International Journal of Mass Spectrometry and Ion Physics最新文献

Electronic relaxation via repulsive excited states in molecular solids is proposed as a mechanism for desorption of large molecules.

Fast heavy ions from the Uppsala EN-tandem accelerator, as well as fission fragments from a ²⁵²Cf-source, have been used to produce quasi-molecular ions of proteins in the molecular weight range 1000–14000. Measured time-of-flight spectra show contributions of neutrals, metastable ion decay products, Coulomb breakup of multiply charged ions and polymer ions. The mechanism for production of these quasi-molecular ions will be discussed within the framework of an ion-track model.

A classical dynamics model is used to investigate nuclear motion in solids due to bombardment by energetic atoms and ions. Of interest are the mechanisms of ejection and cluster formation both of elemental species such as Ni_n and Ar_n and molecular species where we have predicted intact ejection of benzene-C₆H₆, pyridine-C₅H₅N, napthalene-C₁₀H₈, biphenyl-C₁₂H₁₀ and coronene-C₂₄H₁₂. The results presented here show that the energy distributions of the parent molecular species, e.g. benzene, are narrower than those of atomic species, even though the ejection processes in both cases arise from energetic nuclear collisions. The bonding geometry also influences the ejection yield and angular distribution. The specific case of π-bonded and σ-bonded pyridine on a metal surface is discussed with comparisons between the calculated results and experimental data. These calculations provide a means of interpreting SIMS, FABMS and possibly even PDMS experimental data.

Desorption ionization (DI) involves the transfer of material from a condensed phase to a collision-free environment (ref. 1,2). Tandem mass spectrometry (ref. 3), used with desorption ionization, improves the signal-to-noise ratio for spectra of individual analytes present in complex matrices, provides evidence that fragmentation in DI is typically due to gas phase dissociations of energized but intact molecular ions after they have left the surface, and allows the compositions of desorbed ions to be characterized. A complementary approach to improving analytical performance and obtaining further information on the species and processes of desorption ionization is to be found in the examination of the sample in the presence of matrix materials. Some matrices act as reagents which yield an appropriate ionized form of the analyte (ref. 4), either during or prior to energization of the sample, while others serve to isolate analyte molecules and reduce intermolecular analyte reactions (ref. 5). Particularly complex matrices are those encountered when examining samples directly from chromatographic materials or in their natural state, for example, crude extracts of plant materials. Examples of analyses in these situations are given.

Ammonium chloride acts as a valuable matrix material which, even at sample dilutions of 10³, can cause an increase in both absolute secondary ion yields and in spectral persistence (ref. 6,7,8). This matrix has beneficial effects in SIMS, FAB and LD mass spectra and has the advantage of being totally transparent except under high flux conditions. It is shown to decrease ion internal energies, presumably by providing a sputtered ion with a shield of solvating molecules which are readily lost as NH₃ and HCl, thereby carrying away excess energy. Cluster ions [(NH₄)_n+1Cl_n]⁺ are observed in FAB and shown by MS/MS to undergo ready desolvation. These cluster ions are remarkable for the absence or low intensity of clusters where the total number of anions and cations is a prime number and for the high intensity of clusters which may be made up of regular arrays of atoms, e.g., 3×3×3 or 3×3×5.

A qualitative model of desorption ionization, advanced some years ago (ref. 9), accommodates the observations reported here using MS/MS and matrix effects. The chief features of this model are (i) isomerization (loss of identity) of the input energy, (ii) desorption of preformed ions or intact molecules, (iii) ion/molecule reactions such as cationization occurring in the selvedge region, (iv) dissociation of energetic (metastable) ions well-removed from the surface. In most cases just a few types of ionic species are sputtered from the surface and their unimolecular chemistry determines the chief features of the desorption ionization mass spectrum.

Applications of ²⁵²Cf-PDMS to a variety of chlorophyll problems are described. This form of heavy particle induced desorption mass spectroscopy is highly suitable to the mass measurement of nonvolatile, thermally unstable chlorophylIs and chlorophyll derivatives. ²⁵²Cf-PDMS has been applied to the characterization of the products of the classical chlorophyll allomerization reaction, and to the synthesis of linked chlorophyll special pairs designed to mimic the in vivo P700 photoreaction center. The applications of ²⁵²Cf-PDMS to the characterization of chlorophyll aggregates and to the examination of the redox properties of the chlorophylls are also described.

The yield of molecular ions desorbed from a valine sample by fast heavy ions (³²S, ¹⁶O, ¹²C) has been investigated as a function of the primary ion parameters mass, energy and charge state. The measurement indicate that primary ions initiate the desorption only within a thin surface layer (a few molecular layers).

A summary of the ion formation processes in laser mass spectrometry is presented. Particular emphasis is placed on the process of solid state chemical ionization. Recent results on the direct analysis of materials using LMS are also presented.

Heavy ion induced plasma desorption mass spectrometry was used with a micro-beam in a scanning mode. A 10 μm diameter 84 MeV ⁸⁴Kr⁺⁷ beam was used for the desorption. A target of NaCl and KCl microcrystals was scanned relative to the beam. Mass spectra were obtained as a function of position on the sample.