribution is expected. However, the resolution of this particle size measurement technique for detecting fine details in particle morphology is limited, especially for non-spherical particles, due to its reliance on the assumption of spherical shapes and the calculation of a volume-based particle size distribution, as the diffraction of light is proportional to the volume of the particle.

Dynamic image analysis: DIA excels in providing detailed information on particle size and shape. The method’s resolution is high, allowing it to detect and characterize even small differences in particle morphology, such as circularity, elongation, and aspect ratio. Any other irregularities can also be detected. Due to its number-based particle size distribution, under- and oversized particles and outliers are detected. DIA can cover size ranges from the micrometer up to the millimeter range, making it a versatile particle size analysis method for applications requiring detailed morphological analysis.

Laser diffraction: Laser diffraction is known for its rapid analysis speed and high throughput. It can process large sample volumes quickly, making it ideal for environments where speed is crucial, such as in quality control settings in manufacturing. This particle size analysis method is typically automated and requires minimal operator intervention, further enhancing its suitability for high-throughput applications.

Dynamic image analysis: DIA is generally slower compared to LD, as it involves capturing and processing a large number of images to ensure statistical significance. The throughput of DIA is lower, and the particle size analysis method often requires more sample preparation and operator oversight, which can be more time-consuming.

Laser diffraction: LD produces a volume-based distribution, which shows the percentage of the total volume of particles within specific size ranges. This particle size analysis technique usually represents the particle size distribution (PSD) as a graph, with particle size on the x-axis and either the volume percentage or cumulative volume percentage on the y-axis. Although less common, distributions based on the number or surface area of particles are also available, depending on the software and application. These give different perspectives on the particle size, emphasizing smaller particles (number-based) or the overall surface area (surface-based).

D₁₀, D₅₀, D₉₀ are the most common percentiles in the distribution. Moreover, information on the mean particle size, median, mode, and distribution width can be assessed from the distribution. However, because LD assumes spherical particles, it may not accurately represent the size of irregularly shaped particles, leading to potential misinterpretations of the data when using this particle size measurement technique.

Dynamic image analysis: DIA provides a wealth of information, including particle size distribution and detailed morphological data such as shape factors, circularity, and elongation.

The primary outputs from a dynamic image analysis include a number-based particle size distribution. Moreover, it provides detailed information on particles’ size and shape characteristics. This additional information can be particularly valuable in understanding the behavior of particles in different applications. Moreover, it allows for real-time analysis of particles as they are processed, providing immediate feedback. However, the complexity of the data can also make it more challenging to interpret without proper expertise. 

Laser diffraction: Laser diffraction is favored in various industries for its ability to provide rapid, accurate, and reproducible measurements of particle size distribution across a wide range of sizes. This particle size analysis method’s non-destructive nature and ability to handle both powders and liquids make it a valuable tool in many industrial applications. Sample recovery is occasionally possible. It can measure a broad range of particle sizes, typically from sub-micron to millimeters. LD is widely used in industries where particle shape is less critical, and where quick, reliable size distribution data is essential. Common applications include quality control of cement, pharmaceuticals, and metal powders. 

Dynamic image analysis: DIA is particularly useful in applications where particle shape and morphology are crucial, such as in the analysis of food particles, pharmaceuticals (e.g. where shape may affect dissolution rates, compressibility, granulation processes, etc…), and abrasives. It is also advantageous in research settings where detailed particle characterization is required. Due to the number-based distribution, this particle size analysis technique is excellent at detecting under- or oversized particles in the sample, or even individual outliers, making it valuable in both R&D and QC settings.

Laser diffraction: The main limitation of LD lies in its assumption of spherical particles, which can lead to inaccuracies when dealing with irregularly shaped or agglomerated particles. Additionally, LD can struggle with very fine or very coarse particles or complex mixtures, especially if they are outside the optimal size range for the equipment. In the case of multi-modal distributions, where multiple particle populations exist, the interpretation can be challenging.

Dynamic image analysis: DIA’s limitations include its relatively lower throughput and higher time requirement for analysis. Additionally, DIA systems can be more complex and expensive to operate, requiring skilled operators to ensure accurate data collection. Interpreting complex shape data and ensuring it accurately represents the sample can be challenging, especially in heterogeneous mixtures.

Both laser diffraction and dynamic image analysis have their strengths and weaknesses, making them suitable for different applications, depending on the specific requirements of the analysis. Laser diffraction is the particle size analysis method of choice for rapid, high-throughput analysis across a broad size range, particularly when shape is not a critical factor. In contrast, dynamic image analysis as one of the more detailed particle size analysis methods, offers unparalleled insight in particle morphology, making it invaluable in applications where a deep understanding of particle shape is essential.

In practice, the choice between LD and DIA often depends on the balance between the need for speed and the need for detailed morphological information. For comprehensive particle characterization, these particle size measurement techniques are sometimes used in conjunction, providing a fuller picture of particle properties.

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• Dynamic Image Analyzer: DIA
• Laser Diffraction Particle Size Analyzer: Litesizer DIF