International Journal of Mass Spectrometry and Ion Physics最新文献

A new experimental arrangement for angle-resolved mass spectrometry is described featuring a needle which delivers collision gas to the source slit of a reversed geometry mass spectrometer. In this experiment the scattering angle is defined by pre-collision electrical deflection of the parent ion beam, as contrasted with post-collision selection of the direction of the daughter ion beam. Advantages are rapid access to any desired angle, coupled with an improvement in ion intensity and in angular resolution evident from measured breakdown curves. Collision-induced dissociation spectra of the methanol molecular ion, recorded over a range of scattering angles, agree well with previous results. Experiments which record ions that survive collisions, i.e., scatter without fragmentation, indicate that scattering of polyatomic ions through substantial angles (0–6°) makes a significant contribution to the phenomena underlying angle-resolved mass spectrometry.

The reaction of the model system of C₃H₆ and Cu to produce cationized species C_n,H_m,Cu⁺ in the spark source mass spectrometer proceeds through neutral organic species and Cu⁺. On this basis reactions of Cu⁺, Ag⁺ and Au⁺ with neutral methyl, ethyl and propyl ethanoate, butanone, butanal, 2,4-pentanedione, ethanoic acid and ethanoic anhydride are compared with reactions of similar compounds with laser-generated Cu⁺ in an ion cyclotron resonance spectrometer. In the wake of the spark more fragmentation of the cationized organic species is observed. The simple cationized molecule is not observed if the metal ion has low-lying excited states.

Reactant-ion distributions in ion mobility spectrometry (IMS) with a membrane inlet system have been studied using ion mobility spectrometry/mass spectrometry (IMS/MS). The membrane was able to exclude the water and ammonia components of laboratory air from the carrier gas of the instrument, but not carbon dioxide. Nonporous dimethylsilicone membranes were found to exclude these components more effectively than microporous polypropylene membranes.

Ab initio molecular orbital calculations (6-31G*/MNDO) of the C₃H₄^+· potential energy surface (electronic ground state) reveal the following features. The molecular ions of allene (1) and propyne (2) are separated by substantial potential energy barriers which preclude easy interconversion. The minimal energy requirement path for the 1 ⇌ 2 isomerization proceeds via two successive 1,2-hydrogen migrations and involves the as yet unknown stable, linear C₃H₄^+· ion 5. Isomerization via a direct 1,3-hydrogen migration is, if it is involved at all, energetically less favoured. The ion 5 also serves as the central intermediate for ring closure to the cation radical of cyclopropene (4), which itself is the actual precursor for loss of H^· thus generating the cyclopropenylium ion (3). The complete geometric data of the various C₃H₄^+· isomers and the transition states connecting them are reported.

The gas-phase clustering reactions NO₂⁻· (C₂H₅ONO₂)_n−1 + C₂H₅ONO₂ = NO₂⁻· (C₂H₅ONO₂)n (n = 1–3) NO₃⁻· (C₂H₅ONO₂)_n−1 + C₂H₅ONO₂ = NO₃⁻· (C₂H₅ONO₂)n (n = 1 and 2) have been studied by means of a high-pressure mass spectrometer at temperatures from 200 to 380 K and at total pressures of 0.5–2 torr. It is demonstrated that under the conditions used here these reactions, with the exception of NO₂⁻ + C₂H₅ONO₂ = NO₂⁻· C₂H₅ONO₂, achieve thermodynamic equilibrium above 1 torr. On the basis of the temperature dependence measurements of the equilibrium constants, thermodynamic data have been determined. The following ΔH_n−1⁰,_n values were obtained: NO₂⁻·(C₂H₅ONO₂)_n; ΔH_0,1⁰ =−20.9 (indirectly obtained), ΔH_1,2⁰ = −8.5 and ΔH_2,3⁰ = −7.3 kcal mol⁻¹; NO₂⁻·(C₂H₅ONO₂)_n; ΔH_0,1⁰ = −17.2 and ΔH_1,2⁰ = −7.2 kcal mol⁻¹.

Semiempirical INDO MO calculations were performed for NO₂⁻· (C₂H₅0NO₂)_n = _1,2 and NO₃⁻·(C₂H₅ONO₂)_n = _1,2 to test the structures and binding energies of these systems. The experimental observations are discussed in terms of the structure stabilities of the solvated ions.

A large range of elements has been evaluated for use as cathode, anticathode and anode materials in an oxygen cold-cathode discharge. The criterion for suitability of a particular material/electrode combination was based on the ability to extract from the source an ion beam suitable for high dynamic range SIMS. The effect of materials on long and short term stability and total beam current was investigated. The best materials were found to be A1 (all electrodes), Ni/Fe (cathode) and Ti (anticathode). Magnesium was found to be totally unsuitable as a cathode material in an oxygen discharge and caused extreme instability in both the extracted beam and the discharge. The reasons for beam current instability are related to departure from cylindrical symmetry in the source, and are discussed qualitatively.

A triple-sector mass spectrometer is described in which the ion beam traverses successively a magnetic sector and two electric sectors. The advantages of this arrangement in suppressing artifact peaks and in enabling fast and slow reactions of ions formed in the region between the magnetic sector and the first electric sector are described. The instrument is used to study the fragmentation of CH₄²⁺ ions of low internal energy formed by charge stripping. It is shown that whilst unstable CH₄²⁺ ions react to give CH₂²⁺ and C²⁺, metastable CH₄²⁺ ions give the charge separation reaction leading to CH₃⁺ and H⁺. The lifetime of some CH₄²⁺ ions formed by charge stripping of CH₄^+· is several microseconds.

The fragmentation mechanism of n-butane by low-energy electron bombardment has been studied by means of the ab initio MO method. Optimized geometries of possible n-butane cation conformers, reaction intermediates and fragments have been calculated using the energy gradient technique. The results suggest that the fragmentation to C₁ + C₃ is more favorable than that to C₂ + C₂, when the electron impact energy is at most only a few eV above the ionization threshold. The base peak at m/z 43 has been calculated to be due to the 2-propyl cation. In the course of fragmentation to C₁ + C₃ proton tunneling is expected.

The effective work functions (Φ⁺ and Φ⁻) for the emission of positive and negative ions (M⁺ and X⁻), respectively, from an inhomogeneous surface of a binary salt (MX) such as an alkali-metal halide are investigated on the basis of experimental results achieved in our previous work. Each of the work functions is generally expressed as a function of the thermochemical values of the constitutive elements. Φ⁺ = I + D + D⁰ − 2D⁺ Φ⁻ = A − D − D⁰ + 2D⁻ where I is the first ionization energy of the electropositive atom M, A the electron affinity of the electronegative atom X, D the dissociation energy of the molecule MX, D⁰ the desorption energy (or the heat of sublimation) of MX, and D^±the desorption energy of M⁺ or X⁻.

Electron-impact luminescence spectra were measured for cyclohexanol, acetic acid, pyridine, pyrimidine, benzonitrile, aniline, nitrobenzene, p-methoxy-nitrobenzene, 1-chlorobutane, 1-chloropentane, tetrahydrofuran and anisole. The spectra were analyzed and compared with the literature data of mass spectrometry. It was found that fragment emissions of OH(A²Σ⁺-X²Π), CN(B²Σ-X²Σ), NO(B²Π-X²Π) and HCl⁺(A²Σ-X²Π) can be used for the identification of neutral fragments of OH and H₂O, HCN, NO and HCl, respectively.