3.3. Microplastic analyses
A Bruker IFS66 Fourier transform-infrared (FT-IR) spectrometer with a mercury cadmium telluride
detector operating in the 4000–600 cm−1 wavenumber range was used for object identification. A Specac
DC2 Diamond compression cell (2 mm in diameter) was used to allow transmission of the IR beam to the
detector. Bruker’s OPUS 5.5 spectroscopy software was used for measurement, processing and evaluation
of the IR spectra. Matches with quality index greater than or equal to 0.7 were accepted. Matches with
quality index less than 0.7, but greater than or equal to 0.6 were individually inspected and interpretedbased on the closeness of their absorption frequencies to those of chemical bonds in the known polymers.
Matches with quality index less than 0.6 were rejected. We added the spectra of potential contaminants,
such as those from waterproofs and laboratory gloves used during sample collection and processing,
to the Plymouth University FT-IR library in order to eventually eliminate any contamination from the
data. No matches with these materials were found in any of the samples. In addition, we checked all
residue sediment for the larger plastic particles described in Van Cauwenberghe et al. [13]. None were
seen. Protocols were implemented to prevent plastic contamination from the processing environment
and controls to monitor air and water supplies were taken during all processing phases.
There has only been one previous report of microplastics in the deep sea, which documented very
low abundance [13]. The previous study employed different separation methods to those used here and
critically did not enumerate microplastic fibres, which are the most numerous type of particle present in
shallow water habitats and in some biota [5]. We ran a preliminary trial to compare methods used by both
studies, by splitting some of our deep-sea sediment samples and analysing them using both approaches.
We found that our approach and our recording of microfibres yielded substantially greater abundance
of microplastic particles, whereas the other method [13] underestimated the microplastic concentration
(electronic supplementary material).