Molecular Composition And Structure Analysis Of Dissolved Organic Matter In Natural Water: Methodology And Application

Dissolved organic matter(DOM)widely exists in various environmental water,which participates in migration and transformation process of pollutants in water,and produce important ecological environment effects.However,the molecular composition and structure of DOM are still unclear.Based on high-resolution mass spectrometry,including Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)and Orbitrap mass spectrometry,this dissertation establishes the characterization methods for molecular composition and structure of DOM in natural water,respectively,and study the molecular structure characteristics of DOM from different sources.(1)A new approach was established by using co-precipitation and solid phase extraction(SPE)to characterize molecular composition of DOM in seawater.The separation and enrichment process of seawater DOM was also achieved by co-precipitation of iron hydroxide.The total organic carbon yield of DOM by SPE was increased by 25%.Compared with the traditional SPE method,the number of assigned molecular formulas identified by-ESI FT-ICR MS were increased by 51%.In addition,the method has good selectivity on tannin-like compounds which are easily lost in traditional SPE methods.Therefore,the method is expected to be a conventional evaluation strategy for DOM extraction of environmental water.(2)Systematic investigations were performed for the fragments ions of DOM from different water sources,and the molecular structure similarity of DOM from different water sources was demonstrated.A very large proportion(about 75-90%)of fragment ions of DOM from different water sources is identical.The differences of molecular composition in DOM from different sources are responsible for the mass spectral differences in tandem mass spectrum analysis.Therefore,the compounds with same molecular formula have extremely similar molecular structures in DOM from different sources.This study provides important data support to explain the carbon storage mechanism of marine refractory DOM by using mass spectrometry.(3)A novel method for molecular structure characterization of DOM was established based on tandem mass spectrometry and structure prediction software(CFM-ID).The molecular composition of SRFA is firstly obtained by using UPLC-Orbitrap MS,and a combined search database including multiple compound databases(including HMDB,Mass Bank and Pub Chem,etc.)is formed.Secondly,the corresponding relationship between one parent ion and the fragment ions is determined based on the similarity between parent ion cluster and fragment ion clusters.Finally,the best candidate compounds were obtained by structure prediction software(CFM-ID).Four model compounds were used to verify the feasibility of the method.This method is expected to be applied to the molecular structure identification of other complex samples,such as petroleum and lipids.(4)The established method was applied to the molecular structure characterization of SRFA.A series of monomer compounds in DOM with specific structures were identified.For the compounds with different double bond equivalent(DBE)values,the ring structure in DOM gradually cracks with the decrease of DBE.For the compounds with the same DBE value,these compounds have great similar structure,which are carboxylic acid compounds formed by one or more alkyl chains connecting benzene rings.In addition,some sub-structural units,such as carboxyl group,benzene ring,aliphatic bond,ether bond and carbonyl group,commonly exist in the molecular structure of SRFA.This result indicates that lignin is one of the important sources of DOM in natural water.