A Vacuum Ultraviolet Photoionization Time-of-Flight Mass Spectrometer for Investigation of Free Radical Reaction

A home-made vacuum ultraviolet (VUV) photoionization time-of-flight mass spectrometer (VUVPI-TOFMS) was assembled to study atmospheric radical reactions. In the work , a commercial VUV discharge lamp with photon energy at 10.6 eV was installed as light source and a cage-shaped photoionization source was employed to efficiently extract and focus ions. With this spectrometer, a limit of detection (LOD) of 0.03 mu g/L (S/N = 3) for benzene detection was obtained. Then a microwave discharge flow tube was employed as reactor to produce free radicals and model their atmospheric reactions. The concentration of the methyl radical was measured with its self-reaction kinetics within the flow tube. As a representative example , the reaction of CH3 and NO was investigated by VUVPI-TOFMS. Both the stable species and radicals involved in the reaction were probed and the reaction rate constant at room temperature and 300 Pa pressure was measured to be k(CH3+NO) = 1.2 x10(-12) cm(3)/(molecule.s).