The Novel Methods Of Molecular Structure Of Micro/Nanoplastics By Mass Spectrometry And Their Application In Food Packaging Detection

Under the impact of weathering,ultraviolet light and microorganisms,plastics can be broken into microplastics smaller than 5 mm or even nanoplastics smaller than 100 nm.Due to the tiny particle size,micro/nanoplastics(MNPs)can not only migrate to food through food packaging materials and be ingested by human body,but also easily be enriched by direct inhalation,ingestion of food chain,skin contact etc.,posing the potential threat to human health.In order to assess the hazards of MNPs accurately,it is essential to establish appropriate characterization methods of molecular structure.Up to now,microscopy and spectroscopy have been the mainstream technologies in this field.However,microscopy can only be used to observe the morphology of MNPs and the structure information of samples which be tested cannot be provided.Spectroscopic method is used to determine the MNPs by absorption bands of characteristic functional groups,but complex pretreatment is still required to eliminate water or fluorescent substances in the samples before analysis.Compared with microscopy and spectroscopy,the detection method based on mass spectrometry(MS)is more sensitive,and effectively makes up for the shortcomings of the current technologies in the characterization of MNPs.However,it still lacks a method for achieve accurate characterization of the composition and structures and sequence backbones of a variety of MNPs at the molecular level.In this project,novel methods had been established to accurately characterize the molecular structure of MNPs by fourier transform ion cyclotron resonance mass spectrometry(FTCIR MS)and ultraviolet photodissociation(UVPD)mass spectrometry.First of all,sample preparation conditions which suitable for FTICR MS characterization were systematically developed by response surface method using polystyrene(PS)as the model before analysis and characterization.Under these conditions,the mass signal-to-ratios of each sample were significantly improved.Further combined with ultra-high resolution FTICR MS,the composition and structure of PS,polyethylene terephthalate(PET),polycaprolactam(PA6)and polymethyl methacrylate were determined according to the mass difference of peaks with equal spacing 104.06 Da,192.05 Da,113.08 Da and 100.05 Da,respectively.The end group composition and ionization mode were determined successfully based on the mass difference between characteristic ion peaks and type of peaks,respectively.Then,the sequence backbones of MNP need to be elucidated by tandem mass spectrometry(MS/MS),and the structural characterization of MS/MS largely dependent on the dissociation strategy.PET,PA6,polyhexamethylene adipamide(PA66),and polyhexamethylene sebacamide(PA610)were used as samples in this work.Compared with collision-induced dissociation(CID)and high-energy dissociation(HCD),which are commonly used to characterize polymers,the number of fragment ions generated by ultraviolet photodissociation(UVPD)was increased by an average of 5 times and 1.9 times,and the sequence coverage was increased by an average of 5 times and 1.8 times,so the sequence backbone and end-group structure of MNPs could be characterized accurately.In addition,the differences of sequence backbones of polyamide analogues including PA6,PA66,and PA610 could also be distinguished accurately by UVPD.In contrast,the molecular structure of MNPs could not be characterized accurately by CID and HCD due to the lack of structural information seriously.Finally,the MNPs which were prepared by food packaging including foam,water bottle,tea bag,etc.,were successfully characterized by above novel methods.The results of characterization were consisted with corresponding industrial synthetic materials,further proving the wide applicability of the two methods.In this study,the composition structure of MNPs could be characterized by MALDI-FTICR MS preliminarily and the sequence backbone of MNPs could be further elucidated by UVPD-MS/MS,which proves the great potential of MS in the field of structural characterization of MNPs.In the future,with the in-deep study of MNPs from food packaging,the combination of use of above methods will play the important role in the characterization and hazard assessment of MNPs.