Double Focusing Sector Field Mass Research Articles

Electron impact study of carbon disulphide micro-clusters

The production of carbon disulphide micro-clusters (CS 2) n , n⩽11, by adiabatic gas expansion of mixture of CS 2 vapour and Ar is studied by using an electron impact ion source and a double focusing sector field mass spectrometer. The appearance potentials for clusters n⩽9 decreased as n increased but was not a simple function of 1/ n. The appearance pressures of observed clusters showed a slowly increasing dependence on cluster size. Results of measurements of appearance potentials and appearance pressures of selected clusters are described for a range of operating conditions.

Precise sulfur isotope ratio measurements in trace concentration of sulfur by inductively coupled plasma double focusing sector field mass spectrometry

T. Prohaska, C. Latkoczy and G. Stingeder, J. Anal. At. Spectrom., 1999, 14, 1501 DOI: 10.1039/A901973A

Determination of rare earth elements U and Th in environmental samples by inductively coupled plasma double focusing sectorfield mass spectrometry (ICP-SMS)

The excellent capability of high resolution inductively coupled plasma sectorfield mass spectrometry for measurements of the rare earth elements Th and U is demonstrated by investigating different materials such as geological matrices (sediments, soils), plant tissues and marine animal tissues. Appropriate digestion of the samples resulted in complex matrices, especially in the case of silicate containing samples. The elemental loss in silicate residues of plant material was found to be up to 30% and therefore required HF-containing digestion methods. The high concentration of matrix elements leads to spectral interferences, which are investigated by measuring the elements with different mass resolution. High mass resolution is shown to be a prerequisite for accurate determination of Sc and Y, respectively. Furthermore, effects of non spectral interferences are investigated and could be properly corrected for by using 115 In as internal standard. Moreover, the capability of a microconcentric nebulizer in combination with a membrane desolvation unit compared to a conventional microconcentric nebulizer is discussed with respect to suppression of spectral interferences. Oxide interferences could be reduced to a negligible amount, whereas it could be observed that high salt freight leads to a blockage of the membrane.

Absolute total and partial electron impact ionization cross sections of hexamethyldisiloxane

We studied the electron impact ionization of hexamethyldisiloxane (HMDSO), Si 2O(CH 3) 6, which is widely used in plasma-enhanced polymerization applications. Appearance energies and absolute partial cross sections for the formation of fragment ions with relative intensities >1% of the most abundant ion, the Si 2O(CH 3) 5 + fragment ion, were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. Dissociative ionization was found to be the dominant process. The main ionization channel removes a complete methyl group to produce the fragment ion Si 2O(CH 3) 5 + with a cross section of 1.7 × 10 −15 cm 2 at 70 eV. The stoichiometric and isotope composition of the various fragment ions was determined by using the high resolution ( m/Δ m = 40,000) of the mass spectrometer used in these studies. The methyl ion is formed with considerable excess kinetic energy, whereas all other fragment ions are formed with essentially no excess energy. The experimental total single ionization cross section of HMDSO (2.2 × 10 −15 cm 2 at 70 eV impact energy) is in good agreement with the result of a semiempirical calculation (2.1 × 10 −15 cm 2 at the same energy).

Speciation of organic selenium compounds by reversed-phase liquid chromatography and inductively coupled plasma mass spectrometry. Part I. Sector field instrument with low mass resolution

The speciation of selenocystine, selenocystamine, selenomethionine and selenoethionine was studied by on-line coupling of high performance liquid chromatography with inductively coupled plasma mass spectrometry with hydraulic high pressure nebulization (HHPN) for sample introduction employing a double-focusing magnetic sector field mass spectrometer in order to achieve the utmost sensitivity. Reversed-phase chromatography was applied to separate the organic Se compounds. The eluent was 30 mm ammonium buffer adjusted to pH 3.0 in methanol-water (5+95 v/v). The operating conditions for chromatographic separation (pH of the mobile phase, methanol content of the mobile phase, pH of the sample and sample uptake rate) were carefully optimized. With the developed procedure, detection limits below 0.02 ng ml –1 were achieved, so that even low pg levels could be detected when applying sample volume of 200 μl.

Dissociative ionization of silane by electron impact

We studied the electron impact ionization of silane (SiH 4) which is widely used in the plasma deposition of different siliconcontaining thin films. Absolute partial cross-sections for the formation of all fragment ions were measured in a high resolution double focusing sector field mass spectrometer with a modified ion extraction stage for electron energies from threshold to 100 eV. No evidence for the formation of stable parent SiH 4 + ions was found in agreement with previous experimental investigations. The single positive fragment ion formation is the dominant ionization process. We observed the following product ions: SiH 3 +, SiH 2 +, SiH +, Si +, H 2 +, and H +. The agreement between our measured absolute partial ionization crosssections and two earlier data sets obtained by different techniques is generally good for the silicon-containing fragment ions taking into account quoted uncertainties of ± 10% to ± 20%, but less satisfactory for the formation of atomic and molecular hydrogen ions which were found to be produced with significant excess kinetic energies, particularly in the case of H +. A comparison of the total SiH 4 ionization cross-section derived from the measured partial ionization cross-sections and a calculated cross-section based on the Binary-Encounter-Bethe (BEB) model showed excellent agreement in the energy range above 30 eV.

Analysis of GaAs using a combined r.f. glow discharge and inductively coupled plasma source mass spectrometer

A radiofrequency (r.f.) glow discharge ion source was coupled to a double-focusing sector field mass spectrometer with reverse Nier–Johnson geometry. The glow discharge cell powered by a 13.56 MHz generator was connected directly to the interface of the mass spectrometer. The r.f. glow discharge ion source operates optimally at an argon pressure of 2.5 hPa and radiofrequency powers of 30 W. With increasing argon pressure more complex mass spectra were observed due to the higher molecular ion formation rate. The analytical performance of r.f. glow discharge mass spectrometry was investigated for the trace elemental analysis of semi-insulating gallium arsenide crystals. Using ICP–MS after matrix separation for a better quantification of multielement determination of trace impurities, detection limits comparable to r.f. GDMS in the low ng/g concentration range are obtained.

Electron impact ionization of tetramethylsilane (TMS)

We studied the electron impact ionization of tetramethylsilane (TMS), Si(CH 3) 4, which is utilized in plasma polymerization applications, using various mass spectrometric techniques. Absolute partial cross-sections for the formation of the parent ion and 15 fragment ions were measured in a high resolution double-focusing sector field mass spectrometer from threshold to 90 eV. Complementary measurements of mass spectral cracking patterns of TMS were also carried out using two quadrupole mass spectrometers. Dissociative ionization of TMS was found to be the dominant process. The Si(CH 3) 3 + ion is by far the most abundant fragment ion with a maximum cross-section of about 1 × 10 −15cm 2. In contrast, the maximum parent ionization cross-section is less than 2% of that value. On the basis of the measured partial ionization cross-sections, mass spectral cracking patterns and appearance energies of the various ionic products, we propose a collision-induced decomposition scheme for the TMS molecule. Three major decomposition channels were identified which all involve the total or partial removal of a methyl group. The agreement of our measured absolute partial ionization cross-sections with earlier data obtained by a different technique is generally poor.

Determination of iodine using a special sample introduction system coupled to a double-focusing sector field inductively coupled plasma mass spectrometer

A procedure for the determination of iodine (especially 129I) in biological materials using double-focusing sector field mass spectrometry with an inductively coupled plasma ion source is described.

Interaction of electrons with C 60+ and C 60−: post-ionization, dissociation and kinetic energy release

We report on the interaction of energetic electrons with size- and charge-selected fullerene ions in the second field-free region of a double-focusing sector-field mass spectrometer. C 60 2+ and C 60 + are formed from C 60 + and C 60 −, respectively. The kinetic energy released in the electron-induced formation of the fragment ions, C 58 2+ and C 58 +, is measured and compared to the kinetic energy released in metastable dissociations.

Appearance and ionization energies of Cz+60-2m and Cz+70-2m ions (with z and m up to 4) produced by electron impact ionization of C60 and C70, respectively

Electron impact ionization of C60 and C70 has been studied as a function of electron energy in the threshold region using a crossed molecular beam/electron beam double focusing sector field mass spectrometer system. Appearance energies have been determined (using a special threshold extrapolating procedure) for Cz+60-2m and Cz+70-2m ions with z ranging from 1 to 4 and m from 0 to 4, respectively. Whereas the different charge states of the parent ions of C60 and C70 have the same appearance energies within the experimental error bars, the fragment ions of C70 show slightly higher values than their C60 counterparts due to an increased kinetic shift. The presently deduced ionization energies for the C60 parent ions are in excellent agreement with previously reported reliable data for Cz+60 ions (previous data are available only up to z is 3). In contrast, these experimental values disagree strongly with a theoretical calculation by Yannouleas and Landman predicting an approximately linear dependence of the ionization energy on the charge state z, i.e., the experimental values deviate strongly from linearity with increasing charge.

Dynamics of Formation and Evaporation of Mixed Alkali Halide Nanocrystals: A Case of Comparable Lattice Energies

Abstract : The relative intensity distribution of the (Rb(n)K(14-n)I(13)(+) mixed nanocrystals containing a 'magic' number of 14 metal cations and 13 iodide anions is examined. These nanocrystals were generated through sputtering of mixed RbI/KI solids using fast atom bombardment, and analyzed by use of a double-focusing sector-field mass spectrometer. The relative intensities of the observed peaks is found to have a 'bimodal' distribution with a maximum near n=7. This observation is explained by assuming: (a) a rate of formation that is proportional to the number of isomers that can be formed for each mixed nanocrystal, and (b) a rate of dissociation that is insensitive to the composition of the mixed nanocrystal. The later assumption leads to the conclusion that the internal energies of these mixed nanocrystals are similar and independent of their composition. The conclusion is supported by the results of Monte Carlo simulation annealing as well as by the similarity of the known lattice energies of KI and RbI bulk crystals.

Dissociative ionization of an O 2 molecule in Ar nO 2 clusters: a new method for monitoring excited states and for studying the dynamics of energy transfer processes in clusters?

The dynamics of energy- and charge-transfer from cluster atoms to an embedded impurity molecule is studied in Ar n O 2 clusters excited and ionized by electron impact ionization. It is shown that the first excited state of teh Ar +* n cluster ion corresponding to the trapped n = 1 exciton (i.e., the 3Σ u Ar * 2 excimer) is quenched by the presence of an oxygen molecule. A simultaneous excitation- and charge-transfer from this state leads to a dissociative ionization of the oxygen molecule within the cluster. The energy released in this process is sufficient for the formation of an O + atomic ion in the first excited state O + ( 2D). The Ar n O + cluster ions so produced decay in the metastable time regime (i.e., several microseconds after the ionization) by the evaporation of up to five Ar monomers. The resulting fragmentation pattern, measured in a double-focusing sector-field mass spectrometer, is compared with results from molecular dynamics simulations. Two fragmentation mechanisms are discussed, i.e.: (i) a mechanism where the charge-transfer reaction O + ( 2D) + Ar → O + Ar + is occurring within the cluster, which is followed by a sequence of curve-crossing transitions leading to the release of kinetic energy and to subsequent fragmentation of the cluster; (ii) a mechanism which involves the radiative decay of the O +( 2D) metastable excited state embedded within the cluster, followed by the contraction of ArO + bonds and by the vibrational excitation and subsequent fragmentation of the cluster.

Production and properties of CO2 cluster anions

Stoichiometric and non-stoichiometric negatively charged CO2 cluster ions have been produced in a crossed neutral cluster/electron beam ion source. The abundance and stability of these ions have been studied with a double focussing sector field mass spectrometer. The observed abundance anomalies (“magic numbers”) in the mass spectra of (CO2) − and (CO2) n O− ions correlate with corresponding small and large metastable fractions of these ions (for loss of one CO2 unit). Variation of the measured metastable fractions as a function ofn are related to corresponding changes in the monomer binding energies. In addition, we have observed for the first time (CO2) n O 2 − ions (i.e. at electron energies above 8 eV with an energy resonance at about 14 eV) and we discuss possible production mechanisms for these ions. Relative electron attachment cross sections have been determined in the energy regime O<E≦20 eV for (CO2) − , (CO2) n O− and (CO2) n O 2 − withn=1 to 20. The shape of the cross section function for (CO2) n O− is strongly dependent on the cluster sizen.

Metastable fragmentation of (C4H1)nC4H+7 cluster ions induced by delayed isomerization reactions of C4H+7

The metastable fragmentation of the cluster ions (C4H10)nC4H+q (q=7 to 10) produced by the electron impact ionization of a neutral n-butane cluster beam has been studied with a double focusing sector field mass spectrometer. For q&amp;gt;7, the usual statistical single-monomer evaporation is the only observable metastable fragmentation channel. Surprisingly, the (C4H10)nC4H+7 cluster ions were found to exhibit a more complex metastable decay pattern, involving not only the single-monomer evaporation, but also two additional fragmentation channels leading to evaporation of 2 to 4 and 4 to 7 (C4H10) monomers, respectively. After studying the dependence of the parent and the fragment ion currents on the electron energy, we concluded that the C4H+7 ionic chromophore of the (C4H10)nC4H+7 cluster ion is produced after internal H− transfer between n-C4H10 and some primary fragment ion (e.g., C2H+5, C2H+3, C3H+3, CH+3, C3H+4 ) of n-butane within the cluster. A mechanism based on the energy storage in metastable isomers of the C4H+7 chromophore ion is suggested to explain the unusual metastable fragmentation pattern of (C4H10)nC4H+7 cluster ions.

Metastable fragmentation of Ne +4 cluster ion initiated by excimer decay

A new metastable decay reaction of the Ne + 4 cluster ion produced by electron-impact ionization has been studied with a double-focusing sector field mass spectrometer. Appearance energy measurements of this decay reaction Ne + 4 → Ne + 2 + 2Ne provide quantitative support in favour of the suggested decay model, i.e. the radiative decay of an excimer dimer embedded in the rare-gas cluster ion is the origin of this new, unusual metastable fragmentation channel leading to delayed ejection of more than one monomer. Moreover, the dependence of the metastable fraction on the time interval between ionization and fragmentation obtained supports this model qualitatively.

Production and stability of oxygen cluster cations and anions, revisited

Stoichiometric and non-stoichiometric, positive and negative oxygen cluster ions (n up to 70) have been produced in a crossed neutral beam/electron beam ion source. The abundance and stability of the ions formed have been analyzed with a double focussing sector field mass spectrometer in a series of experiments. Positive and negative ion mass spectra observed exhibit distinct abundance anomalies, however, at different cluster sizes. Abundance maxima and minima correlate with correspondingly small and large metastable fractions of (O2) + and (O2) − ions, respectively. (O2) + ions may also lose up top=(n−1) monomers by collision induced dissociation with monotonously decreasing probability with increasingp. Metastable fractions determined for (O2) − ions produced with appr. zero eV electrons are in general larger than those for ions produced with appr. 7 eV electrons. (O2) − ions are also observed to decay via autodetachment, with lifetimes increasing with increasing cluster size. Finally, here we were able to prove that an apparent loss of the monomer fragment O (and higher homologues) observed in the metastable time regime is due to ordinary metastable monomer evaporation in the acceleration region. Moreover, we will also present here some new data and interpretation concerning the electron attachment cross section function for O2 clusters.

Metastable decay of Ar n + involving single monomer evaporation and the loss of peculiar numbers of monomers

Production and stability of Ar n +* ions (withn up to 420) formed by electron impact ionization of a supersonic Ar cluster beam were investigated with a double focussing sector field mass spectrometer. The present study confirms previous magic number determinations up to the 4th icosahedral shell. A systematic study of metastable dissociations (monomer evaporation, magic number evaporation) for singly charged cluster ions as a function of cluster size, internal excitation energy and time elapsed since ion formation gives new insight into the ionization process and subsequent reactions of the ions formed. At a well-defined threshold energy ofca. 28 eV, the magic number loss mechanism occurs simultaneously with the well known single monomer evaporation process which proceeds at all energies. The new mechanism is the first known example of cluster ion metastability showing an exponential dependence on time, providing further evidence that the precursor parent cluster ion is produced in a specific energy state.

Metastable decay of singly charged argon cluster ions Ar +n*

Metastable decay of Ar + n * (with 2 ⩽ n ⩽ 83) produced by electron impact ionization of a supersonic Ar cluster beam has been investigated qualitatively (decay mechanism) and quantitatively (metastable fraction) with a double focussing sector field mass spectrometer. Through the use of an additional MS/MS technique it has been possible to distinguish between fissioning and sequential decay mechanisms involving the overall loss of two argon atoms in a metastable dissociation reaction. A systematic study of the apparent metastable reaction rates (ranging from −1 to −10 6 s −1) as a function of cluster size, internal excitation energy (cluster ion production conditions) and time elapsed since the cluster ion production reveals further details of the metastable dissociation process in accordance with recent theoretical predictions.

Electron impact ionization cross sections of molecules: Part I. Experimental determination of partial ionization cross sections of SF 6: a case study

Electron impact ionization of SF 6 has been studied as a function of electron energy from threshold up to 180 eV using a Nier type ion source in combination with a double focusing sector field mass spectrometer (reversed geometry). Absolute partial ionization cross section functions for the production of SF + 5, SF + 4, SF + 3, SF + 2, SF +, S + , F +, SF 2+ 4 and SF 2+ 2 have been determined using an improved experimental method based on ion trajectory calculations. In addition, the total ionization cross section function of SF 6 has been obtained. The present data are compared with previous experimental and theoretical results in order to evaluate the reliability and accuracy of the method used.