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Interest in analyzing the presence of microplastics in the environment and food chains is growing, but which technique is best for your needs or those of your customers? This is a common question today. For example, should I be using a technique such as pyrolysis coupled with a GC/MS, or would a spectrophotometer coupled with a microscope be more suitable?
To consider this, it is worth examining the sort of information the user may be seeking. At the broadest level, the user may seek information about the total (bulk) mass of the plastics and additives present. Alternatively, they may be interested in detailed information about the individual particles including size, morphology, and chemical identity. Since no one technique can give all the possible answers, a user must assess which is right for them.
There are really two broad categories of techniques, each providing the user with different yet complementary information. The GC/MS and suitable sample introduction techniques can provide highly accurate and detailed information about the total mass of polymers, additives, and other substances present in the sample. They cannot, however, provide any insight into the particles themselves, including particle number, size, surface area, etc.
Spectroscopic techniques, such as infrared (IR) and Raman micro-spectroscopy, can provide detailed information about the individual particles (size, shape, chemical identity, etc.) but they cannot provide total mass information. Although some researchers also predict total mass following spectroscopic investigation, these results are only indicative. All the microscopes used for spectroscopy today only measure two dimensions of a particle, the length and width. The height, however, cannot be measured. As a result, estimations of mass in these techniques rely on assumption of the height, the elliptical nature of the particle, and the mass of the polymer. Thus, the GC/MS-based thermal techniques and spectroscopic techniques provide the user with complementary information for complete characterization of microplastics.
GC/MS may be coupled with a pyrolyzer or a thermogravimetric analyzer. Pyrolysis involves a microfurnace connected directly to the injection port of the gas chromatograph (GC). During flash pyrolysis, the high-molecular weight compounds are thermally decomposed to smaller fragments (pyrolyzates) and deposited directly on the GC column for separation, which can then be passed to the mass spectrometer (MS) for detection. A thermogravimetric analyzer (TGA) may be used to conduct thermal extraction desorption (TED), also coupled with a GC/MS. This technique offers additional benefits to pyrolysis in providing more homogeneous samples, increasing automation of the analysis and potentially providing greater robustness.
Among the spectroscopic techniques, FTIR microscopes (both imaging and non-imaging), Raman microscopes, and laser direct infrared (LDIR) spectroscopy are the commonly used techniques. Each of these will be addressed in their own section of this article.
Agilent gas chromatography systems coupled with mass spectrometry detectors (GC/MS)—with innovative sample introduction techniques—are used for thermal analysis and quantification of microplastics.
Sample preparation requirements are a key area of differentiation. GC/MS techniques require comparatively little sample preparation. Spectroscopic techniques, however, may require extensive sample preparation. As these techniques are examining individual particles, the removal of any materials that are not of the target types is important, as it can drastically slow analysis or worse, hide the level of detail sought.
While sample preparation for clean drinking water is relatively simple, that for wastewaters and other complex mixtures may consist of some form of filtration and density separation to remove the solids, plus various digestions to remove organic matter, protein material, etc. All of these must be conducted, however, in a way to retain and minimize damage to the target microplastics particles. It should be noted that this type of sample preparation is largely common among all the spectroscopic techniques.
Laser direct infrared imaging (LDIR) is a new technique for IR spectroscopy. It combines a tunable quantum cascade laser (QCL) as the IR source with rapidly scanning optics. It can be used in two modes: frequency parked with rapid scanning over a large area, or position parked with rapid sweep through the entire available wavelength range with resolution at the diffraction limit. LDIR detects microplastic particles by rapid imaging of the area using IR light rather than visible cameras to determine the location, size, and shape of particles. Spectra can then be obtained from individual particles and compared to the onboard library, with results presented in real time.
LDIR overcomes some of the key limitations of FTIR systems such as eliminating the need to collect data in empty spaces. This results in significantly faster analysis times and it can also be fully automated. A QCL operates at lower power than lasers used in Raman, hence fluorescence and sample damage pose no risk. An electrically cooled detector eliminates the need for liquid nitrogen, yet it has the highest resolution of any IR system and can detect particles as small as 10 µm. It is, however, relatively new technology. This, coupled with the use of the fingerprint region of the IR spectrum, only means that relevant libraries of data are less developed than other systems.
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Microplastics particles can be colour coded by polymer type for easy identification on the screen.
The 8700 LDIR chemical imaging system provides an automated workflow for microplastics analysis in environmental samples.
For further information on the 8700 LDIR chemical imaging system and microplastics analysis:
Fourier transform infrared (FTIR) instruments range from simple (and lower-priced) single point microscopes, through to much more expensive focal plane array (FPA) microscopes that can acquire spectra simultaneously for a number of pixels over a larger area. FTIR is a mature and well-understood technology, and there is significant relevant literature and extensive spectral libraries available. Time of analysis, however, can be prohibitive. Even the largest FPA systems can only image areas of less than 1mm2 at a time, and these areas must be mosaiced and the data stitched together. As a result, data acquisition on a typical 13 mm filter area would exceed 3 hours. After acquisition, a vast amount of data must then be separately processed to obtain results. Depending on the area imaged and system used, this can take many hours.
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The Agilent 8700 LDIR for microplastics overcomes many of the key limitations of FTIR. For further information on LDIR and microplastics analysis:
In contrast to FTIR and LDIR, Raman imaging uses the Raman effect rather than IR transmission or reflection. Many Raman systems can detect particle sizes down to 1 µm, where the very best IR systems are limited to the diffraction limit of the light (>10 µm). Another benefit is the ability to automate the analysis using visible light pictures to locate and describe the particles, then obtaining spectra only from the particles and ignoring the empty space in between. A key limitation—the Raman effect is relatively weak, so these systems use higher-powered lasers. Sample damage may occur, or the sample may exhibit fluorescence, drowning out the useful Raman signal. While each can be mitigated using lower powered lasers and/or by attenuating the signal, this comes at the cost of speed.
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The Agilent 8700 LDIR for microplastics overcomes many of the key limitations of Raman imaging. For further information on LDIR and microplastics analysis:
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