Harald Oser

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.

Production of the NO photofragment in the desorption of RDX and HMX from surfaces

A promising scheme for the remote detection of nitrate-based explosives, which have low vapor pressure, involves two lasers: the first to desorb, vaporize, and photofragment the explosive molecule and the second to create laser-induced fluorescence in the NO fragment. It is desirable to use for the first a powerful 532 nm frequency-doubled Nd:YAG laser. In this study, we investigate the degree of photofragmentation into NO resulting from the irradiation of the explosives RDX and HMX coated on a variety of surfaces. The desorption step is followed by femtosecond laser ionization and time-of-flight mass spectrometry to reveal the fragments produced in the first step. We find that modest laser power of 532 nm desorbs the explosive and produces adequate amounts of NO.

Editorial The Future of Sensors and Instrumentation for Human Breath Analysis

This Special Topics Issue provides an overview of currently explored sensing techniques and methods that are being researched and developed in academic and industrial settings.

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.

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.

Laser-Based Standoff Detection of Surface-Bound Explosive Chemicals

Avoiding or minimizing potential damage from improvised explosive devices (IEDs) such as suicide, roadside, or vehicle bombs requires that the explosive device be detected and neutralized outside its effective blast radius. Only a few seconds may be available to both identify the device as hazardous and implement a response. As discussed in a study by the National Research Council, current technology is still far from capable of meeting these objectives. Conventional nitrocarbon explosive chemicals have very low vapor pressures, and any vapors are easily dispersed in air. Many pointdetection approaches rely on collecting trace solid residues from dust particles or surfaces. Practical approaches for standoff detection are yet to be developed. For the past 5 years, SRI International has been working toward development of a novel scheme for standoff detection of explosive chemicals that uses infrared (IR) laser evaporation of surfacebound explosive followed by ultraviolet (UV) laser photofragmentation of the explosive chemical vapor, and then UV laser-induced fluorescence (LIF) of nitric oxide. This method offers the potential of long standoff range (up to 100 m or more), high sensitivity (vaporized solid), simplicity (no spectrometer or library of reference spectra), and selectivity (only nitrocompounds).

Real-Time Detection of IED Explosives with Laser Ionization Mass Spectrometry

The ESSEX system (Extremely Sensitive and Selective Explo- sives detector) is a system that can be used for standoff or remote detection of suicide bombers at a distance of up to a few meters via the trace amounts of explosives present in vapor phase around explosives with a high vapor pressure. The ESSEX detection method is based on laser ionization mass spectrometry (LI-MS). Direct sampling of air with explosives in vapor phase will allow detection of explosives with relatively high vapor pressure, such as TATP (suspected to have been used in the recent suicide bombings in Lon- don as well as numerous suicide bombings in Israel) and EGDN (normally found in dynamite explosives such as the ones used in the Madrid bomb- ings). The LI-MS method has a potential to detect all or nearly all explosives. For explosives with low vapor pressure (e.g. RDX, HMX, PETN, AN) it is probably necessary to sample particles as well. Other aspects of the future potential for this method are that the number of explosives that the method targets can be increased as needed. The method is very suited for data fusion and processing, making it possible to incorporate artificial intelligence (AI) into the system. The uniqueness of LI-MS methods lies in its low detection limit, possibility of real-time detection, its versatility regarding the number of possible substances to detect and its unique selectivity, leading to very few false alarms. The ESSEX concept is appropriate for indoor and outdoor applications, it is benign to humans and property and can be applied in an unnoticeable way. Laser ionization mass spectrometry and specifically reso- nance enhanced multi photon ionization (REMPI) is an ultra-sensitive, highly selective analytical technique that can identify and quantify vapor-phase con- stituents at parts-per-trillion (ppt) levels. The REMPI technique combines the principles of optical spectroscopy and mass spectrometry to provide a two- dimensional detection scheme that yields a high degree of chemical sensitivity

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.