Double-Angling-Subspace Enabled Laser-Induced Fluorescence Method for Determining the Types and Mass Ratio of Marine Microplastics.

Currently, the laser-induced fluorescence method faces challenges in reliably determining the types and mass ratios of marine microplastics due to overlapped fluorescence spectra of different microplastics. To address this issue, this paper proposes a double-angling-subspace (DAS) method to differentiate the overlapped fluorescence spectra. The key idea is to span subspaces with vectors converted by known fluorescence spectra, followed by calculating the angle between vectors and subspaces. Specifically, it is found that the angle between the vectors converted from fluorescence spectra of unknown microplastics and their projections on the subspaces, as well as the angle between these vectors and the vectors spanning the subspaces, is indicative of microplastic types. The vector of an unknown microplastic belongs to the subspace spanned by the vectors converted by the known microplastics, and the mass ratios of unknown samples can be determined by analyzing the linear correlation between the vectors of both unknown and known microplastics. The reliability of the proposed DAS method is validated with real marine microplastic samples.