Electron Impact Ion Source Research Articles

Production and decay of highly-charged fullerene ions

Highly-charged fullerene ions C60z+ and C70z+ with charge states up to z=7 have been produced in an electron impact ion source of a two sector field mass spectrometer by using ion source operating conditions similar to those used in EBIT sources. The stability of these ions was investigated quantitatively in the two field free regions of the mass spectrometer. It was found that besides C2 evaporation the dominant fission process for ions with charges larger than +2 is the loss of a charged C2+ unit via a super-asymmetric charge separation reaction C60z+ → C58(z−1)++C2+ and C70z+ → C68(z−1)++C2+, respectively. The most important finding from these studies is that this super-asymmetric dissociation reaction proceeds via a three stage reaction sequence involving an electron transfer reaction at the second stage between a receding C2 unit and the remaining highly-charged fullerene cage.

A new concept tandem thermal dissociator/electron impact ionization source for RIB generation

An innovative thermal dissociation/electron impact ionization positive ion source is presently under design at the Oak Ridge National Laboratory for potential use for generating RIBs at the Holifield Radioactive Ion Beam Facility (HRIBF). Because of the low probability of simultaneously dissociating and efficiently ionizing the individual atomic constituents with conventional, hot-cathode, electron-impact ion sources, the ion beams extracted from these sources often appear as a mixture of several molecular sideband beams. In this way, the intensity of the species of interest is diluted. We have conceived an ion source that combines the excellent molecular dissociation properties of a thermal dissociator and the high efficiency characteristics of an electron impact ionization source. If the concept proves to be a viable option, the source will be used as a complement to the electron beam plasma ion sources already in use at the HRIBF. The design features and principles of operation of the source are described in this article.

A positive (negative) surface ionization source concept for radioactive ion beam generation

A novel, versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer ( φ≅ 5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered by continually feeding a highly electropositive vapor through the ionizer matrix. The use of this technique to effect low work function surfaces for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB 6 while enhancing the probability for negative ion formation of atomic and molecular species with low to intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam (RIB) applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in the use at the Holifield Radioactive Ion Beam Facility (HRIBF). The design features and operational principles of the source will be described in this report.

High sensitivity mass spectrometer system for contaminant measurement in high purity gases

A mass spectrometer beam system using a double focusing sector analyzer and an electron impact ion source has been developed for trace analysis. The molecular beam, formed through a focusing glass capillary array, serves as the gas inlet of the system. Closed cycle cryopumps and ion pumps are used to generate the high vacuum. System roughing is achieved using sorption pumps. Clean and oil free vacuum was obtained by nonmechanical pumping. System normal mode sensitivity is about 107 counts/s/Torr. System low pressure mode sensitivity can be three orders of magnitude higher when sample pressures are below 30 mTorr. A parts-per-billion range system detection capability was accomplished. Slow response time for adsorptive species measurements is a major drawback of the sample inlet system.

Optical properties of planar immersion objectives in electron impact ion sources

The ion optical properties of an immersion ion source have been calculated in the presence of electron space charge. The ion source was an electron impact ionizer with planar geometry and a crossed beam configuration, assuming a well defined atomic beam. We present the focal properties and the third-order aberration coefficients, as a function of both the electron beam current and the ion extraction geometry. The image position is relatively unchanged by the electron beam or extraction geometry; however, there are significant changes to the magnification. The aberrations indicate that it is possible to preserve a spatial resolution in a typical imaging ion source of over a 6 mm field of vision. .

A combined thermal dissociation and electron impact ionization source for radioactive ion beam generationa)

The probability for simultaneously dissociating and efficiently ionizing the individual atomic constituents of molecular feed materials with conventional, hot-cathode, electron-impact ion sources is low and consequently, the ion beams from these sources often appear as mixtures of several molecular sideband beams. This fragmentation process leads to dilution of the intensity of the species of interest for radioactive ion beam (RIB) applications where beam intensity is at a premium. We have conceived an ion source that combines the excellent molecular dissociation properties of a thermal dissociator and the high ionization efficiency characteristics of an electron impact ionization source that will, in principle, overcome this handicap. The source concept will be evaluated as a potential candidate for use for RIB generation at the Holifield Radioactive Ion Beam Facility, now under construction at the Oak Ridge National Laboratory. The design features and principles of operation of the source are described in this article.

A high-efficiency positive (negative) surface ionization source for radioactive ion beama)

A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer (φ≡5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered to φ≡1.43 eV by continually feeding cesium vapor through the ionizer matrix. The use of this technique for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB6 while enhancing considerably the efficiency for negative surface ionization of atoms and molecules with intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in use at the Holifield radioactive beam facility. The design features and operational principles of the source will be described in this report.

A combined thermal dissociation and electron impact ionization source for radioactive ion beam generation (abstract)a)

The probability for simultaneously dissociating and efficiently ionizing the individual atomic constituents of molecular feed materials with conventional, hot-cathode, electron-impact ion sources is low and consequently, the ion beams from these sources often appear as mixtures of several molecular sideband beams. This fragmentation process leads to dilution of the intensity of the species of interest for radioactive ion beam (RIB) applications where beam intensity is at a premium. We have conceived an ion source that combines the excellent molecular dissociation properties of a thermal dissociator and the high ionization efficiency characteristics of an electron impact ionization source that will, in principle, overcome this handicap. The source concept will be evaluated as a potential candidate for use for RIB generation at the Holifield Radioactive Ion Beam Facility, now under construction at the Oak Ridge National Laboratory. The design features and principles of operation of the source are described in this article.

A high-efficiency positive (negative) surface ionization source for radioactive ion beam (abstract)a)

A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed and fabricated which will have the capability of generating both positive- and negative-ion beams without mechanical changes to the source. The source utilizes a highly permeable, high-work-function Ir ionizer (φ≡5.29 eV) for ionizing highly electropositive atoms/molecules; while for negative-surface ionization, the work function is lowered to φ≡1.43 eV by continually feeding cesium vapor through the ionizer matrix. The use of this technique for negative ion beam generation has the potential of overcoming the chronic poisoning effects experienced with LaB6 while enhancing considerably the efficiency for negative surface ionization of atoms and molecules with intermediate electron affinities. The flexibility of operation in either mode makes it especially attractive for radioactive ion beam applications and, therefore, the source will be used as a complementary replacement for the high-temperature electron impact ionization sources presently in use at the Holifield radioactive beam facility. The design features and operational principles of the source will be described in this report.

Mobilities of Aromatic Ions Drifting in Helium

Aromatic ions are produced in an electron impact ion source and injected into the flow tube section of a selected-ion flow-drift tube (SIFDT) apparatus. The reduced mobilities of C6H5+, C6H6+, C6H7+, C12H12+, C12H11+, naphthalene (C10H8+) cation, and biphenyl (C12H10+) cation drifting in helium are measured as a function of the characteristic parameter E/N (the ratio of the electric field to the number density of the helium buffer gas atoms in the drift tube). The reduced mobilities of the benzene dimer ions (C12H12+ and C12H11+) are compared with the measured mobilities of naphthalene and biphenyl cations to correlate the mobilities with ionic structures. An angle-averaged collision cross section of the ions drifting in helium is obtained from the geometric structure of these ions and is used to compute the zero-field mobility. The predicted zero-field mobilities are found to compare well with the experimentally measured mobilities.

Surface ionization mass spectrometry of drugs in the thermal and hyperthermal energy range — a comparative study

Thermal and hyperthermal surface ionization (SI) mass spectra of nicotine, caffeine and lidocaine were obtained using a rhenium oxide surface. Thermal surface ionization was studied on an oxidized surface positioned inside an electron impact ion source, while hyperthermal surface ionization (HSI) was obtained upon seeding the compounds into a hydrogen or helium supersonic molecular beam that scattered from the rhenium oxide surface. Both HSI and SI provide rich, informative and complementary mass spectral information. The results indicate that SI follows thermal dissociation processes on the surface prior to the desorption of the ion, while in HSI no thermal equilibrium is established and the ionization process is impulsive, followed by mostly unimolecular ion dissociation. HSI mass spectra are similar to electron impact mass spectra in the fragment ion masses, but the observed relative intensities are different. HSI is a softer ionization method compared to SI, and enables the degree of ion fragmentation to be tuned so that it can be minimized to a low level at low molecular kinetic energy. In SI, limited control over the degree of fragmentation is possible through the surface temperature. The analytical mass spectrometric applications of SI and HSI are briefly mentioned.

New Approach for the Analysis of Acidic Pesticides in Water by LC/MS with a Particle Beam Interface

A new method for the determination of 18 acidic pesticides in water by liquid chromatography/mass spectrometry is presented. A liquid-solid extraction procedure for the isolation of the pesticides from the aqueous samples was employed. The method, which avails of a micro-flow rate particle beam interface, offers improved performance for the analysis of heat-sensitive compounds such as phenoxy acid herbicides. The new technique combines the benefits of a reduced mobile-phase flow rate (1-5 μL/min) with a modified vaporization surface inside the ion source of the mass spectrometer. The electron impact ion source was covered with a Teflon layer, which has been proven to minimize compounds decomposition and adsorption. The improved vaporization of thermally unstable substances, accomplished by the modified surface, allows a higher signal response and an accurate reproduction of the original chromatographic profile. Electron impact ionization allows almost unambiguous identification of the analytes. The liquid chromatography, carried out with a C18 packed capillary column, was optimized to allow the injection of larger sample volumes, thus improving method sensitivity for trace level pollutants. The detection limits were in the range of 0.1-1 ppb, which is a considerable improvement in the analysis of these compounds with mass spectrometric detectors.

Electron impact mass spectrometry of alkanes in supersonic molecular beams

The electron impact mass spectrometry of straight chain alkanes C8H18-C40H82, squalane, methylstearate, 1-chlorohexadecane, 1-bromohexadecane, and dioctylphthalate was studied by sampling them with supersonic molecular beams. A fly-through Brink-type electron impact ion source was used, utilizing a vacuum background ion filtration technique based on differences between the kinetic energy of the supersonic beam species and that of thermal molecules. The 70-eV electron impact mass spectra of all the alkanes were characterized by a pronounced or dominant molecular weight peak together with all the fragment ions normally exhibited by the standard thermal 70-eV EI mass spectra. In contrast, the NIST library of most of these molecules did not show any molecular weight peak. By eliminating tile intramolecular thermal vibrational energy we gained control over the degree of molecular ion fragmentation by the electron energy. At an electron energy of 18 eV the molecular ion dissociation was further reduced considerably, with only a small absolute reduction in the peak height by less than a factor of 2. The effect of vibrational cooling increased with the molecular size and number of atoms. Pronounced differences were observed between the mass spectra of the straight chain triacontane and its branched isomer squalane. Similar mass spectra of octacosane (C28H58) achieved with 70-eV EI in a supersonic molecular beam were obtained with a magnetic sector mass spectrometer by using an electron energy of 14 eV and an ion source temperature of 150 °C. However, this ion source temperature precluded the gas chromatography-mass spectrometry (GC-MS) of octacosane. The GC-MS of alkanes was studied with an ion trap gas chromatograph-mass spectrometer at an ion source temperature of 230 °C. Thermal peak tailing was observed for C20H42 and heavier alkanes, whereas for C28H58 and heavier alkanes the severe peak tailing made quantitative GC-MS impractical. In contrast, no peak tailing existed even with C40H82 for GC-MS in supersonic molecular beams. The minimum detected amount of eicosane (C20, H42) was shown to be 60 fg. This was demonstrated by using single ion monitoring with the quadrupole mass analyzer tuned to the molecular weight peak of 282 u. The coupling of electron impact mass spectrometry in supersonic molecular beams with hyperthermal surface ionization and a fast GC-MS inlet is briefly discussed.

Isomerism and Beckmann Rearrangement Reactions of α-Benzilmonoxime. A Comparative Kinetic and Stability Study

Abstract The isomerism of α-benzilmonoxime I is investigated by the measurements of their mass spectra and the integrated ion current curves at temperature 200°C. Evidences of isomerisation and Beckmann rearrangement reactions of I under electron impact source are given. The high stability of solid I on storage is explained on the basis of hydrogen bonding. A comparative stability constant as measured kinetically in term of half life period between I and benzophenone oxime IV is investigated.

A differential retarding potential method for improving ion-beam kinetic energy resolution

We present a differential retarding potential (DRP) method for improving the kinetic energy resolution of a reactant ion beam for scattering experiments. This method allows ion-molecule reaction absolute total cross-section measurements to be performed down to thermal energies using the simple electrostatic aperture ion lenses of a tandem quadrupole mass spectrometric ion-molecule reaction apparatus, even though the reactant ions are formed originally with a broad kinetic energy distribution. To illustrate the principle of the DRP method, examples are given for its application to reactant ion beams prepared in an electron impact ion source and in an ion-molecule reaction ion source.

Low-Pressure Inductively Coupled Plasma Ion Source for Molecular and Atomic Mass Spectrometry

A low-pressure inductively coupled plasma system is described which can be used for both molecular and atomic mass spectrometry. Element-selective detection of organotin, organolead, organoiron, and organohalide compounds was possible using this source coupled with gas chromatography. Detection limits were as follows: tetraethyllead, 13 pg; tetrabutyltin, 35 pg; ferrocene, 33 pg; iodobenzene, 25 pg; bromobenzene, 50 pg; and chlorobenzene, 500 pg. By decreasing the power and plasma gas flow it was possible to sustain a helium plasma using only the carrier gas from the gas chromatograph, and mass spectra were obtained for chlorobenzene, bromobenzene, and iodobenzene that were similar to those obtainable using an electron impact ionization source

Factors controlling precision and accuracy in isotope-ratio-monitoring mass spectrometry.

The performance of systems in which picomole quantities of sample are mixed with a carrier gas and passed through an isotope-ratio mass spectrometer system was examined experimentally and theoretically. Two different mass spectrometers were used, both having electron-impact ion sources and Faraday cup collector systems. One had an accelerating potential of 10kV and accepted 0.2 mL of He/min, producing, under those conditions, a maximum efficiency of 1 CO2 molecular ion collected per 700 molecules introduced. Comparable figures for the second instrument were 3 kV, 0.5 mL of He/min, and 14000 molecules/ion. Signal pathways were adjusted so that response times were <200 ms. Sample-related ion currents appeared as peaks with widths of 3-30 s. Isotope ratios were determined by comparison to signals produced by standard gases. In spite of rapid variations in signals, observed levels of performance were within a factor of 2 of shot-noise limits. For the 10-kV instrument, sample requirements for standard deviations of 0.1 and 0.5% were 45 and 1.7 pmol, respectively. Comparable requirements for the 3-kV instrument were 900 and 36 pmol. Drifts in instrumental characteristics were adequately neutralized when standards were observed at 20-min intervals. For the 10-kV instrument, computed isotopic compositions were independent of sample size and signal strength over the ranges examined. Nonlinearities of <0.04%/V were observed for the 3-kV system. Procedures for observation and subtraction of background ion currents were examined experimentally and theoretically. For sample/ background ratios varying from >10 to 0.3, precision is expected and observed to decrease approximately 2-fold and to depend only weakly on the precision with which background ion currents have been measured.

Fragment Ion Formation in Resonance Enhanced Multiphoton Ionization (REMPI) of n-Propyl Phenyl Ether in a Supersonic Jet

Resonance enhanced multiphoton ionization (REMPI) mass spectra of different conformational isomers of n-propyl phenyl ether at 30 K give the same fragmentation patterns. Laser REMPI excitation spectra exhibit three principal conformers in a supersonic free jet expansion. The most abundant species in the jet happens also to contribute the longest wavelength 0,O band. SCF calculations suggest that the lowest energy structure corresponds to a gauche geometry in which all the carbons except that of the methyl group are essentially coplanar with the oxygen. This is confirmed by experimental observation of a predicted blue shift for a prominent vibrational overtone when the propyl group is partially deuterated (0,/3-d2 or a,a,T,T,T-d5). Time-of-flight mass spectra of deuterated analogues of each of three conformers exhibits propene expulsion to yield PhOH'+ and PhOD'+ (the principal fragment ions) in which all seven alkyl hydrogens have become randomized within the chain. REMPI of individual conformers (via intermediacy of the lowest vibrational levels of their excited singlet electronic states) therefore gives the same outcome as does field ionization or electron impact source mass spectra. The predominant decomposition mechanism of the radical cation involves an ion-neutral complex, nPrOPh*+ -. (iPr+ PhO'), in which the hydrogens of the iPr+ undergo rapid internal transpositions prior to the ultimate decomposition step. Ab initio computations on a model system concur with the experimental inference that this mechanism operates regardless of the conformation of the precursor neutral.

Determination of Acidic and Basic/Neutral Pesticides in Water with a New Microliter Flow Rate LC/MS Particle Beam Interface

A new method for the determination of 32 base/neutral and 13 acidic pesticides in water, which has been proven to be particularly amenable to high-performance liquid chromatography, is discussed. A liquid/solid extraction procedure for the isolation of pesticides from aqueous samples is employed. The LC/MS analysis is carried out with a LC-18 reversed-phase packed capillary column coupled with a Hewlett-Packard 5989A quadrupole mass spectrometer using a new microliter flow rate particle beam interface. Mobile-phase flow rates, ranging between 1 and 5 μL/min, pass into the electron impact ion source of the mass spectrometer

Formation and fragmentation of C n + clusters produced from a C60/C70 mixture in an electron-impact ion source

Various fullerene ions are generated in a standard plasma ion source from a vaporized mixture of C60/C70. Except C60+ and C70+, the fullerene ions are formed by fragmentation of C60 or C70 excited by electron impact. Information on the structure and stability of the fullerene ions is obtained by studying unimolecular dissociation and collision-induced fragmentation of C60+, C58+ and C56+ in H2 and Ar target gases.