Threshold Photoionization Ion Trap Time-of-Flight Mass Spectrometry for Real-Time Detection of Explosives

We propose to develop an ultrasensitive threshold photoionization mass spectrometer (TPI MS) system for explosives detection that has the following attributes: (1) simultaneous multispecies detection of parent molecules, (2) real-time response at the part-per-trillion (ppt) level, (3) compact, rugged, field deployable instrument that can be remotely operated. The key components of the detection system are: (1) a high output KR light source operating at 116.5 nm (10.65 eV) and 123.6nm (10.03 eV) for achieving threshold photoionization, (2) a compact quadrupole ion trap (QIT) for accumulating ions, and (3) a time-of-flight mass spectrometer (TOFMS) for recording complete mass spectra for all ions collected between pulse out periods. A commercially available radiofrequency Kr discharge lamp will be modified for vacuum use and coupled to the QIT to effect in situ photoionizaton. The light source delivers about 10(15) VUV photons/s. The flow of sample gas into the QIT will be adjusted to maintain 10(-3) -10(-2) torr pressure. These conditions correspond to an ion production rate of 3-30 s (-1) for a ppt concentration (assuming photoionization cross section of 10(-16 cm2). This level of detectability corresponds to <1 attogram for typical nitro-,nitroso-,and aromatic molecules, with 1 sec response time. The photoinization energy is sufficiently high to also ionize nearly all amines, halons, and heteroatom containing molecules, and most CFCs; yet it is of sufficiently low energy to avoid fragmentation of these molecules and to avoid ionization of common atmospheric constituents such as N2, O2, H2O, CO2, CO, Ar, and other rare gases. These attributes permit enormous simplification in the analysis of complex mixtures and an increase in the dynamic range for detection of trace constituents compared to currently available high performance mass spectrometers.