| Halogenated organics are the products of human civilization advancement.Most of them are toxic and persistent.They have invaded our atmosphere,soil,ocean,and water environment.Compound specific isotope analysis(CSIA)is an important technology for tracing groundwater pollution,quantifying degradation and identifying degradation pathways,and is the only technology for providing direct evidence of microbial degradation.The CSIA-Cl technology mainly adopts mass spectrometry technology.At present,the commercial mass spectrometers for CSIA mainly have the following problems: one is the extracting efficiency of the ion beam produced from the ion source low.It is difficult to meet the efficient extraction of chloride ions.The other is the accuracy of conventional mass analyzers such as time-of-flight,quadrupole,and ion trap mass analyzers low.Commercial magnetic mass spectrometers due to its versatility,complex structure,and high cost,applied to the CSIA-Cl technology need to improve the instrument hardware and software.The third is the methods used in the reaction ionization process for isotope analysis.It is necessary to solve a series of key issues in the extraction of isotope information,and to establish appropriate reaction conditions,ionization mode,ionization energy and cathode filament current optimizated parameters to achieve online stripping and analysis of chlorine isotope fragments.Based on the elucidation of the ionization mechanism of organic monomer chlorine isotope,the mass spectrometry technology for high accuracy CSIA-Cl was researched.The main research contents are as follows:(1)Development of electron impact-chemical ionization composite ion sourceIon source is an important part of mass spectrometer to ionize sample molecules.In order to improve the ion extraction efficiency of the electron impact(EI)ion source and achieve high accuracy analysis of chlorine isotopes,a high extracting efficiency EI ion source was developed.Ion optical model of the EI ion source was constructed.The semi-cylindrical repeller electrode and the semi-cylindrical ionization chamber together formed a cylindrical ionization chamber.Due to the small potential difference between them,a potential lens effect was constructed,and the ion beam spatial divergence decreased.The ion beam extracted from the ion source was high efficient.The internal electric field distribution and ion deflection focusing theory were researched.Theoretically,the ion extraction efficiency could reach more than 80%.Based on theoretical analysis,an ion source capable of extracting ions with high efficiency was developed.The developed ion source was tested.The oxygen ion current intensity in negative ionization mode could reach more than 20 nA.Its high-efficiency molecular ionization and ion extraction capabilities could meet the requirements of high accuracy CSIA-Cl.(2)Development of quadrupole field ion transmission systemThe ion transmission system is a key component in the mass spectrometer that transmits the ion beam from the ion source to mass analyzer,and its performance directly determines the parameters such as the detection limit of the instrument.In order to improve the transmission efficiency of the ion beam from the high-pressure ion source to the high-vacuum mass analyzer.The quadrupole field ion transmission theory was researched,and a quadrupole field ion transmission system for the ion transmission region of a mass spectrometer was developed.Compared with the conventional radio frequency(RF)electronic system,the RF electronic system adopted the closed-loop negative feedback pattern to control the RLC resonance amplified voltage,which had stable performance and strong load capacity.The RF electronic system was tested.The peak-to-peak voltage Vp-p stability of the resonance signal reached 2.5‰ within two hours,and the Vp-p was continuously adjustable in the range of 50 ~ 1000 V when the load quadrupole capacitance was 34.79 pF and resonant frequency 2.01 MHz.It was applied to a laboratory-developed mass spectrometer and adopted benzene,toluene,and xylene to test the performance of the mass spectrometer.The detection limits of the mass spectrometer reached 0.07,0.05,and 0.04 ppbv,respectively,and the linear correlation coefficients were 0.9958,0.9962,and 0.9973,respectively.The analysis of volatile organic compounds(VOCs)in human exhaled gas and newly decorated rooms showed that the developed quadrupole field ion transmission system could meet the requirements of mass spectrometer ion transmission system.(3)Research on sector magnetic field mass analyzer and multi-faraday cup ion detector technologyMass analyzer is a key component of mass spectrometer to separate ions according to their different mass-to-charge ratios.In order to solve the problem that traditional analyzers such as time-of-flight,quadrupole,and ion trap could not realize high accuracy CSIA-Cl,a technology adopted fan-shaped magnetic field mass analyzer to realize high accuracy CSIA-Cl was proposed.The chlorine isotope analysis accuracy affected by magnetic field magnetic induction intensity fluctuations,repeller potential fluctuations,and initial energy divergence factors were investigated.The magnetic field magnetic induction intensity was 218.5Gs,and the repeller potential-5000 V was the theoretical optimized parameter of the instrument.The multi-faraday cup ion detector adopted the suppression electrode was developed which could suppress the sputtered secondary electrons from faraday cups and improve the performance of the instrument.The magnetic induction intensity stability of the mass analyzer was tested.The magnetic induction intensity-current linear correlation coefficient reached 0.99976.The accuracy of the magnetic induction intensity of the magnetic field could reach 0.01% in repeated experiments,which satisfied the high accuracy CSIA-Cl.(4)Development of Instrumentation control system and instrumentation integrationVacuum and electronic control systems are necessary functions to ensure the normal operation of the instrument.In order to achieve stable control of the vacuum environment of the instrument cavity,a control system adopted feedback pattern to controll molecular pumps and mechanical pumps was developed,which was realized by reading the parameters of the vacuum gauge to ensure the normal operation of the instrument.The serial transmission bus RS-485 was used as the signal transmission standard,the ion source and high potential power supply control system were developed.The software had current / voltage threshold limitation,current / voltage step adjustable functions,and two modes of setting parameters: manual and automatic scanning which could ensure the safe operation of the instrument and fast parameter adjustment.The whole instrument was assembled and integrated,which laid the foundation for the next step of the instrumentation.(5)Research on chlorine analysis of organic monomersIn the process of isotope analysis,it is necessary to solve the problem of isotopic interference in the process of extracting isotope information.In this paper,an analysis method of organic monomer chloride was proposed.Taking PCB138,one of the organic monomer chlorides as the analyzed object,under the positive electron impact and the negative chemical ionization mode,the different mass spectrums were compared.Under the negative chemical ionization mode,the conditions of ionization energy and cathode filament current parameters were optimized,and the optimized ionization energy of 70 eV and cathode filament current of 60?A,the PCB138 detection limit was 0.09 ppmv,and the linear correlation coefficient was 0.99544.Finally,several PCB mixtures were analyzed and the negative chemical ionized method was established.The chemical model could satisfy the online stripping and analysis of PCB isotopic fragments.Adopted the negative chemical ionization mode to study the accuracy of trichloroethylene TCE isotope analysis in pure water,and adopted the peak area integrated method to calculate the accuracy of chlorine isotope analysis.The average relative accuracy of the six experimental data was 19.06‰.Finally,TCE doped in tap water in the laboratory was tested,and the theoretical foundation was laid for the mass spectrometer developed for the analysis of organic monomers chloride. |
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