Mass resolution of linear quadrupole ion traps with round rods

Abstract

Adding weak nonlinear fields to a quadrupole field can improve the resolution with mass-selective ejection of ions by up to a factor of 2.7. The optimum ratio r/r0 is 1.14 to 1.16, which differs from the optimum ratio for a mass filter of 1.128-1.130.

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

“…This (Figure 3) and for the round rod set ( Figure 6) shows that the cylindrical electrodes give losses in transmission from 65% to 50% and to losses in resolution from R 0.1 = 360 to 250, approximately, for the ion source under consideration. These data are in a qualitative agreement with the results of detailed studies [25,27,30] of peak shapes with round rods.…”

“…The result is that the peak width Δq is simultaneously decreased (narrowed) from both sides of the peak. The choice of the optimal ratio r/r 0 of the mass filter can be done by using the data from the publications [25,27,30] (however, it should be noted that the most symmetric mass peak profile for the dipole excitation corresponds to the value r/r 0 = 1.126 [27]). …”

“…For a round rod set with r/r 0 = 1.13, A 3 = 0.2095 and there are also higher harmonics corresponding to 14, 18, and 22 pole terms at the excitation frequencies. The use of round rods also contributes 12-pole and 20-pole terms to the main quadrupole rf potential [25,27]. These also are not taken into account in Equations 1-3.…”

“…Ion motion in linear quadrupoles with additional higher order multipole fields and with dipole excitation can be described by the following equations [25]: …”

“…The use of dipole resonant excitation of the ions was first proposed by Fischer in 1959 [13]. Much later, dipole excitation of ion oscillation found broad application in linear and 3D ion traps for mass selective detection of ions [14][15][16][17][18][19][20][21][22][23][24][25]. The oscillation frequencies of ions in a quadrupole field, ω u , are given by ω u = (2n + β u )Ω/2, where u is x or y, n = 0, ± 1, ± 2 … and Ω is the angular frequency of the main quadrupole rf.…”