Development Of Miniature Flight Time Mass Spectrometry And Radiofrequency Field Enhanced Chemical Ionization

The volatile organic compounds?VOCs?in the environment are widely distributed and have high toxicity,especially some malodorous gases,which cause great harm to human health and ecological environment.Therefore,it is of great significance to develop an accurate and highly sensitive method and instrument for rapid measurement of VOCs.The on-line mass spectrometry technology has the advantages of high quality resolution,good sensitivity and fast analysis,which is one of the main techniques for on-line monitoring and accurate analysis of complex samples.This paper studies the most critical mass analyzer and ionization source in the on-line mass spectrometry,and aims to develop the high sensitive miniature TOFMS technique for environmental VOCs field.According to the ion optical theory,this thesis has successfully developed a vacuum ultraviolet?VUV?photoionization TOFMS,the size and weight of the instrument is 30×33×20 cm and 17 kg.the instrument adopted the vertical acceleration and double reflector technology,to ensure that under the condition of limited size,higher resolving power.After debugging and optimizing,the resolution of the instrument at m/z 78 is 1663.In terms of ionization source,the light source adopts the VUV krypton?Kr?discharge lamp with a photon energy of 10.6 eV,and the ionization mode is single photon ionization?SPI?.Finally,the detection limit of benzene?LOD?was 250 ppbv,and it was successfully used in site rapid analysis of CS₂ in the high temperature environment.The SPI ionization source based on the VUV lamp is a good soft ionization source,but the low luminous flux and photon energy make its ionization efficiency and ionization range serious limited.In addition,the volume of miniature TOFMS limits the sample size,which further limits the detection sensitivity of the instrument,and it is difficult to meet the detection requirements of trace VOCs in the environment.In order to further improve the sensitivity of the instrument,a new type of Radio-Frequency Enhanced Chemical Ionization source?RF-RCI?was developed based on the VUV-SPI ionization source.First,through the SIMION 8.0 simulation,optimized of the structure and performance parameters of the ionization source,the theory demonstrated that under a certain pressure,RF-ECI ionization strengthened ion collisions between the neutral molecules and ions,radial motion of ions has also extended the molecular ion reaction time,effectively improve detector sensitivity of the miniature mass spectrometer.Further experiments showed that,compared with the traditional DC field,the introduction of radio frequency electric field increases the sensitivity of benzene,toluene,hydrogen sulfide and other compounds by two or three orders of magnitude.At the same time,the introduction of rf field O₂⁺reagent ion has realized the chemical ionization of charge transfer and effectively expanded the detection range of the SPI based VUV.