Towards monitoring of small microplastics via the rapid identification and quantification of microplastics by hyperspectral quantum cascade laser infra red (QCL-IR) imaging

, Primpke Sebastian, Godejohann Matthias, Gerdts Gunnar.

The investigation and monitoring of microplastics (MP) is of emerging concern for legislation and societies. Especially the analysis of small MP (below 100 µm) creates additional challenges and pitfalls. To determine particle size and chemical composition in one measurement FTIR imaging provides a relatively fast method for complete filter areas. Here, modern systems measure field sizes of 12 x 12 mm² (pixel resolution = (5.5 – 11 µm) within 4 hours but need a permanent liquid nitrogen supply 24 hours. Here, we present a novel approach to measure such filter areas within less than 1 hour. To achieve this measurement speed a quantum cascade laser infra red (QCL-IR) based microscope, Daylight Solution Spero QT, was used. Compared to existing FTIR imaging systems it has several advantages. The use of liquid nitrogen is obsolete while 230.400 spectra are measured simultaneously on an area of 2 × 2 mm² using a focal plane array detector with a pixel resolution of 4.2 µm within 1 minute including data storage. A smaller spectral range of 1800 – 950 cm−1 is available compared to FTIR imaging while a larger number of spectra is collected. To investigate the performance of the instrument and to foster harmonization with existing studies, it was validated against known polymer samples, the database was optimized for automated analysis similar to FTIR imaging and finally the results compared to those achieved by FTIR imaging. The QCL-IR-based microscope achieved more detailed results with a higher sensitivity for smaller particles compared to FTIR imaging in much shorter measurement times. Here, the measurement of a 12 x 12 mm area took only 36 minutes. By combining automated analysis with hyperspectral QCL-IR based imaging, we realized a novel fast and versatile tool to identify MP allowing the measurement of large sample sets within current laboratory routine times.

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