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

Abstract

Resolution improvements in dipolar resonant excitation have been examined in a round-rod quadrupolar collision cell for values of the Mathieu characteristic exponent ␤ equal to n/p, where n and m are small integers (prime ␤ values) versus other ␤ values where n and p are not small (ordinary␤ values). The trajectories of ions moving in the time-varying electric fields of a quadrupole with and without buffer-gas molecules were calculated to determine the relationship of prime and ordinary ␤ values to frequency resolution for resonant ion excitation and ejection. For prime ␤ values, the ion trajectory in the hyperbolic quadrupole field will be exactly periodic with a period of at most 4 p/⍀, where ⍀ is the angular frequency of the main drive radio-frequency ( . By scanning the RF voltage, trapped ions may be ejected one m/z ratio at a time out of the confining electric field for detection. While the ions are confined, ion-molecule reactions may be carried out. Additionally, the kinetic energy of the trapped ions can be increased by applying a dipolar AC potential that is resonant with the secular frequency of the motion of ions ( 0 ) of one or more m/z values. This increased kinetic energy can be used to selectively eject the ions from the trapping field, or to add internal energy to the confined ions through collisions with a neutral gas added to the trap. This dipolar resonant excitation has been a common method for performing mass-selective ejection and fragmentation of ions confined in Paul traps and, more recently, in linear ion traps [2][3][4]. The theory of instability space, as it relates to auxiliary AC excitation in a linear and 3D quadrupole trap for quadrupolar resonant excitation, has been addressed [5][6][7]. The trajectories of ions in a linear ion trap with dipolar auxiliary excitation have been examined by Franzen [8]; those for hyperbolic and circular quadrupole rods with and without collision gas by Collings et al. [2].In tandem mass spectrometry (MS/MS), resolution of the product ions is sometimes crucial as many fragments have similar m/z values [9 -15]. Unambiguous product-ion assignment often requires better than unit-mass resolution to resolve "isobaric" interferences, which are particularly acute when the analyte is the minor component. Two factors that degrade m/z resolution after dipolar resonance excitation are increasing collisions with buffer-gas molecules [2] and increasing dipolar voltage amplitude [16]. 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...

Mass selectivity of dipolar resonant excitation in a linear quadrupole ion trap

“…Because these simulations require that ions reach an electrode for ejection, they might most suitably be applied to radial ejection through a trapping electrode. Nevertheless the simulations are in qualitative agreement with experimental observations for both radial and axial ejection from linear traps; at low scan speeds the dipolar voltage should be small and the resolution is highest, the resolution is greater with excitation at q = 0.85 than at lower q , and a low pressure buffer gas can increase the mass resolution . In future we would like to extend this work to study the effects of space charge on S ( q ).…”

“…The low scan speed for high resolution requires the excitation amplitude to be decreased to a few mV . A nitrogen buffer gas pressure of a few mTorr increases the mass resolution with dipolar excitation …”

Nonlinear Ion Harmonics in the Paul Trap with Added Octopole Field: Theoretical Characterization and New Insight into Nonlinear Resonance Effect

“…For the normal resonance, only one resonant pattern, either excitation or absorption, can be observed in one spectrum [34]. The ion excitation and deexcitation at nonlinear resonance can accelerate and delay the ion ejections in the dipolar resonance experiments [30]. In a linear ion trap, when the external AC frequency is scanned around the nonlinear resonance condition, simultaneous increment and decrement of the bandwidth of the frequency response profiles (FRP, i.e., ion ejection time versus the external AC frequency) have been observed.…”

“…For instance, in the MS/MS experiments, the loss of daughter ions because of the nonlinear resonance has been knows as the black holes or black canyons [29]. But in the dipolar resonance experiments, the effect can accelerate the ion ejection and improve the mass resolution [30]. In addition, the nonlinear resonance may result in peak splitting effect in the quadrupole mass filter operated in the second stability region [31].…”

Dipole Excitation: A New Method for Mass Analysis with a Quadrupole Mass Filter

“…For practical reasons, the values of β x and β y should be selected as a simple ratio like β = K/N, where K and N are integers and K < N: in this case, the ion oscillations are periodic with the period 2πN and the dipole resonance is sharper [28].…”