Five Practical Steps to Successful Laser Diffraction

1. Sample Preparation

Proper sample preparation is fundamental to accurate laser diffraction results. This begins with good representative sampling. For inhomogeneous bulk materials, mass segregation can complicate the process. External forces like transportation or vibration can force smaller particles into interstitial spaces, collecting at the bottom of a container. Alternatively, forces from filling or feeding operations cause large particles to collect at the bottom of the container. Remixing or subsampling from multiple locations begins to address the problem.

However, dividing samples is the best solution to minimize errors in subsequent particle size measurements. Figure 1 shows the qualitative variation based on sampling technique.

Figure 1 describes the qualitative variation associated with different sampling techniques. The effects of poor sampling have a tremendous effect on the subsequent particle size analysis. Sample division with a device like the Retsch PT100 is a preferred laboratory practice for repeatable and reproducible data.

A major advantage of the SYNC is a simple exchange of wet and dry delivery modules without tools. The FLOWSYNC offers a wide range of chemical compatibility and an optional internal sonication probe.

Wet Measurements

Wet measurements can be tricky with an array of choices for dispersants, mixers, and procedures. Microtrac has published guides to help select a carrier fluid and disperse and stabilize samples. Figure 2 shows a basic dispersion test for a 5-micron graphite powder. Initially, it may seem that isopropanol would be an appropriate choice but notice the adhesion of agglomerates to the beaker sidewalls. The sample cell is made from a similar glass composition to the beaker, so we can imagine this dispersion easily fouling the cell. Also important is to notice the profound effect of a non-ionic surfactant solution to wet the graphite and create a stable dispersion. The optional sonication probe inside the FLOWSYNC fluid path helps maintain the sample stability during experimentation. Lastly, users must consider particle density and expected size as these are used to determine an effective flow rate for fully effective recirculation. For example, the fluid forces to accelerate submicron polymers are significantly less than larger, denser metal powders.

Figure 2 shows three different sample preparation options for a 5-um graphite powder.
From left to right: Isopropanol – Deionized Water – 2% Surfactant Solution (DI water)
The solid sample quantity and liquid volumes are identical. A gentle swirling of the beaker is the only mixing provided; no external sonication was applied. The surfactant solution is the preferred sample preparation option, exhibited by the stable dispersion. Observing a drop of the samples on a microscope slide helps visualize the dispersion quality.

These strategies are intended to avoid common mistakes in wet particle size analysis.

Dry Measurements

In some cases, there is no suitable liquid, the particles are secondary structures like granules, or the results would be more meaningful to users if the materials were measured in a dry state. The TURBOSYNC has similar published guidelines to optimize methodology. The choice of dispersion pressures and sample quantities are simplified, so successful measurements are possible with anything from a fine staticky to a large flowable powder. Figure 3 shows an example of a pressure titration performed on a fine glass powder. Microtrac application specialists work with end users to advise sample preparation based on users' objectives or expectations.

Figure 3 shows particle size distributions for different dispersion conditions for a fine glass powder. Dry measurement of 500 mg at 0 psi dispersion (red), 100 mg at 0 psi (green), 100 mg at 1.5 psi (orange), and wet measurement (blue). The similarity of the yellow and blue curves indicates the 1.5psi pressure is the preferred adjustment.

2. Software Parameters

Microtrac laser diffraction analyzers produce size distributions based on an innovative modified Mie scattering theory. Basic information about the particles to be measured, and the carrier liquid are inputs to the calculation of particle size distribution. This algorithm produces accurate data for both spherical and irregular particles and accommodates the various modes of laser-light-solid interaction.

Figure 4 shows the basic software inputs in DIMENSIONS LS.

For example, the DIMENSIONS LS method architect is customizable for a spherical metal additive manufacturing powder, or an irregular transparent active pharmaceutical ingredient. A comprehensive reference list of refractive indices for solids and liquids is embedded in the software. There are also options to input custom values, including adsorption coefficients.

The calculation workspace of the software allows users to compare different shape and transparency models using existing data. This time-saving feature provides feedback to fine-tune methods or engage in more complex analyses. The software also indicates when the signal strength is sufficient, and enough samples have been added. Easy-to-follow guidelines exist for the "loading index", which is analogous to the amount of laser light obscuration by solids.

3. Interpreting Laser Diffraction Results

Interpreting laser diffraction results can be challenging, especially for those new to the technique. Particle size distributions are representable in a variety of ways. The graphic and tabular forms can show the cumulative or frequency distribution of particle sizes. With basis options of number, area, and volume it’s important to keep all factors in context for comparison of data, assessment of process control, or conclusion for a research project. An understanding of simple statistical concepts like mean, median, and mode are powerful tools for interpretation.

Such concepts relate to conditions at the particle level like disintegration, swelling, sedimentation, or agglomeration. Recognizing these conditions is the users' practical feedback to things like changing the distribution basis, adjusting the amount of surfactant, choosing a dry delivery module, or increasing pump speed for satisfactory results. It is also important practice to ensure repeatability and reproducibility of measurement, easily accomplished with DIMENSIONS LS statistical tools. Figure 5 is an example of how to use Microtrac software to interpret results for an electronics-grade synthetic diamond powder (SYNC 1R2B, FLOWSYNC, IPA). Relevant markers like d10, d50, and d90 are easily identified along with correlation to sieve results.

Figure 5 is a snapshot of the DIMENSIONS LS data comparison tools that simplify data interpretation.

4. Benefits of Incorporating Dynamic Imaging Analysis (DIA) with Laser Diffraction

The characterization of particulate systems, once dominated strictly by size measurements, is evolving. DIA defines particle size and morphology and provides detailed information about the physical properties of materials. These key properties and the resulting manufactured product can change drastically with no significant differences reported in the Laser Diffraction size distribution.

SYNC provides particle shape analysis in the same measurement run with laser diffraction for both wet and dry measurements, then analyzed in a single software platform. There are no separate modules, measurement cells, or software, which makes lab analysis a lot more productive.

Image analysis can rapidly identify problems and significantly reduce troubleshooting time. Particles in a flowing stream, backlit by a high-speed strobe light, are photographed by a high-resolution digital camera to create a video file of images of the flowing particles. More than 30 size and shape parameters are acquired for every particle. DIMENSIONS LS software includes filter functions to search, display, and evaluate particles with specific properties or a combination of properties. Enhancing diffraction experiments with a secondary orthogonal technique means easier inspection of sample cell cleanliness, detection of bubbles or contamination, discovery of oversized particles, and classification of high aspect ratio samples. 

Detection of Oversized or Irregular Shaped Particles

Laser Diffraction techniques assume all the particles are spherical and most often outlier particles that are oversized or irregularly shaped will not be detected with laser diffraction alone. The reason could be due to insufficient quantities of such particles in the sample batch. This could affect end-product performance like additive manufacturing for the quality of metal powders. Figure 6 shows how Sync’s DIA can detect oversized particles when blended with laser diffraction. In addition, you can use DIA data alone from Sync measurements to compare particle shapes from two different samples to determine the presence of oversized particles as shown in Figure 7.

Figure 6: The laser diffraction data on the left shows a typical particle size distribution. However, the plot on the right shows the detection of oversized particles when blending dynamic imaging with laser diffraction from a single measurement.

Figure 7: Shape analysis of metal powders shows two metal powder samples with a median (d50) of 34 µm and 37 µm. The shape analysis proves that one sample consists almost exclusively of spherical particles, while the other contains a high proportion of irregularly shaped particles.

Conclusion

Laser diffraction is a widely used particle size measurement technique. Successful use of this technique requires a fundamental focus on proper sample preparation, software parameter selection, and interpretation of data. Incorporating (DIA) simultaneous orthogonal technique enhances analytical capabilities. Routine equipment maintenance is beneficial to accuracy and repeatability, as well as instrument uptime.