No abstract

Ion mobility spectrometry (IMS) has become the most widely used technology for trace explosives detection. A key task in designing IMS systems is to balance the explosives detection performance with size, weight, cost, and safety of the instrument. Commercial instruments are, by and large, equipped with radioactive (63)Ni ionization sources which pose inherent problems for transportation, safety, and waste disposal regulation. An alternative to a radioactive source is a corona discharge ionization source, which offers the benefits of simplicity, stability, and sensitivity without the regulatory problems. An IMS system was designed and built based on modeling and simulation with the goal to achieve a lightweight modular design that offered high performance for the detection of trace explosives using a corona ionization source. Modeling and simulations were used to investigate design alternatives and optimize parameters. Simulated spectra were obtained for 2,4,6-trinitrotoluene (TNT) and cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX) and showed good agreement with experimentally measured spectra using a corona ionization source. The reduced mobilities for TNT and RDX obtained with corona ionization were 1.53 and 1.46 cm(2)/(V s), respectively, and this agreed well with literature values.

The effect of space charge on the performance of an Ion Mobility Spectrometry (IMS) system becomes more important as the system is made smaller. We use the SIMION software package with the Statistical Diffusion Simulation (SDS) module and SIMION's new capability to solve the Poisson equation to study the effect of space charge on ion loss and resolving power in IMS systems. We consider IMS systems ranging in length from 50 mm to 150 mm and in diameter from 8.33 mm to 50 mm with a fixed electric field of 50 V/mm. We also examine a system with a length of 50 mm, a diameter of 16.7 mm, and an electric field of 16.7 V/mm. We assume that any charge density can be injected into the IMS system, and we have obtained expressions that predict the ion loss and resolving power of IMS systems as a function of input charge density and drift tube aspect ratio (length/diameter).

Accurately computing molecular Raman spectra would enable rapid development of inexpensive and extensive Raman libraries. This is especially beneficial for chemicals that are regulated, toxic, or otherwise difficult to handle. Numerous quantum mechanical methods have been developed that enable computation of Raman spectra. Here, we study the B3LYP exchange correlation functional with various combinations of basis sets, polarization functions, and diffuse functions to determine which combination best computes the Raman spectra for explosive and nonexplosive molecules. In comparing spectra, three metrics were utilized: the root mean square error, the earth mover's distance, and the weighted cross-correlation average. The earth mover's distance and weighted cross-correlation metrics are shown to have significantly greater power at detecting spectral similarities and differences than the root mean square error. Across all methods and molecules examined, B3LYP/6-311++G(d,p) was found to provide the best match between measured and computed Raman spectra. Spectra generated at the B3LYP/6-311++G(d,p) level were found to be accurate enough to correctly identify each molecule out of a set of measured molecular spectra.

Internal disruptions occurring in an inductively driven discharge are suppressed by lower hybrid waves that drive the plasma current in the same direction as the inductively driven current. Upon suppression, in the central region a finiteamplitude, non-growing m = 1 oscillation is observed and strong electron heating is measured.