µSpectroscopy in microplastics research: How to choose bewteen FTIR, Raman, O-PTIR, LDIR, FLIM....?
- Oskar Hagelskjær
- Mar 10
- 5 min read
In the field of microplastics research, the selection of analytical instruments to detect microplastics is actually quite broad. When opting for a new instrument, you're immediately met with an important, first choice: go for a destructive, mass-based method (Ex. Py-GC-MS, amongst others) or a non-destructive particle-based method?
Whille mass-based techniques are able to measure even nanoplastics, let's say that your particular application requires the analysis of individual particles, and therefore, you decide to opt for a particle-based approach. Now, you'll have to choose between quite a few techniques, including:
i) Fourier-Transform Infrared (FTIR).
ii) Raman.
iii) Optical Photothermal Infrared spectroscopy (O-PTIR).
iv) Laser direct infrared (LDIR).
v) Fluorescence spectroscopy such as Fluorescence-lifetime imaging microscopy (FLIM) and dye-stain derived Polymer Identification Model (PIM).
Making the right choice now depends on a set of parameters unique to your application, and of course, economic situation. An important aspect of particle-based spectroscopic analysis, is the spatial limit of detection (LOD) of the method in question. For a quick overview, the figure below compares spatial LOD and price of the techniques/instruments in question (excluding fluorescence spectroscopy techniques). I have to add though, that FTIR and Raman prices are strongly underestimated here, and are today, at least twice as expensive as displayed in the figure.
Mostoften, the choice stands between FTIR and Raman. As verified tools for microplastics detection, FTIR and Raman are both vibrational spectroscopy methods, that in slightly different ways (we will not go into details) detect molecular bonds, producing spectra that function as 'chemical fingerprints'.
Fluorescence spectrometry methods also produce 'chemical fingerprints' through different approaches, including:
FLIM (Fluorescence Lifetime Imaging Microscopy): measures fluorescence decay time rather than intensity, providing contrast based on the molecular environment or composition.
PIM (Polymer Identification Model): applies machine learning to fluorescence spectra, enabling the classification of polymer types based on their characteristic fluorescence signatures.
However, these fluorescence-based techniques are not as developped as FTIR and Raman, and are not (yet) classified as verified techniques for microplastics detection. An FTIR instrument is approximately half the price of a Raman system, and while Raman microspectroscopy can reach particles down to 1 µm in diameter (in contrast to 10-20 µm with FTIR), that doesn't necesarilly mean that Raman is the better choice, and this is principally due to Raman's Achilles' heel: fluorescence interference.
Autofluorescence, emitted by a wide host of compounds (when irradiated with laser) including most biogenic organic matter and plastic pigments, mimics the Raman signal (inelastic scattering of photons) and is often much more intense. As a result, fluorescence interference saturates the detector and 'drowns' the Raman signal in noise, rendering it completely useless.
In my personal experience though, the presence of pigmentation in microplastic pasticles below ~50 µm in environmental samples, is incredibly rare, as pigmentation leahces over time. Smaller particles have higher relative surface area compared to their volume, inciting accelerated leaching rates.
In addition, most biogenic organic material can be sucessfully removed by properly executed sample pre-treatment. In any case, biogenic organic matter poses an issue not only due to fluorescence interference, but also due to filter saturation and incident particle agglomeration and coverage.
Therefore, pretreatment is also often a requirement when applying FTIR in the analysis of environmental samples. With newer FTIR models however, spetral mapping elliminates the issue of particle agglomeration (although not coverage), given that the agglomerating particles are not of the same polymer type. However, this is unlikely to occur in environmental samples, where microplastics often make up less than 1% of particles; even after pretreatment.
Conclusively, if you plan to analyze only microplastics >20 µm, you are better off with an FTIR instrument. Although some claim, that modern FTIR instruments are capable of measuring particles down to 10 µm in diameter, I would advice against doing so, as the spatial resolution of many of these systems is 5 µm/pixel. Therefore, you are most likely going to be underestimating the number of particles within the 10-20 µm size fraction.
Alternatively, there are two new infrared microspectroscopy systems appearing on the market with sub 10 µm claims; namely LDIR and O-PTIR.
Both are rather expensive, but O-PTIR takes the cake, at ~400-500k€ with LDIR at more than half that at ~200k€. Personally, I have very little to no experience with these techniques, but as far as I have gathered, LDIR does not differ significantly from modern top-tier FTIR intrstruments, other than producing data with a much shorter spectral range, limiting you to the fingerprint region of the infrared spectrum. Correct me if I'm wrong!
O-PTIR combines Raman and infrared spectroscopy to detect infrared absorption at the spatial resolution of a Raman instrument, intrinsically limited by the wavelength of the laser in question - being submicron. While O-PTIR offers the advantages of Raman without the issue if fluorescence interference, the technique still relies on image acquisition to segment and scan microplastic candidates - just like Raman. I will add that Raman and O-PTIR mapping is technically possible but it takes a very long time to cover a representative area, making it impractical in environmental microplastics research.
Technical characterisics are important, but to incorporate a new instrument into a constructive, practical workflow, the accompanying software must be well-suited to accomedate microplastics analysis. Manual acqusition is not a viable means of collecting data so you need an automated approach!
For FTIR systems there are several pieces of software available, with one of the most prominent being the siMPle software suit developped by some of the leading microplastics researchers in Europe. Additionally, all instrument manufacturers include their own proprietary software solutions.
Only for some Raman microscopes, namely the alpha300 R by WITec and the QONTOR by Renishaw, data acquisition freeware exists; namely the GEPARD software pakcage which offers several advantages of which I am actually not fully aware. I'd love to hear the comments of anyone who has experience with GEPARD!
Another prominent piece of freeware for particle-based Raman acquisition is the TUM-ParticleTyper 2 which features the adaptive de-agglomeration option that cleverly separates agglomerated particles based on convexity values using Otsu’s algorithm, prior to spectral acquisition! (see the Fig. below).
Personally, I have a lot of experience with the LabSpec6 software suite, on which I have based my workflow. To get the most out of the software, we have developped our own code to merge unintentionally partitioned edge-particles, while combining spectral data using the Spectragryph software suite and morphological data produced by the ParticleFinder software package itself. However, I'm open to exploring alternative methods that may be more comprehensive.
It seems today, that every microplastics research team has their own unique approach to data acquisition and treatment, and it's always interesting to learn how others approach their data, because it's no secret that environmental microplastics research is riddled with compromises! I firmly believe that neural network-driven computer vision software capable of segmenting individual particles prior to spectral acquisition, is the future for particle based Raman and O-PTIR microspectroscopy.
To round off - when you are looking into buying a new instrument, take all of these pieces of information into consideration, but most importantly, send one of your true environmental samples to the manufacturer and let them run a few analyses - or even better, go there and do it yourself. Having the right instrument to fit your analysis needs is so incredibly important!
Oskar Hagelskjær
Founder and CEO
Microplastic Solution
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