Atmospheric pressure chemical ionization source. 1. Ionization of compounds in the gas phase

A novel chemical ionization source for organic mass spectrometry is introduced. This new source uses a glow discharge in the flowing afterglow mode for the generation of excited species and ions. The direct-current gas discharge is operated in helium at atmospheric pressure; typical operating voltages and currents are around 500 V and 25 mA, respectively. The species generated by this atmospheric pressure glow discharge are mixed with ambient air to generate reagent ions (mostly ionized water clusters and NO+), which are then used for the ionization of gaseous organic compounds. A wide variety of substances, both polar and nonpolar, can be ionized. The resulting mass spectra generally show the parent molecular ion (M+ or MH+) with little or no fragmentation. Proton transfer from ionized water clusters has been identified as the main ionization pathway. However, the presence of radical molecular ions (M+) for some compounds indicates that other ionization mechanisms are also involved. The analytical capabilities of this source were evaluated with a time-of-flight mass spectrometer, and preliminary characterization shows very good stability, linearity, and sensitivity. Limits of detection in the single to tens of femtomole range are reported for selected compounds.     

Atmospheric pressure matrix-assisted laser desorption/ionization in transmission geometry

In both atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) and vacuum MALDI, the laser typically illuminates the analyte on the front side of an opaque surface (reflection geometry). Another configuration consisting of laser illumination through the sample backside (transmission geometry) has been used in conventional MALDI; however, its use and the number of reports in the literature are limited. The viability of transmission geometry with AP MALDI is demonstrated here. Such a geometry is simple to implement, eliminates the restriction for a metallic sample holder, and allows for the potential analysis of samples on their native transparent surfaces, e.g., cells or tissue sections on slides.     

An effective analytical model of selective laser melting

Selective Laser Melting (SLM) is a laser-based powder bed Additive Manufacturing process that can produce near net shape products with metallic powders according to Computer-Aided Design models. In this paper, a useful analytical model of SLM is proposed by investigating the energy requirement of the process. Results from the experiments on SLM of pure nickel and pure tin are reported. By compiling process parameter data from various literatures and experiments, this model has shown to enable predictions of the energy input required to process different metallic materials to an order of magnitude despite the many assumptions made. Possible explanations for the deviation in predictions and actual energy inputs are also discussed.     

Electrosonic spray ionization. A gentle technique for generating folded proteins and protein complexes in the gas phase and for studying ion-molecule reactions at atmospheric pressure

Electrosonic spray ionization (ESSI), a variant on electrospray ionization (ESI), employs a traditional micro ESI source with supersonic nebulizing gas. The high linear velocity of the nebulizing gas provides efficient pneumatic spraying of the charged liquid sample. The variable electrostatic potential can be tuned to allow efficient and gentle ionization. This ionization method is successfully applied to aqueous solutions of various proteins at neutral pH, and its performance is compared to that of the nanospray and micro ESI techniques. Evidence for efficient desolvation during ESSI is provided by the fact that the peak widths for various multiply charged protein ions are an order of magnitude narrower than those for nanospray. Narrow charge-state distributions compared to other ESI techniques are observed also; for most of the proteins studied, more than 90% of the protein ions can be accumulated in one charge state using ESSI when optimizing conditions. The fact that the abundant charge state is normally as low or lower than that recorded by ESI or nanospray indicates that folded protein ions are generated. The sensitivity of the ionization technique to high salt concentrations is comparable to that of nanospray, but ESSI is considerably less sensitive to high concentrations of organic additives such as glycerol or 2-amino-2-(hydroxymethyl)-1,3-propanediol (Tris base). Noncovalent complexes are observed in the case of myoglobin, protein kinase A/ATP complex, and other proteins. The extent of dissociation of protein ions in ESSI is comparable to or even smaller than that in the case of nanospray, emphasizing the gentle nature of the method. The unique features of ESSI are ascribed to very efficient spraying and the low internal energy supplied to the ions. Evidence is provided that the method is capable of generating fully desolvated protein ions at atmospheric pressure. This allows the technique to be used for the study of ion-molecule reactions at atmospheric pressure and examples of this are shown.     

Electron-impact ionization of sulfur in the gas phase

The procedure and results of studying the yield of positive ions formed via electron-impact dissociative ionization of sulfur are described. The ionization energy of the ground molecule and appearance energies of fragmentary ions are derived from the ionization efficiency curves. The dynamics of formation of molecular sulfur ions in the temperature range of 300–670 K are studied. The energy dependences of the efficiency of formation of ions S _n for n = 1–6 are analyzed, and the appearance energies are determined. The total ionization cross section of sulfur is determined using a monoenergetic electron beam.     

Atmospheric pressure chemical ionization source. 2. Desorption-ionization for the direct analysis of solid compounds

The flowing afterglow-atmospheric pressure glow discharge (APGD) ionization source described in part 1 of this study (in this issue) is applied to the direct analysis of condensed-phase samples. When either liquids or solids are exposed to the ionizing beam of the APGD, strong signals for the molecular ions of substances present on their surfaces can be detected without compromising the integrity of the solid sample structure or sample substrate. As was observed for gas-phase compounds in part 1 of this study, both polar and nonpolar substances can be ionized and detected by mass spectrometry. The parent molecular ion (or its protonated counterpart) is usually the main spectral feature, with little or no fragmentation in evidence. Preliminary quantitative results show that this approach offers very good sensitivity (detection limits in the picogram regime are reported for several test compounds in part 1 of this study) and linear response to the analyte concentration. Examples of the application of this strategy to the analysis of real-world samples, such as the direct analysis of pharmaceutical compounds or foods is provided. The ability of this source to perform spatially resolved analysis is also demonstrated. Preliminary studies of the mechanisms of the reactions involved are described.     

A REMPI method for the ultrasensitive detection of NO and NO2 using atmospheric pressure laser ionization mass spectrometry

We report on the development of a quasi-simultaneous highly selective method for NO and NO2 detection at the ultratrace level. Atmospheric pressure laser ionization (APLI), recently introduced by our group, is used to detect both compounds at low parts per trillion by volume (pptv) mixing ratios. APLI is based on resonance-enhanced multiphoton ionization mass spectrometry. Two-color pump-probe experiments employing a single excimer pumped dye laser combination allow for the ultrasensitive measurement of NO and NO2 within a narrow range of maximum pumping efficiency of the laser dye Coumarin 120. NO is detected via excitation of the long-lived A 2sigma+ (nu' = 1) level at 215.36 nm and subsequently ionized with 308-nm radiation provided by the excimer pump laser. NO2 is ionized after double resonant excitation of the A2B1 and 3psigma manifolds in a (1 + 1' + 1(')) process using 431.65 + 308 nm. The selectivity of the NO measurement exceeds 2,000 with respect to NO2 and N2O5. For NO2, a selectivity of >3,000 with respect to N2O5 and organic nitrates is observed. The current APLI detection limit of NO and NO2 is 0.5 and 5 pptv, respectively, with a 20-s integration time.     

An analytical model for estimating deformation in laser forming

Laser forming of metal sheets offers the advantages of requiring no external forces and thus reduces cost and increases flexibility. This paper presents an analytical model to estimate the angle bent during the laser forming of a sheet. Plastic deformation is considered during both heating and cooling and is calculated based on a history-dependent incremental stress–strain relationship. On the basis of the proposed model with known temperature distributions, the bending angle induced by laser can be calculated. Comparison of the present model with experiment data is provided to demonstrate the accuracy of the present model under both TGM and BM.     

Polyaniline-based highly sensitive microbial biosensor for selective detection of lindane

A highly sensitive, selective, and rapid, whole-cell-based electrochemical biosensor was developed for detection of the persistent organochlorine pesticide γ-hexachlorocyclohexane (γ-HCH), commonly known as lindane. The gene linA2 encoding the enzyme γ-hexachlorocyclohexane (HCH) dehydrochlorinase (LinA2), involved in the initial steps of lindane (γ-HCH) biotransformation, was cloned and overexpressed in Escherichia coli . The lindane-biodegrading E. coli cells were immobilized on polyaniline film. The rapid and selective degradation of lindane and concomitant generation of hydrochloric acid by the recombinant E. coli cells in the microenvironment of polyaniline led to a change in its conductivity, which was monitored by pulsed amperometry. The biosensor could detect lindane in the part-per-trillion concentration range with a linear response from 2 to 45 ppt. The sensor was found to be selective to all the isomers of hexachlorocyclohexane (HCH) and to pentachlorocyclohexane (PCCH) but did not respond to other aliphatic and aromatic chlorides or to the end product of lindane degradation, i.e., trichlorobenzene (TCB). The sensor also did not respond to other commonly used organochlorine pesticides like DDT and DDE. On the basis of experimental results, a rationale has been proposed for the excellent sensitivity of polyaniline as a pH sensor for detection of H(+) ions released in its microenvironment.     

Real-time, ultralow concentration detection of analytes in solution by infrared intracavity laser absorption

Two intracavity laser absorption techniques for ultralow concentration detection of chemicals in solution are compared. The first consists of a laser diode in a grating extended cavity, which produces a linear calibration curve for parts in 10(9) (ppb) concentrations corresponding to 17 nM. By replacing the grating with a highly reflective mirror, parts in 10(12) (ppt) concentration detection is achieved, which corresponds to 340 pM. We report, to our knowledge for the first time, ppt detection of analyte concentration in liquid solution demonstrating good agreement between theory and experiment.     

Selective detection of volatile aromatic compounds using a compact laser ionization detector with fast gas chromatography

We report results for a new gas chromatography detector that is comparatively sensitive and far more selective for aromatic compounds than the traditional photoionization detector. The detection means is multiphoton ionization at atmospheric pressure. The ionization source in these experiments is a diode-pumped passively Q-switched microchip laser operating at 266 nm. Experiments were conducted with the detector interfaced to a fast gas chromatograph. For <20 s elution time, limits of detection were <1 pg for toluene, ethylbenzene, xylenes, and isopropylbenzene; the limit of detection for benzene is approximately 10 pg. Detector response was linear over 5 orders of magnitude, including these low levels. Negligible signals were observed for nonaromatic ketones, aldehydes, ethers, and cycloalkanes at levels as high as 0.1 microg (10 mg/L concentration). Detector efficiency after fast GC separation was 0.002% when using a detector cell with a radius of 1.1 cm and a purge gas flow of 500 mL/min. The advantages of this detector are further illustrated by the fast GC analysis of fuel samples.     

Atmospheric pressure laser-induced acoustic desorption chemical ionization mass spectrometry for analysis of saturated hydrocarbons

We present atmospheric pressure laser-induced acoustic desorption chemical ionization (AP/LIAD-CI) with O(2) carrier/reagent gas as a powerful new approach for the analysis of saturated hydrocarbon mixtures. Nonthermal sample vaporization with subsequent chemical ionization generates abundant ion signals for straight-chain, branched, and cycloalkanes with minimal or no fragmentation. [M - H](+) is the dominant species for straight-chain and branched alkanes. For cycloalkanes, M(+?) species dominate the mass spectrum at lower capillary temperature (<100 °C) and [M - H](+) at higher temperature (>200 °C). The mass spectrum for a straight-chain alkane mixture (C(21)-C(40)) shows comparable ionization efficiency for all components. AP/LIAD-CI produces molecular weight distributions similar to those for gel permeation chromatography for polyethylene polymers, Polywax 500 and Polywax 655. Coupling of the technique to Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) for the analysis of complex hydrocarbon mixtures provides unparalleled mass resolution and accuracy to facilitate unambiguous elemental composition assignments, e.g., 1754 peaks (rms error = 175 ppb) corresponding to a paraffin series (C(12)-C(49), double-bond equivalents, DBE = 0) and higher DBE series corresponding to cycloparaffins containing one to eight rings. Isoabundance-contoured plots of DBE versus carbon number highlight steranes (DBE = 4) of carbon number C(27)-C(30) and hopanes of C(29)-C(35) (DBE = 5), with sterane-to-hopane ratio in good agreement with field ionization (FI) mass spectrometry analysis, but performed at atmospheric pressure. The overall speciation of nonpolar, aliphatic hydrocarbon base oil species offers a promising diagnostic probe to characterize crude oil and its products.     

Atmospheric pressure photoionization mass spectrometry. Ionization mechanism and the effect of solvent on the ionization of naphthalenes

The ionization mechanism in dopant-assisted atmospheric pressure photoionization and the effect of solvent on the ionization efficiency was studied using 7 naphthalenes and 13 different solvent systems. The ionization efficiency was 1-2 orders of magnitude higher with dopant than without, indicating that the photoionization of the dopant initiates the ionization process. In positive ion mode, the analytes were ionized either by charge exchange or by proton transfer. Charge exchange was favored for low proton affinity solvents (water, hexane, chloroform), whereas the addition of methanol or acetonitrile to the solvent initiated proton transfer. In negative ion mode, the compounds with high electron affinity were ionized by electron capture or by charge exchange and the compounds with high gas-phase acidity were ionized by proton transfer. In addition, some oxidation reactions were observed. All the reactions leading to ionization of analytes in negative ion mode are initiated by thermal electrons formed in photoionization of toluene. The testing of different solvents showed that addition of buffers such as ammonium acetate, ammonium hydroxide, or acetic acid may suppress ionization in APPI. The reactions are discussed in detail in light of thermodynamic data.     

Terahertz ionization of highly charged quantum posts in a perforated electron gas

"Quantum posts" are roughly cylindrical semiconductor nanostructures that are embedded in an energetically shallower "matrix" quantum well of comparable thickness. We report measurements of voltage-controlled charging and terahertz absorption of 30 nm thick InGaAs quantum wells and posts. Under flat-band (zero-electric field) conditions, the quantum posts each contain approximately six electrons, and an additional ~2.4 × 10(11) cm(-2) electrons populate the quantum well matrix. In this regime, absorption spectra show peaks at 3.5 and 4.8 THz (14 and 19 meV) whose relative amplitude depends strongly on temperature. These peaks are assigned to intersubband transitions of electrons in the quantum well matrix. A third, broader feature has a temperature-independent amplitude and is assigned to an absorption involving quantum posts. Eight-band k·p calculations incorporating the effects of strain and Coulomb repulsion predict that the electrons in the posts strongly repel the electrons in the quantum well matrix, "perforating" the electron gas. The strongest calculated transition, which has a frequency close to the center of the quantum post related absorption at 5 THz (20 meV), is an ionizing transition from a filled state to a quasi-bound state that can easily scatter to empty states in the quantum well matrix.     

Atmospheric pressure photoionization for ionization of both polar and nonpolar compounds in reversed-phase LC/MS

Atmospheric pressure photoionization can provide high ionization efficiency simultaneously to both polar and nonpolar compounds delivered in reversed-phase solvent. The method to achieve this utilizes toluene as a dopant and simply requires that the solvent flow be limited so that reactions between toluene photoions and the organic component of the solvent are not driven to completion. Under these conditions, toluene photoions remain in the source for ionizing nonpolar compounds via charge exchange (electron transfer), while protonated solvent ions are available for proton-transfer reactions with polar molecules. The reagent ion mixture is then suitable for ionizing a wide range of both polar and nonpolar compounds. The critical effect of solvent flow rate is demonstrated here with results for a test analyte, 9-methylanthracene, which may be ionized by either charge exchange or proton transfer. For a solvent of 50:50 methanol/water (v/v), lowering the flow from 200 to 50 microL min-1 results in a 10x increase in charge exchange ionization efficiency--further flow reductions provide even greater enhancements. This method is compatible with sample delivery by direct infusion and micro- and narrow-bore LC, as well as conventional LC using a flow splitter.     

Two-step matrix application technique to improve ionization efficiency for matrix-assisted laser desorption/ionization in imaging mass spectrometry

A novel matrix application protocol for direct tissue mass spectrometry is presented. Matrix-assisted laser desorption/ionization is a popular ionization procedure for direct tissue analysis and imaging mass spectrometry. Usually, matrixes are applied by dispensing droplets through either pipettes or automated dispensing machines, or by airbrushing. These techniques are very simple, but it was difficult to obtain uniform matrix crystals on the tissue surface, and nonuniform crystals degrade the spectrum qualities. Here we report a new matrix application protocol, which is a combination of spraying and dispensing droplets, and we have succeeded in overcoming these problems in conventional matrix applications on tissue surfaces. We call our new technique the "spray-droplet method". In this technique, tiny matrix crystals formed by spraying act as seeds for crystal growth. Our technique leads to matrix spots that are filled homogeneously with minute crystals. Such matrix crystals dramatically improve peak intensity and signal-to-noise ratio. In an example on a rat brain section, the number of detectable peaks was increased and signal intensity of m/z 5440 in our method was approximately 30.6 times higher than that in conventional methods. We used this spray-droplet method with a chemical ink-jet technology for matrix deposition to succeed in MALDI imaging of signals, which were undetectable from the conventional matrix applications.     

光面式高压氢气泄漏快速可视化检测方法及数值仿真研究

针对高压氢泄漏扩散的检测问题,国内外学者利用实验和数值模拟等方法进行了相关研究,提出了一些检测方法,这些检测方法结果可靠,但是普遍存在反映速度慢的不足。本文提出了一种光面式高压氢气泄漏快速可视化检测方法,利用半导体激光器发射激光,通过柱面透镜组及反射镜后形成光面（laser sheet）照射储氢罐,利用安装在储氢罐上方特定位置的CCD相机拍照采集图像并传输至计算机,经数字图像处理后对高压氢气泄漏情况进行鉴别。基于该方法开展仿真实验,建立高压氢气射流的分层流动模型,实验结果与前人研究的实验公式吻合,证明了该方法的可行性;开展了泄漏口直径为1 mm 时3,5,7 MPa压强的仿真实验,以及压强为5 MPa时泄漏口直径为0.5,1,1.5 mm的仿真实验,获得了高压氢气泄漏口直径与特定高度、气射流边界层直径的解析关系,以及高压氢气压力与射流边界层直径的解析关系,为实现高压氢气泄漏快速准确检测提供了有力支撑。     

An analytical model for the prediction of load distribution in highly torqued multi-bolt composite joints

This paper presents the development of an enhanced analytical approach for modelling the load distribution in multi-bolt composite joints. The model is a closed-form extension of a spring-based method, where bolts and laminates are represented by a series of springs and masses. The enhancement accounts for static friction effects between the laminates, a primary mechanism of load transfer in highly torqued bolted joints. The method is validated against detailed three-dimensional finite element models and where possible, experimental results. The effect of varying bolt-torque and bolt-hole clearance on the load distribution in a three-bolt, single-lap joint is investigated and the method proves to be robust, accurate and highly efficient. Finally, the method is employed in a parameter study, where increasing bolt torque levels can be used for achieving a more even load distribution in multi-bolt joints.     

Microfabricated gas chromatograph for the selective determination of trichloroethylene vapor at sub-parts-per-billion concentrations in complex mixtures

A complete field-deployable microfabricated gas chromatograph (μGC) is described, and its adaptation to the analysis of low- and subparts-per-billion (ppb) concentrations of trichloroethylene (TCE) vapors in complex mixtures is demonstrated through laboratory testing. The specific application being addressed concerns the problem of indoor air contamination by TCE vapor intrusion. The μGC prototype employs a microfabricated focuser, dual microfabricated separation columns, and a microsensor array. These are interfaced to a nonmicrofabricated front-end pretrap and high-volume sampler module to reduce analysis time and limits of detection (LOD). Selective preconcentration and focusing are coupled with rapid chromatographic separation and multisensor detection for the determination of TCE in the presence of up to 45 interferences. Autonomous operation is possible via a laptop computer. Preconcentration factors as high as 500?000 are achieved. Sensitivities are constant over the range of captured TCE masses tested (i.e., 9-390 ng), and TCE is measured in a test atmosphere at 120 parts-per-trillion (ppt), with a projected LOD of 40 ppt (4.2 ng captured, 20 L sample) and a maximum sampling + analytical cycle time of 36 min. Short- and medium-term (1 month) variations in retention time, absolute responses, and response patterns are within acceptable limits.