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

Abstract

Abstract. Trajectory calculations are used to investigate peak shapes and ion transmission with a proposed new method of mass analysis with a quadrupole mass filter. Dipole excitation is applied to either the x or the y electrodes, or both, to create bands of instability within the first stability region. With excitation between the y electrodes (near β y = 0), ions are removed from the low mass side of a peak, and with ion excitation in x (near β x = 1), ions are removed from the high mass side. The mass resolution can be approximately doubled with comparatively little loss in ion transmission. Ion motion in an ideal quadrupole field and in the field of a quadrupole constructed with round rods has been studied. With an ideal quadrupole field, excitation in y is found to give better peak shape and resolution than excitation in x. With quadrupoles constructed with round rods, excitation in y is found to remove ions from both the low and high mass sides of a peak. The additional higher order multipoles introduced to the quadrupole potential by the use of round rods couple the x motion to the y motion so that exciting the y motion also excites ions in x. Thus, only excitation in y is necessary. Both with an ideal quadrupole field and quadrupoles constructed with round rods, the resolution can be increased ca. ×2 with little loss of transmission.

Stable X-islands of quadrupole mass filter

“…5,15 Dipole resonant excitation on small frequencies leads to the appearance of instability bands on the (a, q) stability diagram along iso-b lines. [16][17][18] With linear quadrupoles, an instability band along x ¼ const line is formed when a small dipole rf auxiliary voltage is applied between the X pair of electrodes and y ¼ const with dipole excitation between the Y rods. These instability bands together with the general stability diagram allow the QMF transmission to be increased for the same resolution value.…”

Modeling dipolar excitation for quadrupole mass filter

“…The early works were devoted mainly to DC dipole excitation in QMF. 8–10 It was found that the most effective resonant dipole excitation is along the β y line near the boundary β y = 0. 8 This effect occurs due to the low frequency oscillations in the y direction at the stability boundary for the fundamental frequency β Ω / 2.…”

“…8–10 It was found that the most effective resonant dipole excitation is along the β y line near the boundary β y = 0. 8 This effect occurs due to the low frequency oscillations in the y direction at the stability boundary for the fundamental frequency β Ω / 2. For example, if β y = 0.01, then ω y = Ω / 200.…”

Quadrupole mass filter operation with dipole direct current and quadrupole radiofrequency excitation

“…Resonant excitation of ions in a three‐dimensional ion trap was first described by Fisher . Dipole resonant excitation leads to the appearance of instability bands in the ( a , q ) stability diagram along the iso‐ β lines of the characteristic exponents β x , β y or β z . With linear quadrupoles an instability band along β x = const is formed when a small dipole rf auxiliary voltage is applied between the x pair of electrodes and β y = const with dipole excitation between the y rods.…”