Linear ion traps in mass spectrometry

Mass selective axial ejection of ions from linear quadrupoles with added octopole fields is described. Quadrupoles with 2.0% and 2.6% added octopole fields have been tested and compared with a conventional quadrupole. The effects of trapping ions at different q values, excitation voltage, scan direction, balanced and unbalanced rf voltages on the rods, and dc applied between the rods have been investigated. The highest scan speeds and best resolution are obtained with resonant excitation and ejection at high q (q ϭ 0.8). With axial ejection, the quadrupole with a 2.0% added octopole field provides mass resolution and ejection efficiencies similar to a conventional rod set. Quadrupole, dipole, and simultaneous dipole-dipole excitation between the x and y rod pairs were compared, and no advantage was found with quadrupole or dipole-dipole excitation. The effects of scan speed were investigated and a resolution at half height of about 1600 is possible at scans speed up to 5000 Th/s. In 1999, Hager [3] reported a new method of mass scanning with linear quadrupoles, called mass selective axial ejection (MSAE). The method uses the fringing field at the exit of the quadrupole. In the fringing field the otherwise independent ion motions in x, y, and z are coupled. The quadrupole is operated in rf-only mode with ions flowing continuously into the quadrupole. The end plate or exit lens is set at a stopping potential relative to the quadrupole rod offset. Within the fringing field, radial energy of the ions is converted into axial energy and ions can surmount the exit lens barrier to reach a detector. Ions with larger radial displacements in the fringing field receive greater axial kinetic energy increases than ions with smaller radial displacements. Thus, some form of radial excitation of ions is required for ion ejection.The same concept was then used for axial ejection of ions trapped in a linear quadrupole [4]. Experiments were done with a triple quadrupole mass spectrometer.Fragment ions produced in tandem mass spectrometry were trapped in the collision cell (Q2) or the last mass analyzing quadrupole (Q3), by applying stopping potentials to lenses at the ends of the quadrupoles. For axial ejection, ions were excited at their resonant frequencies by applying fixed-frequency auxiliary ac voltages to two or all four rods of a quadrupole or to the exit lens. By scanning the trapping rf amplitude, ions of different m/z ratios came into resonance and were ejected to produce a mass spectrum.The mass resolution (mass resolving power) at half height (R 1/2 ) with axial ejection from Q2 (4 ϫ 10 Ϫ4 torr) was typically about 1000 at m/z 609 with a scan rate of 1000 Th/s. Slower scans, 5 Th/s, resulted in much higher resolution, R 1/2 ϭ 6000 at m/z 609. It was found that, because of the lower pressure in Q3 (3 ϫ 10 Ϫ5 torr), R 1/2 ϭ 6000 could be obtained at the higher scan speed of 100 Th/s. Experiments with fragment ions formed in Q2 and then trapped and axially ejected from Q3 showed 16 times greater sensitivity than a conventi...

mentioning

confidence: 99%

Mass selective axial ion ejection from linear quadrupoles with added octopole fields

Moradian

Douglas

2008

“…The most widely discussed distortion is the "stretched" ion trap [2], which has the end cap electrodes moved out so that the distance to the end cap, z 0 , is increased over that of an ideal field, z 0 ϭ r 0 ⁄ ͙ 2, where r 0 is the distance from the center to the ring electrode. It has been argued that the addition of higher order multipole fields of the correct sign to 3-D traps improves MS/MS efficiency [1c, 1f, 2], and allows faster ejection at the stability boundary [2,3], to give higher scan speeds and improved mass resolution.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]). …”

mentioning

confidence: 99%

“…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]). …”

mentioning

confidence: 99%

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

Michaud

Frank

Ding

et al. 2005

Modeling of ion motion and experimental investigations of ion excitation in a linear quadrupole trap with a 4% added octopole field are described. The results are compared with those obtained with a conventional round rod set. Motion in the effective potential of the rod set can explain many of the observed phenomena. The frequencies of ion oscillation in the x and y directions shift with amplitude in opposite directions as the amplitudes of oscillation increase. Excitation profiles for ion fragmentation become asymmetric and in some cases show bistable behavior where the amplitude of oscillation suddenly jumps between high and low values with very small changes in excitation frequency. Experiments show these effects. Ions are injected into a linear trap, stored, isolated, excited for MS/MS, and then mass analyzed in a time-of-flight mass analyzer. Frequency shifts between the x and y motions are observed, and in some cases asymmetric excitation profiles and bistable behavior are observed. Higher MS/MS efficiencies are expected when an octopole field is added. MS/MS efficiencies (N 2 collision gas) have been measured for a conventional quadrupole rod set and a linear ion trap with a 4% added octopole field. Efficiencies are chemical compound dependent, but when an octopole field is added, efficiencies can be substantially higher than with a conventional rod set, particularly at pressures of 1.4 ϫ 10 Ϫ4 torr or less. . The most widely discussed distortion is the "stretched" ion trap [2], which has the end cap electrodes moved out so that the distance to the end cap, z 0 , is increased over that of an ideal field, z 0 ϭ r 0 ⁄ ͙ 2, where r 0 is the distance from the center to the ring electrode. It has been argued that the addition of higher order multipole fields of the correct sign to 3-D traps improves MS/MS efficiency [1c, 1f, 2], and allows faster ejection at the stability boundary [2,3], to give higher scan speeds and improved mass resolution.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]).There is also interest in trapping and exciting ions for MS/MS at the relatively low pressures typical for operation of the last mass analyzing quadrupole in triple quadrupole systems, ca. 3 ϫ 10 Ϫ5 torr [7]. Addition of higher multipoles to a linear ion trap might be expected to provide benefits similar to those seen with 3-D traps. Douglas and coworkers [8] have shown that an octopole field can be added to a linear quadrupole by using rod sets with rods equally spaced from the central axis but with one pair of rods different in diameter than the other pair, as shown in Figure 1. The electric potential within this rod set is given to a good approximation bywhere x is the distance from the center towards a smaller rod, y is the distance from the center towards a larger rod, r 0 is the distance from the center to any rod, and U and V rf are the amplitudes...

“…Under certain conditions, ion motion in any inhomogeneous AC field can be described by the pseudo or effective potential theory [3][4][5][6][7][8][9]. If the ion loses kinetic energy due to inelastic collisions with neutral gas particles, it will move to areas of low pseudopotential located near the center [10].…”

Section: Ion Guiding and Collisional Focusingmentioning

confidence: 99%

Computer simulations of a new three rods ion optic (TRIPOLE) with high focusing and mass filtering capabilities

Salazar

Masujima

2007

A novel three rod (tripole) ion optic to which three AC voltages with symmetrically delayed phase shifts were applied to each electrode. We studied its ion guiding, focusing, and mass filtering capabilities by SIMION ver. 7.0 computer simulations. An electric field mathematical model was developed to calculate the pseudopotential of the tripole radial AC force. The tripole showed stable ion guiding for wide ranges of AC amplitude; better collisional focusing than hexapole and octapole and similar focusing as quadrupole (rod pole). Also, the ion optic clearly showed interesting mass filtering potential when the phase shift was asymmetrically delayed. The symmetric shape of the pseudopotential field explained the tripole ion guiding and focusing capabilities. For mass filtering, the pseudopotential was asymmetric and its effect was balanced with DC voltage to separate the ions, depending in their masses. The resolution was much lower than quadrupole but useful when rough filtering was required. (J Am Soc Mass Spectrom 2007, 18, 413-421)