International Journal of Mass Spectrometry and Ion Physics最新文献

An ion source is described in which molecules having an ionization potential less than 11.8 eV are selectively photoionized. It is shown that this method considerably improves the air contaminant detection limit of a standard quadrupole mass spectrometer and has great potential in the analysis of complex mixtures of trace gases.

A method is described which allows the determination of absolute unimolecular rate constants from ion lifetime measurements using the field ionization technique. For this purpose the molecules are heated to ∼ 800 K prior to ionization by a high electric field. The internal energy distribution of the molecular ions generated in this way is mainly determined by the thermal energy distribution which can be calculated with fair accuracy. The additional small amount of energy transferred during the ionization process can be estimated by comparing the experimental and calculated temperature dependence of the molecular ion. If ions energized in this manner undergo a unimolecular decomposition, the rate of formation of a given fragment can be determined in the time range 10⁻¹¹-10⁻⁵ s after ionization. Knowledge of the internal energy distribution of the precursor and the rate of formation allows the determination of the rate constant as a function of the internal energy. The accuracy of this method is limited by uncertainty in the determination of the energy distribution and the potential distribution in the source. Approximate rate constants are reported for methyl loss from ionized t-butylbenzene and diethylether in the range 10⁴-10⁹ s⁻¹. Reasonable agreement with RRKM calculations and—as far as available—with photo-electron-photoion coincidence measurements is achieved. It is demonstrated that the method allows the determination of very steep k(E) functions, which do not lend themselves to other methods.

Using the technique of laser ionization mass spectrometry, a study of multiphoton ionization (MPI) of atomic iodine and cesium iodide was conducted with a tunable dye laser. One of the objectives was to determine if this technique provides more selective ionization of the components of such a mixture than does conventional electron bombardment. It was found that under intense photon irradiation, CsI fragments to I⁺ by a multistep process. The first step is dissociation of CsI which is followed by multiphoton ionization of the liberated atomic iodine. Because of this mechanism, the atomic iodine in a CsI + I mixture with less than 10% iodine cannot be detected by MPI at the available photon wavelengths (2800–3000 Å). However, operation at wavelengths which preclude dissociation of CsI should greatly improve the selectivity. The cross-section for two-photon excitation of iodine at 3047 Å was determined to be 4 × 10⁻⁵⁰ cm⁴ s⁻¹.

The knowledge of the abundance distribution and intensities of molecular ions has been of special interest for mass spectrographic trace analysis. A double-focussing mass spectrograph has been used to measure the intensities of carbide, oxide and dimer cluster ions for the IVa, IIIb and VIb elements, iron, nickel, silver, tantalum, bismuth and uranium. For some elements a typical alternating abundance distribution of carbide ions, MC_n⁺, was found which is comparable with the distribution of carbon cluster ions in a high temperature plasma.

Whereas singly-charged molecular ions with relative abundances of ⩽ 10⁻² were observed the doubly-charged molecular ions could be measured to the order of magnitude of ⩽ 10⁻⁴.

Triply-charged molecular ions of the investigated elements were not formed in the mass spectra (detection limits ⩾ 10⁻¹⁰). We can conclude that either triply-charged molecular ions are not formed in a high temperature plasma or that their lifetime is lowered with the time-of-flight through the mass spectroscopic system.

In this paper a model for isotopic fractionation in the thermal ionization source is presented. The samples, normally loaded as salts, are assumed to evaporate, in general, as binary vapors of two chemically different forms. The molecular species in the vapor might either dissociate before ionization and the metal might then be ionized; or, alternatively, molecular ions might be generated which then dissociate into metal ions. Whereas isotopic effects during ionization are negligible, such effects have to be considered for the dissociation process.

The dependence of the observed isotope ratio on the chemical form of the loaded sample and on the temperature of the ionization can be explained with this model, whereas the time dependence and the effects of reverse or enhanced fractionation of the observed isotope ratio are readily explained by a generalized Rayleigh distillation equation.

The application of the fraetionation model to the normalization of observed isotope ratios to an internal standard ratio shows the principal limits for the accuracy of normalization. The commonly used normalization techniques and their inherent errors are considered in the light of the fractionation model and an improved normalization formula is presented which uses the concept of the “apparent mass”. Finally, the model is used to propose experimental methods for the accurate determination of non-normalizable isotopic ratios.

Contour maps of metastable peaks are produced by computer-assisted data acquisition and plotted as a function of ion currents of daughter vs. parent ions. Their presentation, evaluation and practical use is demonstrated by analyzing charge separation and other degradation reactions of doubly-charged cations and by identification of sequential reactions in consecutive field-free regions of a double-focusing instrument.

The 70 eV mass spectra of unsubstituted cyclic polyethers, “crown ethers”, from 12-crown-4 (12 membered ring) to 21-crown-7 (21 membered ring) are reported. These compounds exhibit very nearly identical spectra. Preferred secondary structures for these compounds from solution studies are used to explain specific unimolecular rearrangements; that is, the mass spectra of these “three dimensional molecules” is discussed.

Ammonium chloride increases the absolute signal intensity for the intact cations of organic ammonium, pyrylium and phosphonium salts when admixed in 10−10³-fold excess. This signal enhancement results in improved signal-to-noise ratios and is achieved without addition of extraneous ions to the spectrum. The ionization efficiency (secondary ions produced per incident primary ion) is similar for neat and ammonium chloride diluted samples. However, the ionization yield (secondary ions produced per sample molecule) is enhanced by the matrix by four orders of magnitude. Sensitivity, calculated here for a desorption ionization method, is 0.5−1 × 10⁻¹⁰ C μg⁻¹, and is less than that of electron ionization and chemical ionization sources. Spectra of submicrogram amounts of organic species in ammonium chloride are stable and can be recorded continuously for many hours, as compared to less than an hour for the neat compounds. Even subnanogram amounts of sample diluted in ammonium chloride give spectra which last a number of minutes. Matrix-diluted samples may be withdrawn from the spectrometer and stored for days before being reanalyzed without incident. With this matrix, the technique is essentially nondestructive. Total ionization yields for matrix-diluted samples are estimated at 0.01-0.1%. Primary-ion fluxes impinging on matrix-diluted samples exceed static SIMS limits without evidence of beam damage.

The effusion method together with mass spectral analysis is used to study ion/molecule equilibria, involving the ThF₅⁻ ion, and equilibria between neutral species, involving NaThF₅ molecules. The experimental data obtained made it possible to calculate the enthalpies of formation of ThF₅⁻:

• 1.

  Δ H_298,f⁰, (ThF₅⁻) = −2432.4 ±12.5 kJ mol⁻¹ and NaThF₅:

• 2.

  ΔH_298,f⁰ (NaThF₅) = −2339.7 ±13.8 kJ mol⁻¹ and estimate the following enthalpies:

• 3.

  ΔH_298,f⁰ (LiThF₅) = −2373.2±18.0 kJ mol ⁻¹ li]ΔH_298,f⁰ (KThF₅) = −2381.5±15.5 kJ mol⁻¹

• 4.

  ΔH_298,f⁰ (RbThF₅) = −2393.2 ±20.9 kJ mol⁻¹

• 5.

  ΔH_298,f⁰ (CsThF₅) = −2393.7 ±17.2 kJ mol⁻¹