Ion rotating motion in a gas‐filled radio‐frequency quadrupole ion guide as a new technique for structural and kinetic investigations of ions

Abstract

An ion rotating excitation mode of operation of a segmented gas-filled radio-frequency quadrupole (RFQ) ion guide for a high-resolution orthogonal time-of-flight (TOF) mass spectrometer is described. It is shown theoretically, by computer simulation and experimentally, that ion rotating excitation in a gas-filled RFQ has several advantages over other types of ion oscillation excitation. The main advantages are an approximately twofold increase in average ion kinetic energy for the same maximal deviation from the RFQ axis and therefore an increase of about this factor of average internal excitation energy of ions, and the extended mass range of fragment ions that can be observed. The new method of ion decomposition by ion rotation around the axis of an RFQ ion guide was experimentally implemented and tested using a home-built`SIN-COS' generator to supply the excitation voltage. This generator enables control of phase shift and amplitude of excitation voltages applied to quadrupole rods smoothly from the data acquisition program running on a PC. Copyright # 2001 John Wiley & Sons, Ltd.As a result of the orthogonal injection of ions, 1,2 the coupling of electrospray ionization (ESI) to time-of-flight mass spectrometry (TOFMS) has now become an attractive technique, especially for structural investigation of biomolecules. Tandem mass spectrometry (MS/MS) capabilities in this case are very important and, within the last few years, the literature on ESI-TOFMS has begun to focus on MS/MS techniques with hybrid instruments. Commonly, fragmentation of ions in these systems is achieved by traditional methods for collision-induced dissociation (CID). 3±5 However, a collision cell which is often also a radio-frequency ion guide provides the intrinsic possibility of inducing of collision-induced fragmentation of selected ions by resonant excitation of ion oscillations at the auxiliary angular frequency:where q M is the Mathieu parameter, o is the fundamental angular radio frequency (RF) of the voltage applied to the quadrupole ion guide (collision cell), q is the ion charge, m is its mass, V rf is RF-amplitude, and r 0 is the radius of the radiofrequency quadrupole (RFQ), i.e. the minimal distance from axis to rods. These oscillations may be considered on average as harmonic due to the quadratic dependence of averaged effective RFQ potential (or ion`potential' energy) on the distance r av from the RFQ axis:where t v is the ion velocity relaxation time, and F is the maximal field strength at the considered point. The last approximate equality is valid for low enough gas pressure in the RFQ (when ot v ) 1) and in the vicinity of its axis. The motion of an ion in the RF-only quadrupole ion guide can be excited through the application of an auxiliary voltage on one set of pole pairs (dipolar excitation) or on two sets of these (quadrupolar excitation). In the presence of a background neutral gas, the excited ion motion will result in an increase in the number and energy of collisions. As kinetic energy is transferred to ion inter...

Linear ion traps in mass spectrometry

“…This is very similar to excitation of a damped harmonic oscillator (Landau & Lifshitz, 1960, p. 77). For Figure 4, the damping force of Equation 2.24 was used, Both the x and y motion can be excited together by applying dipole excitation simultaneously between the x rod pair and y rod pair (Raznikov et al, 2001). If the two excitation voltages differ in phase by p/2 (908), circular motion of the excited ions is produced, and the average kinetic energy is twice that produced by simple dipole excitation.…”

A New Method to Study the Kinetics of Ion Decay in a Radio Frequency Quadrupole with Resonance Rotational Excitation

“…The second term accounts for the contribution of the excited RF motion of the ion (which is orthogonal to the longitudinal drift of the ion) to T x . We have shown previously 11 that, in the case of RRE, the average kinetic energy of the ion is given by Here, U rot is the amplitude of the rotational-excitation voltage and R 0 is the inscribed RFQ radius.…”

“…All installation developments and experimental measurements were performed on a home-built high-resolution o-ToF MS instrument described in detail earlier. 10,11 The scheme of the vacuum interface, including two RFQ ion guides, is represented in Figure 1. The first RFQ ion guide, with a buffer-gas pressure of about 0.7 torr, was used as an ion-molecule reactor (MIR) in our previously published methods of ion excitation and decomposition.…”

“…[6][7][8][9] This work is closely related to a gas-phase peptide sequencing technique, 6 because the fragment-ion mass spectrum depends on the decay rate constants of the corresponding fragmention channels. We have found it convenient to study these kinetic processes on-line using a high-resolution orthogonal time-of-flight mass spectrometer (o-ToF MS) 10,11 which enables precise and rapid registration of mass spectra over a wide range of m/z values. This paper is dedicated to the development of a new method for the measurement of decay rate constants of complex gas-phase organic ions produced by an electrospraytype ion source.…”

Computer simulation of the gap-tripole ion trap with linear injection, 3D ion accumulation, and versatile packet ejection

“…If the axially trapped ion loses kinetic energy due to inelastic collisions with neutral gas particles, it would be confined into the areas at low pseudopotential in the gap [37]. Multipole LITs and ion guides with phase-shifted rf voltages have already been used [38,39], but none of them has included a gap for ion confining.…”