Development And Application Of TOFMS For Online Monitoring Of Dioxin Precursors

Municipal solid waste (MSW) incineration treatment is widely used due to itshigh volume reduction, economic space occupation and realizable resource utilization.However, the emissions generated in the process of incineration namedpolychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) with strongteratogenic and mutagenic effects posed a serious threat to the environment andhuman health. The concentration of dioxin in the flue gas is too low to detect directly.The dioxin precursors correlation method provides a new approach for rapid detectionof dioxin. Time of flight mass spectrometry is suitable for determination of dioxinprecursors for its rapidity and accurate qualitative.Based on the SIMION simulation, a vacuum ultraviolet (VUV) lamp with photonenergy of10.6eV was used as the photon ionization source for soft ionization. Underthe optimal conditions, the achieved mass resolution of the TOFMS was3000(m/z78).A sampling-enrichment-thermal desorption-inlet system was developed for thetrace detection of dioxin precursors in incineration plants. Three chlorobenzenes (cbs)(MCBz,1,3-DiCBz,1,2,4-TrCBz) and three chlorophenols (cps)(o-MCPs,2,4-DiCPs,2,4,6-TrCPs) as typical dioxin precursors were detected to investigate theperformance of the system. As a result, the LODs (limit of detection) of the sixcompounds were4pptv、5pptv、14pptv、20pptv、30pptv、24pptv respectively, andresponse time was within200s. The enrichment factors of o-MCPs,2,4-DiCPs and2,4,6-TrCPs by using the enrichment inlet system were60,249,503.Continuous and automatic sampling-enrichment-thermal desorption-inlet wasachieved combined with solenoid valves and controlling software. Based on the20hours’ continuous monitoring of the three cps, the RSD of signal intensities were16.7%,11.01%,14.88%and RSD of response time is11.16%,11.9%,11.72%.Furthermore, in order to avoid the reduction of instrument sensitivity caused by theparticle in flue gas which would pollute the VUV lamp window, a calibration gas wasintroduced into the ion source from another entrance, the real-time online calibration of the signal intensity was realized by calibrating the ratio of the current cycle signalintensity and the previous cycle signal intensity. The RSD of benzene, toluene andp-xylene were1.04%,2.17%and1.33%after calibration, respectively.Light ionization source combined with a high-power RF windowless lamp wasdeveloped, and the ionization performances of different discharge gases as N2, Kr, Ar,He and air were investigated. The ionization mechanism was analyzed by comparingRF lamp ionization result with VUV lamp result for detecting acetone and chloroformwhose ionization energy (IE) are greater than10.6eV and less than10.6eVrespectively, as a result, organic compounds with ionization energy higher than10.6eV were effectively ionized by RF ionization but fragments generated when detectingcompounds whose ionization energy lower than10.6eV. An excellent linear responsewas observed for benzene, toluene, p-xylene in the range of20ppbv to5ppmv. Thesignal intensity of RF lamp-TOFMS of1,2,4-TrCBz and2,4,6-TrCPs are18times and15times the signal intensity of VUV lamp-TOFMS respectively.