Mass selective axial ion ejection from a linear quadrupole ion trap

Abstract

The electric fields responsible for mass-selective axial ejection (MSAE) of ions trapped in a linear quadrupole ion trap have been studied using a combination of analytic theory and computer modeling. Axial ejection occurs as a consequence of the trapped ions' radial motion, which is characterized by extrema that are phase-synchronous with the local RF potential. As a result, the net axial electric field experienced by ions in the fringe region, over one RF cycle, is positive. This axial field depends strongly on both the axial and radial ion coordinates. The superposition of a repulsive potential applied to an exit lens with the diminishing quadrupole potential in the fringing region near the end of a quadrupole rod array can give rise to an approximately conical surface on which the net axial force experienced by an ion, averaged over one RF cycle, is zero. This conical surface has been named the cone of reflection because it divides the regions of ion reflection and ion ejection. Once an ion penetrates this surface, it feels a strong net positive axial force and is accelerated toward the exit lens. As a consequence of the strong dependence of the axial field on radial displacement, trapped thermalized ions can be ejected axially from a linear ion trap in a mass-selective way when their radial amplitude is increased through a resonant response to an auxiliary signal. . Both approaches yield high quality mass spectra and offer advantages over conventional three-dimensional ion traps, such as greater ion capacity, higher trapping efficiencies, less mass discrimination, and reduced effects of space charge [1,3]. In certain cases, linear ion trap mass spectrometers can be incorporated into the ion path of triple quadrupoles yielding an instrument that combines the strengths of both platforms [1,4]. Ions are trapped radially in these devices by the RF quadrupole field and axially by static DC potentials at the ends of the quadrupole rod array, in contrast to conventional Paul traps in which ions are trapped by a three-dimensional RF quadrupole field. Radial mass-selective ion ejection occurs when the RF voltage is ramped in the presence of a sufficiently intense auxiliary AC voltage. The auxiliary AC resonance-ejection voltage is applied radially and the ions emerge from the linear ion trap through slots cut in the quadrupole rods [3]. Radial ejection requires that the RF field be of high quality over the entire length of the ion trap [3] in order to preserve mass spectral resolution, since resolution depends on the fidelity of the secular frequency of the trapped ions. Thus, very high mechanical precision is required in fabrication of the quadrupole rods in order to maintain the same secular frequency over the length of the device. Of course, the greater the length of the linear ion trap, the more difficult it is to maintain the high degree of mechanical precision. In addition, considerable care must be taken to ensure that the ions that are intended to be ejected radially from the linear ion trap are isolated as ...

Study of the enhancement of dipolar resonant excitation by linear ion trap simulations

“…One method to increase resolution is to excite the ions in such a way to reduce the frequency composition and the periodicity of the ions' motion in the electric field. In the present study, we used simulation software to determine the trajectories of ions [17] moving in a time-varying electric field and colliding with buffer-gas molecules in a linear quadrupole reaction cell with the instrumentally common and practical round-rod geometry.It is a common misconception that an ion trajectory within the quadrupolar field is necessarily aperiodic. As we will show below, provided that the Mathieu char-…”

“…However, increasing the ␤ value can have deleterious effects on the minimum m/z value of ions trapped; experimentally, operating at a higher ␤ value may lead to problems pertaining to ion storage, due to larger overall radii and higher secular frequencies of ions. As described in two manuscripts by Londry and Hager and Stott et al, a method was developed for increasing FRP resolution by varying the DC potential difference between the quadrupole rods and exit lens of a linear ion trap as a function of ion mass [17,20].…”

Evolution of instrumentation for the study of gas-phase ion/ion chemistry via mass spectrometry

“…However, the presence of RF barriers at the ends of a linear ion trap can compromise other aspects of an MS n experiment if they cannot be created and removed at appropriate points in the sequence of events. For example, high RF barriers at the ends of the array can adversely affect ion injection into the ion trap and can degrade the performance of mass analysis using mass selective axial ejection (MSAE) [42]. Therefore, subsequent work with this apparatus has used superposition of RF on containment lenses because this approach is more readily adapted to software control during the course of an MS n experiment.…”

A pulsed triple ionization source for sequential ion/ion reactions in an electrodynamic ion trap

“…After another cooling step, the third ionization source was triggered and the second type of reagents ions were injected into Q3 under the same conditions described for the first ion/ion reaction. After another mutual storage period, all the product ions resulting from the second ion/ion reaction were allowed to cool in Q3 for 50 ms before they were subjected to mass selective axial ejection (MSAE) [37] using a supplementary RF signal at frequency 380 kHz. The spectra shown here are typically the averages of 20 to 100 individual scans.…”