Measuring Microparticles

This week in the lab I have been extracting and processing microplastics from my most recent sampling trip. The extraction and processing steps require nearly consistent hands-on activity, so pictures can be quite difficult. Therefore, I am going to talk about how we measure microplastics in this week's blog.

A few weeks ago, I talked about an image analysis software that I can use to digitally measure microplastic particles, so long as I have a reference image and keep the same focal length from the reference image for the microplastic image. Once I import a picture into the imaging software, I can use zoom tools to get a much closer look at the particles and then use the segmented line tracing tool to trace the length of a particle, shown in the image as the pale yellow line with white squares. Microplastics are quite small, and therefore, measurement errors can substantially change the size of a particle. For my work, my undergraduate student, Emily, measures all microplastic dimensions three times and averages the measurements to reduce measurement error. You can see in this week's photo that there was very slight variation in the length measurement (I accidentally clicked measure twice, which is why there are four measurements). Based on the quick measurements I made for this particle, the length is approximately .158 cm, or 1580 micrometers, so a particle on the larger end of the microplastics size scale (< 5 mm). However, length measurements alone do not quantify the size of a particle. Instead, microplastic size is often calculated using the square root of the length times width measurement, which leads to a substantial difference for microfibers like the one pictured.

Why is microplastic size classification important? Microparticles are often consumed by animals in the water column and in the sediment, and researchers have shown that there is a relationship between the sizes of animals' mouths and the size of particles they consume. While a nematode may not consume a microplastic that is 1580 micrometers long if the nematode is perpendicular to the microfiber pictured, it may consume the fiber if it approaches head on, since the particle is only 40 micrometers wide. Therefore, classifying microplastic size accounting for both length and width, provides more realistic information regarding the particles possible harm to marine organisms. Using the holistic size classification system also enables researchers to understand how a particle ends up on different sized sieves. For example, I wash all samples through a 500 micrometer sieve stacked on top of a 63 micrometer sieve. Based on the particle's length alone, one could assume that it is retained on the 500 micrometer sieve. Yet, this particle was found on the 63 micrometer sieve, which provides more support for the calculated size (251.39 micrometers) than the measured size, since the measured size suggests it would either be retained on the 500 micrometer sieve, or would be small enough to fall through the 500 and 63 micrometer sieves. 

We are currently working to understand how the size of microplastic particles change as we move farther away from wastewater treatment plants. My goal is to have data for publication by this Spring, and I cannot wait to eventually share my work with all of you. I will not be posting next week, but look for a new blog the last week of the year.

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