Search

A new FAst Scan Submillimeter Spectroscopic Technique (FASSST) for analytical applications is described. It is based on voltage tunable Backward Wave Oscillators (BWOs), afast scan to 'freeze' frequency instability, and optical calibration methods. FASSST, due to its high resolution, is used to record the gas phase rotational spectra of mixtures of molecules (e.g. aromatic or heterocyclic compounds). This technique provides an unambiguous identification of the individual component compounds. The combination of FASSST with gas chromatography is expected to be an excellent method for qualitative and quantitative analysis.

We have demonstrated the detection and discrimination of explosive residues on a variety of surfaces using laser-induced breakdown spectroscopy and partial least squares discriminant analysis. The correlation between the hardness of a substrate and the ability to discriminate residues on that substrate is discussed. We have also shown that while using the full spectra in the chemometric model improves the classification of sample types in the model, the use of intensities and ratios specific to explosive residues improves the classification of sample types that are not part of the model. In the first case, differences in the laser-induced plasma (and resulting spectral features) attributable to the laser-material interaction contribute to the sample discrimination, while using intensities and ratios specific to explosive residues focuses on the chemical composition of the sample for discrimination. Combining the results of the two models for a given sample set decreases the overall false positive rate for all test samples. [ABSTRACT FROM AUTHOR]

Laser-induced breakdown spectroscopy (LIBS) is a promising technique for real-time chemical and biological warfare agent detection in the field. We have demonstrated the detection and discrimination of the biological warfare agent surrogates Bacillus subtilis (BG) (2% false negatives, 0% false positives) and ovalbumin (0% false negatives, 1% false positives) at 20 meters using standoff laser-induced breakdown spectroscopy (ST-LIBS) and linear correlation. Unknown interferent samples (not included in the model), samples on different substrates, and mixtures of BG and Arizona road dust have been classified with reasonable success using partial least squares discriminant analysis (PLS-DA). A few of the samples tested such as the soot (not included in the model) and the 25% BG:75% dust mixture resulted in a significant number of false positives or false negatives, respectively. Our preliminary results indicate that while LIBS is able to discriminate biomaterials with similar elemental compositions at standoff distances based on differences in key intensity ratios, further work is needed to reduce the number of false positives/negatives by refining the PLS-DA model to include a sufficient range of material classes and carefully selecting a detection threshold. In addition, we have demonstrated that LIBS can distinguish five different organophosphate nerve agent simulants at 20 meters, despite their similar stoichiometric formulas. Finally, a combined PLS-DA model for chemical, biological, and explosives detection using a single ST-LIBS sensor has been developed in order to demonstrate the potential of standoff LIBS for universal hazardous materials detection. [ABSTRACT FROM AUTHOR]

The ability to detect trace amounts of explosives and their residues in real time is of vital interest to Homeland Security and the military. Previous work at the US Army Research Laboratory (ARL) demonstrated the potential of laser-induced breakdown spectroscopy (LIBS) for the detection of energetic materials. Our recent efforts have been focused on improving the sensitivity and selectivity of LIBS for residue explosives detection and on extending this work to the standoff detection of explosive residues. One difficulty with detecting energetic materials is that the contribution to the oxygen and nitrogen signals from air can impede the identification of the explosive material. Techniques for reducing the air entrainment into the plasma—such as using an argon buffer gas or a collinear double-pulse configuration—have been investigated for this application. In addition to the laboratory studies, ARL’s new double-pulse standoff system (ST-LIBS) has recently been used to detect explosive residues at 20 m. The efficacy of chemometric techniques such as linear correlation, principal components analysis (PCA), and partial least squares discriminant analysis (PLS-DA) for the identification of explosive residues is also discussed. We have shown that despite the typical characterization of LIBS as an elemental technique, the relative elemental intensities in the LIBS spectra are representative of the stoichiometry of the parent molecules and can be used to discriminate materials containing the same elements. Simultaneous biohazard and explosive residue discrimination at standoff distances has also been demonstrated. [ABSTRACT FROM AUTHOR]

Because methyl formate (HCOOCH3) is abundant in the interstellar medium and has a strong, complex spectrum, it is a major contributor to the list of identified astrophysical lines. Because of its spectral complexity, with many low lying torsional and vibrational states, the quantum mechanical (QM) analysis of its laboratory spectrum is challenging and thus incomplete. As a result it is assumed that methyl formate is also one of the major contributors to the lists of unassigned lines in astrophysical spectra. This paper provides a characterization, without the need for QM analysis, of the spectrum of methyl formate between 214.6 and 265.4 GHz for astrophysically significant temperatures. The experimental basis for this characterization is a set of 425 spectra, with absolute intensity calibration, recorded between 248 and 408 K. Analysis of these spectra makes possible the calculation of the Complete Experimental Spectrum of methyl formate as a function of temperature. Of the 7132 strongest lines reported in this paper, 2523 are in the QM catalogs. Intensity differences of 5%–10% from those calculated via QM models were also found. Results are provided in a frequency point-by-point catalog that is well suited for the simulation of overlapped spectra. The common astrophysical line frequency, line strength, and lower state energy catalog is also provided. [ABSTRACT FROM AUTHOR]