In previous studies of marine debris or microplastics (b5 mm), various types of semi-synthetic celluloses (e.g.
rayon) are ubiquitous in some field investigations. However, it is hard to distinguish semi-synthetic and natural
celluloses clearly even using the spectroscopic method. In this study, 8 semi-synthetic and 4 natural celluloses
were employed as the test materials to simulate the environmentally relevant samples. Our results showed
that these original commercial products exhibited obvious physical (e.g., color) and chemical (e.g., spectra)
changes after UV weathering and agent (H2O2 and KOH) digestion treatments. The changes of 4 characteristic
bands (1735, 1425/1419, 1105, 1060–1053/1030–1027 cm−1) were evaluated. We found that the band at
1105 cm−1 which is assigned to the C\\O antisymmetric in plane stretching band only existed in natural fibers
even after the weathering and digestion treatments. The mixture of semi-synthetic and natural fibers from the
real field samples was also easily distinguished using the characteristic band at 1105 cm−1. Our results suggest
that the characteristic band at 1105 cm−1 could be an ideal reference to distinguish natural and semi-synthetic
fibers in fieldmicroplastic investigations. Wealso proposed a practical method to enhance the library of polymer
spectra and improve the accuracy of semi-synthetic microplastic identification.