A new linear ion trap mass spectrometer

Abstract

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.Quadrupole mass spectrometers 1 have been in wide use for many years. Generally, these devices are constructed from four parallel rods within which a two-dimensional quadrupole field is established. Mass selection is achieved by appropriately choosing a combination of radio-frequency (RF) and direct-current (DC) voltages such that ions within a very narrow mass-to-charge window are stable over the length of the quadrupole. The mass selection criteria, however, leads to the major drawback of beam-type scanning mass spectrometers, namely that most of the ions generated from the source are wasted. Conventional ion trap mass spectrometers, on the other hand, operate with a three-dimensional (3D) quadrupole field. These instruments are capable of very high efficiencies since the time to fill the ion trap and generate a complete mass spectrum can be very short.2 A problem with 3D ion traps is that they generally have poor trapping efficiencies, typically 1±10%, for externally generated ions primarily due to their small volume. This small volume also results in a limited dynamic range since there is a maximum charge density beyond which response becomes nonlinear with respect to ion number and the quality of the mass spectra deteriorates. The magnitude of this problem can be reduced by measuring and appropriately adjusting ion number prior to analysis, 3 but this reduces duty cycle and ultimately sensitivity.Linear ion traps have been known since the introduction of the quadrupole storage ring. 4,5 Ions have also been trapped ...

Mass selective axial ion ejection from a linear quadrupole ion trap

“…These fringing fields, positioned at both ends of the quadrupole, are most often deleterious to RF/DC quadrupole performance [5] as well as radial ejection from a linear ion trap [3]. In the fringing region, the diminution of the quadrupole field, the increased significance of higher-order terms in the multipole expansion of the potential and coupling of radial and axial fields lead to significant changes in the secular frequency [6,7] and thus ejection at unexpected stability coordinates.Mass-selective axial ejection (MSAE) of ions from linear quadrupole ion traps takes advantage of the RF fringing fields to convert radial ion excitation into axial ion ejection [1,2] in a manner analogous to resolving RF-only mass spectrometers [8,9]. Trapped ions are given some degree of radial excitation via a resonance excitation process, and in the exit fringing-field, this radial excitation results in additional axial ion kinetic energy that can overcome the exit DC barrier [1,2].…”

“…Mass-selective axial ejection (MSAE) of ions from linear quadrupole ion traps takes advantage of the RF fringing fields to convert radial ion excitation into axial ion ejection [1,2] in a manner analogous to resolving RF-only mass spectrometers [8,9]. Trapped ions are given some degree of radial excitation via a resonance excitation process, and in the exit fringing-field, this radial excitation results in additional axial ion kinetic energy that can overcome the exit DC barrier [1,2].…”

“…I ons trapped within a linear quadrupole ion trap can be extracted mass selectively either axially [1,2] or radially [3]. 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].…”

Ion excitation in a linear quadrupole ion trap with an added octopole field

“…There is increasing interest in using linear quadrupoles as ion traps, both as stand alone mass analyzers with radial [4] or axial [5] ejection, or in combination with other mass analyzers (for a recent review see [6]). …”

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

“…With the introduction of linear quadrupole ion trap mass spectrometers [35,36], interest has been directed to the implementation of ion/ion reactions in such devices. Linear ion traps have an inherent efficiency advantage over 3D ion traps with respect to coupling with ion sources, detectors, and other devices.…”