Bipolar Ionization Source for Ion Mobility Spectrometry Based on Vacuum Ultraviolet Radiation Induced Photoemission and Photoionization

A novel bipolar ionization source based on a commercial vacuum-UV Kr lamp has been developed for ion mobility spectrometry (IMS), which can work in both negative and positive ion mode. Its reactant ions formed in negative ion mode were predominantly assigned to be O-3-(H2O)(n), which is different from that of the Ni-63 source with purified air as carrier and drift gases. The formation of O-3-(H2O)(n) was due to the production of ozone caused by ultraviolet radiation, and the ozone concentration was measured to be about 1700 ppmv by iodometric titration method. Inorganic molecules such as SO2, CO2, and H2S can be easily detected in negative ion mode, and a linear dynamic range of 3 orders of magnitude and a limit of detection (S/N = 3) of 150 pptv were obtained for SO2. Its performance as a negative ion source was investigated by the detection of ammonium nitrate fuel oil explosive, N-nitrobis(2-hydroxyethyl)amine dinitrate, cyclo-1,3, 5-trimethylene-2,4, 6-trinitramine, and pentaerythritol tetranitrate (PETN) at 150 degrees C. The limit of detection was reached at 45 pg for PETN, which was much lower than the 190 pg using Ni-63 ion mobility spectrometry under the same experimental condition. Also, its performance as an ordinary photoionization source was investigated in detecting benzene, toluene, and m-xylene.