Central-field intermolecular potentials from the differential elastic scattering of H2(D2) by other molecules

Differential elastic scattering cross sections for the systems H2+ O2, SF6, NH3, CO, and CH4 and for D2+ O2, SF6, and NH3 have been obtained from crossed beam studies. In all cases, rapid quantum oscillations have been resolved which permit the determination of intermolecular potential parameters if a central-field assumption is adopted. These potentials were found to be independent of both the isotopic form of the hydrogen molecule, and the relative collision energy. As a result of this, and the ability of these spherical potentials quantitatively to describe the measured scattering, it is concluded that anisotropy effects do not seem to be important in these H2(D2) systems

Femtosecond Laser Photoionization Time-of-Flight Mass Spectrometry of Nitro-aromatic Explosives and Explosives Related Compounds

The ultrafast laser-induced photoionization and photodissociation processes of the nitroaromatic containing explosive and explosive related compounds (ERCs) nitrobenzene (NB), 1,3-dinitrobenzene (DNB), m-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT) have been investigated at three laser wavelengths and power densities using a time-of-flight mass spectrometer. Examination of the mass spectra of these compounds reveals the enhanced formation of the molecular ion [M+] when ultraviolet (332 nm) and visible (495 nm) light is used relative to infrared (795 nm) radiation. In addition, at 795 nm and a power density of 3.5 × 1014 W/cm2, the presence of a competition between multiphoton ionization (MPI) and Coulomb explosion (CE) channels is revealed by peak shape analysis, and is thought to be operative under these conditions for all of the molecules investigated

Initial Development of a Continuous Emission Monitor for Dioxins

Under contract DE-AC26-98FT-40370, SRI International has completed the third phase of a planned three-phase effort to develop a laboratory prototype continuous emission monitor (CEM) for dioxins and furans generated during the incineration of waste materials at DOE remediation sites. The project was initiated on July 29, 1998 with the technical effort completed in October 2001. During this research effort, SRI has made numerous improvements in our jet-REMPI instrument. These improvements have involved characterization and optimization of the molecular cooling in the gas jet, implementation of a custom-fabricated, four pulsed valve assembly, new data acquisition and display software, and preliminary development of a wavelength and mass calibration approach. We have also measured the REMPI excitation spectra of numerous organic compounds that are likely to be present in the exhaust stream of a waste incinerator. These spectra must be well characterized in the laboratory to understand any potential interferences that might arise when monitoring for dioxin and furan congeners. Our results to date continue to validate the original concept of using jet-REMPI as the detection method in a dioxin CEM. Using only commercial components with minor modifications, we have already demonstrated a detection sensitivity in the low ppt range with sufficient chemical specificity to separately detect two closely related congeners of dichlorodibenzodioxin present in a mixture. To demonstrate the utility of this methodology outside of the controlled conditions of the laboratory, we performed a series of pseudo-field experiments at the US Environmental Protection Agency's National Risk Management Research Laboratory, Research Triangle Park, NC. The instrument used for those studies was built by SRI under contract with US EPA, and was an exact duplicate of the SRI system. This duplication allowed the experiments to be conducted without transporting the SRI system to the EPA site. Using the jet-REMPI system in conjunction with a combustion flow reactor, the joint SRI-EPA team discovered several new, and unexpected, chemical species in the exhaust stream of a pure methane flame. Based on our work in this project, we have developed a number of concepts for instrumental improvements that will substantially increase our sensitivity while maintaining the exceptional selectivity required of a dioxin CEM. In addition, we have developed several system configurations with varying degrees of functionality that can be further developed and deployed for process monitoring, surrogate measurements, and potentially, as a dioxin control CEM. Due to the extremely demanding regulatory compliance monitoring requirements involving both congener specificity and sub-part-per-trillion sensitivity with near real-time speed, we believe it is not as yet possible to specify a system configuration for a true dioxin compliance monitor. While a true TEQ compliance monitor is not yet possible using the jet-REMPI approach, the technique may prove useful as a surrogate, or indicator monitor. This application would involve continuous measurement of surrogate compounds, such as lowly chlorinated dioxins and furans, whose concentrations have been previously correlated with the TEQ. Such an instrument would not require the extreme sensitivity of a compliance monitor although the high degree of chemical selectivity would remain important

Novel mass spectrometric instrument for gaseous and particulate characterization and monitoring

SRI International will develop a unique new instrument that will be capable of providing real-time, quantitative, chemical characterization of gaseous and particulate pollutants generated from DOE waste cleanup activities. The instrument will be capable of detecting and identifying volatile organic compounds, polynuclear aromatic hydrocarbons, heavy metals, and transuranic species released during waste cleanup activities. The instrument will be unique in its ability to detect and quantify in real-time these diverse pollutants in both vapor and particulate form. The function of the sampler is to obtain a sufficient and representative quantity of air pollutants from a wide range of waste cleanup activities, both vapor and particulate. In addition, the sampler must deliver the vapors and particles to the detection instrument with minimal material loss