Measuring metal powders on the Mastersizer 3000(+) in accordance with ASTM B822-20

What is ASTM B822-20?

ASTM International, previously known as the American Society for Testing and Materials, is a global organization that develops and publishes standards across a wide range of industries and analysis techniques. The designation B822-20 has been devised to establish standard methods for analysing the particle size distribution (PSD) of metal powders and similar compounds using light scattering techniques such as laser diffraction.

How is this designation relevant for the Mastersizer?

Within ASTM B822-20, there are several recommendations for developing methods for light scattering instruments such as those in the Mastersizer 3000 range. The standard also outlines best practises for instrument verification and performance, as well as recommended PSD parameters for reporting and sharing data.

In this document, we will chronologically review the relevant sections of the ASTM B822-20 designation highlighting how Mastersizer 3000 and 3000+ users can ensure they are following the best practises recommended by ASTM. Specifically, we will focus on how this relates to the operation and software features of the Mastersizer. For other elements of the ASTM B822-20 standard, such as repeatability and reproducibility testing, it is the user’s responsibility to carry out the necessary sensitivity studies and design experiments to assess method robustness.

Section 6 – Interferences

When measuring metal powders using a Mastersizer in a wet dispersion system, the goal is for particles to scatter the incident light, which is then interpreted by an optical model to generate a particle size distribution (PSD). However, the presence of air bubbles in the liquid dispersant can also produce light scattering, which may be misinterpreted by the analysis model and erroneously included in the PSD.

To avoid air bubbles affecting the particle PSD, it is crucial to use bubble-free circulating liquids. The Mastersizer software has an optional degassing step that can be programmed to occur after filling the dispersion unit and prior to analysis which is available when running both standard operating procedures (SOPs) and manual measurements (Fig. 1). Degassing can also be programmed to happen after ultrasound has been used.

Particle agglomeration and settling during a laser diffraction measurement in liquids can affect the reported PSD. To assess the extent of these effects across different sample types, it is important to follow method development guidance. This typically involves performing stirrer speed and ultrasound titrations, in other words, taking multiple measurements under varying conditions and then comparing the resulting PSDs. By doing so, you can determine whether changes in variables such as stirrer speed or ultrasound have led to agglomeration or sedimentation.

Determining the correct sample concentration is also critical for accurate laser diffraction measurements. The Mastersizer reports optical concentration using obscuration, defined as the percentage of laser power reduced compared to the background measurement after the sample is introduced. To establish the optimal obscuration range for different sample types, an obscuration titration is recommended. This process helps identify the minimum concentration needed for a good signal-to-noise ratio while avoiding excessive sample addition, which can cause multiple scattering and lead to an overestimation of fine material.

The SOP Architect tool, available to Mastersizer 3000+ Pro and Ultra users, can automate stirrer speed, ultrasound and obscuration titrations, with integrated machine learning algorithms can suggest optimal SOP settings tailored to your materials (Fig. 2).

Section 7 – Apparatus

The Mastersizer 3000+ range has instrument configurations available to suit a range of customer requirements when it comes to the size range and level of automation of analyses. Please get in touch with us if you have questions about instrument configurations or would like further support deciding which option will work best for your materials.

When it comes to how your laser diffraction instrument interprets the light scattering data, all Mastersizer instruments have the option to use Fraunhofer or Mie theory (spherical or non-spherical options available) to resolve the PSD. The Fraunhofer approximation provides a good solution for large opaque particles, such as coffee and coarse sediments whereas we recommend Mie theory when the PSD has a large proportion of material <50 μm and the particles have a degree of transparency. For more detail on optical models and the optical properties requirements of Mie theory see this recorded webinar.

Section 8 – Reagents and Materials

The ASTM designation stresses the importance of using good quality, particle-free reagents for laser diffraction measurements. We stress that it is the responsibility of Mastersizer users to check the chemical compatibility, purity and solubility of their reagents and samples. The wet configuration of the Mastersizer requires an aqueous or solvent resistant measurement cell, the latter offering compatibility with a wide range of solvents. The tubing that connects the wet dispersion accessory to the optical bench must then also be compatible with the solvent used and Malvern Panalytical can provide a range of grades of Tygon tubing as outlined in our chemical compatibility guide.

For dry measurements, the user must ensure their compressed air source is filtered and humidity controlled according to the site requirements for the Mastersizer (see user manual and basic guide).

Section 9 – Sampling and Sample Size

The amount of sample required for individual measurements on a Mastersizer will depend on the PSD of the material and dispersion method used. We provide guidance on ensuring you achieve representative sampling which you can access from within the most recent versions of the Mastersizer software (v. >5.0) and we have similar documentation available on our website on how to approach sampling from bulk materials.

Section 10 – Calibration and Standardization

To ensure consistent instrument performance, as recommended by ASTM B822-20, Mastersizer users can run a performance verification using the Quality Audit Standard (QAS) materials supplied with the instrument. Additional QAS samples can be purchased from the Malvern Panalytical store, or users may source certified reference materials from other suppliers. Internal standards can also be prepared for intermediate precision or reproducibility testing. The frequency of these performance verification checks can be scheduled using the maintenance reminder function (Fig. 3).

Section 11 – Procedure

The Mastersizer performs an automatic alignment before each new measurement, followed by a background scattering check. It is essential to confirm that the background does not indicate poor system cleanliness, misalignment, or dispersant contamination prior to measuring the background signal. Recommended limits for the background profile are 100 scattering units on detector 1 and 20 units on detector 20. These thresholds can be set as alarms within the SOP and measurement settings, alerting the user if specifications are exceeded before proceeding (Fig. 4). The background should display a smooth decay from the highest signal on detector 1 to negligible levels on higher detectors, with no humps or significant fluctuations during the live background inspection stage. Built-in data quality guidance within the Mastersizer 3000 software flags poor background data after measurement. In the latest software versions for Mastersizer 3000+ Pro and Ultra models, this guidance is also available during the background inspection stage, enhancing user confidence in data quality before the sample is introduced.

ASTM B822-20 notes that measurement duration can influence laser diffraction results. We recommend determining the optimal measurement time during method development by evaluating repeatability and reproducibility across a range of durations. For wet dispersion measurements, 10 seconds is typically sufficient, though longer times may be required for coarse or polydisperse samples. For dry measurements, the duration should allow the entire sample to pass through the system to account for potential segregation as powders flow into the venturi. This can be achieved by enabling obscuration filtering and setting extended measurement times that automatically time out when obscuration falls outside the defined range.

How samples are dispersed in wet laser diffraction measurements can impact the PSD due to differences in the state of dispersion and potential for dissolution. Mastersizer users who are running wet dispersion methods can follow the guidelines in the ASTM B821 dispersion guide. Customers must ensure that they have the appropriate chemical compatibility of wet cell and tubing if using dispersants other than deionized water.

Other recommendations from ASTM B822-20 can be supported during Mastersizer measurements. For example, within the Mastersizer measurement and SOP settings for wet dispersion, a 20s pre-measurement delay can be programmed. Similarly, a stabilization time can be programmed for a dry dispersion to ensure the sample has flowed for sufficient time before the measurement starts (Fig 5).

To ensure your laser diffraction measurements comply with this section of the ASTM designation, cleaning recommendations must be followed. To support this within the SOP and manual measurement settings of the Mastersizer software, cleaning cycles and durations can be programmed to ensure they are the same each time a measurement is made. This combined with the background alarm and data quality guidance can ensure frequent and procedural cleaning of the Mastersizer measurement cell and dispersion accessories.

We also provide specific cleaning advice should material build up on the cell windows (both wet and dry) as the cell windows may need to occasionally be cleaned manually. The frequency of this will be application dependent but maintenance reminders can be programmed by the user to remind user to perform the manual cleaning process (Fig. 6).

With the Aero S dispersion unit, the cleaning cycle removes any excess material on the tray by using a high feed rate. The duration of this can be optimized by the customer for their application. The customer will have to manually brush down any components where they see build-up of their samples over time. The frequency of manual cleaning will be sample dependent and reminders to do this can be programmed in the software via maintenance reminders (Fig. 7).

Section 12 – Report

The Mastersizer software offers highly flexible and customizable reporting, ensuring all ASTM B822-20 requirements can be met. Figure 8 shows an example of a custom report that includes the parameters and information specified as minimum requirements in ASTM B822-20. These include the volume-weighted mean (D[4,3]), the optical properties of the dispersant and sample, the optical model used (Mie or Fraunhofer), and whether the measurement was performed in liquid or air. To create a custom report, you can either edit an existing template by adjusting tables and charts or build a new report from scratch using different widget types.

Summary

By applying the recommendations in ASTM B822-20 and utilizing the capabilities of the Mastersizer 3000 and 3000+ software, users measuring metal powders and similar particulate materials can ensure robust method development, reliable instrument performance, and compliant reporting. Following these best practices not only improves data quality and confidence but also supports consistency and comparability across measurements.