International Journal of Mass Spectrometry and Ion Physics最新文献

Probability theory is used to describe the effect of the cross-sections for collisional ion-loss of daughter ions on fragmentation yield in collisional-activation mass spectrometry (CA/MS). Expressions for the parent-ion and total daughter-ion intensities are derived using the usual two-step model for the collisional-activation (CA) process and also for the more general sequential excitation model proposed previously. A smaller cross-section for the loss of daughter ions by collision as compared to that of the parent is found to affect the experimental CA data (daughter vs. parent intensities) in a similar way to sequential excitation. Independent determination of relative ion-loss cross-sections for daughter ions seems to be needed to obtain information concerning the dynamics and energetics of a CA process from CA experiments.

Two low power filaments (thin wire rhenium-and LaB₆) have been designed to allow a chemical-ionization ion source to operate below 100°C. Both filaments simplified analysis of the thermally sensitive compound flurazepam using negative chemical-ionization mass spectrometry. The Lab₆ cathode is susceptible to poisoning from solvents with temporary loss of thermionic emission. This effect has been studied using various organic solvents.

A method for thermalizing electrons is presented which involves the acceleration of electrons away from a hot filament with crossed electric-magnetic (EXB) fields in a magnetron configuration. It is shown that during a part of the resulting cycloidal path the electron velocity is significantly less than the initial velocity. The electron accelerates as it leaves the surface at a rate only slightly less than if there were no magnetic field, which prevents charge build-up at the surface. As the electron traverses the cycloid it is decelerated during the 2nd, 3rd and 4th quadrants, then re-accelerated as it approaches the end of the 4th quadrant to regain its original velocity. The minimum velocity occurs during the 4th quadrant, and corresponds to an electron temperature of 200–500°C for the electric and magnetic fields commonly encountered in the ion sources of magnetic sector mass spectrometers. An ion source modeled from this principle is presented.

In TPEPICO mass spectrometry, a threshold photoelectron detector is employed to transmit zero kinetic energy electrons. The steradiancy analyzer normally employed allows a small but significant number of non-zero kinetic energy electrons to be transmitted as well. A method has been developed to estimate the contribution to measured breakdown graphs of energetic electrons transmitted through the detector. This technique consists of convoluting the known instrument function and the HeI photoelectron spectrum (PES) with various postulated breakdown curves until the experimental and calculated breakdown curves match. This method has been successfully applied to propane and the C₄H₈ isomers, but it cannot be applied when the HeI PES does not represent the internal energy distribution in the energy range being examined. This is the case for CF₃I; for such cases it is possible to construct a variable-energy PES if the HeI PES, the real breakdown graph and the instrument function are known.

Secondary-ion mass spectra of the dipeptides Gly-Gly, Gly-Leu, Phe-Gly, Meth-Phe, the tripeptides Gly-Gly-Gly, Leu-Leu-Leu, Gly-Leu-Ala, Gly-Pro-Ala, the tetrapeptides Phe-Gly-Phe-Gly, Pro-Phe-Gly-Lys, and the octapeptide Asp-Arg-Val-Tyr-Ile-His-Pro-Phe (angiotensin II) were recorded using bare silver targets and targets covered with mixtures of the peptides and glycerol. Spectra were compared with those obtained by chemical ionization (CI) with isobutane as a reactant gas, electron impact ionization (EI), and field desorption (FD). Secondary ionization (SI) was established to be less destructive than EI and CI and to yield more characteristic fragment ions than FD without loss of information on the molecular mass M; e.g. the abundance of the protonated angiotensin II molecule MH⁺ (1046 mass units) was measured to be 9.2% of that of the base peak. SI leads mainly to fragmentation of the peptide bonds. This permits application of SIMS to sequence analysis of minute quantities of oligopeptides.

The energy distributions of ions produced in a spark source mass spectrometer were recorded for different trace element ions and for the total ion current. Both electrodes of pure doped graphite, and with 10% potassium salts added, were examined. The results show that the measured energy of an ion is mainly determined by the m/z ratio and that the energy of the total ion current depends on the sample composition. It was also clear that if the accelerating voltage is set to obtain maximum transmission of the total ion current, the fraction of the heavy elements that is discriminated at the β-slit depends on the matrix composition if the pass band of the instrument is narrow. These effects influence considerably the accuracy in the analysis of variable samples.

The synchronous passer system of a JEOL spark source mass spectrometer was modified so that the ion beam could be deflected at a variable time after the start of the first of a series of discharges. The transmitted part of the ion beam was analyzed by photoplate detection.

Time-resolved measurements revealed a different behaviour between different ionic species of the same m/z value. The singly-charged atomic ions were delayed by (150±100) ns with respect to the multiply-charged ions. Molecular ions were found to be delayed by about 700 ns (C_n⁺) or even one or more μs (S_n⁺, SC_n⁺) relative to multiply-charged ions.

The duration of the production/extraction process of multiply- and singly-charged ions was well defined in time, while this process could last for several μs in the case of polyatomic ions.

Protonated formic acid and CH₃O₂⁺ produced in the reaction of O₂⁺ with CH₄ both undergo the same isotopic exchange reactions with D₂O and show the same dissociation pattern, indicating that both have the structure

Highly porous targets for SIMS were prepared by thermal decomposition of metal compounds on foils of the respective metals. Compounds and foils of silver, gold and tantalum were compared with respect to secondary-ion (SI) intensities measured after spreading solutions of a test substance, phenylalanine hydrochloride, on the porous metal surface. Highest SI intensities were found with silver surfaces obtained after thermal decomposition of silver carbonate. SI intensities depend on primary-(argon) ion flux, on the concentration of the spread solution, and on additives, such as proton donators. It was found that porous targets perform much better than smooth ones with respect to avoiding rapid depletion of substances from the area of the target hit by the primary ions. Further improvement was achieved in some cases by covering the target with a mixture of the sample and some involatile solvent, such as glycerol.