Electron Impact Ion Source Research Articles

Effect of magnetic field in electron-impact ion sources and simulation of electron trajectories

Effect of magnetic field has been studied for the electron-impact ion sources coupled with a quadrupole mass spectrometer. Computer simulation of electron trajectories showed better collimation and more helical rotations under stronger magnetic field, suggesting enhanced ionization efficiency. However, the magnetic field showed different effect on the mass spectrometer signals for different types of ion sources. For the cross beam ion source, where ion beam direction is perpendicular to the magnetic field, the mass spectrometer signals decreased when the magnetic field strength was higher than 20G. For the axial ion source, where both the electron beam and ion beam are on the same axis of the magnetic field, the mass spectrometer signals increased almost linearly with the applied magnetic field intensity. The axial ion source with a magnet intensity of 800G showed about five times improvement in mass spectrometer sensitivity compared with the cross beam ion source.

Simultaneous determination of three residual barbiturates in pork using accelerated solvent extraction and gas chromatography–mass spectrometry

A new method was developed for the rapid extraction and unequivocal determination of barbital, amobarbital and phenobarbital residues in pork. The isolation of the analytes from pork samples was accomplished by utilizing an accelerated solvent extractor ASE 300. The procedure was automatically carried out in series for fat removing and extraction, respectively with n-hexane and acetonitrile pressurized constantly at 10.3 MPa for 30 min. After evaporation, the extracts were cleaned up on a C(18) solid phase extraction (SPE) cartridge and the barbiturates were eluted with hexane-ethyl acetate (7:3), evaporated on a rotary evaporator and derivatized with CH(3)I. The methylated barbiturates were separated on a HP-5MS capillary column and detected with a mass detector. Electron impact ion source (EI) operating in time program-selected ion monitoring mode (SIM) was used for identification and external standard method was used for quantification. Good linearity was obtained in the range from 0.5 microg/kg to 25 microg/kg. Average recoveries of the three barbiturates spiked in pork ranged from 84.0% to 103.0%, with relative standard deviations from 1.6% to 12%. The limit of detection (LOD) was 0.5 microg/kg for the three barbiturates (S/N>or=3). The quantification limit (LOQ) was 1 microg/kg for the three barbiturates (S/N>or=10).

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.

Correlated phase space distributions of ions in an orthogonal time-of-flight mass spectrometer

A prototype orthogonal time-of-flight (oTOF) mass spectrometer equipped with a thermal ionization (TI) source and an electron impact (EI) source was designed and constructed. Low energy ion beams (∼10 eV) were generated and ejected by a two-stage acceleration configuration into the TOF mass analyzer operating in the linear mode. Interfaces between the sources and the analyzer were optimized so as to reduce the contribution of the turn-around time on the final resolving power of the instrument. The concept of space-velocity correlated ion beams is described in this context. Experiments with variable detector positions were performed and spectral line widths of ∼2 ns were achieved at the end of a 45 cm long linear oTOF MS with ∼3.7 keV energy ions. A new theoretical approach is developed for linear phase space distributions of ions at the start of the TOF experiment. Analytical solutions for first and second-order focusing instruments are presented.

Mass Spectrometry Investigations on Electrolyte Degradation Products for the Development of Nanocomposite Electrodes in Lithium Ion Batteries

In the continuing challenge to find new routes to improve the performance of commercial lithium ion batteries cycling in alkyl carbonate-based electrolyte solutions, original designs, and new electrode materials are under active worldwide investigation. Our group has focused on the electrochemical behavior of a new generation of nanocomposite electrodes showing improved capacities (up to 3 times the capacity of conventional electrode materials). However, moving down to "nanometric-scale" active materials leads to a significant increase in electrolyte degradation, compared to that taking place within commercial batteries. Postmortem electrolyte studies on experimental coin cells were conducted to understand the degradation mechanisms. Structural analysis of the organic degradation products were investigated using a combination of complementary high-resolution mass spectrometry techniques: desorption under electron impact, electrospray ionization, and gas chromatography coupled to a mass spectrometer equipped with electron impact and chemical ionization ion sources. Numerous organic degradation products such as ethylene oxide oligomers (with methyl, hydroxyl, phosphate, and methyl carbonate endings) have been characterized. In light of our findings, possible chemical or electrochemical pathways are proposed to account for their formation. A thorough knowledge of these degradation mechanisms will enable us to propose new electrolyte formulations to optimize nanocomposite-based lithium ion battery performance.

Molecular beam mass spectrometer equipped with a catalytic wall reactor for in situ studies in high temperature catalysis research

A newly developed apparatus combining a molecular beam mass spectrometer and a catalytic wall reactor is described. The setup has been developed for in situ studies of high temperature catalytic reactions (>1000°C), which involve besides surface reactions also gas phase reactions in their mechanism. The goal is to identify gas phase radicals by threshold ionization. A tubular reactor, made from the catalytic material, is positioned in a vacuum chamber. Expansion of the gas through a 100μm sampling orifice in the reactor wall into differentially pumped nozzle, skimmer, and collimator chambers leads to the formation of a molecular beam. A quadrupole mass spectrometer with electron impact ion source designed for molecular beam inlet and threshold ionization measurements is used as the analyzer. The sampling time from nozzle to detector is estimated to be less than 10ms. A detection time resolution of up to 20ms can be reached. The temperature of the reactor is measured by pyrometry. Besides a detailed description of the setup components and the physical background of the method, this article presents measurements showing the performance of the apparatus. After deriving the shape and width of the energy spread of the ionizing electrons from measurements on N2 and He we estimated the detection limit in threshold ionization measurements using binary mixtures of CO in N2 to be in the range of several hundreds of ppm. Mass spectra and threshold ionization measurements recorded during catalytic partial oxidation of methane at 1250°C on a Pt catalyst are presented. The detection of CH3∙ radicals is successfully demonstrated.

Acaricidal activity against Panonychus citri of a ginkgolic acid from the external seed coat of Ginkgo biloba

An acaricidal substance extracted from the external seed coat of Ginkgo biloba L. was identified by UV (ultraviolet), IR (infrared), EI-MS (electron impact ion source mass spectrometry), (1)H NMR (nuclear magnetic resonance) and (13)C NMR as 6-[(Z)-10-heptadecenyl]-2-hydroxybenzoic acid (compound 1). Laboratory bioassay on citrus red mite, Panonychus citri (Mcg), showed that compound 1 possessed the following properties. (i) Powerful contact toxicity with an LC(50) of 5.2 mg litre(-1) after 24 h that was similar to that of pyridaben (LC(50) = 3.4 mg litre(-1)) and significantly superior to that of omethoate (LC(50) = 122 mg litre(-1)). Furthermore, its LC(90) was 13.4 mg litre(-1) after 24 h, which is significantly superior to both pyridaben (LC(90) = 69.6 mg litre(-1)) and omethoate (LC(90) = 453 mg litre(-1)). (ii) Quick-acting acaricidal activity. At identical concentrations, compound 1 was much faster-acting than pyridaben or omethoate. (iii) Compound 1 had strong corrosive action on the cuticle of P. citri but no phytotoxicity to plants.

Microscopic high-speed liquid-metal jets in vacuum

The operation of microscopic high-speed liquid-metal jets in vacuum has been investigated. We show that such jets may be produced with good stability and collimation at higher speeds than previously demonstrated, provided that the nozzle design is appropriate and that cavitation-induced instabilities are avoided. The experiments with a medium-speed tin jet (u ∼ 60 m/s, Re=1.8×104, Z=2.9×10−3) showed that it operated without any signs of instabilities, whereas the stability of high-speed tin jets (d=30 μm, u=500 m/s, Re=5.6×104, Z=4.7×10−3) has been investigated via dynamic similarity using a water jet. Such a 500-m/s tin jet is required as the anode for high-brightness operation of a novel electron-impact X-ray source.

Ultra-high resolution mass spectroscopy of boron cluster ions

Boron clusters have recently received considerable attention as a possible solution to the throughput dilemma associated with ultra-low energy (sub keV) p-type source drain extension implants required by cutting edge complimentary metal–oxide semiconductor (CMOS) technology. Boron cluster ion beams contain many masses due to the binomial distribution of the two naturally occurring isotopes (masses 10 and 11) of boron. The broadness of the mass distribution peak in the dispersive plane is further complicated by a plurality of ion states, due to the varying number of hydrogen atoms remaining attached to the borohydride molecule when it is ionized.The B18Hx+ cluster ion mass spectrum from an electron impact ionization source will be analyzed in detail. An ultra-high resolution mass spectrum, exhibiting 1 AMU resolution of a mass 220 cluster ion will be shown. It will be compared to high-resolution spectra of decaborane (B10H14) cluster ions obtained from natural abundance decaborane and from isotopically enriched material.The deconvolution of the binominal distribution from ion states present in the cluster ion beam reveals the hydrogen distribution function. The hydrogen distribution functions as well as the binomial distributions will be presented and discussed. Physical models will be presented that explain the origin of hydrogen distribution function for these high mass borohydride cluster ions.This ultra-high mass resolution is usually unavailable to the ion implant community, however our 120° mass analyzing magnet and the extremely low emittance of the ion beam extracted from the ClusterIon® source coupled with a variable width beam defining aperture and variable width mass defining slits allow for superior mass resolution.

Interpretation of butyltin mass spectra using isotope pattern reconstruction for the accurate measurement of isotope ratios from molecular clusters

The fragmentation patterns of butyltin compounds (mono-, di-, and tributyltin) in an electron impact ion source were studied using an isotope pattern reconstruction algorithm with emphasis on isotope ratio measurements from molecular clusters. For this purpose, standards of natural tin isotope abundance and a (119)Sn-enriched mixture of the three compounds were both ethylated and propylated using sodium tetraalkylborates. The corresponding mass spectra of the various tetraalkyltin compounds prepared were obtained by GC/MS after their extraction with hexane. The results showed that pure interference-free molecular clusters were obtained only for certain R(3)Sn(+) ions where no isobaric overlap with R(2)SnH(+) ions occurred (e.g. BuEt(2)Sn(+) overlaps with Bu(2)SnH(+)). These ions are ideal candidates for accurate Sn isotope ratio measurements, while isotope pattern perturbing interferences are observed for other molecular fragments down to Sn(.)(+). Isotope pattern reconstruction algorithm thus can be used as an analytical tool to ensure the absence of molecular interferences--a requirement for accurate isotope ratio measurements from molecular clusters. The relevance of these studies for the determination of butyltin compounds in environmental samples by isotope dilution GC/MS is also discussed.

Membrane Inlet-based Portable Time-of-flight Mass Spectrometer for Analysis of Air Samples

A miniaturized time-of-flight mass spectrometer with an electron impact ionization source and sheet membrane introduction has been developed. The advantages and features of this mass spectrometer include high sensitivity, simple structure, low cost, compact volume with field portability, and ease of operation. A mass resolution of 400 at m/z 78 has been obtained with a 25 cm flight path length. Under optimized conditions, the detection limits for the volatile organic compounds (VOCs) studied were 0.2-10 ppm by volume with linear dynamic ranges greater than three orders of magnitude. The response times for various VOCs using a silicone membrane of 127 <TEX>$\mu$</TEX>m thickness were in the range 4.5-20 s, which provides a sample analysis time of less than 1 minute. These results indicate that the membrane introduction/time-of-flight mass spectrometer will be useful for a wide range of field applications, particularly for environmental monitoring.

Quantitative Analysis of 39 Polybrominated Diphenyl Ethers by Isotope Dilution GC/Low-Resolution MS

A GC/low-resolution MS method for the quantitative isotope dilution analysis of 39 mono- to heptabrominated diphenyl ethers was developed. The effects of two different ionization sources, electron impact (EI) and electron capture negative ionization (ECNI), and the effects of their parameters on production of high-mass fragment ions [M - xH - yBr](-) specific to PBDEs were investigated. Electron energy, emission current, source temperature, ECNI system pressure, and choice of ECNI reagent gases were optimized. Previously unidentified enhancement of PBDE high-mass fragment ion [M - xH - yBr](-) abundance was achieved. Electron energy had the largest impact on PBDE high-mass fragment ion abundance for both the ECNI and EI sources. By monitoring high-mass fragment ions of PBDEs under optimized ECNI source conditions, quantitative isotope dilution analysis of 39 PBDEs was conducted using nine (13)C(12) labeled PBDEs on a low-resolution MS with low picogram to femtogram instrument detection limits.

Vibrationally resolved rate coefficients and branching fractions in the dissociative recombination of O2+

We have studied the dissociative recombination of the first three vibrational levels of O(2) (+) in its electronic ground X (2)Pi(g) state. Absolute rate coefficients, cross sections, quantum yields and branching fractions have been determined in a merged-beam experiment in the heavy-ion storage ring, CRYRING, employing fragment imaging for the reaction dynamics. We present the absolute total rate coefficients as function of collision energies up to 0.4 eV for five different vibrational populations of the ion beam, as well as the partial (vibrationally resolved) rate coefficients and the branching fractions near 0 eV collision energy for the vibrational levels v=0, 1, and 2. The vibrational populations used were produced in a modified electron impact ion source, which has been calibrated using Cs-O(2)(+) dissociative charge transfer reactions. The measurements indicate that at low collision energies, the total rate coefficient is weakly dependent on the vibrational excitation. The calculated thermal rate coefficient at 300 K decreases upon vibrational excitation. The partial rate coefficients as well as the partial branching fractions are found to be strongly dependent on the vibrational level. The partial rate coefficient is the fastest for v=0 and goes down by a factor of two or more for v=1 and 2. The O((1)S) quantum yield, linked to the green airglow, increases strongly upon increasing vibrational level. The effects of the dissociative recombination reactions and super elastic collisions on the vibrational populations are discussed.

The effect of N2O on the isotopic composition of air‐CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.

Liquid-metal-jet anode x-ray tube

We describe a novel electron-impact x-ray source based on a high-speed liquid-metal-jet anode. Thermal power load calculations indicate that this new anode concept potentially could increase the achievable brightness in compact electron-impact x-ray sources by more than a factor 100 compared to current state-of-the-art rotating-anode or microfocus sources. A first, successful, low-power proof-of-principle experiment is described and the feasibility of scaling to high-brightness and high-power operation is discussed. Some possible applications that would benefit from such an increase in brightness are also briefly described.

Microsolvation of the indole cation (In+) in a nonpolar environment: IR spectra of In+–Lncomplexes (L = Ar and N2, n ≤ 8)

Infrared photodissociation (IRPD) spectra of complexes composed of the indole cation (In+ = C8H7N+) and several neutral ligands (L = Ar and N2) were recorded in the vicinity of the N–H stretch vibration (ν1) of bare In+ in its 2A″ electronic ground state. The analysis of systematic size-dependent ν1 band shifts and photofragmentation branching ratios in the spectra of In+–Arn (n ≤ 5) and In+–(N2)n (n ≤ 8) provides information about the stepwise microsolvation of In+ in a nonpolar hydrophobic environment, including the existence of structural isomers and the determination of ligand binding energies. The IR spectra of the In+–L dimers reveal two transitions, which are attributed to ν1 fundamentals of the H-bound and π-bound isomers on the basis of their complexation-induced ν1 frequency shifts, Δν1. In both cases, the H-bound isomer is found to be more stable than possible π-bound isomers. The Δν1 shifts are used to derive the first experimental estimate for the proton affinity of the indolyl radical (∼920 ± 30 kJ mol−1). The IR spectra of In+–Arn (n ≤ 5) suggest that the preferred microsolvation path for this cluster system begins with the formation of the H-bound In+–Ar dimer core, which is further solvated by (n − 1) π-bound ligands. In contrast, the spectra of In+–(N2)n with n ≤ 8 suggest that this cluster grows by the formation of an In+–(N2)2 trimer core with two H-bound N2 ligands, to which (n − 2) π-bound N2 molecules are attached. The In+–Ln complexes were generated in an electron impact (EI) ion source, which predominantly produces the most stable isomer of each cluster ion. For several In+–Ln complexes, the geometry of the most stable isomer produced in this ion source differs drastically from the structures previously observed by resonant photoionization of the corresponding neutral precursors, demonstrating the severe restriction of photoionization techniques (given by the Franck–Condon principle) for the spectroscopic characterization of cluster ions. Most of the In+–Ln complexes investigated exhibit a distinct ionization-induced change in the preferred substrate–ligand recognition pattern.

Surface-Induced Dissociation of Polyatomic Hydrocarbon Projectile Ions with Different Initial Internal Energy Content

The specific effect of initial internal energy of CH 5 +, C 2 H 4 +, C 2 H 5 +, and C 2 H 6 + ions on the extent of surface-induced fragmentation was investigated. The ions were prepared either in a gas discharge Colutron type (using a hydrogen-methane mixture at 0.2-0.5 Torr) or in a low-pressure Nier-type electron impact ion source using various target gases. Whereas projectile ions from the Colutron source are thermalized due to collisions in the high pressure environment and thus contained a negligible amount of internal energy, projectile ions from the Nier-type ion source resulting from direct electron impact ionization reactions have internal energies up to several eV. Their internal energy content was estimated using break-down curves and photoelectron spectra from literature. Results obtained here show that their different initial internal energy content had a considerable effect on the extent of fragmentation of the surface-excited projectile ions: ions with initial internal energy fragmented at much lower collision energies than internally relaxed projectile ions. It appears that the initial internal energy content of the projectile ions is entirely preserved in the projectile ion during the ion/surface collision, and thus is available in the subsequent dissociative processes as additional energy to the internal energy acquired by the projectile ion in the surface-excitation process.

Focus on Laboratory Equipment

Focus on Laboratory Equipment

Collisions of slow polyatomic ions with surfaces: the scattering method and results

Surface-induced dissociation (SID) and reactions following impact of well-defined ion beams of polyatomic cations C 2H 5OH +, CH 4 +, and CH 5 + (and its deuterated variants) at several incident angles and energies with self-assembled monolayers (SAM), carbon surfaces, and hydrocarbon covered stainless steel were investigated by the scattering method. Energy transfer and partitioning of the incident projectile energy into internal excitation of the projectile, translational energy of products, and energy transferred into the surface were deduced from the mass spectra and the translational energy and angular distributions of the product ions. Conversion of ion impact energy into internal energy of the recoiling ions peaked at about 17% of the incident energy for the perfluoro-hydrocarbon SAM, and at about 6% for the other surfaces investigated. Ion survival probability is about 30–50 times higher for closed-shell ions than for open-shell radical cations (e.g., 12% for CD 5 + versus 0.3% for CD 4 +, at the incident angle of 60° with respect to the surface normal). Contour velocity plots for inelastic scattering of CD 5 + from hydrocarbon-coated and hydrocarbon-free highly oriented pyrolytic graphite (HOPG) surfaces gave effective masses of the surface involved in the scattering event, approximately matching that of an ethyl group (or two methyl groups) and four to five carbon atoms, respectively. Internal energy effects in impacting ions on SID were investigated by comparing collision energy resolved mass spectra (CERMS) of methane ions generated in a low pressure Nier-type electron impact source versus those generated in a Colutron source in which ions undergo many collisions prior to extraction and are essentially vibrationally relaxed. This comparison supports the hypothesis that internal energy of incident projectile ions is fully available to drive their dissociation following surface impact.

Liquid-metal-jet anode electron-impact x-ray source

We demonstrate an anode concept, based on a liquid-metal jet, for improved brightness in compact electron-impact x-ray sources. The source is demonstrated in a proof-of-principle experiment where a 50 keV, ∼100 W electron beam is focused on a 75 μm liquid-solder jet. The generated x-ray flux and brightness is quantitatively measured in the 7–50 keV spectral region and found to agree with theory. Compared to rotating-anode sources, whose brightness is limited by intrinsic thermal properties, the liquid-jet anode could potentially be scaled to achieve a brightness &amp;gt;100× higher than current state-of-the-art sources. Applications such as mammography, angiography, and diffraction would benefit from such a compact high-brightness source.