A virtual tour: How we determine the number of particles in our microplastic powders

When handling microplastic reference materials, the significance of an accurate scale is crucial! After moving our reference material laboratory to a location separate from the University of Toulouse, we required numerous scientific instruments, including a top-quality laboratory scale.

We opted for a PCE Instruments PCE-ABT 220. The scale's resolution of 0.1 mg and precision of ± 0.5 mg is a necessity for determining the number of particles per mass unit in dry, neat (powder) samples.

PCE Instruments

For larger fragments (500-1000 µm), we distribute the particles directly onto a filter membrane to measure their mass.

Black polyethylene (PE) fragments placed on a white paper membrane.

The filter is gently transferred from the scale to a small USB microscope designed specifically for this task.

Multiple images of the paper membrane are captured to cover the entire filter membrane at a resolution of approximately 10 µm/pixel..

With the current settings, the microscope produces 4x4 images at a resolution of 10 µm per pixel. An example of this type of micrograph is shown below.

Micrograph captured under reflected white light, displaying black PE fragments from ~0.5 to 1.0 mm in Feret diameter, at a resolution of 10 µm/pixel.

The images are subsequently combined to create a high-resolution mosaic. This mosaic is manually edited to distinguish agglomerated particles. Once the image is processed, the morphological details of each individual particle are extracted, as shown in the GIF below.

The GIF demonstrates a mosaic of black PE fragments (500-1000 µm), and how individual particles are annotated in the mask overlay.

For each sample, we analyze four subsamples on which we determine the mean number of particles and their size distribution per mass unit.

For fragments smaller than 500 µm, it is impossible to weigh individual subsamples. Therefore, we take a measurable quantity of powder and suspend it in an ethanol solution. -Similar to how we produce EasyMP™ samples.

3.9 mg of dry, neat (powder) of fluorescent green PE fragments from approximately 40-100 µm in diameter. The powder is weighed in a 100 mL glass bottle, before 100 mL of ethanol (95 vol.%) is added.

Knowing the dry mass of powder and the volume of ethanol solution, we are able to extrapolate to particle numbers per mass unit of dry powder.

To capture images of particles of this size, we utilize state-of-the-art CADFLI (critical angle darkfield imaging) microscopy. The image below demonstrates a mosaic composed of 50 phtomicrographs.

Mosaic composed of 50 photomicrographs captured under CADFLI illumunation of fluorescent green PE fragments. The samples makes up one of four subsamples used to determine the particle count and size disitributoin per mass unit.

We hope you enjoyed this virtual tour of the process behind our reference materials, and we're happy to answer any questions you may have.

Oskar Hagelskjær, Ph.D, CEO Microplastic Solution oskar@microplasticsolution.com