Development Of A Time-of-flight Mass Spectrometer For Online Measurement Of Atmospheric Active Radicals And Its Application

Because of their ability to react with the most components in atmosphere, the hydroxyl radical(OH) and peroxy radical (HO2) become the main oxidant and catalyst in atmospheric chemicalreactions, which are associated with the formation of acid rain, urban smog, haze weather andsecondary aerosol, etc. However, the real-time, on-line, continuous detection of OH and HO2radicals have been an extremely challenging task due to their characteristics of low concentrationand short life. Chemical ionization mass spectrometry (CIMS) can be used for radical measurementby first converting radicals into stable molecules titrimetrically and then detecting the ions generatedby chemical ionization of the stable molecules with mass spectrometer. Compared to other detectionmethods, CIMS has attracted wide attention of the atmospheric chemists for its advantages of lowdetection limits, fast measuring speed, high accuracy and little interference.An atmospheric pressure negative-ion orthogonal acceleration time-of-flight mass spectrometer(TOFMS) operated in linear mode was developed for real-time, on-line detection of OH, HO2radicals and their derivatives PANs, which conquered the disadvantages of inefficient use of ionsand long measuring period in quadrupole mass spectrometer. The achieved mass resolution ofTOFMS exceeded1170at m/z96with a790mm long field free drift region.Based on the characteristics of atmospheric active radicals, an atmospheric pressure chemicalionization source with orthogonal dual tube structure was adopted in the instrument, whichminimized the interference between the reagent gas ionization and the titration reaction. For OHradical detection, a63Ni radioactive source was fixed inside one of the orthogonal tubes to generatereactant ion of NO3-from HNO3vapor. OH radicals was first titrated by excess SO2to formequivalent concentrations of H2SO4in the other orthogonal tube, and then reacted with NO3-ions inthe chemical ionization chamber, leading to HSO4-formation. The concentration of atmospheric OHradicals can be directly calculated by measuring the intensities of the HSO4-product ions and theNO3-reactant ions. For HO2radical detection, we converted it into OH radicals in reaction with NOand got the concentration of atmospheric HO2radicals by OH detection. The achieved limits ofdetection (LODs) of the present system are less than2105molecules cm-3for OH, based on5seconds integration, and less than5106molecules cm-3/5ppt for HO2/PANs, based on1secondintegration.The detection of explosives has become a worldwide issue in the public safety field. Toexamine the ability of the atmospheric pressure negative-ion TOFMS for in situ, on-line detection of trace explosives, an acetone-enhanced negative photoionization (AENP) source based on a10.6eVvacuum ultraviolet (VUV) lamp was developed. In the AENP source, acetone molecules absorbed10.6eV photons and were ionized by single photon ionization to emit photoelectrons. Thephotoelectrons reacted with O2, CO2, etc. in the atmosphere to produce mainly CO3-negativereactant ions. With the AENP source, common explosives, DINA, Tetryl, TNT and RDX, could bedetected sensitively, and LOD of2pg for TNT with a linear range of3orders of magnitude wasachieved.