What Does Your Particle Size Data Mean?

Laser diffraction is a mature technology with extensive capabilities. Highly automated and easy to use, modern laser diffraction systems efficiently meet the requirements for particle size data analysis in applications ranging from research and development through quality control.

Appropriate dispersion of a sample ahead of measurement is essential, and remains a potential source of error. This is particularly true for systems containing the finer particles that are increasingly the focus of industrial interest. Dispersion ensures that the particle size data generated is representative and relevant to the application.

Imaging technology can be extremely helpful in assessing the efficacy of a dispersion process during laser diffraction measurement. Images of a sample support the development of a method that delivers complete dispersion without causing primary particle damage and provide valuable insight during troubleshooting. Imaging accessories that can be used in-line with laser diffraction analysis systems make it easier to exploit the synergies between these two important techniques. This article examines the role of imaging in particle size analysis and how it can help answer the question: “What does your particle size data mean?”

Introducing laser diffraction

Laser diffraction systems calculate the particle size distribution of a sample by first measuring the intensity of light scattered by the sample as it passes through a collimated laser beam. Large particles scatter light at relatively narrow angles to the incident beam, while smaller particles scatter at much wider angles. Laser diffraction analyzers record the angular dependence of the scattered light intensity. This data is used to determine particle size distribution for the entire sample by comparing it to a mathematical model that predicts how particle scattering changes with particle size.

The appeal of laser diffraction derives from its flexibility and ease of use. Today’s automated systems feature pushbutton operation and deliver exemplary measurement reproducibility and robustness. However, this level of efficiency and analytical productivity is not possible without sophisticated method development techniques in place.

For most applications, the desired output from a laser diffraction measurement is the primary particle size distribution for the sample. This means that any agglomerates within the sample must be reliably and reproducibly dispersed ahead of measurement. To bring this process under control, users must understand how the measured particle size is dependent on the dispersion energy applied to the sample.

The merits of liquid sample dispersion

Liquid sample dispersion is a gentle and effective sample preparation method for particle size analysis, especially when handling materials that are very fine (less than 20 μm in size) or are sticky/cohesive. Although slower than dry powder dispersion methods, liquid sample dispersion is applicable to a wider range of sample types due to the variety of liquid dispersants, surfactants and stabilizers that are available and which ensure that reproducible dispersion is achieved. The application of agitation and ultrasound can also be used to aid dispersion of agglomerates. Liquid sample dispersion is the reference method against which dry dispersion methods are validated due to its gentle nature and wide applicability.

In most applications, it is critical to ensure that the sample is completely dispersed by the proposed wet dispersion method. This must be achieved without causing particle damage. Imaging is valuable because it allows analysts to see how the sample is changing during the dispersion process and to reliably differentiate between agglomerate breakup and particle fracture. The focus on accelerating method development has therefore prompted instrument developers to devise ways to integrate imaging and laser diffraction technology.

The following case studies demonstrate the importance of imaging and the utility of the Hydro Sight in-line particle imaging accessory (Malvern Instruments, Westborough, Mass.) in method development and, more generally, for the interpretation of laser diffraction particle size data.

Case study 1: tracking dispersion with in-line imaging technology

A sample of powdered toner was predispersed in deionized water with a surfactant and then added to the liquid dispersion accessory of the Mastersizer 3000 laser diffraction analyzer (Malvern Instruments) for full dispersion and particle size measurement. Figure 1 shows an image of the sample and the associated laser diffraction particle size distribution data at an early stage of the dispersion process. Loosely bound agglomerates are being dispersed as a result of the stirring and pumping action of the dispersion unit. Residual agglomerates are clearly visible in the image and in the laser diffraction particle size distribution data.

Figure 1 – Particle imaging confirms the presence of agglomerates in the sample, which are also evident from the particle size distribution data.

As sample dispersion proceeds, the number of particles in the sample increases and there is a simultaneous shift in the size distribution toward smaller particle sizes. These changes can be tracked using real-time obscuration and particle size data provided by the laser diffraction analyzer software. Obscuration, a measure of the percentage of light that is blocked by the sample, will increase as the number of particles in a sample increases.

The dispersion index (DI) parameter calculated by the imaging accessory provides complementary real-time data to aid dispersion tracking. This is a measure of the extent of disorder within the sample images. As the agglomerates within the sample disperse the disorder within each image frame increases, and, thus, so does the DI. The relative standard deviation (RSD) of the DI value, a measure of the variability in the degree of disorder within each image over a rolling frame set, decreases as the sample completely disperses and reaches a stable state of dispersion.

Figure 2 shows the toner sample proceeding to complete dispersion as a result of successive periods of stirring and ultrasound. The plateau in the DI and RSD curves suggests that complete dispersion has been attained. This finding is supported by an image of the suspension and the associated laser diffraction data (Figure 3), which shows that the sample has a monomodal particle size distribution. Here, imaging validates the conclusions drawn from the laser diffraction data.

Figure 2 – Monitoring dispersion index (DI—green curve) and relative standard deviation (RSD—blue curve) helps users determine when complete, stable dispersion is achieved.
Figure 3 – Images of the dispersed toner sample validate that the sample is fully dispersed, as evidenced by the monomodal particle size distribution.

Case study 2: troubleshooting sample dispersion

In a second study, the particle size distribution reported by laser diffraction for a fine pharmaceutical powder was measured using liquid and dry dispersion methods. Figure 4, an overlay of the particle size distributions reported by each method, shows that the particle size distribution measured using liquid dispersion is shifted to smaller particle sizes compared to that reported using dry dispersion. This leads to an important question: Are the differences in the reported results caused by particle breakup during the liquid dispersion method or by incomplete dispersion of the sample during dry dispersion? This question was particularly relevant in this case because the sample contained needle-shaped particles.

Figure 4 – Wet (red) and dry (green) particle size distribution data suggests that dry dispersion does not completely break up all agglomerates within the sample.

Imaging was used to investigate the nature of the agglomerates present in the sample, with the goal of optimizing the liquid dispersion method. As in case study 1, DI was tracked as a function of ultrasonication time to establish the conditions required for a stable state of dispersion to be achieved. These results (not shown) suggested that dispersion was complete after an ultrasonication time of 2 minutes. In this instance, because the particles within the sample were observed to be needle-shaped, the elongation data produced by the imaging system was also analyzed (see Figure 5).

Figure 5 – Changes in particle elongation during wet dispersion confirm that agglomerates are being broken up successfully to form needle-shaped primary particles that are undamaged by ultrasonication.

Needle-shaped particles tend to be fragile and prone to breakage, a phenomenon that can be tracked using particle elongation data. Elongation is calculated as [1-particle width/particle length]. Needle-shaped particles have an elongation ratio close to 1, whereas regular-shaped particles, such as spheres, have an elongation ratio near 0. If the selected dispersion method causes the breakup of needle-shaped particles present in a sample, then the elongation would be expected to decrease, since needles will be broken into shorter, more regular-shaped pieces.

The data in Figure 5 shows an increase in elongation during sonication. Here, imaging provides direct evidence that the change in particle size reported by laser diffraction during sonication of the sample is due to the dispersion of clumps of needles and that needle breakup is not observed. This, in turn, confirms that the larger particle size distribution reported using dry dispersion is attributable to incomplete dispersion.

Exploiting the synergies of laser diffraction and imaging

Laser diffraction analyzers deliver high precision and accuracy at the push of a button. The ability to obtain relevant data relies on effective method development and troubleshooting. During these steps it can be extremely helpful to see the particles to rationalize observed behavior. Thus, a method is required that is able to integrate imaging and laser diffraction analysis to answer the question: What does your particle size data mean?

Dr. Paul Kippax is product group manager, and Dr. Deborah Huck-Jones is product manager, Analytical Imaging, Malvern Instruments, Grovewood Rd., Malvern, Worcestershire, WR14 1XZ, U.K.; tel: +44 (0) 1684 892456; fax: +44 (0) 1684 892789; e-mail: [email protected];www.malvern.com

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