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.

Characterization of TATP gas phase product ion chemistry via isotope labeling experiments using ion mobility spectrometry interfaced with a triple quadrupole mass spectrometer.

Identification of the fragment ion species associated with the ion reaction mechanism of triacetone triperoxide (TATP), a homemade peroxide-based explosive, is presented. Ion mobility spectrometry (IMS) has proven to be a key analytical technique in the detection of trace explosive material. Unfortunately, IMS alone does not provide chemical identification of the ions detected; therefore, it is unknown what ion species are actually formed and separated by the IMS. In IMS, ions are primarily characterized by their drift time, which is dependent on the ion׳s mass and molecular cross-section; thus, IMS as a standalone technique does not provide structural signatures, which is in sharp contrast to the chemical and molecular information that is generally obtained from other customary analytical techniques, such as NMR, Raman and IR spectroscopy and mass spectrometry. To help study the ion chemistry that gives rise to the peaks observed in IMS, the hardware of two different commercial IMS instruments has been directly coupled to triple quadrupole (QQQ) mass spectrometers, in order to ascertain each ion׳s corresponding mass/charge (m/z) ratios with different dopants at two temperatures. Isotope labeling was then used to help identify and confirm the molecular identity of the explosive fragment and adduct ions of TATP. The m/z values and isotope labeling experiments were used to help propose probable molecular formulas for the ion fragments. In this report, the fragment and adduct ions m/z 58 and 240 of TATP have been confirmed to be [C3H6NH·H](+) and [TATP·NH4](+), respectively; while the fragment ions m/z 73 and 89 of TATP are identified as having the molecular formulas [C4H9NH2](+) and [C4H9O2](+), respectively. It is anticipated that the work in this area will not only help to facilitate improvements in mobility-based detection (IMS and MS), but also aid in the development and optimization of MS-based detection algorithms for TATP.

Characterizing the gas phase ion chemistry of an ion trap mobility spectrometry based explosive trace detector using a tandem mass spectrometer.

A commercial-off-the-shelf (COTS) ion trap mobility spectrometry (ITMS) based explosive trace detector (ETD) has been interfaced to a triple quadrupole mass spectrometer (MS/MS) for the purpose of characterizing the gas phase ion chemistry intrinsic to the ITMS instrument. The overall objective of the research is to develop a fundamental understanding of the gas phase ionization processes in the ITMS based ETD to facilitate the advancement of its operational effectiveness as well as guide the development of next generation ETDs. Product ion masses, daughter ion masses, and reduced mobility values measured by the ITMS/MS/MS configuration for a suite of nitro, nitrate, and peroxide containing explosives are reported. Molecular formulas, molecular structures, and ionization pathways for the various product ions are inferred using the mass and mobility data in conjunction with density functional theory. The predominant product ions are identified as follows: [TNT-H](-) for trinitrotoluene (TNT), [RDX+Cl](-) for cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), [NO(3)](-) for ethylene glycol dinitrate (EGDN), [NG+NO(3)](-) for nitroglycerine (NG), [PETN+NO(3)](-) for pentaerythritol tetranitrate (PETN), [HNO(3)+NO(3)](-) for ammonium nitrate (NH(4)NO(3)), [HMTD-NC(3)H(6)O(3)+H+Cl](-) for hexamethylene triperoxide diamine (HMTD), and [(CH(3))(2)CNH(2)](+) for triacetone triperoxide (TATP). The predominant ionization pathways for the formation of the various product ions are determined to include proton abstraction, ion-molecule attachment, autoionization, first-order and multi-order thermolysis, and nucleophilic substitution. The ion trapping scheme in the reaction region of the ITMS instrument is shown to increase predominant ion intensities relative to the secondary ion intensities when compared to non-ion trap operation.

Structure and dynamics of water dangling OH bonds in hydrophobic hydration shells. Comparison of simulation and experiment.

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

Are long-chain alkanes hydrophilic?

Although short n-alkane chains are classic examples of hydrophobic solutes, mounting evidence points to a hydrophilic crossover for the hydration free energies (DeltaG) of sufficiently long n-alkane chains. Experimental and simulation results for the hydration of n-alkanes from methane (C1) to docosane (C22) are combined with fundamental thermodynamic relations to elucidate intermolecular contributions to DeltaG. Theoretical bounds on the influence of solute conformation on DeltaG are inferred by considering the hydration of idealized linear (all-trans) and globular (spherical) model solutes. More detailed theoretical extrapolations of experimental and simulation results imply that the water-mediated free energy change associated with collapsing an all-trans C100 chain is on the order of -100 kJ/mol and thus that n-alkane chains of this length and longer may be hydrophilic (DeltaG < 0).