Resonance Ejection from the Paul Trap: A Theoretical Treatment Incorporating a Weak Octapole Field

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...

Section: Bistable Behaviormentioning

confidence: 95%

Section: Bistable Behaviormentioning

confidence: 99%

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

Michaud

Frank

Ding

et al. 2005

“…The simulation approach is especially powerful for system optimization and fine feature tuning. On the other hand, theoretical modeling provides relatively general and better understandings of the fundamental mechanisms behind phenomena [23,[42][43][44][45][46][47][48][49][50][51][52][53][54][55]. Ion motion in the presence of buffer gas has been investigated theoretically by solving the ion motion differential equation with the background buffer gas treated as a damping term [53].…”

mentioning

confidence: 99%

Ion trap mass analysis at high pressure: A theoretical view

Song

Smith

et al. 2009

The mass-selective manipulation of ions at elevated pressure, including mass analysis, ion isolation, or excitation, is of great interest for the development of mass spectrometry instrumentation, particularly for systems in which ion traps are employed as mass analyzers or storage devices. While experimental exploration of high-pressure mass analysis is limited by various difficulties, such as ion detection or electrical discharge at high-pressure, theoretical methods have been developed in this work to study ion/neutral collision effects within quadrupole ion traps and to explore their performance at pressures up to 1 Torr. Ion trapping, isolation, excitation, and resonance ejection were investigated over a wide pressure range. The theoretically calculated data were compared with available experimental data for pressures up to 50 mTorr, allowing the prediction of ion trap performance at pressures more than 10 times higher. (J Am Soc Mass

“…In practical LITs, higher-order fields are inevitable for many reasons, such as the truncation of the electrodes, the use of simplified electrodes, and the existence of the ion ejection slots [30,[33][34][35][39][40][41][42]. Moreover, it is well known that ITs with some positive even-order fields might improve performance [33,[39][40][41][42][43][44][45].…”

Section: The Effects Of Higher-order Fieldsmentioning

confidence: 99%

“…Moreover, it is well known that ITs with some positive even-order fields might improve performance [33,[39][40][41][42][43][44][45]. For example, the addition of octopole [40][41][42][43][44][45] or dodecapole fields [30,33,39] can compensate for the detrimental field caused by the slot and, therefore, the mass resolution and mass accuracy are enhanced.…”

Section: The Effects Of Higher-order Fieldsmentioning

confidence: 99%

Theoretical Study of Dual-Direction Dipolar Excitation of Ions in Linear Ion Traps

Dang

Wang

et al. 2016

Abstract. The ion enhanced activation and collision-induced dissociation (CID) by simultaneous dipolar excitation of ions in the two radial directions of linear ion trap (LIT) have been recently developed and tested by experiment. In this work, its detailed properties were further studied by theoretical simulation. The effects of some experimental parameters such as the buffer gas pressure, the dipolar excitation signal phases, power amplitudes, and frequencies on the ion trajectory and energy were carefully investigated. The results show that the ion activation energy can be significantly increased by dual-direction excitation using two identical dipolar excitation signals because of the addition of an excitation dimension and the fact that the ion motion radius related to ion kinetic energy can be greater than the field radius. The effects of higher-order field components, such as dodecapole field on the performance of this method are also revealed. They mainly cause ion motion frequency shift as ion motion amplitude increases. Because of the frequency shift, there are different optimized excitation frequencies in different LITs. At the optimized frequency, ion average energy is improved significantly with relatively few ions lost. The results show that this method can be used in different kinds of LITs such as LIT with 4-fold symmetric stretch, linear quadrupole ion trap, and standard hyperbolic LIT, which can significantly increase the ion activation energy and CID efficiency, compared with the conventional method.