Identifying and measuring individual micrometre-sized fibres in environmental samples by light and confocal microscopies
Marina Corte Tedesco, Mark A. Oakley Browne
Abstract
Plastic and cellulose fibres contaminate the ecosystem. These are increasingly distinguished spectroscopically and their mass is estimated geometrically by melding data about their density, length, diameter and breadth. This is problematic because the shape and porosity of fibres vary, and spectroscopy cannot distinguish amongst celluloses. Light and electron microscopies can measure and distinguish amongst celluloses using transversal and longitudinal morphological features (e.g. twists, nodes, size, shape, pores) but they require physical sections that are not possible for micrometre-sized fibres. While forensic scientists have proposed using confocal microscopy to virtually section fibres, it is unknown if the virtual images can be used to distinguish and measure fibres. Here, we show that confocal microscopy can distinguish features and identify cellulosic fibres (cotton, linen, rayon-viscose, ramie, jute) but the virtual size of cellulosic and plastic fibres increased when the confocal image is compiled. By physically sectioning individual pristine and weathered cellulosic (cotton, rayon-viscose) and plastic (polyester, polyamide) fibres and then measuring transversal sections imaged by light and confocal microscopies, we revealed (i) the widths of fibres measured from confocal images were more accurate than their heights and areas; (ii) transversal areas estimated by the equation of an ellipse were more accurate than those estimated with a circular equation. Experimental addition of dibutyl phthalate polystyrene xylene to fibres showed it physically shrank the width of fibres of rayon-viscose and polyamide by 4–5% but not cotton and polyester. Finally, we used our confocal technique to identify cellulosic fibres found amongst oyster-beds and showed many were predominantly cotton. Our results are discussed in relation to improving the methods to better estimate the type, number, mass and volumes of fibres in environmental surveys.