The Development Of Mass Spectrometry Photoionization/Photochemical Ionization Sources And Its Applications For Online VOCs Analysis

The variety and rapid temporal and spatial changes of volatile organic compounds（VOCs）cause serious harm in ecological,environmental,and health fields.Therefore,the research and development of rapidly,accurately and highly sensitive VOCs detection methods and technologies is of great significance.In online detection,photoionization mass spectrometry（PI-MS）has advantages such as simple and easy spectral analysis,high sensitivity,and fast detection speed,and it is increasingly applied in complex sample online monitoring and rapid analysis.Vacuum ultraviolet krypton discharge（VUV-Kr）lamp is commonly used as the light source in PI-MS,but its relatively low photon energy and flux limit the detection sensitivity of the mass spectrometer and the analysis range of the samples.This paper focuses on researching photochemical ionization methods and technologies to improve ionization efficiency and detection range,developing ion funnel focusing in photoionization methods and technologies to enhance transmission efficiency,and developing photoionization/photochemical ionization-ion funnel focusing ion source to improve the detection sensitivity of the mass spectrometer and expand the analysis range of the samples.Therefore,highly sensitive and online detection of VOCs can be realized in the fields of ecological environment and life health.This dissertation consists of five parts as follows:1.Development of a reflectron time-of-flight mass spectrometer with vertical accelerationA high-performance vertical acceleration reflectron time-of-flight mass spectrometer（TOFMS）has been developed,with its main components including an injection system,an ionization source based on a VUV-Kr lamp,an ion transmission system combining radio frequency quadrupole（RFQ）and electrostatic lens（ESL）,a reflectron time-of-flight mass analyzer combined with second-order spatial focusing theory,a Chevron structured MCP ion detector,a data acquisition and processing system,a vacuum system,and an electrical control system.After tuning and optimizing the performance parameters of the TOFMS instrument and testing its indicators,the developed TOFMS has a resolution exceeding 5,800 at m/z=92,and the detection limit for common volatile organic pollutants such as benzene,toluene,and p-xylene is as low as pptv level.In addition,the instrument has good stability,with a relative standard deviation of continuous monitoring for 14 hours is less than1.5%,which could meet the needs of long-term stable work.2.Research on photochemical ionization methods and techniquesThe photon energy of VUV-Kr lamp is 10.6 e V,and its photon flux is 10¹²photons/s,which makes it difficult to achieve efficient ionization and high sensitivity mass spectrometry detection of compounds with high ionization energy or low photoionization cross-sections.Therefore,to improve the ionization efficiency of compounds with high ionization energy or low photoionization cross-sections,a new ionization method called photochemical ionization was proposed based on the PI source and chemical ionization mechanisms.A new type of photoionization-induced NO⁺chemical ionization（PNCI）component was developed.This component uses nitric oxide（NO）as the reagent gas and generates stable and sufficient NO⁺reagent ions through VUV photoionization.Subsequently,photoinduced chemical ionization of the sample molecules is achieved through ion-molecule complexation and hydrogen anion（H^-）extraction reactions.This process effectively improves the ionization efficiency,detection range,and instrument sensitivity for the sample under investigation.The component was optimized and debugged at an ionization region pressure of 800 Pa,a reaction zone electric field strength of 8.20 V/cm,and a transmission zone electric field strength of 33.33 V/cm.It not only enables the detection of formaldehyde with high ionization energy,but also increases the signal intensity of low photoionization cross-section acrolein by 86 times.The limit of detection（LOD）for the detected compounds can be as low as 10 pptv within 60seconds,and the linear range covers three orders of magnitude,with a linear correlation coefficient（R²）greater than 0.99.3.Research on methods and techniques of ion funnel focusing in photoionizationThe direct current electric field of traditional photoionization sources cannot effectively utilize the sample ions generated within the source,limiting further improvement in instrument detection sensitivity.Therefore,to improve the transmission efficiency of sample ions inside the photoionization source,a new ionization method called ion funneling photoionization is proposed based on the PI source and combined with ion focusing technology.A new ion funnel focused photoionization（IFPI）component was developed and optimized.This component generates sample ions through VUV photoionization and utilizes the focusing and transmission capabilities of the radio frequency ion funnel to efficiently deliver sample ions to the mass analyzer for analysis.This process significantly improves the transmission efficiency of the sample ions and the detection sensitivity of the instrument.After optimizing and tuning the component,at an ionization region pressure of 400 Pa and a radio frequency peak-to-peak value of 45 V,the detection sensitivity for benzene,toluene,and p-xylene is increased up to 37 times.The limit of detection（LOD）for the detected compounds can be as low as 7.0 pptv within 5seconds,and the linear range is 2-3 orders of magnitude with linear correlation coefficients（R²）greater than 0.99.4.Development of a photoionization/photochemical ionization-ion funnel focusing ion sourceA new ion funnel radio frequency enhanced photoelectron-induced H₃O⁺chemical ionization（IFPICI）source has been developed based on the PI/PCI source and combined with RF ion funnel focusing technology.This ionization source uses saturated water vapor as the reagent gas and takes advantage of the ion funnel’s radio frequency enhancement to accelerate photoelectrons,generating high-intensity H₃O⁺reagent ions.Subsequently,the sample molecules undergo proton transfer reactions with H₃O⁺reagent ions,resulting in highly efficient ionization.This process not only improves the ionization efficiency and transmission efficiency of the sample,but also enhances the detection sensitivity of the mass spectrometer.The IFPICI source has been optimized and tested.At an ionization region pressure of 300 Pa and an RF voltage amplitude of 65.2 V for the ion funnel,compared with the ion funnel focusing technology in photoionization,the detection signal intensity of five compounds including isoprene,p-xylene,dimethyl sulfide,acetone,and acetic acid is improved by up to 10.9 times.The lowest limit of detection（LOD）for the detected compounds within 5 seconds can be as low as 1.65 pptv,and the linear range covers 4-5 orders of magnitude,with linear correlation coefficients（R²）all greater than 0.99.5.Application research of time-of-flight mass spectrometry for online detection of VOCsThis paper describes the development of a photoionization/photochemical ionization time-of-flight mass spectrometry,which was applied to the online detection and real-time quantitative analysis of VOCs in three different fields:the aldehydes and benzenes VOCs in vehicles,trace benzene series in the atmospheric environment,and small-molecule volatile organic metabolites in the breath of healthy volunteers.The experiments showed that this technique has superior performance,high detection sensitivity,and stability,and can meet the long-term online detection requirements for trace VOCs.