Study On Fragmentation Of Proton Transfer Reaction Mass Spectrometry And Establishment Of Model Of Spectral Analysis

Proton transfer reaction-mass spectrometry(PTR-MS)utilises the characteristics that water's proton affinity is between the main components of air and most volatile organic compounds(VOCs).By making proton transfer reactions between VOCs and H₃O⁺,PTR-MS can detect VOCs with specificity and high sensitivity,and applied successfully to the qualitative and quantitative analysis of trace VOCs in ambient air.Therefore,it is applied to many fields including environment research,food control,medical application,security application et al.Although PTR-MS is considered as a soft ionization technique,there are still a considerable number of VOCs which have only fragmented ion products and no protonated ions in PTR-MS.This brings difficulties to the spectral analysis and limits the adaptability of the technology.In this work,the research of PTR-MS and its application method were carried out.While completing the national science instrument program,the fragmentation issues in PTR-MS were deeply studied from the product distributions,fragmentation mechanism and spectral analysis.The main work of this thesis is as follows:1.The key components of the ion source and the PTR-MS instrument were developed.A data preprocessing method suitable for spectral analysis of PTR-MS was established.PTR-MS was used to analysis the exhaled breath of lung cancer patients and the headspace of the lung cancer cell lines,and three potential disease markers were identified.Through the above work,it also provided a stable experimental platform for the study of the fragmentation.2.Through the experiments,the distribution patterns of the fragment ions of 10kinds of linear alkanes(C₆?C₁₅)were analysed,and the chemical formulas of these fragment ions were determined as[C_nH_2n+1]⁺,[C_nH_2n-1]⁺and[C_nH_2n-3]⁺.The trend of the distribution of the fragment ions with the increase of E/N value(E is the electric field and N is the number density of molecules)was studied.In addition,by analysing the data of fragment ions,the chemical formulas of the fragment ions of various compounds such as alkenes,alcohols,aldehydes,and alkylbenzenes in PTR-MS were determined.Moreover,it also deepened the understanding of the types of fragment ions and their distribution in PTR-MS.3.Using n-undecane as a model molecule,the protonation of linear alkanes and their dissociation reactions in PTR-MS were systematically studied.Density functional theory(DFT)calculations were carried out to study the protonation sites of n-undecane,which have two cases,the protonation of the C-H bond and the protonation of the C-C bond.Combined with the experiments,the effect of the protonation sites on the fragmentation of linear alkanes in PTR-MS was discovered,that is,the protonation of C-H bonds results in the loss of H₂,and the protonation of C-C bonds results in the dissociation of C-C bonds.Based on the comprehensive analysis of collision energy and protonation sites,the mechanism of the fragmentation of linear alkanes in PTR-MS was clarified.Four types of dissociation reactions of these compounds in PTR-MS were systematically proposed,namely dehydrogenation of protonated alkanes,C-C bond cleavage of protonated alkanes,dehydrogenation reaction of carbocations,and?-Site cleavage of carbocations.4.According to the research results on the distribution of fragment ions in this work,a spectrum analysis model of PTR-MS based on the principle of chemical mass balance was proposed.By establishing the relationship equation between fragment ions and VOCs,the multiple linear regression method was used to calculate the contribution of VOCs to fragment ions.The model was applied to the identification of biomarkers,which achieved in-depth analysis of characteristic ion and further identified potential disease markers.Furthermore,it provided new ideas and methods for the qualitative and quantitative analysis of VOCs in the complex mixtures by PTR-MS.