Applications of rf fields and collision dynamics in atomic mass spectrometry

Abstract

Today, inhomogeneous, time dependent electric fields, created with suitable electrode arrangements, are used routinely in sophisticated machines for studying the structure and reactivity of atomic and molecular ions, clusters, and charged nanoparticles. This contribution first illustrates recent progress made in this field by briefly reviewing some fundamental experiments, performed with radiofrequency (rf) based electrode arrangements. The examples have been selected according to their potential relevance for mass spectrometry. They cover the study of integral and differential cross sections in octopoles, the determination of low temperature rate coefficients in multi-electrode systems, and laser induced reactions in traps. In addition to applications in basic research, the past decades have seen many activities in integrating ion guides and traps into instruments developed for analytical purposes. In early applications, rf fields were mainly utilized for confinement, transfer, mass selection, and phase space compression, while currently inelastic or reactive collisions with suitable neutral buffer gases are used in addition for improving the sensitivity and versatility of the instruments. In the field of atomic mass spectrometry the method is called the collision and reaction cell technique or mass spectrometry with a dynamic reaction cell. The strategy is to systematically utilize ion–molecule reactions for changing the mass of isobaric species or polyatomic ions interfering with the element of interest. It is the aim of this paper to tentatively analyze some of the related applications and to evaluate the potential of reaction dynamics and rf-based devices for improving the sensitivity in elemental and isotope analysis. It is concluded that for processes where energy is required (e.g., for collision induced dissociation or endothermic reactions), one should make use of a guided ion beam with well defined kinetic energy, while in the case of exothermic reactions, a thermalized ensemble, especially at cryogenic temperatures, is probably the best choice. Generally applicable methods are the change of mass by deuteration or ternary association reactions with rare gases.