Electron Impact Desorption Research Articles

Positron-annihilation-induced ion desorption fromTiO2(110)

We have investigated the positron-stimulated desorption of ions from a ${\text{TiO}}_{2}(110)$ surface. Desorbed O${}^{+}$ ions were detected in coincidence with the emission of annihilation $\ensuremath{\gamma}$ rays. The energy dependence of the ion yields shows that the O${}^{+}$ ions were detected at energies much lower than the previously reported threshold for electron impact desorption corresponding to the excitation energy of Ti($3p$) core electrons. These results provide evidence that core-hole creation by positron annihilation with electrons in the core levels leads to ion desorption.

Speciation analysis in on-line aerosol mass spectrometry

Ambient aerosol particles affect both the earth's climate and human health. Both effects depend on the chemical composition of the particles including the binding state of specific elements. Modern on-line aerosol mass spectrometry is capable of measuring aerosol composition with high temporal resolution, avoiding artifacts often introduced by classical off-line methods. The two most common types of on-line aerosol mass spectrometers, laser desorption/ionization mass spectrometers (LDI-MS) and thermal desorption electron impact ionization mass spectrometers (TD-EI-MS) provide reliable information on the most common ambient inorganic and organic aerosol species with high temporal resolution. However, for less common aerosol species identification with both types of instrument is frequently associated with large uncertainties. Here, we provide an overview of the element speciation capabilities of current on-line aerosol mass spectrometry for both carbonaceous and non-carbon-containing aerosol species. We describe limitations and other issues for this type of on-line aerosol analysis.

Identification of Li-Based Electrolyte Degradation Products Through DEI and ESI High-Resolution Mass Spectrometry

The nature and composition of gel-like organic films forming during the cycling of Li-based cells functioning through a conversion reaction process were investigated. Besides infrared techniques, both desorption electron impact (DEI) and electrospray ionization (ESI) mass spectrometry were used to study the large amounts of films obtained after extended cycling at 55°C. We give direct evidence for the formation, depending on the type of electrolytes used that differ by the nature of either the Li-based salt or solvents (dimethyl carbonate, propylene carbonate, ethylene carbonate, and their mixtures), of either phosphate-ending PEG-(polyethylene glycol) type chains, PEG chains or polypropylene glycol chains with n values ranging from 1 to 9, and also trimethyl phosphate. The reaction schemes involving either electrochemical or chemical processes are proposed to describe the formation of such species. © 2004 The Electrochemical Society. All rights reserved.

Study of Wall Recycling and Conditioning on the Hanbit Mirror Device

The wall recycling effect dominantly appears in the ICRH discharge with ω < ωci in the HANBIT plasma. The methods and evaluation of wall conditioning are described. The progress of wall conditioning is monitored with neutral pressure and plasma parameters. Electron cyclotron resonance–discharge cleaning(ECR-DC) is applied to improve wall conditioning, and then electron impact desorption(EID) by filament heating is utilized in order to desorb the impurities from the wall. The impurities are analyzed quantitatively by quadrupole mass spectrometer(QMA). We also install new baking system by Halogen lamp radiation with 2 kW in the HANBIT central cell. It is also observed that Hα emission reduces after lamp heating. The evolution of neutral pressure profiles are carefully evaluated during discharge and monitored discharge cleaning effect after several hundred of radio frequency(rf) shots. The partial pressure of light impurities much reduced after rf discharges The line integrated density and edge density much decreased after rf shots, while edge temperature increases. After ECR-DC, also line density decreases, but edge temperature much increases. Plasma beta goes up more than three times after 250 rf shots.

Electron-impact desorption of metastable particles from CO films

The impact of monochromatic low-energy electrons (4–52 eV) is found to induce desorption of metastable particles (MP) from multilayer films formed by condensing CO gas on a Pt(111) substrate held at 20 K. The results are compared with metastable production from gaseous CO. Metastable CO (CO*) is identified as the major desorbed species, its desorption is discussed in terms of intramolecular to molecule-surface vibrational energy transfer. Primary electronic excitation proceeds via the states, a′3Σ+, d3Δ, e3Σ−, I1Σ−, and D1Δ which contribute to MP desorption directly or by first decaying in high vibrational levels of the a3Π configuration. The energy threshold of 8.0 eV for CO* desorption indicates that CO in low vibrational levels of the a3Π state does not desorb. Electronic excitation occurs either near the surface of the CO film or in the bulk. In the latter case, excitonic motion to the surface can be followed by CO* desorption.

Synthesis, extraction and electronic structure of Ce@C 2 n

In view of the growing interest in endohedral lanthanide fullerenes, Ce, as a typical+ 4 oxidation state lanthanide element, has been systematically studied. The synthesis, extraction and electronic structure of Ce @ C 2 n are investigated. Soot containing Ce@C 2 n was synthesized in high yield by carbonizing CeO 2-containing graphite rods and are back-burning the CeC 2-enriched cathode deposit in a DC arc plasma apparatus. Ce@C 2 n dominated by Ce@C 82, can be efficiently extracted from the insoluble part of the soot after toluene Soxhlet extraction by pyridine at high temperature and high pressure in a closed vessel. About 60% Ce@C 2 n (2n = 82, 80, 78, 76) and 35% Ce@C 82 can be enriched in the pyridine extract. This fact is identified by desorption electron impact mass spectrometry (DEI MS). The electronic structure of Ce@C 2 n is analyzed by using X-ray photoemission spectroscopy (XPS) of pyridine-free film. It is suggested that the encapsulated Ce atom is in a charge state close to+ 3 and was effectively protected from reaction with water and oxygen by the enclosing fullerene cage. Unlike theoretical expectation, the electronic state of Ce@C 82 is formally described as Ce +3@C 3− 82.

High yield synthesis and extraction of La@C 2n

A higher yield synthesis for lanthanofullerenes has been studied by activating the La 2O 3 containing graphite rod in situ and back-burning the graphite-rich cathode deposit. La@C 2n are efficiently extracted by high temperature toluene (180°C) in a closed vessel, in which a new species La@C 74 is added to the members of the soluble lanthanofullerenes. The toluene extraction is first characterized by desorption electron impact mass spectrometry. The influence of anode components on synthesis is also analyzed by the XRD technique. Furthermore, the EPR spectra change with temperature are also studied. The assignment of octet II peaks in EPR is also discussed.

A REVIEW: ORGANIC GEOCHEMICAL STUDIES BY LASER PYROLYSIS MASS–SPECTROMETRY AND HOW THEY COMPARE WITH FLASH PYROLYSIS MASS–SPECTROMETRY STUDIES

Various methods of laser pyrolysis have been used by researchers to investigate at a molecular level the chemistry of dispersed organic matter. An understanding of molecular composition is important for evaluations of oil generation potential and maturation levels of source rocks. Laser-based pyrolysis-mass spectrometry studies as applied to petroleum source rocks and coals are reviewed. Experiments undertaken to date involve three main techniques: (i) laser ionisation-mass spectrometry; (ii) laser pyrolysis-gas chromatography-mass spectrometry; and (iii) laser desorption-electron im-pact-mass spectrometry. These techniques have had variable degrees of success for detecting structurally significant hydrocarbon products. Typically assessed through comparison with conventional flash pyrolysis data, the relative merits of the different techniques are described.The most powerful analysis is obtained through the combined use of chromatographic separation and mass spectrometry. The emergence of the laser pyrolysis-gas chromatography-mass spectrometry technique offers enormous potential for various geochemical applications. Laser pyrolysis in conjunction with a microscope (micropyrolysis) allows analysis of specific organic entities, thereby avoiding the masking of individual properties through bulk analysis. However, laser micropyrolysis-gas chromatography-mass spectrometry has not been widely accepted because of the complexities inherent in interfacing the required hardware devices and the high cost of laser instruments compared with conventional pyrolysers. Chromatography also adds cost with regard to both instrumentation and analysis time. Unfortunately, the ability of laser ionisation to provide insights into molecular structure is severely limited by a predominance of carbon cluster ions from highly carbonaceous materials.In contrast, the laser desorption-electron impact ionisation routine yields large ion concentrations of aliphatic and aromatic products. Sufficient information has been obtained with this method to distinguish source rocks and coals of different organic composition.

Characterization of N-benzylcarbazole and its metabolites from microsomal mixtures by tandem mass spectrometry.

The metabolism of N-benzylcarbazole (NBC) was studied in vitro using hamster hepatic microsomes to establish whether the corresponding amide is formed. This work was carried out in order to see if the extremely low pka characteristic of such a benzylic amine would allow the formation of the carbonyl derivative. No amide formation was observed. However, a number of metabolic products were detected using HPLC, including the oxidative debenzylation products, namely carbazole and benzaldehyde, together with 2 phenolic isomers of NBC. These products were tentatively characterized by their UV spectra using a rapiscan UV detector connected to HPLC equipment. The structural characterization of these 4 metabolites, together with unchanged substrate, was carried out using desorption electron impact tandem mass spectrometry (DEI-MS/MS) on a hybrid instrument with EBQ1Q2 configuration.

Sublimation of pure C60fullerene and of C60adsorbed on MgO or graphite powders

AbstractThe sublimation of pure C60and the thermal desorption of C60fullerene deposited on MgO and graphite samples have been analyzed by mass spectrometry. The experiments have been performed in the Desorption Electron Impact mode (DEI) using a heating current between 40 and 1200 mA. The desorption from a substrate as well as the C60sublimation is a first order process with an activation energy of about 200 kJ mole−1. This value confirms the computational results obtained for van der Waals interaction. The peak broadening in the case of C60/graphite can be explained by a strong interaction between C60and the graphite substrate which is also predicted theoretically.

Mass spectrometric analysis of prenyl phosphates and their glycosylated forms.

Three different mass spectrometric method suitable for the analysis of polyprenyl and dolichyl phosphates and their glycosylated forms are described. Fast atom bombardment mass spectrometry (FAB MS) of glycosyl monophosphopolyprenols produces negative ions characteristic of the intact molecule. Tandem mass spectrometry of (M-H)- anions allows the determination of masses of both glycosyl and lipid moieties. Thus, for example, FAB-MS/MS of a mixture of native glycosyl monophosphopolyprenols isolated from ethambutol-treated Mycobacterium smegmatis enabled us to detect two novel pentosyl monophosphopolyprenols. Two other methods are proposed for the analysis of prenyl phosphates, as these compounds do not produce fragments in FAB-MS/MS at low collisional energy. By Desorption Electron Impact ionization (DEI) an intense (M-H3PO4)+ ion as well as fragments corresponding to the successive loss of isoprene residues (68 Da) can be observed. Alternatively, Desorption Chemical Ionization yields ions corresponding to the loss of 66, 78 and 98 Da (i.e. of a part or the entire phosphate moiety) of a prenyl phosphate molecule. Tandem mass spectrometry of the (M-H-98)- ion gives a series of intense fragments differing by 68 mass units over the whole mass range.

Analysis of wall conditioning by using plasma impact desorption in the tandem mirror GAMMA 10

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID ( ΔP PID ) was hydrogen ( M = 2) and a small amount of impurity as CO ( M = 28), CH 3 ( M = 15), H 2O ( M = 18) and CO 2 ( M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔP PID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔP PID and the charge-exchange flux was investigated.

Characterization of oligomers of 3,3,3-trifluoro-1-phenylpropyne and 1-phenylpropyne by mass spectrometry

Oligomers of PhCCCF 3 and of PhCCCH 3 have been prepared by heating the alkynes with dibenzoyl peroxide as an initiator. The degree of oligomerization, the average molecular weights and the nature of the end groups were determined using a combination of mass spectrometric techniques including desorption electron impact, fast atom bombardment, field desorption and MS/MS. The contrasting end-group patterns have been interpreted in terms of relative rates and HOMO/SOMO energy gaps.

Comparison of hot cathode and cold cathode ionization gauges

Ionization gauges are commonly used for pressure measurement from the lowest achievable pressures to 10−2 Torr. Two types exist: In the hot cathode gauge (HCG) the source of the ionizing electrons is a thermionic cathode. In the cold cathode gauge (CCG) it is a circulating space charge current of electrons trapped in crossed electric and magnetic fields. Indications of both types of gauge are gas dependent. Relative sensitivities for different gases are not the same for different HCGs or CCGs, or for HCGs compared to CCGs. Both types of gauge are susceptible to contamination. The filament heating power of the HCG is sufficient to cause local outgassing of adsorbed material, and possibly large pressure errors. Each has certain spurious effects. The lower limit of the HCG is set by x‐ray photocurrent and electron impact desorption. The CCG is subject to discontinuities in the current versus pressure characteristic. Starting of the discharge in a CCG may be delayed at ultrahigh vacuum (UHV). Pumping speeds o...

Massenspektroskopie von Thiogibberelllnen

Mass Spectroscopy of Thiogibberellins.The results of the positive ion (electron impact, desorption electron impact, low voltage electron impact, positive ion desorption chemical ionization, — EI, DEI, LEI, PIDCI), as well as negative ion (electron attachment — EA) mass spectra of 4 different types of thio gibberellins 6–9 are compared, and the fragmentation pathways are discussed.

Analysis of adsorbed gas on the interior-surface of the GAMMA 10 tandem mirror by the electron-impact desorption technique

An analysis for adsorbed gas on wall surfaces using an electron-impact desorption (EID) technique was developed and applied to the evaluation of wall conditioning in the GAMMA 10 tandem mirror. The partial pressure rise due to EID (ΔPEID) was measured with a quadrupole mass analyzer installed near an electron emitting filament inserted in the central cell of GAMMA 10. The analysis using this method was performed during the wall-conditioning period and the ΔPEID related to light impurities (M=15, 28, 44) was observed to decrease by an order of magnitude with the progress of the wall conditioning. It was also observed that a good correlation exists between the ΔPEID and the partial pressure rise induced by the plasma discharge (ΔPPID). The desorption efficiency of typically adsorbed gas molecules ξ was estimated by a simple balance equation for the partial pressures. A significant reduction of ξ due to the wall conditioning was observed.

Electron stimulated desorption and conversion of N 2 adsorbed on a tungsten (110) plane

The response of γ-N 2 on W(110) to electron impact has been investigated. The desorption products are principally neutral N 2 and small amounts of neutral N, which may come from decomposition of N 2 in the mass analyzer. N + 2 was not detected. N + is seen for electron energies above 60 eV. Approximately 32% of the initial saturated γ-N 2 layer (in terms of N atoms) is converted by electron impact to chemisorbed atomic N at E _ = 150 eV, corresponding to N/ W ≈ 0.50. It was possible, by thermal desorption after electron impact for different times to follow both the appearance of N and the desorption of N 2. It turns out that the cross section for conversion to adsorbed atomic N is 2.5 × 10 −18 cm 2, virtually independent of N 2 coverage remaining. The ESD signal for N 2 is not proportional to the amount of N 2 remaining on the surface. The data indicate that along with desorption there is conversion to a electron impact desorption inactive state (not atomic N) which however is either N 2 or desorbs thermally like N 2. UPS measurements show that such a new molecular state, with 5σ1π shifted to higher binding energy by ∼ 1 eV is formed by electron impact. The desorption cross section is 1 × 10 −17 cm 2, the conversion to inactive N 2, 8.5 × 10 −18 cm 2. The threshold for N 2 desorpt E - = 7–8 eV, suggesting creation of a 5σ −1 state as the desorption mechanism near threshold. Conversion to adsorbed N occurred at E - ⩾ 12.5 eV within our detection limit (XPS of the N 1s level of atomic N) suggesting a low energy intramolecular N 2 excitation. N + is not formed from chemisorbed N but by Coulomb explosion of N 2, probably after creation of a 3σ −2 hole state. The weak binding of N 2 and the resultant large N 2-W separation are postulated to account for the absence of N + 2 : at the large N 2-W distance in the ground state vertical transitions lead to the attractive part of any ionic curves reached, so that N + 2 is propelled toward the surface and neutralized before it can desorb as an ion. Similarities and differences with the ESD behavior of CO are rationalized in terms of the differences in 5σ and 2π orbital distributions in the two cases.

Laser desorption electron impact: application to a study of the mechanism of conjugation of glutathione and cyclophosphamide

Toward the objective of producing ion radical species from involatile and thermally labile samples, we have combined laser desorption of neutral molecules with electron impact ionization on a time-of-flight mass analyzer with a delayed draw-out pulse. The analytical capabilities of this method are tested in the analysis of isotope labels in the involatile product in a mechanistic study of both the chemical and the enzyme catalyzed reactions of cyclophosphamide with glutathione.

Ion angular distribution of species desorbed from single crystal surfaces by electron impact

The measurement of the angular distribution of desorbing positive ions produced by electron impact desorption (ESDIAD) is of fundamental importance in understanding molecular structure in the chemisorbed layer. In this short review, two applications of ESDIAD to structural problems in the adsorbed layer will be described. Examples of strong chemisorption and weaker physical adsorption effects will be discussed. In addition, interactions between adsorbed species, leading to changes in bonding geometry will be described. The apparatus used for this work allows digitized acquisition of ion angular distributions in the absence of background effects due to soft X-ray emission stimulated by electron impact.

Chemical Identification of a Tumor-Derived Angiogenic Factor

Neoplasms produce substances that induce blood vessel formation (angiogenesis). Fractions from ethanol extracts of the Walker 256 carcinoma were isolated by silica column chromatography and C18 reversed-phase high-performance liquid chromatography. Two of the isolated fractions induced neovascularization when tested in the rabbit corneal micropocket assay. One of the fractions was identified as nicotinamide by desorption-electron impact mass spectrometry, nuclear magnetic resonance spectroscopy, and gas chromatography-mass spectrometry. The second active fraction contained nicotinamide as part of a more complex, as yet unidentified, molecular arrangement. Microgram quantities of commercial nicotinamide induced neovascularization in the corneal micropocket assay and in the chick chorioallantoic membrane assay.