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SPECTROSCOPY


trawl), 1-2mm and 2-5mm. Te next step consists of the collection of one Raman spectrum at the centre of each particle, based on its previously determined position, with two accumulations of an integration time of 10s. Te particles are illuminated with a 785nm wavelength laser source for optimum signal/fluorescence ratio, using a 10x magnification objective lens. Te Raman spectra cover the 200-1,700cm−1 range with a spectral resolution of about 4cm−1


, using a grating with 300 grooves


per mm. Care is taken to maintain the laser power below a certain threshold to avoid structural or chemical medication of the polymers.


Chemical identification is finally obtained from commercial Raman spectra libraries (KnowItAll Informatics Systems, Bio-Rad, Raman ID Expert). Out of the 962 particles collected in the surface water, 75% were chemically characterised. Microplastics of different polymer nature (polyethylene 48%, polypropylene 12%, polystyrene 11%) accounted for 71% of the whole sample. Other identified particles corresponded to inorganic minerals, mainly calcium carbonate (2%) and quartz (2%). Te non-identified particles exhibited either


Raman spectra with no correspondence with databases, saturated spectra (due to strong fluorescence background) or an absence of signal. Tis workflow, based on selective and targeted Raman analysis, providing automated localisation, counting and characterisation of environmental microplastic particles, hugely reduces the total analysis time compared to a full point-by-point raster scanning approach. As the proposed method is time effective, a large sample size can be analysed in a reasonable amount of time, which prevents the downsizing to subsamples as is done in most studies.


TOP LEFT: Florian Formanek, head of applications at Horiba Scientific, posing in front of a LabRam HR Evolution Raman microscope at Horiba’s European Research Centre in France


REFERENCE [1] Frere L., Paul-Pont I., Moreau J., Soudant P., Lambert C., Huvet A., Rinnert E. (2016). A semi-automated Raman micro-spectroscopy method for morphological and chemical characterisations of microplastic litter. Marine Pollution Bulletin, 113(1-2),


Catalina David is with Horiba and Emmanuel Rinnert is with Ifremer. www.horiba.com


Proportion of particles identified and not identified in seawater samples


www.scientistlive.com 43


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