From anthropogenic sources to natural oceanic emissions, the concentration of methane in the atmosphere has more than doubled in the last 200 years. Since methane represents a global warming potential 23 times an equivalent mass of carbon dioxide, monitoring this species is of great interest. In terms of anthropogenic emissions, landfills represent a significant source of atmospheric methane. We have developed an in-house algorithm for extracting methane concentrations directly from FTIR spectra for gas samples from a local landfill. In this work, we will describe the method and present some preliminary measurements.
Nitric oxide (NO) is a major chemical byproduct of many photochemically active nitrogen-containing compounds. As a prototypical free radical with a very well characterized high-resolution spectrum, NO provides a standard spectroscopic fingerprint for indirect quantitative analysis and detection of a number of low vapor pressure nitroaromatic compounds in air through either direct photochemical decomposition of a parent molecule or from its relatively high vapor pressure chemical constituents. In this paper, we will discuss applications of picosecond laser spectroscopy for measurements and detection of NO and the nascent NO generated from photolysis of nitrobenzene. We will give a general overview of our tunable picosecond laser and detection system that we routinely use for probing and exciting the NO gamma band. This broad wavelength tuning capability of our laser allows us to set up pump-probe type experiments for detecting blue shifted rovibronic bands and probing the relative population distribution for NO. In all cases, experiments were performed using UV laser pulses of duration less than 20 ps. Also, we studied the effect of N2 collisions on the photoframentation spectrum of nitrobenzene in 1000 mbar of N2 buffer gas.
Polyisobutylene is an industrial polymer that is widely used in a number of applications including the manufacture of military grade explosives. We have examined the vapor emanating from a series of different molecular weight samples of polyisobutylene using high resolution Quantum Cascade Laser FM spectroscopy. The vapor phase spectra all exhibit a rovibrational structure similar to that for the gas phase isobutylene molecule. We have assigned the structure in the 890 cm-1 and 1380 cm-1 regions to the isobutylene ν28 and ν7 fundamental bands respectively. These spectroscopic signatures may prove useful for infrared sensing applications. Here we will present the infrared signatures along with recent GCMS data from a sample of C4, utilizing solid-phase microextraction vapor collection fibers, which confirm the presence of isobutylene as one of the volatile bouquet species in RDX-based explosives.
The explosive material known as Composition C4, or simply C4, is an RDX based military grade explosive.
RDX itself possesses a negligible vapor pressure at room temperature suggesting it is not a good target for conventional
instruments designed to detect vapor phase chemical compounds. Recent research with canines has indicated that a
better approach for detecting explosive vapors such as C4 is to focus on a characteristic mixture of impurities associated
with the material. These characteristic mixtures of impurity vapors are referred to by canine researchers as the explosive
bouquet and are fairly unique to the specific energetic material. In this paper, we will examine and report rotationally
resolved infrared spectral signatures for the known compounds comprising the explosive bouquet for C4 based
explosives including isobutylene, 2-ethyl-1-hexanol and cyclohexanone.
The concentration of Ammonia excreted through human skin has recently been measured using a gas
chromatograph equipped with a flame ionization detector (GC-FID) by a group from Nagoya, Japan. These emissions,
referred to as ammonia skin gas, were determined to be 1.7±.4 ng/cm3 for healthy subjects in the study. To achieve
greater molecule specificity, sensitivity, as well as add a real time capability, we are investigating the potential of a mid
IR laser spectrometer, consisting of a Pb-salt diode laser coupled with a low volume 75 meter Herriott gas sample cell, to
perform real time ammonia diagnostic measurements. Here we will present a series of preliminary ammonia skin gas
measurements obtained with this mid IR laser system.
Termed Special Nuclear Material (SNM) by the Atomic Energy Act of 1954, fissile materials, such as 235U and 239Pu, are
the primary components used to construct modern nuclear weapons. Detecting the clandestine presence of SNM
represents an important capability for Homeland Security. An ideal SNM sensor must be able to detect fissile materials
present at ppb levels, be able to distinguish between the source of the detected fissile material, i.e., 235U, 239Pu, 233U or
other fission source, and be able to perform the discrimination in near real time. A sensor with such capabilities would
provide not only rapid identification of a threat but, ultimately, information on the potential source of the threat. For
example, current detection schemes for monitoring clandestine nuclear testing and nuclear fuel reprocessing to provide
weapons grade fissile material rely largely on passive air sampling combined with a subsequent instrumental analysis or
some type of wet chemical analysis of the collected material. It would be highly useful to have a noncontact method of
measuring isotopes capable of providing forensic information rapidly at ppb levels of detection. Here we compare the
use of Kr, Xe and I as "canary" species for distinguishing between 235U and 239Pu fission sources by spectroscopic
methods.
Research with canines suggests that sniffer dogs alert not on the odor from a pure explosive, but rather on a set of far
more volatile species present in an explosive as impurities. Following the explosive trained canine example, we
have begun examining the vapor signatures for many of these volatile impurities utilizing high resolution
spectroscopic techniques in several molecular fingerprint regions. Here we will describe some of these high
resolution measurements and discuss strategies for selecting useful spectral signature regions for individual
molecular markers of interest.
Vapor phase sensing and detection of TNT-based explosives is extremely challenging due in part to the low
vapor pressure of TNT. We believe one effective strategy for optically based sensing of TNT-based explosives involves
focusing not on the spectral signature for pure TNT, but rather on a more volatile series of compounds that are present in
TNT as impurities. To date we have catalogued and reported a number of rotationally resolved infrared transition
frequencies for nitrobenzene, toluene, o-nitrotoluene, and m-nitrotoluene in the 14 micron region. Here we describe the
use of an in-house spectral calibration program that while designed for calibration of Pb-salt diode laser spectra, is quite
general and could be utilized for many spectroscopic detection and/or analysis applications. Finally, a sensing
measurement for a volatile organic impurity related to RDX-based explosives such as C4 is presented and discussed.
Explosive compounds such as RDX, and HMX present significant challenges to optically based sensors. This
difficulty is due in part to the low vapor pressures these compounds possess. One approach for sensing explosives
that circumvents the low explosive vapor pressure problem, involves focusing on the trace amounts of relatively
high vapor pressure impurities that will be present in the vapor signature. In order to effectively detect these volatile
impurities, the spectral signature databases must be readily available. One of our goals therefore, is the generation
of a database of high resolution spectral signatures for these volatile organic impurities. Some rather formidable
spectroscopic measurement challenges have been encountered while working to extend the spectral signature effort
to the 3 micron region. Here we will outline progress to date, with a focus on the volatile organic compounds
formaldehyde, acetaldehyde, nitromethane, acetone, isobutene, and cyclohexanone.
One of the technological challenges associated with trace vapor detection of explosive materials are the relatively low
vapor pressures exhibited by most energetic materials under ambient conditions. For example, the vapor pressure for
TNT is ~10 ppbv at room temperature, a concentration near the Limit of Detection for many of the technologies
currently being deployed. In the case of improvised explosive devices, the clandestine nature of the device further
serves to exacerbate the vapor pressure issue. Interestingly, the gold standard in explosives detection remains the trained
canine nose. While there is still some debate as to what the dog actually smells, recent studies have indicated the alert
response is triggered, not by the vapor presence of a specific explosive compound but, by a characteristic bouquet of
odors from chemical impurities used to manufacture and process the explosives. Here we present high resolution
infrared data for several of these volatile organic compounds in the 700 cm-1 region required for real time optical sensing
of energetic materials.
We report the structural characterization of the gas phase adducts HCN and CH3CN with BF3. Both have symmetric top structures with the nitrogen end of the R-CN toward the boron, reminiscent of the well known dative bond chemistry of BF3 with nitrogen donors. The B-N bond lengths and N-B-F angles, however, are intermediate between those expected for van der Waals or covalent interactions. Moreover, in CH3CN-BF3, where comparison with x-ray crystallographic studies is possible, the gas phase adduct shows a markedly longer bond length and smaller N-B-F angle. We show that in a series of related BF3 and BH3 adducts, the bond length and bond angle can, in fact, be tuned almost continuously between the covalent and van der Waals limits. By analogy with classic crystallographic work by Buergi and Dunitz and coworkers, we discuss how members of such a series can be interpreted as snapshots along a generalized reaction path for the formation of the dative bond. Finally, in the context of such a path, we examine the evolution of other (non-structural) properties of the BF3 adducts as the donor-acceptor bond formation proceeds.
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