Microplastics Analysis

Microplastic particles can be categorized into primary and secondary particles. Primary MP particles (MPPs) are those which have been specifically produced for industrial use, for instance as peeling particles in cosmetic products.

Secondary MPPs are formed by physical, biological and chemical degradation of macroscopic plastic parts and are the main source of micro particles released into the environment. They are mainly formed by the degradation of improperly disposed plastic waste, tire abrasion and washing of synthetic textiles.

They are found in riverbeds, the arctic ice, natural fertilizers, soils and even drinking water show noticeable amounts of MPPs. During the last decades, microplastics have even found their way into the human food chain. In short, the ubiquity of microplastic particles makes them an enormous challenge to our environment.

While the threat to marine life is mostly understood, its full extent cannot be assessed at present. However, uptake by marine organisms and fish leads to contamination of the human food chain by microplastics. As MPPs may contain problematic plasticizers and can also adsorb other organic pollutants, the long-term effects are fairly unpredictable.

Although millimeter-sized particles are already considered MPPs and can be distinguished with the naked eye, light microscopy should be mentioned first as the most basic technique to detect microplastics. 

However, this approach does not provide a chemical identification, which is particularly important to investigate the influence and origin of the detected MPPs. FT-IR and Raman spectroscopy offer the possibility to identify unknown polymer particles within minutes and are fully compatible with microscopic techniques.

About FT-IR Spectroscopy

 As the name suggests, infrared radiation interacts with the particle, yielding information by the absorption of certain wavelengths. If you want to know more about IR spectroscopy, look here. FT-IR’s biggest benefit is the exceptional reliability and its straightforward application. It virtually analyzes all polymers, including dark and fluorescent materials. By comparing sample data with reference data libraries unknowns can be identified and false positives minimized.

About Raman Spectroscopy

Raman spectroscopy relies on the inelastic scattering of light from a coherent light source (e.g. laser). Raman spectroscopy is not as widespread as IR spectroscopy, mainly because Raman measurements often require expert knowledge. Especially dark or fluorescent polymers are difficult to analyze or require special techniques. However, when it comes to spatial resolution, Raman microscopy clearly has the advantage, since it offers the analysis of MPPs down to the nanometer range.

It is virtually impossible to find a definitive answer to this question, because Raman and FT-IR spectroscopy are complementary techniques. From a spectroscopic point of view, this means that a complete data set can only be obtained if both techniques are used together. In practice, however, this is rarely the case.

Both techniques offer clear pros and cons and it is usually the application that decides which technique is preferred. Currently, even researchers are still discussing the best approach. If you have questions about which methods is the most suitable for your application, contact our microplastic experts and together we will find a suitable solution.

Depending on the sample you can either use transmission (contact free, IR light completely passes MP) or attenuated total reflectance (ATR, needs contact, IR light slightly penetrates MP surface). Measurements in reflection are also possible (contactless, IR light must pass MP twice) but will not be discussed at this point.

Transmission measurements are the standard approach but require special filters that allow IR light to freely pass to the detector. Depending on preference you can choose from Teflon (PTFE) membranes, metal mesh, silicon and aluminum oxide filters, which all have specific advantages and disadvantages. However, aluminum oxide filters are quite popular and thus will be used as an example on our website and in our videos. 

ATR on the other hand doesn’t need complex sample preparations or special filters. Microplastics can be directly analyzed on standard nitrocellulose filters and even on top of sediments or other complex matrices.In the case of drinking water or beverage analysis, the liquid is filtered through an appropriate filter material and subsequently analyzed. If your analyzing river or sea water, material like wood, sand or seaweed needs to be removed by density separation. 

For this, salt solutions of various concentrations are used. The prepared samples should be dried thoroughly before subjected to IR analysis. In some cases, enzymatic digestion and/or treatment with H2O2 prior to sample filtration may be necessary to remove organic and biological contaminants.

The easiest way is to first detect interesting particles with visual methods and then characterize them point by point with chemical mapping. This “point-and-shoot” approach is very feasible but can require a lot of time if manual search is applied. 

As a result, automated visual identification is a key requirement for an effortless workflow in microplastic analysis by FT-IR mapping. After measurement, clear identification is readily available by infrared spectral reference libraries for all commonly found polymers.

Although an automatic visual detection reduces human error, this method poses the risk of missing smaller particles as their contrast might be low. To eliminate the human factor almost completely, FT-IR imaging is the safer approach. FT-IR or focal-plane array (FPA) imaging is the state-of-the-art solution to microplastic analysis. It is faster and offers a higher spatial resolution compared to single-point mapping analysis.

Usually, imaging analyzes a whole filter loaded with microplastic particles in one session. Evaluation is carried out by chemical information only, the chance to miss smaller particles, that have low visual contrast, is significantly reduced. Please watch our video to learn more about FT-IR imaging of microplastics.

Again, it is hard to find a definitive answer to this question. Researchers such as the pioneering Alfred-Wegener-Institute and Aalborg University are relying on FPA technology. However in some cases, in which lower concentrations of microplastics are characterized, mapping experiments offer higher efficiency.

As experts in vibrational (micro) spectroscopy with a long-standing experience in MP analysis by FT-IR we support you in finding the most fitting solution to your demands in microplastic investigations. Don’t hesitate to contact us if you need more information.

For the analysis of microplastics using Raman, it is important that neither the examined particles nor the used filter material exhibit fluorescence. Additionally, due to unintentional sample heating, Raman is not readily able to analyze black plastics and rubbers. Although Raman analysis requires significantly more parameter adjustments, depending on the nature of the sample, the preparation itself is very similar to that of IR analysis.

Drinking water must be filtered through an appropriate filter material (e.g. gold-coated polycarbonate) and unwanted material like wood, sand or seaweed must be removed prior by density separation with salt solutions of various concentrations. 
If the MP are loaded with organic and biological pollutants (e.g. plasticizers or algae) enzymatic digestion and/or treatment with H₂O₂ might be necessary before sample filtration.

The first option would be to visually analyze the sample and look for individual microplastic particles. Contrast enhancement tools (e.g. dark field illumination) help detect MP particles, but using automatic visual analysis is the safer and quicker approach. After localization, the particles are automatically measured and analyzed. The identity of found particles is easily clarified using Raman spectral reference libraries for all commonly found polymers.

However, there is a risk that colorless MPs with low contrast will be overlooked. In cases where human error is to be eliminated, automated Raman imaging is the key to success. Instead of analyzing single MPs on a filter, the whole filter can be scanned with a very narrow measuring grid. This will increase the time needed for the analysis tremendously but afterwards, identification solely relies on the chemical contrast. By this, MPs are reliably quantified and the chance for human error is greatly reduced.

Again, it is hard to find a definitive answer to this question. Mapping as well as imaging are viable approaches for a comprehensive MPP analysis, each offering unique advantages.

 If time is of the essence, mapping is the more suitable approach. As experts in vibrational (micro) spectroscopy with a long-standing experience in MPP analysis, we support you in finding the best solution to microplastic analysis.