J. E. Rodriguez

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

A prototype generator for creating a continuous stream of explosive vapor was referenced quantitatively both to a standard weight from the National Institute of Standards and Technology (NIST) and to the response of an ion mobility spectrometer. Vapors from solid explosive, in a precision bore glass tube at constant temperature, diffuse into an inert gas flow. Mass output rates were determined by (1) sample temperature, and (2) sample tube dimensions (length and cross-sectional area). A reference to NIST was achieved gravimetrically though a microbalance calibrated with a reference weight; mass output rates were obtained for 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX) and pentaerythritol tetranitrate (PETN) at three or more oven temperatures between 79 degrees C and 150 degrees C. The mass output rate was stable over hundreds of hours of continuous operation and the output was adjustable from a few picograms per second to several nanograms per second through variation of the oven temperature. An independent calibration of the vapor generator for TNT at 79 degrees C using an ion mobility spectrometer matched exactly the gravimetric-based findings. In most instances, measured mass output rates compared favorably with theoretically calculated mass output rates, with discrepancies in a few cases resulting primarily from uncertainties in terms (vapor pressures and diffusion coefficients) used to perform the calculations. Agreement is generally not good for PETN, where molecular decomposition contributed to higher than expected measured mass outputs.

Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry

Mobility spectra for positive ions, created from a 63Ni foil in purified air at ambient pressure (660 Torr) with 0.15 ppm moisture, were obtained with a drift tube with a discrete drift ring design at 250 °C as electric fields for components were individually and independently varied. Peak area, peak width, baseline intensity, drift times, and reduced mobilities (Ko) were used to measure the function and performance of each component and findings were interpreted using a model for the transport of thermalized ions in weak electric fields at ambient pressure. Transit times and intensities for ions in drift tubes at ambient pressure can be understood through a detailed knowledge of the fields local to a component and derivations from theory of ion transport. Prolonged ion residence in the drift region resulted in ion transformations even for highly purified gases of low moisture at high temperature. These findings suggest that mobility spectra may be obtained with uniformly high quality and reproducibility ...

Chemical class information in ion mobility spectra at low and elevated temperatures

Mobility spectra for organic compounds at temperatures of 50C and 175‐250C were categorized by chemical class using back-propagation neural networks with the successful classification even of chemicals not familiar to the networks. Network performance suggested that chemical class information in spectra at 50C differed from that in spectra at high temperatures, prompting a detailed analysis of regions where chemical class information was located. These regions, or drift times in the mobility spectra, were identified at each temperature using a method of incrementally removing portions of spectra so that the value or structural content of the subtracted region could be seen in comparisons of network performance. At high temperatures (175‐250C), chemical class information was contained in fragment ions located in a narrow region of the spectra with reduced mobilities of 3.06‐2.11 cm 2 V 1 s 1 corresponding to the drift times near and encompassing the reactant ions peaks. In contrast, spectra at low temperature (50C) were classified through fragment ions that resided in a broad region of drift time between protonated water clusters and product cluster peaks. This corresponded to reduced mobilities of 1.8‐1.2 cm 2 V 1 s 1 . These findings suggest that fragmentation in ion mobility spectrometry and other atmospheric pressure chemical ionization based methods, with moisture <1 ppm, may be more common than previously understood. Class specific fragmentation reactions for ions at low temperature have never been described in IMS and became evident only when mobility spectra were formatted with a logarithmic axis for ion intensity.

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 °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.

Dissociation of proton bound ketone dimers in asymmetric electric fields with differential mobility spectrometry and in uniform electric fields with linear ion mobility spectrometry.

The kinetics for the decomposition of the symmetrical proton-bound dimers of a series of 2-ketones (M) from acetone to 2-nonanone have been determined at ambient pressure by linear ion mobility spectrometry (IMS) and by differential mobility spectrometry (DMS). Decomposition, M2H(+) →MH(+) + M, in the IMS instrument, observed under thermal conditions over the temperature range 147 to 172 °C, yielded almost identical Arrhenius parameters Ea = 122 kJ mol(-1) and ln A = 38.8 for the dimers of 2-pentanone, 2-heptanone, and 2-nonanone. Ion decomposition in the DMS instrument was due to a combination of thermal and electric field energies at an effective ion internal temperature whose value was estimated by reference to the IMS kinetic parameters. Decomposition was observed with radio frequency (RF) fields with maximum intensities in the range 10 kV cm(-1) to 30 kV cm(-1) and gas temperatures from 30 to 110 °C, which yielded effective temperatures that were higher than the gas temperature by 260° at 30 °C and 100° at 110 °C. There was a mass dependence of the field for the onset of decomposition: the higher the ion mass, the higher the required field at a given gas temperature, which is ascribed to the associated increasing heat capacity with the increasing carbon number, but similar, internal vibrations and rotations.

Planar Drift Tube for Ion Mobility Spectrometry

Abstract The drift tube in ion mobility spectrometry is the component of central importance, where sample vapors are ionized and where ions are separated on the basis of gaseous mobility in a comparatively weak electric field. Construction of drift tubes is labor intensive and costly when built with precision machined components and an alternative design, fabricated using photolithography methods with planar drift plates, is described. Diagnostics of performance including response toward electric fields, peak shape of mobility spectra, determined values for reduced mobility coefficients, and patterns of response to changing vapor concentrations were consistent with conventional drift tubes. Neither ion losses nor band broadening were unexpectedly large though ultimate measures of performance were limited by the unavailability of drift tube components with rectangular profiles and by an imperfect reaction region design. The cost of making drift tubes is calculated as roughly 10% of that for conventional designs.

Ion Mobility Spectrometry of Gas-Phase Ions from Laser Ablation of Solids in Air at Ambient Pressure

A mobility spectrometer was used to characterize gas-phase ions produced from laser ablation of solids in air at 100 °C and at ambient pressure with a beam focused to a diameter of <0.2 mm at energy of 6 mJ/pulse and wavelength of 266 nm. Metals, organic polymers, glass, graphite, and boron nitride exhibited characteristic mobility spectra with peaks at drift times between 8.75 and 12.5 ms (reduced mobility values of 2.19 to 1.53 cm2/Vs). Ion intensities increased initially and then decreased with repeated laser shots through drilling of the solid, and persistence of signal was proportional to hardness. A single comparatively narrow peak for negative ions was observed in mobility spectra for all materials and this was mass-identified as O2−. These ions were formed in air from reactions of oxygen with electrons emitted from the ablation step. Positive ions ablated directly from the solid were masked in ion mobility spectrometry/mass spectrometry (IMS/MS) studies by ionization of moisture and impurities. Positive ions from solids were seen only in the IMS analyzer at elevated temperature and low moisture. Under such conditions, materials were classified from mobility spectra alone with principal component analysis.

Volatile Organic Compounds in Headspace over Electrical Components at 75 to 200°C – Part 1. Identification of Constituents and Emission Rates

The identity and emission rates of volatile organic compounds (VOCs) in headspace vapors over electronic components were determined at temperatures from 75 to 200°C using gas chromatography/mass spectrometry. The emission of VOCs may provide a basis to detect the onset of the overheating of electronic components in confined atmospheres near electronic bays on airplanes and submarines before smoldering or ignition. VOCs found in headspace vapors over components, including resistors, capacitors, diodes, transistors, and insulation from wires of a transformer, were composed of simple mixtures of substances with 6 to 10 carbon number from chemical families including ketones, aldehydes, substituted benzenes, alcohols, and phenols. Composition of the vapors was characteristic but not exclusive of a particular electrical component, except for phenols and methylstyrene, which were found only in a single component. Emission rates were expressed as nanogram of chemical per gram of component per minute, and increased from a low of 0.001 ng/g-min for nonanal from transformer wire at 100°C to a maximum of 2.5 ng/g-min at 150°C for isophorone from a resistor. Patterns of persistence with repeated sampling of headspace for components at 200°C over 5 hr suggested that VOCs arose from impurities in plastics rather than from thermal decomposition of the polymer.