How a new technique to improve the chemical identification of microplastics was created
To examine microplastics in any environmental setting, such as the ocean, a river, in soil, in the air, or anywhere else, a researcher would follow the same 3 steps.
Firstly, samples need to be collected. There are many ways to do this and it largely depends on the research question as to what the best methodology is. For example, to examine microplastics in air (as I have been focusing on for the last couple of years), you can use a passive sampler to collect atmospheric deposition (essentially a rain collector), or an active sampler which extracts suspended microplastics directly from the air.
Secondly, the samples would have to be processed. For example, if a researcher had a soil sample, they would have to extract the microplastics from the mass of soil, which is no simple task! Typically, the microplastic component of a sample would make up less than 1% of the total mass. Therefore, many techniques are required to extract the microplastics such as sieving, density separation, or acid, alkali, and enzyme treatments, and often a combination of these.
Thirdly, once the individual microplastics are the only things remaining in the sample (more or less), the microplastics are then counted, described, and chemically analysed.
It should be mentioned that up until microplastics are chemically analysed and are indeed confirmed as plastic, these should be referred to as “suspected microplastics”. Often other materials produced biologically like chitin, a natural polymer which makes up the exoskeletons of crustaceans and insects, can look almost identical to plastic. Before chemical analysis was more commonplace in microplastic research, up to 70% of particles that visually resembled plastic could be incorrectly identified as a microplastic.
In contemporary microplastic research, Fourier Transform Infrared (FT-IR) spectroscopy has become a widely used chemical analysis method to identify whether a suspected microplastic is indeed plastic, and identify what type of plastic it might be.
In simple terms, this equipment uses an infrared beam to identify a material — be it plastic, or anything else. FT-IR spectroscopy is particularly ideal for microplastic research because different types of plastic absorb and reflect infrared light very differently across the infrared spectrum. These differences are detected by the equipment, and consequently, the most likely plastic can be deduced from the peaks and troughs. Usually, the spectrum for the sample would be compared against a “library”, where the records of many infrared spectrums, for lots of materials, are stored. The software will then give a best-match for the sample spectrum against a library spectrum.
In my PhD research, I collected atmospheric microplastic samples in Singapore and in Cambodia from dry and wet atmospheric deposition. Overwhelmingly, I found fibres in the samples — so much so, that around 90% of all suspected microplastics during visual counts from Singapore samples were fibres.
Although collecting and processing samples was not entirely straightforward, when it came to the chemical analysis of the fibres by FT-IR spectroscopy, we were greeted by greater difficulties! The fibres we found were relatively short (typically around 0.5 mm / 500 microns, or less!), very thin, and were prone to twisting and folding. This meant that it was very hard for us to get the fibres to lie cleanly in the “focal plane of the microscope” — meaning that we would struggle to get a reliable measurement. Also, the fibres would often move about, just slightly, but it would be enough to disrupt the measurement by the FT-IR spectrometer.
This was obviously a problem experienced by other researchers with studies mentioning similar difficulties. The results being poor spectra generated, which led to poor matches to the library and in extreme cases, unreliable findings.
The team that I was working with on this project came up with a unique and simple solution to address this problem. Particularly with the help of Dr Wei-Boon Teo from PerkinElmer Singapore, who has over three decades of experience of infrared spectroscopy.
Our approach involved putting the fibres between two cylindrical metal dies, and crushing the fibres using a hydraulic press with 5 tons of force, for 10 seconds. The benefits of this simple approach were numerous, so much so, I decided to give this method an unofficial name “the Teo Touch” — a joke reference to the force involved and to acknowledge Dr Teo, who was a key figure on creating this technique.
Such benefits included a much wider, thinner, and more uniform plane for the infrared beam, and it also preventing the fibre from moving (since it was completely pulverised). This significantly increased the overall ease, speed and quality of the data we collected. We quantified this by comparing 379 non-pressed fibres, to 259 fibres which had been pressed — i.e., using our “Teo Touch” method.
At this point I should also mention Rebekah Süsserott, whose patience and steady hand was responsible for placing every fibre onto the cylindrical metal dies and was there for all of the FT-IR spectroscopy analyses. She was a key figure for this technique and showed that working with tiny fibres does not have to be difficult — you just need the correct tools… and a bit of perseverance!
To help measure and compare the two methods, we set a threshold of 80% for our sample spectrums to “match” with the library spectrum. Many microplastic studies use a threshold method to ensure that only high-quality data is included in research.
For the combined dataset (Singapore and Phnom Penh), 379 non-pressed fibres were analysed with 193 returning a match score of ≥80%, which was a success rate of 51%. Of the 259 pressed fibres, 254 returned a match score of ≥80%, which was a success rate of 98%.
We also found that the matching was between 5 and 24 percentage points higher on average after using our method. We consistently found that our method improved the data, allowed us to generate data faster, and reduced the number of re-runs required, and spectra which were of poor quality.
Although we focused on fibres, we believe that this method could be used effectively for other microplastic shapes, such as fragments or beads, but it may require a few tweaks as necessary. We welcome researchers to use and experiment with our method, and hope that it serves as an additional technique for the microplastic and microfibre “toolbox”.
For more details on how to use this technique please reach out to me, or find the journal article HERE which covers this in more detail, at the journal of Applied Spectroscopy.
Finnegan A, Süsserott RC, Koh L-H, Teo W-B, Gouramanis C. A Simple Sample Preparation Method to Significantly Improve Fourier Transform Infrared (FT-IR) Spectra of Microplastics. Applied Spectroscopy. April 2022. doi:10.1177/00037028221075065
