Microplastics - plastic particles smaller than 5 mm in size - are a major cause of environmental pollution, and this can no longer be overlooked by society.
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Raman spectroscopy is a non-contact, non-destructive, and chemical-specific technology that has been widely employed throughout the field of microplastic detection. However, more traditional point confocal Raman techniques are limited to single-point detection, impeding the detection speed.
In a study published in the journal Talanta, a research group headed by Professor Bei Li from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS), in partnership with Professor Wolfgang Langbein from Cardiff University, suggested a novel line-scan Raman micro-spectroscopy method for the quick identification of nanoplastics and microplastics.
Based on the key principles of confocal Raman spectroscopy, the focused excitation spot transforms from a convergent point into a convergent line with diffraction-limited width.
The optical setup employs a conjugate imaging design. In the two-dimensional image recorded by the charge-coupled device (CCD), the vertical dimension maps the vertical dimension of the sample along the excitation line, while the spectrum is dispersed along the horizontal dimension.
In this way, a single acquisition provides the spectra for all spatial positions along the excitation line.
The researchers have successfully managed to develop a confocal line-scan Raman micro-spectroscopy system, establish a preprocessing workflow for line-scan Raman spectral data, and apply the factorization into susceptibilities and concentrations (FSC3) to obtain Raman hyperspectral images.
They employed a concave cylindrical lens to generate the excitation line and, using a Powell lens, enhanced the uniformity of energy distribution.
Plastic beads of several sizes were utilized for size and composition identification. The detection of beads with a diameter of 200 nm, which is smaller than the diffraction limit, was identified, thereby illustrating the remarkable sensitivity of the line-scan Raman spectroscopy system.
Moreover, four types of plastic powder samples were utilized for a large-scale area of 40 μm in height and 1.2 mm in length measurement. Impressively, it only took 20 minutes to obtain a 240000-pixel Raman image - which is an extrememly short imaging time.
In comparison with point confocal Raman imaging, the line-scan confocal Raman technology helps increase the imaging speed by around two orders of magnitude.
Line-scan Raman micro-spectroscopy provides non-destructive analysis with high-throughput and high sensitivity. By applying suitable sampling methods like sedimentation or filtration, environmental samples from several sources, such as soil, water, and air, are accessible.
Wu, Q., et al. (2023) Rapid identification of micro and nanoplastics by line scan Raman micro-spectroscopy. Talanta. doi.org/10.1016/j.talanta.2023.125067.