The Role of FTIR Microscopes in Easing the Challenges in Microplastics Research

insights from industryDr. Jennifer LynchResearch BiologistNational Institute of Standards and Technology

Rui Chen, Senior Manager of the Applications group of the Spectroscopy business at Thermo Fisher Scientific, talks to Dr. Jennifer Lynch, Research Biologist of the Chemical Science Division at the National Institute of Standards and Technology and co-director of the center for Marine debris research at Hawaii Pacific University, about the challenges researchers face when studying microplastics, and how FTIR Microscopes can assist in overcoming these challenges.

What kind of challenges are you facing today with different scales of plastic and the techniques needed to analyze them?

I have used both a Raman microscope and the FTIR microscope on the same samples. The two methods are incredibly complimentary because FTIRs detect specific polymers while Raman microscopes see the ones that FTIR finds more difficult to identify.

Better confirmation of the polymer is obtained when using both microscopes on each particle. For example, additional information reveals if something is plastic or another human-made material.

Our research center is relatively new, and we are thinking about what instruments we could purchase if we had the funding. When I dream about what that lab would look like, my vision is to have what I call the trinity of instruments for microplastics - an FTIR microscope, a Raman microscope, and a Pyrolysis-GC-MS.

With microplastics, there is a whole camp of people who want to know how many particles there are, the size distribution of those particles, and the polymer composition of that population of particles. The only way you can get that is by using an FTIR microscope or Raman microscope because they can carry out the individual particle count, sizing, and polymer composition in the same mapped, automated run.

There is then a whole other camp of people, such as toxicologists, who want to know the units of a microgram of polyethylene per gram of tissue, sediment, or sludge. They want the concentration units and a mass fraction.

Particle mass can be estimated based on the particle size measured under the Raman or FTIR microscope and the known density range for each polymer. Due to the diversity of polymers, exact particle depth and density are not known, and many assumptions are made.

Pyrolysis-GC-MS can give you an accurately known concentration, but what it cannot do is tell you how many particles there are and their size because you are burning the entire sample and losing that information.

The trinity would be to analyze every sample using all three methods. The time limits on this would be huge. However, I am usually encouraged to go towards the time-intensive, accurate, and information-intensive routes instead of screening routes where some sort of idea is obtained. It does not need to be incredibly accurate or precise.

How do you go about using these techniques alongside a library identification program when your sample material is non-standardized and subject to such different conditions?

All three of these techniques need further method development. We are nowhere near cookie-cutter science with standard methodology. We are still trying things out, and as you said, there are spectral library limitations.

That is one step that needs to be better looked at and understood. Our lab makes an in-house library with around one hundred different plastic polymers from various sources. They range from scientific standard materials, such as a NIST standard reference material of polyethylene, all the way to shampoo bottles that we brought in from our house. These are consumer goods, and we know that they have many additives and colorants, making them more representative of what we see on the beach and in nature than the NIST standard reference material.

We grade all of our reference polymers across a spectrum of scientific standards. However, even then, people need to know that those scientific standards are usually procured from a pellet producer and that proprietary additives have traditionally been added. No one will know exactly what that is except for the manufacturer. You have to make assumptions about the plastics you are dealing with, whether purchased brand new, from a scientific source, or consumer goods.

You have to assume that there is an additive package in there. The plastic is not going to be 100% pure polyethylene. We have scientific standards. We then purchased some standards from companies that are pellet producers and distributors of plastic. We also receive plastic from some industry partners willing to share with us to help us bring our laboratory up to speed. Then we have consumer goods. We like to have all three kinds of references for each polymer to see how different they come across on an FTIR.

FTIRs do not detect the additives very well. We use that to our advantage because we are more interested in identifying the polymer than additives. Because the FTIR is more blind to the additive, that helps us out. However, we also have a whole research project involved in understanding the chemical additives and how they are leaching out of the plastics and tissues. For this, scientists need to know what the additives are.

Going back to the spectral library searching issue, we recently did a rapid test where we took a few of our samples and ran them through three different software sets that had several library packages in them.

The answers across all three different software packages, which involved various libraries, are dissimilar but all in the same chemical family. For instance, we find many fishing lures, which are made with a very high percentage of salicylates. The salicylates show up on the FTIR, so we can look at salicylate additives as long as they are a high enough percentage.

We want to know which salicylate is the bulk material inside the fish lure. When we run it through one software set, we get a very different hit than when we run it in other software with various libraries because each library has a different set of salicylates included in it. Hence, the top hit is going to be the one that matches the best, but it might not be the right one because that library might be missing that salicylate additive.

That is a challenge for any spectroscopist because your top hit is only as good as your library is, and there are thousands of different salicylate chemicals. For one lab to source all of those and have an in-house library of thousands of salicylates is cost-prohibitive. That is a limitation.

Another limitation is that the FTIR spectrum changes with weathering and, while that does not happen so much that you cannot identify the polymer, it does make the spectrum noisier.

The top hit in a library search might be inaccurate. It does take some manual, visual looking at the spectrum to decide if this top hit is the correct polymer identification - perhaps the top hit is something that would be a liquid phase at room temperature. Therefore, common sense would tell us that it could not have been a polymer in the ocean. There is chemical knowledge we need to use to double-check these top hits.

One of the things that we see in our FTIR spectra that we wish so badly we could differentiate is low-density polyethylene from high-density polyethylene. This is because they are different plastics and produced for different reasons. They also have different resin codes (the chasing arrows sign on the bottom). We want to differentiate those because specific high-density polyethylene, the number twos, are recycled more frequently than the number fours, the low-density polyethylene.

We have different uses and waste streams for these two different polymers that are so similar. We have to be able to differentiate the two if we want to track changes in the environment based on societal changes, changing recycling patterns, or changing policies against specific polymers.

Back in 2018, we published a paper from Jung et al. called Validation of ATR FTIR to identify polymers of plastic marine debris, including those ingested by marine organisms. We could identify low density from high density 85% of the time, but this is not accurate when applied to weathered polyethylene.

High-density polyethylene lacks a particular band in the spectrum, and that band starts to grow as weathering happens. It starts to mimic the low-density polyethylene spectrum. When dealing with weathered polyethylene in the marine environment, which is what we are dealing with, you should not use the FTIR to differentiate the two. That has been a harsh learning lesson because we very much want to be able to do that, and we have had to go to a different instrument for it.

Are you aware of any community effort by different research organizations to pull the data together to move faster?

Chelsea Rockman's lab has created its libraries of non-weathered and weathered plastics called SLOPP and SLOPP(e). She shows that she gets much better, more accurate hits when using her libraries than commercially available ones.

I also know of Open -Specy, a free online open-source spectral library search tool, that Win Cowger from the University of California, Riverside, has developed specifically for the microplastics research community.

I, Chelsea Rockman, Sebastian Primpke’s lab in Germany, and several other labs, have uploaded reference libraries into Open -Specy. It is available online for the community to use, and there are currently around 100 different active users.

It is not perfect, but it will improve as more spectra are added into the library and more people begin to test and break it to figure out how we can improve it more. I think it is an essential tool that a lot of people are excited to have out there.

**Certain commercial equipment, instruments, or materials are identified in this interview to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose

About Dr. Jennifer Lynch

Dr. Jennifer M. Lynch’s research interests are to improve the quality of measurements in the field of marine environmental toxicology and chemistry.  She has performed organic analytical chemistry research for the National Institute of Standards and Technology since 2003.  In 2019 she became the Co-Director of the Hawaii Pacific University (HPU) Center for Marine Debris Research (CMDR). Dr. Lynch is motivated to study pollution exposure and effects in the ocean and educate others through technology transfer to perform quality science that can inform policy and improve environmental measurement.