Identification of microplastics in a large water volume by integrated holography and Raman spectroscopy.

Tomoko Takahashi,Blair Thornton,John Watson,Nicholas Burns,Dhugal Lindsay,Zonghua Liu,Thangavel Thevar,Sumeet Mahajan

doi:10.1364/ao.393643

Abstract

A noncontact method to identify sparsely distributed plastic pellets is proposed by integrating holography and Raman spectroscopy in this study. Polystyrene and poly(methyl methacrylate) resin pellets with a size of 3 mm located in a 20 cm water channel were illuminated using a collimated continuous wave laser beam with a diameter of 4 mm and wavelength of 785 nm. The same laser beam was used to take a holographic image and Raman spectrum of a pellet to identify the shape, size, and composition of material. Using the compact system, the morphological and chemical analysis of pellets in a large volume of water was performed. The reported method demonstrates the potential for noncontact continuous in situ monitoring of microplastics in water without collection and separation.

Highlights

Global awareness of plastic pollution in the marine environment has risen recently
We report a novel method for noncontact identification of microplastic resin pellets that integrates holography and Raman spectroscopy
The laser power for holography was set to 23 mW for PS and 18 mW for poly(methyl methacrylate) (PMMA), which were found to be an optimal power for each target without saturation

Summary

INTRODUCTION

Global awareness of plastic pollution in the marine environment has risen recently. In particular, the impact on ecology is a serious concern, with several reports of plastic waste being discovered in the guts of dead seabirds, turtles, and fish [1,2,3,4]. Using a low electric power compact setup with a collimated laser beam for digital holography and Raman spectroscopy, target recognition and chemical identification are demonstrated for two different types of plastic resin pellets In this approach, the rapid in-line holographic measurements can be used to detect and locate particles in a flow chamber. This approach will enable selective Raman measurements of plastic or targeted particles by prescreening particles in a flow chamber using holographic images This method is suitable for situations where other types of particles are mixed in a flow, such as in situ microplastic analysis in natural environments, in the deep ocean where particles are sparsely distributed, and mostly no more than a single particle would be expected in a measurement volume at a time

Materials

RESULTS AND DISCUSSION

Method

CONCLUSIONS