Ion Intensity Distributions Research Articles

Mixed cluster ions as a structure probe: Experimental evidence for clathrate structure of (H2O)2H+ and (H2O)21H+

First direct experimental evidence for clathrate structures of (H2O)nH+ (n=20,21) is reported based on a technique allowing the number of nonhydrogen-bonded surface hydrogens to be counted. Neutral clusters (H2O)n⋅((CH3)3N)m, prepared in a pulsed nozzle supersonic expansion, are ionized by multiphoton ionization and investigated with a reflectron time-of-flight mass spectrometry technique. The magic numbers (n,m) in the ion intensity distributions of (H2O)n⋅((CH3)3N)m⋅H+ studied under various experimental conditions are well correlated to the stable hydrogen-bonding structures. For the mixed cluster ion (H2O)20⋅((CH3)3N)m⋅H+, the intensity distribution displays an abrupt intensity drop after the magic number at (20,11), while for (H2O)21⋅((CH3)3N)m⋅H+ the magic number appears at (21,10). The findings provide experimental evidence for a stable clathrate structure of (H2O)20H+, with the proton residing on the surface, while for (H2O)21H+, the H3O+ ion is encaged inside the clathrate structure of (H2O)20; the latter structure provides a total of 10 hydrogen-bonding sites for (CH3)3N.

Isomeric tripeptides: A study on structure-spectrum relationship

A systematic investigation of the collisional activation and metastable ion spectra of protonated and deprotonated isomeric tripeptides, obtained in a fast atom bombardment ionization process, confirms the validity of recently described rules for sequencing. It also reveals regularities in the intensity distribution of the immonium ions in connection with the position and specific properties of the corresponding amino acids. These findings can be used to question or confirm a proposed sequence.

An ion-optical bench for testing ion source lenses

An ion-optical bench has been designed and constructed to obtain experimental data on the focusing properties of ion lenses in three dimensions. The essential part of the apparatus is a position-sensitive detector (PSD) which gives output signals proportional to the x and y positions of each ion impact. The position signals can be displayed on an oscilloscope screen and analyzed using a two-parameter pulse—height analyzer, thereby giving a visual picture of the ion beam cross-section and intensity distribution. The PSD itself is mounted on a track and is movable during operation from a position following the ion lens to a distance of 30 cm away. This enables the rapid collection of accurate data on the intensity distribution and divergence angles of ions leaving the source lens. Examples of ion lens measurements are given.

Studies in cluster ion formation. Part I. Fast atom bombardment mass spectrometry of tetraalkylammonium halides: Factors influencing cluster ion size and stability

AbstractPositive and negative ion fast atom bombardment mass spectra of tetraalkylammonium halide salts (NR4X, where X = Cl, Br, I and NR4 = NMe4, NEt4) have been studied and intense cluster ion formation has been observed. The cluster ion intensity distributions were found to show enhancements at certain cluster numbers (n). The negative cluster ions of NMe4X salts showed anomalous ion intensity regions, which differed from both the positive cluster ions of all NR4X salts and also the corresponding negative clusters of NEt4X salts. The influence of anion and cation size on cluster ion formation and abundances has been studied and it has been established that smaller anion and cation size favours the formation of larger cluster ions. The possible structures of the cluster ions exhibiting relative increased stabilities are discussed.

SIMS depth profiling of multilayer metal-oxide thin films — improved accuracy using a xenon primary ion

Multilayer thin films containing silver and copper, sandwiched in a metal oxide, have been depth profiled by secondary ion mass spectrometry (SIMS) using primary ions of differing mass, energy and chemical reactivity. These results were compared for accuracy with those obtained by Rutherford backscattering spectrometry (RBS). The use of O 2 + or O − as primary ions resulted in severe distortion of the silver ion intensity distribution in the SIMS profile of a ZnO/Ag/ZnO thin film on glass; O 2 + bombardment at energies from 3–10 keV resulted in the detection of silver at the glass interface, while the use of O − caused the silver to be detected closer to the outer surface than expected from RBS results. Primary beams of Ar + and Xe + gave progressively more accurate results for the Zn/Ag/Zn distribution; Xe + at 5.0 keV energy produced profiles that agreed within 10% of RBS-derived values. The same beam conditions, used to profile a double silver layer in ZnO, resulted in some discrepancy in the position of the inner layer, compared to RBS results, and this was attributed to an enhanced sputter rate in the oxide under the outside metal film. Depth profiling of TiO 2/Cu/TiO 2 films with oxygen also resulted in significant distortion of the perceived position of the metal layer, and this was again significantly improved using Xe + primary ions of 6–9 keV energy. The distorting effects of oxygen bombardment can be understood in terms of a migration of metal ions in an electrostatic field created by a charged surface. Ionization of the metallic layer is enhanced by the use of oxygen. By contrast, the use of rare gases reduces the production of ions from the metallic layer which can migrate prior to the onset of sputtering.

Characterization of Bacteria by Particle Beam Mass Spectrometry

A technique is described for detecting and characterizing bacteria on a single-particle basis by mass spectrometry. The method involves generation of a particle beam of single whole cells which are rapidly volatilized and ionized in vacuum in the ion source of a quadrupole mass spectrometer. The particle beam can be generated, with minimal sample handling, from a naturally occurring aerosol or from a solution of bacteria that can be dispersed as an aerosol. The mass spectrum is generated by successively measuring the average intensities of different mass peaks. The average intensity is obtained by measuring the ion intensity distribution at the particular mass (m/e) for ion pulses from more than 1,000 bacteria particles. Bacillus cereus, Bacillus subtilis, and Pseudomonas putida samples were analyzed to test the capability of the instrument for differentiating among species of bacteria. Significant ion-intensity information was produced over the m/e range of 50 to 300, an improvement over previous pyrolysis-mass spectrometry results. The complex mass spectra contained a few unique peaks which could be used for the differentiation of the bacteria. A statistical analysis of the variations in peak intensities among the three bacteria provided a quantitative measure of the reproducibility of the instrument and its ability to differentiate among bacteria. The technique could lead to a new rapid method for the analysis of microorganisms and could be used for the detection of airborne pathogens on a continuous, real-time basis.

Ultra-High Mass Spectrometry

Massive cluster ions of CsI (mass-to-charge ratio > 25,000) have been produced, mass analyzed, and detected with a conventional double-focusing mass spectrometer. Variations in the ion intensity distributions depend on the relative lifetime of the ions. These results are of fundamental interest, but they also impact the practical limits of mass spectrometry.

Secondary ion mass spectrometry of metal salts: Polyatomic ion emission

The mass and intensity distribution of cluster ions emitted from several alkali halide (MX) salts bombarded with Ar + and Xe + ions are reported. Cluster ions of the type [M(MX) n] + and [X(MX) n] − are seen where 1≤ n < 100. We present evidence supporting the formation of cluster ions with specific geometric structures. In addition, we find that the stability of the cluster ions is dependent on the surface energy of the cluster, on the size of the constituent atoms, and on the effective lifetimes of the ions.

Fingerprint spectra in Laser Microprobe Mass Analysis of titanium oxides of different stoichiometry

Micrometric particles of powdered titanium oxides of different stoichiometry (TiO 2, Ti 2O 3 and TiO) are examined by Laser Microprobe Mass Analysis (LAMMA). The stoichiometry of the compound can be correlated with the relative intensity distributions of the (positive or negative) cluster and atomic ions. Application of the “valence model” yields similar conclusions. Variations in laser energy density and in ion lens potential also effect the ion intensity distributions.

Secondary ion mass spectrometry depth profiling and simultaneous electrical investigation of MOS structures

A comparative study of the electrical behaviour of MOS structures and the composition of the oxide layer was carried out by means of secondary ion mass spectrometry depth profiling and simultaneous electrical measurements. The width of the SiSiO 2 interface region was determined for wet, dry and HCl oxide layers using a special method for definition of the virtual SiSiO 2 interface of the depth profiles. A good correlation was found between the interface region width and the electrical parameters such as fixed oxide charge density and minimum surface state density for the samples analysed. The secondary ion intensity distributions of different contaminating species show a maximum at a distance of 20–80 Å from the interface. The incorporation of chlorine during HCl oxidation was monitored and a direct identification of migrating sodium ions in the HCl oxide layers was accomplished by comparing 23Na + depth profiles before and after positive bias- temperature stress.

Factors affecting beam shape and quality

The beam shape and quality are affected by hash, ripple, ion intensity distribution and geometrical and operational factors. Hash, which was found to be localized mainly at the sides of the beam (parasitic divergence β), must be stopped by an appropriate diaphram to improve the image quality. The magnitude of β is strongly dependent on the relative hash. Of all the source parameters, the magnetic field is the dominant factor in determining hash. Ripple in the cathode or arc power supplies of the source leads to a modulation of the beam divergence and consequently to an apparent increase of β. The main result is an equivalent loss of transmission when β is eliminated. Installing effective filters in the cathode and arc supplies of the MEIRA separator reduced the ripple by a factor of 10. Other factors found to increase the value of β for long emission slits were faulty alignment of the electrode with the emission slit and non-uniform current density along this slit. Factors affecting ion current density distribution along the slit are described. Preliminary results are given concerning the variation of features of the divergence curve with operational parameters.