Quantitative calibration of vapor levels of TNT, RDX, and PETN using a diffusion generator with gravimetry and ion mobility spectrometry

Formic Acid as a Dopant for Atmospheric Pressure Chemical Ionization for Negative Polarity of Ion Mobility Spectrometry and Mass Spectrometry

Formic acid (FA) is introduced as a potent dopant for atmospheric pressure chemical ionization (APCI) for ion mobility spectrometry (IMS) and mass spectrometry (MS). The mechanism of chemical ionization with the FA dopant was studied in the negative polarity using a corona discharge (CD)-IMS-MS technique in air. Standard reactant ions of the negative polarity present in air are O2-·(CO2)n·(H2O)m (m = 0, 1 and n = 1, 2) clusters. Introduction of the FA dopant resulted in the production of HCOO-·FA reactant ions. The effect of the FA dopant on the APCI of different classes of compounds was investigated, including plant hormones, pesticides, acidic drugs, and explosives. FA dopant APCI resulted in deprotonation and/or adduct ion formation, [M - H]- and [M + HCOO]-, respectively. Supporting density functional theory (DFT) calculations showed that the ionization mechanism depended on the gas-phase acidity of the compounds. FA dopant APCI led to the improvement of detection sensitivity, suppression of fragmentation, and changes in the ion mobilities of the analyte ions for analytes with suitable molecular structures and gas acidity.

Surface Persistence of Trace Level Deposits of Highly Energetic Materials

In the fields of Security and Defense, explosive traces must be analyzed at the sites of the terrorist events. The persistence on surfaces of these traces depends on the sublimation processes and the interactions with the surfaces. This study presents evidence that the sublimation process of these traces on stainless steel (SS) surfaces is very different than in bulk quantities. The enthalpies of sublimation of traces of four highly energetic materials: triacetone triperoxide (TATP), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and 1,3,5- trinitrohexahydro-s-triazine (RDX) deposited on SS substrates were determined by optical fiber coupled-grazing angle probe Fourier Transform Infrared (FTIR) Spectroscopy. These were compared with enthalpies of sublimation determined by thermal gravimetric analysis for bulk amounts and differences between them were found. The sublimation enthalpy of RDX was very different for traces than for bulk quantities, attributed to two main factors. First, the beta-RDX phase was present at trace levels, unlike the case of bulk amounts which consisted only of the alpha-RDX phase. Second, an interaction between the RDX and SS was found. This interaction energy was determined using grazing angle FTIR microscopy. In the case of DNT and TNT, bulk and traces enthalpies were statistically similar, but it is evidenced that at the level of traces a metastable phase was observed. Finally, for TATP the enthalpies were statistically identical, but a non-linear behavior and a change of heat capacity values different from zero was found for both trace and bulk phases.

Investigation of Size-Dependent Sublimation Kinetics of 2,4,6-Trinitrotoluene (TNT) Micro-Islands Using In Situ Atomic Force Microscopy

Kinetic thermal analysis was conducted using in situ atomic force microscopy (AFM) at a temperature range of 15–25 °C to calculate the activation energy of the sublimation of 2,4,6-trinitrotoluene (TNT) islands. The decay of different diameter ranges (600–1600 nm) of TNT islands was imaged at various temperatures isothermally such that an activation energy could be obtained. The activation energy of the sublimation of TNT increases as the diameter of islands increases. It was found that the coarsening and the sublimation rate of TNT islands can be determined by the local environment of the TNT surface. This result demonstrates that a diffusion model cannot be simply applied to “real world” systems for explaining the sublimation behavior and for estimating the coarsening of TNT.

Base-driven sunlight oxidation of silver nanoprisms for label-free visual colorimetric detection of hexahydro-1,3,5-trinitro-1,3,5-triazine explosive

Here we report a label-free method for visual colorimetric detection of hexahydro-1,3,5-trinitro-1,3,5-triazine (HTT) explosive based on base-driven sunlight oxidation of silver nanoprisms (AgNPRs). Under natural sunlight illumination, the surface plasmon of AgNPRs is excited, which populates O₂ antibonding orbitals to generate negative-ion state (O₂^-). The resultant O₂^- with a strong oxidation activity can etch AgNPRs to smaller nanodisks with the aid of NaOH aqueous solution, leading to a blue shift of the absorption peak and color change from blue to pink. However, when HTT is introduced, the resultant O₂^- will be consumed by the nitrite and formaldehyde that are produced from the alkaline hydrolysis of HTT. Under this condition, the etching of AgNPRs does not occur, and the detection solution remains blue. This assay can sensitively detect as low as 1nM HTT, a level which is three orders of magnitude lower than that of gold nanoparticle-based colorimetric assays (2.6μM), and shows linearity in the range of 0.003-3.3μM. The lowest detectable concentration with the naked eye is 0.1μM. Additionally, the present assay exhibits good selectivity, and can be applied in the detection of HTT in natural water and soil samples with recoveries ranging from 90% to 100%.

Improved explosive collection and detection with rationally assembled surface sampling materials

Inorganic sampling cloth chemically modified with phenyl-functional groups for improving the collection and detection of trace explosives.

Gas phase ion chemistry of an ion mobility spectrometry based explosive trace detector elucidated by tandem mass spectrometry

The gas phase ion chemistry for an ion mobility spectrometer (IMS) based explosive detector has been elucidated using tandem mass spectrometry. The IMS system, which is operated with hexachloroethane and isobutyramide reagent gases and an ion shutter type gating scheme, is connected to the atmospheric pressure interface of a triple quadrupole mass spectrometer (MS/MS). Product ion masses, daughter ion masses, and reduced mobility values for a collection of nitro, nitrate, and peroxide explosives measured with the IMS/MS/MS instrument are reported. The mass and mobility data together with targeted isotopic labeling experiments and information about sample composition and reaction environment are leveraged to propose molecular formulas, structures, and ionization pathways for the various product ions. The major product ions are identified as [DNT-H](-) for DNT, [TNT-H](-) for TNT, [RDX+Cl](-) and [RDX+NO2](-) for RDX, [HMX+Cl](-) and [HMX+NO2](-) for HMX, [NO3](-) for EGDN, [NG+Cl](-) and [NG+NO3](-) for NG, [PETN+Cl](-) and [PETN+NO3](-) for PETN, [HNO3+NO3](-) for NH4NO3, [NO2](-) for DMNB, [HMTD-NC3H6O3+H+Cl](-) and [HMTD+H-CH2O-H2O2](+) for HMTD, and [(CH3)3CO2](+) for TATP. In general, the product ions identified for the IMS system studied here are consistent with the product ions reported previously for an ion trap mobility spectrometer (ITMS) based explosive trace detector, which is operated with dichloromethane and ammonia reagent gases and an ion trap type gating scheme. Differences between the explosive trace detectors include the [NG+Cl](-) and [PETN+Cl](-) product ions being major ions in the IMS system compared to minor ions in the ITMS system as well as the major product ion for TATP being [(CH3)3CO2](+) for the IMS system and [(CH3)2CNH2](+) for the ITMS system.

Sublimation kinetics and diffusion coefficients of TNT, PETN, and RDX in air by thermogravimetry

The diffusion coefficients of explosives are crucial in their trace detection and lifetime estimation. We report on the experimental values of diffusion coefficients of three of the most important explosives in both military and industry: TNT, PETN, and RDX. Thermogravimetric analysis (TGA) was used to determine the sublimation rates of TNT, PETN, and RDX powders in the form of cylindrical billets. The TGA was calibrated using ferrocene as a standard material of well-characterized sublimation rates and vapor pressures to determine the vapor pressures of TNT, PETN, and RDX. The determined sublimation rates and vapor pressures were used to indirectly determine the diffusion coefficients of TNT, PETN, and RDX for the first time. A linear log-log dependence of the diffusion coefficients on temperature is observed for the three materials. The diffusion coefficients of TNT, PETN, and RDX at 273 K were determined to be 5.76×10(-6)m(2)/sec, 4.94×10(-6)m(2)/s, and 5.89×10(-6)m(2)/s, respectively. Values are in excellent agreement with the theoretical values in literature.

Continuous flow, explosives vapor generator and sensor chamber

A novel liquid injection vapor generator (LIVG) is demonstrated that is amenable to low vapor pressure explosives, 2,4,6-trinitrotoluene and hexahydro-1,3,5-trinitro-1,3,5-triazine. The LIVG operates in a continuous manner, providing a constant and stable vapor output over a period of days and whose concentration can be extended over as much as three orders of magnitude. In addition, a large test atmosphere chamber attached to the LIVG is described, which enables the generation of a stable test atmosphere with controllable humidity and temperature. The size of the chamber allows for the complete insertion of testing instruments or arrays of materials into a uniform test atmosphere, and various electrical feedthroughs, insertion ports, and sealed doors permit simple and effective access to the sample chamber and its vapor.

Rapid estimation of thermodynamic parameters and vapor pressures of volatile materials at nanoscale

Non-isothermal measurements of thermodynamic parameters and vapor pressures of low-volatile materials are favored when time is a crucial factor to be considered, such as in the case of detection of hazardous materials. In this article, we demonstrate that optical absorbance spectroscopy can be used non-isothermally to estimate the thermodynamic properties and vapor pressures of volatile materials with good accuracy. This is the first method to determine such parameters in nanoscale in just minutes. Trinitrotoluene (TNT) is chosen because of its low melting temperature, which makes it impossible to determine its thermodynamic parameter by other rising-temperature techniques, such as thermogravimetric analysis (TGA). The well-characterized vapor pressure of benzoic acid is used to calibrate the spectrometer in order to determine the vapor pressure of low-volatile TNT. The estimated thermodynamic properties of both benzoic acid and TNT are in excellent agreement with the literature. The estimated vapor pressure of TNT is one order of magnitude larger than that determined isothermally using the same method. However, the values are still within the range reported in the literature. The data indicate the high potential for use of rising-temperature absorbance spectroscopy in determining vapor pressures of materials at nanometer scale in minutes instead of hours or days.

Ion spectrometric detection technologies for ultra-traces of explosives: a review

In recent years, explosive materials have been widely employed for various military applications and civilian conflicts; their use for hostile purposes has increased considerably. The detection of different kind of explosive agents has become crucially important for protection of human lives, infrastructures, and properties. Moreover, both the environmental aspects such as the risk of soil and water contamination and health risks related to the release of explosive particles need to be taken into account. For these reasons, there is a growing need to develop analyzing methods which are faster and more sensitive for detecting explosives. The detection techniques of the explosive materials should ideally serve fast real-time analysis in high accuracy and resolution from a minimal quantity of explosive without involving complicated sample preparation. The performance of the in-field analysis of extremely hazardous material has to be user-friendly and safe for operators. The two closely related ion spectrometric methods used in explosive analyses include mass spectrometry (MS) and ion mobility spectrometry (IMS). The four requirements-speed, selectivity, sensitivity, and sampling-are fulfilled with both of these methods.

Laser-based detection of TNT and RDX residues in real time under ambient conditions

We detect thin films of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-hexanitro-1,3,5-triazine (RDX) by one- and two-laser photofragmentation-fragment detection spectroscopy in real time at ambient temperature and pressure. In the one-laser technique, a laser tuned to 226 nm excites the energetic material and both generates the characteristic NO photofragments and facilitates their detection by resonance-enhanced multiphoton ionization (REMPI) using their A-X (0,0) transitions near 226 nm. In contrast, in the two-laser technique, a 454 nm laser generates the analyte molecule in the gas phase by matrix-assisted desorption, and a second laser tuned to 226 nm both photofragments it and ionizes the resulting NO. We report the effects of laser energy, analyte concentration, and matrix concentration on the ion signal and determine the rotational temperatures of the NO photofragments from Boltzmann, rotational distribution analysis of the REMPI spectra. We achieve limits of detection (S/N = 3) of hundreds of ng/cm(2) for both techniques under ambient conditions with a positive signal identification as low as 70 pg using a single 226 nm laser pulse of approximately 50 microJ.

Detection of volatile indicators of illicit substances by the olfactory receptors of Drosophila melanogaster

Insects can detect a large range of odors with a numerically simple olfactory system that delivers high sensitivity and accurate discrimination. Therefore, insect olfactory receptors hold great promise as biosensors for detection of volatile organic chemicals in a range of applications. The array of olfactory receptor neurons of Drosophila melanogaster is rapidly becoming the best-characterized natural nose. We have investigated the suitability of Drosophila receptors as detectors for volatiles with applications in law enforcement, emergency response, and security. We first characterized responses of the majority of olfactory neuron types to a set of diagnostic odorants. Being thus able to correctly identify neurons, we then screened for responses from 38 different types of neurons to 35 agents. We identified 13 neuron types with responses to 13 agents. As individual Drosophila receptor genes have been mapped to neuron types, we can infer which genes confer responsiveness to the neurons. The responses were confirmed for one receptor by expressing it in a nonresponsive neuron. The fly olfactory system is mainly adapted to detect volatiles from fermenting fruits. However, our findings establish that volatiles associated with illicit substances, many of which are of nonnatural origin, are also detected by Drosophila receptors.

Effect of Zn doping on the sublimation rate of pentaerythritol tetranitrate using atomic force microscopy

A series of Zn ion-doped pentaerythritol tetranitrate (PETN) nanoislands in the form of thin films were prepared on Si substrates using spin coating. The effect of Zn concentrations on the sublimation energy was investigated by atomic force microscopy (AFM). The pure and Zn-doped nanoislands are imaged by AFM in contact mode at room temperature after annealing isothermally for a given time. The volume of the islands starts to decrease after annealing at 45 degrees C for pure PETN, whereas Zn-doped nanoislands start to decrease in height and volume after annealing at 55-58 degrees C. The minimum activation energy is found to be 29.7 Kcal/mol for 1,000 ppm Zn concentration. These studies are important for the long-term stabilization of PETN.

A simple and flexible thin film evaporating device for energetic materials

A thin film evaporation device has been developed to prepare energetic materials at atmospheric pressure. The device is intuitive, simple, and easy to manipulate. The application of the device is demonstrated for pentaerythritol tetranitrate but can be extended to other explosives and organic materials that have low saturation pressures.

Secondary electrospray ionization-ion mobility spectrometry for explosive vapor detection

The unique capability of secondary electrospray ionization (SESI) as a nonradioactive ionization source to detect analytes in both liquid and gaseous samples was evaluated using aqueous solutions of three common military explosives: cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), nitroglycerin (NG) and pentaerythritol tetranitrate (PETN). The adducts formed between the compounds and their respective dissociation product, RDX.NO(2)(-), NG.NO(3)(-), and PETN.NO(3)(-), gave the most intense signal for the individual compound but were more sensitive to temperature than other species. These autoadducts were identified as RDX.NO(2)(-), NG.NO(3)(-), and PETN.NO(3)(-) and had maximum signal intensity at 137, 100, and 125 degrees C, respectively. The reduced mobility values of the three compounds were constant over the temperature range from 75 to 225 degrees C. The signal-to-noise ratios for RDX, NG, and PETN at 50 mg L(-1) in methanol-water were 340, 270, and 170, respectively, with a nominal noise of 8 +/- 2 pA. In addition to the investigation of autoadduct formation, the concept of doping the ionization source with nonvolatile adduct-forming agents was investigated and described for the first time. The SESI-IMS detection limit for RDX was 116 microg L(-1) in the presence of a traditional volatile chloride dopant and 5.30 microg L(-1) in the presence of a nonvolatile nitrate dopant. In addition to a lower detection limit, the nitrate dopant also produced a greater response sensitivity and a higher limit of linearity than did the traditional volatile chloride dopant.

Gas-phase detection of trinitrotoluene utilizing a solid-phase antibody immobilized on a gold film by means of surface plasmon resonance spectroscopy

A multilayered biosensor was constructed and found to detect trinitrotoluene (TNT) in ppb concentrations in air both prior to and after detonation of TNT without use of a liquid phosphate buffered saline (PBS) superstrate. The biosensor surface was fabricated from a monoclonal antibody for TNT covalently bound to an 11,11'-dithio-bis(succinimidoylundecanoate) (DSU) self-assembled monolayer immobilized on a thin gold film bonded to a BK7 glass slide. The binding between the immobilized antibody and TNT antigen was detected using surface plasmon resonance spectroscopy (SPRS). Biosensor specificity for TNT was demonstrated with chemical homologues as well as against an unrelated explosive, RDX.

Detection of the gasoline components methyl tert-butyl ether, benzene, toluene, and m-xylene using ion mobility spectrometers with a radioactive and UV ionization source

For the first time, ion mobility spectrometers (IMS) with radioactive and UV ionization sources in combination with multicapillary columns (MCCs) have been used to determine methyl tert-butyl ether (MTBE), a gasoline additive, in water and nitrogen as well as the monoaromatic compounds benzene, toluene, and m-xylene (BTX). A membrane extraction unit was set up to extract the substances from water, which is simple, effective, and easy to automate for further applications. Thus, the detection of MTBE and BTX of gasoline vapors was accomplished after a preliminary silicone membrane extraction. Two-dimensional data analyses of IMS-chromatograms allow us to separate these substances clearly according to their different retention and drift times. Method detection limits for MTBE were 2 microg/L (UV) and 30 pg/L (63Ni) in nitrogen and 20 mg/L (UV) and 1 microg/L (63Ni) in water. Rather a good reproducibility was achieved with relative standard deviations of between 2.9 and 9%. The method presented in this article has been proven to be suitable for nearly real-time monitoring as the total analysis time is less than 90 s. As an example of application, the detection of MTBE and BTX in a mixture of volatile organic compounds of pure gasoline using the 2-D IMS-chromatogram is presented.

Atmospheric pressure chemical ionization of fluorinated phenols in atmospheric pressure chemical ionization mass spectrometry, tandem mass spectrometry, and ion mobility spectrometry

Atmospheric pressure chemical ionization (APCI)-mass spectrometry (MS) for fluorinated phenols (C6H5-xFxOH Where x = 0-5) in nitrogen with Cl- as the reagent ion yielded product ions of M Cl- through ion associations or (M-H)- through proton abstractions. Proton abstraction was controllable by potentials on the orifice and first lens, suggesting that some proton abstraction occurs through collision induced dissociation (CID) in the interface region. This was proven using CID of adduct ions (M Cl-) with Q2 studies where adduct ions were dissociated to Cl- or proton abstracted to (M-H)-. The extent of proton abstraction depended upon ion energy and structure in order of calculated acidities: pentafluorophenol > tetrafluorophenol > trifluorophenol > difluorophenol. Little or no proton abstraction occurred for fluorophenol, phenol, or benzyl alcohol analogs. Ion mobility spectrometry was used to determine if proton abstraction reactions passed through an adduct intermediate with thermalized ions and mobility spectra for all chemicals were obtained from 25 to 200 degrees C. Proton abstraction from M Cl- was not observed at any temperature for phenol, monofluorophenol, or difluorophenol. Mobility spectra for trifluorophenol revealed the kinetic transformations to (M-H)- either from M Cl- or from M2 Cl- directly. Proton abstraction was the predominant reaction for tetra- and penta-fluorophenols. Consequently, the evidence suggests that proton abstraction occurs from an adduct ion where the reaction barrier is reduced with increasing acidity of the O-H bond in C6H5-xFxOH.

Laser photofragmentation-fragment detection and pyrolysis-laser-induced fluorescence studies on energetic materials

Trace concentrations of energetic materials such as 2, 4, 6-trinitrotoluene (TNT), pentaerythritol tetranitrate (PETN), and hexahydro-1, 3, 5-trinitro-s-triazine (RDX) are detected by laser photofragmentation-fragment detection (PF-FD) spectrometry. In this technique, a single laser operating near 227 nm photofragments the parent molecule and facilitates the detection of the characteristic NO fragment by means of its A (2)Sigma(+)-X (2)Sigma (0, 0) transitions near 227 nm. Fragment detection is accomplished by resonance-enhanced multiphoton ionization with miniature electrodes and by laser-induced fluorescence (LIF) with a photodetector. Experiments are also conducted in the visible region by use of 453.85-nm radiation for photofragmentation and fragment detection. Sand samples contaminated with PETN and RDX are analyzed by a pyrolysis-LIF technique, which involves pyrolysis of the energetic material with subsequent detection of the pyrolysis products NO and NO(2) by LIF and PF-LIF, respectively, near 227 nm. The application of these techniques to the trace analysis of TNT, PETN, and RDX at ambient pressure in room air is demonstrated with limits of detection (signal-to-noise ratio, 3) in the low parts-in-10(9) to parts-in-10(6) range for a 20-s integration time and 10-120 microJ of laser energy at 226.8 nm and approximately 5 mJ at 453.85 nm. An increase in detection sensitivity is projected with an increase in laser energy and an improved system design. The analytical merits of these techniques are discussed and compared with those of other laser-based techniques.