Development Of A Portable Time-of-flight Mass Spectrometry And Its Application In The Measurement Of Volatile Sulfide

Time-of-flight mass spectrometer (TOFMS) features simpler principle, wide range ofmass spec measurement and relatively high resolution, which can deliver massspectrometry information about all the mass numbers in a microsecond time scale.Recently, the portable TOFMS has been used widely in the field of fast-speed on-linedetection for environment and process monitoring for chemical plant.In this thesis, a portable TOFMS system was designed, combined with a compositeionization source using a commercialized vacuum ultraviolet (VUV) lamp. It wasbuilt using the structure of orthogonal acceleration and reflection, with the physicaldimension of40×31×29cm, the weight of27kg, the power consumption of70W,and the resolution of1048at m/z=32. Using a set of capillary for the direct sampleintroduction, the limit of detection (LOD) for benzene was100ppbv; for the sampleintroduction using a PDMS membrane enrichment, the LOD for benzene was10ppbv.The composite ionization source consists of two different modes: a single photonionization mode (SPI) and a photoelectron ionization (PEI) mode. For the SPI mode,the volatile organic compounds (VOCs) could be softly ionized by photons withenergy of10.6eV emitted from the VUV lamp. The molecular ions were thedominated product ions so that a simpler mass spectrum for the analysis of complexsamples can be obtained. For the PEI mode, based on the photoelectric effect, thephotoelectrons were generated by irradiating the metal electrode surface with photons.In the static electric field, photoelectrons were accelerated to increase their energy(>10.6eV), so that the electron impact ionization could be carried out. The portableTOFMS could be switched between SPI and PEI mode quickly. With the use of thisTOFMS, the VOCs such as aldehydes, alcohols and esters in a Chinese liqueur samplewere analyzed, and the results helped to classify three different kinds of Chineseliqueurs.A magnetic field enhanced photoelectron ionization (MEPEI) source was designedto boost the ionization efficiency of photoelectrons, so that the rapid and on-linemeasurement of the reduced sulfur compounds in the atmosphere could beaccomplished. The trajectories of ions and electrons in the ionization source wassimulated by SIMION software. The parameters of ionization source, such as theelectromagnet, the size of static electrode, and the voltage, were optimized. It wasfound that the new MEPEI source allowed the improvement of detection sensitivityfor H2S, SO2and CS2by a factor of5.3,9.4and6.9, respectively; the LODs for H2S,SO2and CS2were0.14mg/m3,0.52mg/m3and0.31mg/m3, respectively, which could meet the detection requirements based on the current industrial emissionstandard.A novel method was developed to analyze the decomposition product SO2in theinsulating gas SF6by the portable TOFMS to alarm the malfunction in a transformersubstation. To solve the problem of the worsen detection sensitivity due to electroncapture by the matrix gas SF6, the sensitivity of TOFMS was improved throughfollowing three methods: enhancement of the photon intensity by using a RF VUVlamp, increase the sample volume by enlarging the membrane area, shortening theacceleration time for the low-energy electrons to reduce the capture of photoelectronsby SF6. This portable TOFMS was employed for the field quantification of SO2insamples from the malfunction transformer substation, and the result was consistedwith the data obtained by standard method in the laboratory. The LOD for SO2in gasSF6was0.2ppm, much lower than the threshold concentration of SO2(12ppmv) inSF6in judging the malfunction of the transformer substation Both of the portabilityand detection sensitivity of this portable TOFMS could fulfill the field applicationcriteria, making this instrument an ideal candidate for the on-line detection ofmalfunction for a transformer substation.