RF ion traps

Three-dimensional quadrupole ion trap Quadrupole (RF) ion traps are the newest of the commercially available mass analyzers, despite having been invented at about the same time as the quadrupole mass filter, nearly 50 years ago. The Paul ion trap... 

More recently, it has been demonstrated that the reagent anions used for either ETD or proton transfer can be derived from the same neutral compound. The radical anions used for ETD, [M]- , are converted into even-electron proton transfer reagent anions. [M + H]-, by changing the potential on the methane Cl source.1 9 This voltage switch can be acheived in milliseconds allowing for rapid sequential ion—ion reactions and opens up the possibility of top-down sequencing of intact proteins in RF ion traps. 

Four of the most powerful methods presently applied to elucidate metal cluster geometric structure will be presented in the following. These are mass-selected negative ion photoelectron spectroscopy, infrared vibrational spectroscopy made possible by very recent advances in free electron laser (FEL) technology, gas-phase ion chromatography (ion mobility measurements), and rf-ion trap electron diffraction of stored mass-selected cluster ions. All methods include mass-selection techniques as discussed in the previous section and efficient ion detection schemes which are customary in current gas-phase ion chemistry and physics [71]. 

Fig. 1.23. The electron diffraction apparatus developed by Parks and coworkers includes an rf-ion trap, Faraday cup, and microchaimel plate detector (MCP) and is structured to maintain a cylindrical symmetry around the electron beam axis [147]. The cluster aggregation source emits an ion beam that is injected into the trap through an aperture in the ring electrode. The electron beam passes through a trapped ion cloud producing diffracted electrons indicated by the dashed hues. The primary beam enters the Faraday cup and the diffracted electrons strike the MCP producing a ring pattern on the phosphor screen. This screen is imaged by a CCD camera mounted external to the UHV chamber. The distance from the trapped ion cloud to the MCP is approximately 10.5 cm in this experiment...

As an example, the reactions of the negatively charged gold dimer and trimer cluster ions with carbon monoxide inside the rf-ion trap are presented in Fig. 1.36 [185]. While no reaction products of Au2 and Aus with carbon monoxide are detected at room temperature, cooling down leads first to the formation of mono-carbonyls and at the lowest temperatures around 100 K to a maximum adsorption of two CO molecules on Au2 and also on Au3 as can be seen from the mass spectra depicted in Fig. 1.36a. In order to deduce the reaction mechanism of the observed reactions, the reactant and product ion... 

The detailed kinetics and energetics of the reactions in the rf-ion trap can be understood by considering that the total pressure inside the ion trap is on the order of 1 Pa, which means that the experiment is operating in the kinetic low-pressure regime. Therefore, a Lindemann-t3rpe mechanism has to be considered for each reaction step, and the reaction rates depend on the buffer gas pressure [187, 188]. As a consequence, the obtained pseudo first order rate constant k contains the termolecular rate constant as well as the concentrations of the helium buffer gas and of the reactants in the case of the adsorption reaction of the first CO molecule (1.1) ... 

Cooperative Coadsorption Effects on Small Gold Clusters. Two examples of cooperative adsorption effects on small gold cluster anions identified in temperature dependent rf-ion trap experiments (see Chemical and Catal3dic Properties of Gas-Phase Clusters for experimental details) will be presented in the following. Au3 does not react with O2 in the ion trap experiment at any reaction temperature [34]. It, however, adsorbs a maximum of two CO molecules at reaction temperatures below 250 K [185]. If the gold trimer is exposed simultaneously to CO and O2 inside the octopole ion trap, still no reaction products are observed at reaction temperatures above 250 K as can be seen... 

Catalytic CO Oxidation by Free Au2. The potential catal3dic activity of Au2 in the CO combustion reaction was first predicted by Hakkinen and Land-man [382]. The subsequent experimental investigation employing an rf-ion trap indeed revealed the catalytic reaction of the gold dimer and, in conjunction with theory, a detailed reaction cycle could be formulated [33]. Also for particular larger gold cluster anions evidence for catalytic CO2 formation has... 

Our research group has developed a wide range of new experimental methods that are designed to be performed on ions stored within radio-frequency (RF) ion traps. In this chapter, we wiU phasize the integration of trap technology within different experimental arrangements in order to perform unique scientific measuranents. We will describe measurements of both trapped-ion electron diffraction (TIED) of metal cluster ions and radiative lifetimes of trapped biomolecular ions. Experiments will be discussed in sufficient detail to permit the advantages afforded by ion trap capabilities to be appreciated. This chapter is not intended as an extensive review of trap-related experimental measurements that are referred to and discussed elsewhere in the volumes of this series. 

Radiofrequency (RE) electric quadrupole mass (really miz) filters represent a considerable majority of analyzers in current use, particularly in trace quantitative analysis for this reason the operating principles of these devices will be discussed in some detail to emphasize their advantages and limitations. (The RF range corresponds to a few MHz.) These analyzers are used as stand-alone (non-tandem) MS detectors, as the components of the workhorse QqQ tandem instruments and as the first analyzer in the QqTOF hybrid tandem instrument. Quadrupole mass filters (Q) are essentially the same device as the RF-only collision cells and ion guides (q) discussed later in this section and are intimately related to the RF ion traps described in Section 6.4.5. In this regard, it can be mentioned that Q and q devices are not called quadrupoles because they are constructed of four electrodes (rods), but because a quadrupolar electric field (see Equation [6.11]) is formed in the space between the rods indeed the three-dimensional (Paul) trap (Section 6.4.5) creates a quadrupolar field using just three electrodes ... 

This reaction has been studied several times in rf ion traps with increasing accuracy. The results are summarized in Fig. 3.22 (see also Refs. 15 and 55). For P-H2 the rate coefHcient for radiative association is 1.7 x 10 cm s at 10 K, while the value for n-H2 is 2.5 times smaller. As discussed in detail in Ref. 22, much more has been learned about such processes, e.g. the competition between complex life time and radiative decay, by comparing ternary and radiative association and by isotopic substitution. 

Gerlich D. (1993) Guided ion beams, rf ion traps, and merged beams State specific ion-molecule reactions at meV energies XVIII. In Andersen T, Fastrup B, Folkmann F, Knudsen H. (eds.). International Conference on Physics of Electronic and Atomic Collisions, AIP, New York, pp. 607-622. 

What are the consequences if a storage device is not operated with parameters which have been called in Ref. 18 safe operating conditions In a recent publication, Mikosch et discussed some experience they made in the unsafe region, where the adiabaticity parameter rj > 0.3 (see Eq. (3.3) in Chapter 3). They observed the evaporation of buffer gas thermalized Cl anions out of a multipole rf ion trap. Working at conditions where the... 

Mikosch J, Priihhng U, Trippel S, Schwalm D, Weidemuller M. Wester R. (2007) Evaporation of buffer-gas-thermahzed anions out of a multipole rf ion trap. Phys. Rev. Lett. 98 223001-1-223001-4. 

The most common RF ion trap is a Paul trap [42], a 3-D quadrupole device in which ions are confined in a small volume of typically a few tens of millimeters [2] between a hyqterbolically shaped inner surface of a ring electrode and two end-cap electrodes, also of hyperbolic shape (Fig. 1). Elach end-cap electrode has a central hole for loading and ejection of irais. As these traps are compact, commercially available, and allow mass-selection of stored ions, they have become an increasingly popular technically simple solution for cryogenic ion spectroscopy. Paul traps have several drawbacks for cold-ion spectroscopy, however inefficient ion injection an intrinsically limited ability to cool ions low storage volume and inconvenient optical access to the ions by laser beams. 

The most commOTily used and perhaps most viable method for cooling the internal degrees of freedom of a large molecular ion is via collisions with a cold buffer gas. Here we describe the specific situation of buffer-gas cooling of ions in an RF ion trap. 

We give examples of implementation of buffer-gas cooling in RF ion traps in Sect. 2.5 and methods of temperature determination of ions in Sect. 2.6, but first we discuss spectroscopic techniques for detecting ion absorption of radiation. 

Rowe, M.A., A. Ben-Kish, B. DeMarco, D. Leibfried, V. Meyer, J. Beall, J. Britton, J. Hughes, W.M. Itano, B. Jelenkovic, C. Longer, T. Rosenband, and D.J. Wineland. 2002. Transport of quantum states and separation of ions in a dual RF ion trap. Quantum Information and Computation 2(4) 257-271. 

---