Inhomogeneous RF Fields: A Versatile Tool for the Study of Processes with Slow Ions

Characteristics of mass selective axial ion ejection from a linear quadrupole ion trap in the presence of an auxiliary quadrupole field are described. Ion ejection is shown to occur through coupling of radial and axial motion in the exit fringing fields of the linear ion trap. The coupling is efficient and can result in extraction of as much as 20% of the trapped ions. This, together with the very high trapping efficiencies, can yield high sensitivity mass spectral responses. The experimental apparatus is based on the ion path of a triple quadrupole mass spectrometer allowing either the q2 collision cell or the final mass analysis quadrupole to be used as the linear trap. Space charge induced distortions of the mass resolved features while using the pressurized q2 linear ion trap occur at approximately the same ion density as reported for conventional three-dimensional ion traps. These distortions are, however, much reduced for the lower pressure linear trap possibly owing to the proposed axial ejection mechanism that leads to ion ejection only for ions of considerable radial amplitude. RF heating due to the high ejection q-value and the low collision frequency may also contribute. Two hybrid RF/DC quadrupole-linear ion trap instruments are described that provide high sensitivity product ion scanning while operated in the linear ion trap mode while also retaining all conventional triple quadrupole scan modes such as precursor ion and neutral loss scan modes. Copyright © 2002 John Wiley & Sons, Ltd.

A new linear ion trap mass spectrometer

Linear ion traps are finding new applications in many areas of mass spectrometry. In a linear ion trap, ions are confined radially by a two-dimensional (2D) radio frequency (RF) field, and axially by stopping potentials applied to end electrodes. This review focuses on linear ion trap instrumentation. Potentials and ion motion in linear multipole fields and methods of ion trapping, cooling, excitation, and isolation are described. This is followed by a description of various mass discrimination effects that have been reported with linear ion traps. Linear ion traps combined in various ways with three-dimensional (3D) traps, time-of-flight (TOF) mass analyzers, and Fourier transform ion cyclotron resonance mass spectrometers are then given. Linear ion traps can be used as stand alone mass analyzers, and their use for mass analysis by Fourier transforming image currents, by mass selective radial ejection, and by mass selective axial ejection are reviewed.

Linear ion traps in mass spectrometry.

The design of a new type of storage ring for heavy ions using electrostatic deflection and focusing devices is described. At low energy, where the velocity is low for heavy ions, electrostatic bends and quadrupoles are more efficient than magnetic ones. Furthermore, electrostatic devices are more compact and easier to construct than magnetic devices. These and other features, e.g. no magnetic fields, makes such storage rings attractive for many atomic-physics experiments, and also for basic research in neighboring fields such as chemistry and biology.

ELISA, and electrostatic storage ring for atomic physics

THE possibility of creating ordered ion beams in high-energy storage rings1,2 by means of electron and laser cooling has opened up a new era in accelerator physics The enhanced luminosity and suppressed momentum spread in such systems create the highest possible phase-space density The first experimental results were obtained by cooling 7Li+ beams to temperatures of a few kelvin or even to sub-kelvin temperatures3,4, and the ordered structures have been studied theoretically5–7 by methods of molecular dynamics Predicted configurations for the lowest ion densities have been observed in low-energy quadrupole storage rings8 and linear traps9 Recently we showed that at slightly higher ion densities helical structures are obtained10 Here we present a series of new experimental results on ordered ion structures in a quadrupole storage ring In order of increasing ion number, a linear chain of ions, a zig-zag structure, helical structures and finally multiple concentric shells could be observed The experimental results agree with molecular dynamics calculations

Multiple-shell structures of laser-cooled 24 Mg + ions in a quadrupole storage ring

The storage of protons and 3He ions in a circular storage‐ring ion trap with background gas collision‐limited loss time constants up to 8 min long under ultrahigh‐vacuum conditions is described. Storage‐ring traps were derived from the linear quadrupole rf mass filter by shaping the electrodes into circular and racetrack configurations. Operation of the traps with a trapping rf frequency Ω large compared to the secular ion oscillation frequency ω inhibited ion loss due to nonlinearities in the trapping fields. Long‐term ion storage in such traps may prove useful for radio‐frequency ion spectroscopic purposes.

D. A. Church

Papers

Storage‐Ring Ion Trap Derived from the Linear Quadrupole Radio‐Frequency Mass Filter