Laser diffraction is a technique that determines a volume-weighted particle size distribution based on the angles at which a dispersion of particles scatter laser light. This technique is commonly used as a quality control test and benefits many industries, as particle size can impact different factors about a material. While all laser diffraction instruments operate using the same basic principles, each manufacturer has a unique optical configuration and proprietary algorithms for generating particle size distributions. PTL currently utilizes laser diffraction instruments capable of liquid and dry analyses from three different manufacturers: Malvern, Beckman Coulter, and Sympatec. Based on the characteristics of a new sample, PTL uses our expertise, in addition to the guidance provided in ISO and USP reference documents, to determine which instrument is most suitable to give accurate and representative results. This post will highlight some of the unique differences that influence instrument selection for a material.
The Malvern Mastersizer
The Malvern Panalytical Mastersizer 3000 is generally the most used instrument at PTL for laser diffraction analyses. It offers a broad particle size range of approximately 10 nm – 3500 µm, which is ideal for polydisperse materials (or materials that have an extensive size distribution). In addition, this instrument can use either Fraunhofer or Mie theory to calculate the distribution, which allows for accurate results about a specific material type. With Mie theory, both the refractive index and the imaginary absorption value can be utilized within the particle size characterization if known. Fraunhofer’s theory does not use these factors and assumes that the particles are large, opaque, and spherical.
Figure 1. Malvern Panalytical Mastersizer 3000.
The Beckman Coulter
The Beckman Coulter LS13 320 utilizes a similar wide size range to the Malvern Panalytical Mastersizer 3000, generally spanning from 40 nm to 2000 µm, and the ability to choose either optical theory. However, on a liquid basis, it incorporates a proprietary technology called “Polarization Intensity Differential Scattering” (PIDS), which enhances low-end particle detection down to 10 nm. This extends the size range in the lower region by accounting for the vertical and horizontal scatter intensity difference. While this is a small value, this additional information has the potential to create a more refined distribution in the submicron region. At PTL, this instrument is ideal for samples that have a significant population in the submicron region, and may be too concentrated for other techniques at our laboratory.
Figure 2. Beckman Coulter LS 13 320.
The Sympatec
The Sympatec HELOS/BR differs from the previous instruments due to its lens and detector setup. For analysis, this instrument uses a Fourier set up (in comparison to a reverse Fourier set up the Malvern 3000 and Beckman Coulter LS 13 320 use) and one of eight interchangeable lenses, each with a distinct size range, making it ideal for more monomodal samples. PTL uses five of the eight available lenses, each occupying a portion of the overall size range of 0.1 µm – 875 µm.
Sympatec is well known for its dry dispersion capabilities, and PTL owns two different dry dispersion attachments, each used for different sample types. The VIBRI attachment is ideal for generally flowable materials, where sample submission quantity does not pose an analysis issue. With the ASPIROS attachment, a very small sample mass is needed to obtain a representative measurement; therefore, this attachment is ideal when sample quantity is low or if safety exposure limits are a concern. In addition, the CUVETTE attachment is the liquid dispersion unit, which requires significantly less material compared to the other liquid attachments at PTL due to the decreased volume of carrier fluid needed for analysis. The Sympatec HELOS/BR prefers to use Fraunhofer theory for calculation models and provides two forms for testing. High Resolution Laser Diffraction (HRLD) is Sympatec’s traditional evaluation mode, which was generally used with their legacy instruments, while the Fraunhofer Enhanced Evaluation (FREE) calculation model is the newest model based on their R-series sensors.
Figure 3. Sympatec HELOS/BR.
Comparison
While each instrument has unique features, all operate on the same principle and provide reliable particle size data on a volume basis. For comparative purposes, PTL analyzed the same reference material on each instrument on a liquid dispersion basis. The reference material manufacturer’s certificate of analysis denotes a specific refractive index, which was used for the Malvern Mastersizer 3000 and Beckman Coulter LS13 320 analyses; however, it was not utilized for the Sympatec HELOS/BR analysis. SEM images of the approximately 15 µm reference material are shown in Figures 4 and 5.
Figure 4. Reference Material at 700X.
Figure 5. Reference Material at 170X.
The particle size distributions and results are shown below from each of the three instruments. For this comparison, we will be looking at the mean results (D[4,3], mean, and VMD circled in red) shown below in Figures 6 through 8.
Figure 6. Malvern Mastersizer 3000 Particle Size Distribution.
Figure 7. Beckman Coulter LS13 320 Particle Size Distribution.
Figure 8. Sympatec HELOS/BR Particle Size Distribution.
The reference material’s acceptance range for the mean result is 15.1-18.7 µm, and results from each of the manufacturers fall within the specified range. Overall, the results are consistent across instruments, regardless of manufacturer.
Conclusion
Laser diffraction is generally recognized as a reproducible technique; however, like many particle sizing techniques, it assumes that all particulate in analysis is spherical. As the shape of a given particle ventures further from a spherical shape, there will likely be added variation between compared results when the same material is analyzed with the same methodology.
For samples with a similar size range and polydispersity, comparable results can generally be expected across all three systems. However, due to variable characteristics of real-world samples, specifications are generally set and confirmed with the specific instrument manufacturer they were developed on.
The differences in laser diffraction instrumentation are especially important to consider during robust method development and validation, which are often performed when a quality control method is the preferred outcome. If the selection is inappropriate for a specific material, these variations can significantly impact the precision and accuracy of the particle size measurements. However, with PTL’s experienced and knowledgeable chemists, we are committed to providing results that meet our clients’ testing needs. Contact us today to get started!
By Amanda Redman, Particle Characterization Chemist III.
