Michael J. Coggiola

Congener-specific detection of dioxins using jet-REMPI

Although 210 chemically different polychlorinated dibenzo-p-dioxin and dibenzofuran congeners can be produced during combustion, it is currently believed that fewer than 20 are toxic enough to warrant monitoring. SRI is developing a continuous emissions monitor to study the emission levels of these most toxic dioxins, leading eventually to an improved understanding of the formation of these molecules and to improved means of monitoring and control. We report here the first results of performing congener-specific detection for two dichloro dibenzo-p-dioxins present in low ppt concentrations in a mixture using the supersonic jet/resonantly enhanced multiphoton ionization time-of-flight mass spectrometer technique. In addition, we present preliminary data on the detection of chlorinated aromatic compounds using a two-color REMPI scheme with the same instrument.

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.

Development of a jet–REMPI (resonantly enhanced multiphoton ionization) continuous monitor for environmental applications

The need for a continuous monitor for environmentally important pollutants at realistic [parts-per-trillion (parts in 10(12))] concentrations measured in real time (minutes) is widely recognized. We developed an instrument that is based on supersonic-jet expansion and cooling, followed by resonantly enhanced multiphoton ionization (REMPI) into a mass spectrometer. This approach furnishes the dual selectivity of narrow-band tuned laser absorption and mass analysis. We initiated a spectroscopic characterization of the jets collisional cooling behavior to optimize the instruments sensitivity and selectivity, made measurements of several aromatic compounds (including polychlorinated dioxins) by use of a one-color REMPI scheme, and demonstrated a two-color excitation scheme.

Distinguishing Methicillin-Resistant and Sensitive Staphylococcus aureus Using Volatile Headspace Metabolites

This proof-of-concept study used solid-phase microextraction fibers (SPME) to collect headspace vapors from a methicillin sensitive Staphylococcus aureus (MSSA) and a methicillin-resistant Staphylococcus aureus (MRSA) strain grown in vitro in liquid growth medium. The collected molecules were separated and identified using gas chromatography and mass spectrometry (GC/MS). Preliminary results distinguished these two strains and provide a foundation for a biomarker library that could one day serve as a diagnostic tool for identifying specific bacterial infections.

Airborne Deployment of an Instrument for the Real-Time Analysis of Single Aerosol Particles

An instrument is described that provides real-time chemical analysis of the composition of individual aerosol particles. A differentially pumped aerosol inlet transfers particles from the ambient atmosphere into the source region of a time-of-flight mass spectrometer where they impact on a heated surface and the resulting vapors are ionized by electron ionization prior to mass analysis. Labora tory calibration studies demonstrated that the instrument was capable of detecting particles with diameters greater than approximately 0.4mu m. The instrument was operated on the NASA DC-8 research aircraft as part of the 1996 SUbsonic aircraft: Contrail and Cloud Effects Special Study (SUCCESS) mission with the intent of studying the chemical composition of upper tropospheric particles. More than 25,000 aerosol particle mass spectra were recorded during 19 mission flights. Although approximately 120 of those spectra showed clear evidence of sulfate, nitrate, and other inorganic materials, the remaining spectra contained only mass peaks consistent with water. Moreover, particles were detected only while traversing clouds. These results are not consistent with expectations of the size, quantity, or composition of upper tropospheric particles. It is likely, however, that a subisokinetic aircraft sampling inlet resulted in the collection of only very large ice particles. This situation would account for both the observed preponder ance of water-only spectra and the apparent lack of particles outside of clouds. Despite the sampling problem, the instrument was able to chemically speciate aerosols directly sampled from a medium altitude aircraft. These represent the first examples of aerosol particles chemically speciated in real time from an airborne platform.

Characteristics of a pulsed, liquid-metal field emission source for use in fast high-current switches

Liquid metal ion sources (LMIS) of the type used for ion beam lithography can also be used to produce large pulses of electrons via field emission. These sources typically consist of a small capillary through which a liquid metal is forced. The application of a voltage between this capillary and an extraction anode causes the liquid metal to form a sharp cone. When the intense field at the tip exceeds the threshold for field emission, a large electron current flows from the tip to the anode. The production of this electron pulse results in the rapid heating of the liquid-metal tip, leading to an explosive pressure increase, and a collapse of the tip. As the field is restored, the tip reforms, with a resultant self-pulsing mode of operation. The pulse repetition rate, the average current, and the current per pulse are all functions of the metal composition, emitter geometry, and applied diode voltage. These characteristics are reported for a variety of conditions, and show that a single emitter tip can produce current pulses in excess of 50 amperes, with a rise time of a few nsec, and a width of 5-50 nsec. Repetition rates vary between 10 Hz and 20 kHz. In addition to the observed electron emission, optical radiation is also produced. The dispersed visible spectrum shows predominantly emission from neutral, atomic species presumably excited by electron impact in the gas phase.

LASER-IONIZATION MASS SPECTROMETRY OF EXPLOSIVES AND CHEMICAL WARFARE SIMULANTS

The objective of the present study was to better understand the photophysics of explosives and chemical warfare simulants in order to develop better performing analytical tools. Photoionization mass spectra were taken using three optical schemes. The first was resonance-enhanced multiphoton ionization (REMPI) using few-ns duration 248 or 266 nm laser pulses. The second scheme was non-resonant multiphoton ionization (MPI) using 100 fs duration laser pulses at wavelengths between 325 and 795. The third approach was single photon ionization (SPI) using few-ns duration 118 nm laser pulses. For all the molecules investigated, mass spectra resulting exposure to ns-duration 248 or 266 nm laser pulses consisted of only low molecular weight fragments. Using fs-duration laser pulses produced more complicated, potentially analyzable, fragmentation patterns, usually with some parent peak. Single photon ionization gave the best results, with mass spectra consisting of almost only parent peak, except for the case of TATP.

Secondary electron yields from Mo and Mo/Cs surfaces by ion impact

Abstract Secondary electron yields resulting from positive ion impact (500 eV–2.5 keV) on a cesiated molybdenum target are reported. Yields of secondary electrons for He + and Ar + projectiles are measured as a function of the cesium/molybdenum target work function, and found to maximize when the work function is a minimum (φ ~ 1.5 eV). The secondary electron yield is always larger for the reduced work function target, however, the magnitude of the increase is dependent on both the ion kinetic energy and identity. Results are also presented for D + 2 impact between 50 eV and 1.0 keV. In this case, backscattered negative ions are also seen in addition to the secondary electrons.

Detection of Explosive and Explosive Related Compounds (ERCs) Using Ultrafast Laser Photoionization Time of Flight Mass Spectrometry

Ultrafast laser photoionization time of flight mass spectrometry has been applied to the detection of explosives and explosive related compounds. The wavelength dependence of the photoionization event and the sensitivity of the technique are discussed.

Compact broadly tunable UV OPO for trace detection using REMPI mass spectrometry

A compact tunable UV OPO laser system has been used for the real-time detection of aromatic hydrocarbon hazardous air pollution vapors at sub-ppb levels using the jet-REMPI (jet-cooled Resonance Enhanced Multi-Photon Ionization) technique. By combining spectral and molecular mass detection, this technique provides high chemical selectivity, allowing species identification even during direct sampling of complex real-world samples. The inherent sensitivity of the jet-REMPI also allows direct real-time detection of trace species without pre-concentration. However, applications of the technique have been confined to the laboratory, requiring a complex and delicate tunable UV laser source and mass spectrometer. The results of applying a less complex, compact low-resolution OPO laser system are presented, with the goal of furthering the development a complete compact jet-REMPI instrument.