Raman Spectrometer In Microplastic Type Analysis
What are "microplastics"?
Microplastics are tiny particles created when larger pieces of plastic break apart. Plastic straws stuck in a turtle's nostril and plastic products stuffed in the stomachs of sea animals are things we can see with our naked eyes, but microplastics that are invisible to the naked eye are even more harmful to the environment.
For the Marine environment, Marine sediment is considered as a "sink" for microplastics accumulation, but there is no unified standard for the detection of microplastics in Marine sediment. In the field of spectroscopy, Fourier infrared spectroscopy and Raman spectroscopy are commonly used as nondestructive analysis methods to characterize the molecular structure of microplastics. Fourier transform infrared spectroscopy can not play a good role in the detection of particles less than 20μm, while Raman spectroscopy can achieve the detection of "microplastics" less than 10μm.
After measuring nearly 200 particles by Raman spectroscope, 41 microplastic samples with particle sizes ranging from 5 microns to 500 microns were identified, including polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), polyamide (PA), polystyrene (PS). Polyethylene terephthalate (PET) and acrylonitrile-butadiene-styrene copolymer (ABS).
Figure 1.Confocal Microscopy of Marine Sediment Samples (A ~ F)
Among them, the confocal microscopic Raman images of PE, PS, PA, ABS, PET and PTFE characteristic particles were selected, as shown in Figure 1. They were labeled as particles D, E, F, G, H and I, respectively. Each microplastic sample retained its original characteristic morphology.
Figure 2. Comparison Between The Raman Spectra of Marine Sediment Samples and The Standard Raman Reference Library (A ~ F)
In addition, the Corresponding Raman analysis diagram of each particle is shown in Figure 2. The functional group information inside the particle substance molecule is characterized from the Raman characteristic peak position. The structural information of each particle substance is determined through comparison and analysis with the established Raman spectrum library of microplastics.
FIG. 3. Comparison Between the Raman Spectra of Particulate and Polyethylene in Marine Sediment Samples
In addition to the characteristic spectrum of the sample mentioned above, one of the particles to be measured in the sample showed additional Raman characteristic peaks at 880 cm-1, 1655 cm-1 and 3010 cm-1 in addition to the characteristic peak position of polyethylene during Raman analysis (Figure 1.5). The characteristic peak at 880 cm-1 is caused by the stretching vibration of C-OH bond, which can be attributed to the vibration of hydroxyl group. The characteristic peaks at 1655 cm-1 and 3010 cm-1 could be classified as lipids.
Raman analysis shows that the spectra of microplastics from the Marine environment do not always match the standard spectra due to the addition of additional compounds to the polymerization or the mixed impurities from organic or inorganic materials. Therefore, in order to improve the efficiency of microplastics analysis, it is important to include the spectrum of microplastics exposed to the environment in the spectral library.
Confocal microscope spectrometer can not only obtain a smaller sample detection limit, but also obtain a clear sample of the original morphology, the real realization of "what you see is what you measure".This analytical method, which can simultaneously observe and detect samples, can not only provide information on the chemical composition of samples, but also provide morphological characteristics of tiny particles, providing data support for relevant research, enabling researchers to explore the source and propagation of microplastics, as well as the availability of organisms.
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