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ABSTRACT: A flow injection analysis (FIA) tandem mass spectrometry (MSMS) method was developed for rapid quantitativeanalysis of 10 different inorganic and organic explosives. Performance is optimized by tailoring the ionization method,declustering potentials, and collision energies for each specific analyte. In doing so, a single instrument can be used to detecturea nitrate (UN), potassium chlorate (PC), 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenylmethylnitramine (tetryl), triacetonetriperoxide (TATP), hexamethylene triperoxide diamine (HMTD), pentaerythritol tetranitrate (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), nitroglycerin (NG) and octohydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) withsensitivities all in the picogram per milliliter range. Atmospheric-pressure chemical-ionization (APCI) was used for all of theexplosives except for UN and PC, where the best sensitivity was achieved using electrospray ionization (ESI). The use ofMSMS mode targeting both precursor and fragment ions provides for two-dimensional separations, thereby eliminating theneed for chromatography, even in complex matrices. By obviating the need for chromatography, the total analysis time isreduced to less than one minute per sample whereas the inter-day and intra-day relative standard deviations of 5% and10%, respectively, compare favorably to existing methods. Use of a single instrument for detection of multiple explosiveclasses also reduces the instrumentation capital cost and footprint for forensic-quality explosives detection. In conclusion,FIA APCI/ESI MSMS is a fast (1 min/sample), sensitive (pg/mL LOQ), and precise (intraday RSD10%) method fortrace explosive detection that can play an important role in criminal and attributional forensics, counterterrorism and environmentalprotection areas, and has the potential to augment or replace several of the existing explosive detection methods.

Dynamic photoacoustic spectroscopy (DPAS) is a trace-gas sensing technique recently developed at MIT Lincoln Laboratory. It is a novel laser-based means of remotely sensing extremely low concentrations of gases.

In the first half of this study, rotational and vibrational constants of six Franck-Condon bright vibrational levels of S1 doubly-substituted (13)C acetylene are determined from laser-induced fluorescence spectra and an updated geometry of the trans conformer of S1 acetylene is obtained. In the second half, we determine the quadratic force field of S1 acetylene on the basis of the harmonic frequencies of four isotopologues of acetylene. The effects of both diagonal and off-diagonal x(ij) anharmonicities are removed from the input harmonic frequencies. Results from both experimental and theoretical studies of various isotopologues of acetylene (including those from the first half of this paper) are used to obtain a set of force constants that agrees well with ab initio calculations. Our set of force constants for S1 acetylene is an improvement over previous work by Tobiason et al., which did not include off-diagonal anharmonicities.

The chemical and physical fates of trace amounts (50 μg) of explosives containing 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and pentaerythritol tetranitrate (PETN) were determined for the purpose of informing the capabilities of tactical trace explosive detection systems. From these measurements, it was found that the mass decreases and the chemical composition changes on a time scale of hours, with the loss mechanism due to a combination of sublimation and photodegradation. The rates for these processes were dependent on the explosive composition, as well as on both the ambient temperature and the size distribution of the explosive particulates. From these results, a persistence model was developed and applied to model the time dependence of both the mass and areal coverage of the fingerprints, resulting in a predictive capability for determining fingerprint fate. Chemical analysis confirmed that sublimation rates for TNT were depressed by UV (330-400 nm) exposure due to photochemically driven increases in the molecular weight, whereas the opposite was observed for RDX. No changes were observed for PETN upon exposure to UV radiation, and this was attributed to its low UV absorbance.

Dynamic photoacoustic spectroscopy (DPAS) is a high sensitivity technique for standoff detection of trace vapors. A field-portable DPAS system has potential as an early warning provider for gaseous-based chemical threats. For the first time, we utilize DPAS to successfully detect the presence of trace aerosols. Aerosol identification via long-wavelength infrared (LWIR) spectra is demonstrated. We estimate the sensitivity of our DPAS system to aerosols comprised of silica particles is comparable to that of SF(6) gas based on a signal level per absorbance unit metric for the two materials. The implications of these measurements are discussed.

Lincoln Laboratory of Massachusetts Institute of Technology has developed a technique known as dynamic photoacoustic spectroscopy (DPAS) that could enable remote detection of trace gases via a field-portable laser-based system. A fielded DPAS system has the potential to enable rapid, early warning of airborne chemical threats. DPAS is a new form of photoacoustic spectroscopy that relies on a laser beam swept at the speed of sound to amplify an otherwise weak photoacoustic signal. We experimentally determine the sensitivity of this technique using trace quantities of SF 6 gas. A clutter-limited sensitivity of ∼100 ppt is estimated for an integration path of 0.43 m. Additionally, detection at ranges over 5 m using two different detection modalities is demonstrated: a parabolic microphone and a laser vibrometer. Its utility in detecting ammonia emanating from solid samples in an ambient environment is also demonstrated.

We present a method of photoacoustic spectroscopy in which a laser beam tuned to an absorption feature of a gas is swept through its plume at the speed of sound. The resulting coherent addition of acoustic waves leads to an amplification of the signal without the need for a resonant chamber, thus enhancing the ability to remotely sense the gas. We demonstrate the concept using a tunable CO2 laser and SF6 gas in conjunction with a microphone. Sound pressure levels of 83 dB (relative to 20 μPa) are generated from a 15-ppm plume.