International Journal of Mass Spectrometry and Ion Physics最新文献

A conventional time-of-flight mass spectrometer has been adapted for pulsed laser desorption. This instrumental configuration enables investigation of “time resolved” mass spectra, i.e. delayed mass analysis following the laser pulse. The energy spread of ions formed after the laser pulse is time dependent. To date this has enabled focussing of ions produced by laser desorption up to 3628 amu. Mechanisms of laser desorption and applications of this instrumental configuration are discussed.

Unusual ionic species created in mass spectrometry can be separated from their congeners, isotopically selected, and supplied with well-defined translational energies. Such advantages of precise characterization counterbalance the disadvantage of small ion flux in synthetic experiments which capture these reagents at surfaces, and analyze the products by sensitive surface analytical techniques. Variation of the energy of the transferred ions is reflected in the products formed at the secondary surface. Preparation times of one hour create sufficient material for analysis by SIMS.

Secondary ion mass spectra obtained by bombardment of low-temperature solids such as the rare gases, nitrogen, oxygen, nitrogen oxides, and others with heavy ions of keV energies contain many cluster ion peaks, often with unusual compositions. A mechanism which accounts for the results in a qualitative way is described. It is proposed that it has general mechanistic implications for keV particle-induced desorption mass spectrometry in spite of the obvious difference between the low-temperature solids and the more usual molecular solids. The mechanism has three principal features. First, it is recognized that keV-energy atoms and ions are quite efficient in causing ionization, secondary electron formation, ionizing fragmentation and electronic excitation of molecules within the molecular solid. These processes occur in addition to homolytic bond cleavage within the collision cascade region and lead to local charging within the insulating solid. Some of the damage centers in the solid are ejected essentially immediately from one of the surface layers during the collision cascade period of the impact event. Second, since the chemical nature of the primary damage centers is the same as in MeV-energy bombardment, so are the chemical reactions and charge transfer processes leading to secondary damage, at least in a qualitative sense. Some of the secondary damage products are ejected during the thermal spike regime which follows the collision cascade. The relative importance of the two batches of the secondary ions is a function of the nature of the bombarding ion. Third, polar and/or polarizable molecules will be the best at aggregating with the various resulting charged species during the ejection from the thermal spike region into the vacuum and the best at staying associated with the charged species during subsequent gas-phase fragmentation of the resulting metastable cluster ions in an “evaporating fractionation” process.

Used in conjunction with powerful separation techniques (GLC and HPLC), and micro-chemical manipulations, fast atom bombardment (FAB) mass spectrometry is a powerful method of structure elucidation. This point is illustrated by the recent solution of two structural problems: (i) the determination of the structure of a toxic octapeptide from the larvae of a sawfly, Lophyrotoma interrupta, as PhCO-D-Ala-D-Phe-L-Val-L-Ile-D-Asp-L-Asp-D-Glu-L-Gln and (ii) the determination of the structure of a peptide toxin isolated from the blue-green algae, Microcystis aeruginosa.

A microscopical theory is presented to describe surface ionization of atomic particles desorbed from solid state by impact of energetic primary ions or atoms. On the basis of this general theory analytical expressions are derived for ionization probabilities of particles desorbed from quiescent substrates, from substrates excited by independent collisions, and from thermally excited substrates. The latter approach, which regards the ionization as induced by an electron-hole plasma at a temperature T_e in the collision cascade region, is found to be consistent with most experiments.

Positive ions observed in the ²⁵²Cf Plasma Desorption Mass Spectrum of behenic acid occur at masses consistent with several cations and cation radical derivatives. Fragmentation mechanisms are suggested by the disparate populations of alkyl, allyl, acyl, ketene and olefinic ions.

Psoralen-nucleosides photocycloadducts have been studied by ²⁵²Cf-Plasma Desorption Mass Spectrometry. The present study involves isolation, high performance liquid chromatography purification and mass spectrometry analysis of the two major monoaddition products formed in the photoreaction of the naturally occurring 8-methoxypsoralen with thymidine. Structural informations have also been obtained concerning photocycloadducts of a new linear psoralen derivative 3-carbethoxypsoralen with thymidine and cytidine. A photobinding of 3-carbethoxypsoralen with adenosine has also been studied. The mass spectra of the adducts have been obtained directly without any derivatization. In all cases they showed molecular and quasimolecular ions in the positive and/or negative mode. It is concluded that ²⁵²Cf-PDMS is a powerful and easy technique for studying xenobiotic-modified nucleic acids.

The energy loss of an MeV/amu ion to the electrons of a substance can be as great as keV/Å. With such an enormous amount of energy available, it is not surprising that a wide variety of phenomena can occur. Among them are track formation and induced desorption of molecular ions. A common ingredient in models of such processes is the requirement that the electronic excitation has a sufficiently long lifetime (∼ picoseconds) that energy can be transferred to atomic motion. Recently, several new phenomena have been observed that indicate that atomic motion/chemical rearrangement can occur even when the electronic excitation is shorter lived (∼ femtoseconds). Two of these processes, which are also induced by MeV ion bombardment, are track damage in heavily doped compound semiconductors and greatly enhanced adhesion of metal films to metallic and semiconducting substrates. Neither of these effects can be easily accomodated within the existing theoretical models; thus, it is possible that an even richer variety of ion induced effects will be discovered. Speculations on the form of a new theoretical approach are presented.

At incident current densities on the order of 10⁻⁵A/cm², a period of ∼1–30 s has been observed for nearly constant secondary ion emission to be established from glycerol sample matrices following the onset of sample irradiation with low energy (∼5–7 keV) metal ions. It is further observed that after prolonged irradiation the intensities of the molecular ions of certain compounds increase significantly shortly before the glycerol is depleted. Further study of these effects may assist in elucidating certain aspects of the matrix's role in the process of particle induced secondary ion emission from liquid samples.

One view of the mechanism of ejection of atomic and molecular ions from salts and minerals, and of molecular ions from electrolyte solutions, is that the sputtering process simply desorbs pre-existing ions without alteration of their charge state. This article briefly discusses some current theoretical aspects of this approach for atomic ion emission, and draws some parallels with molecular ion emission.