Sensitivity and amplitude calibration of a Fourier transform 3D quadrupole ion trap mass spectrometer

Abstract

With a three-dimensional (3D) quadrupole ion trap running in a Fourier transform operating mode, the detected signal is an image of the collective motion of the confined ions. Consequently, it is assumed that the image signal is the sum of the axial trajectories of the simultaneously confined ions. The resulting frequency spectrum after Fourier transformation comprises frequency peaks at the axial secular frequencies of the confined species according to their mass/charge ratio. With a singly confined species, the maximal amplitude of the image signal is proportional to the amplitude of the secular axial frequency peak. The matrix method is employed to express the axial trajectory sampled at the confinement field period. In that case, the expression of the image signal, as well as its maximal amplitude, is calculated as a function of the trap operating conditions and initial axial positions and axial velocities of the ions. The initial position and velocity distributions are connected to the injection mode. With the steady ion flow injection mode (SIFIM) and an initial phase of the confinement field equal to kπ, the maximal amplitude of the image signal is proportional to either the sum of the initial axial positions or the number of confined ions and the mean value of the initial axial positions. By simulation, amplitude fluctuation of the frequency peak is then calculated for a number of ions ranging between some tens to some thousands of ions injected by SIFIM. The peak amplitude fluctuations induced by the fluctuations of the number of ions are seven times greater than those induced by the fluctuations of the distribution of the initial axial positions.

Computer Modeling of an Ion Trap Mass Analyzer, Part I: Low Pressure Regime

“…In addition to modules for the direct integration of the Mathieu equation (MA) and the field interpolation method, ISIS also incorporates a simulation program for studies of the quadrupolar resonance SPQR [4,5], rf, and DC fields during the axial modulation [16], kinetic energy effects, and frequency absorption in resonantly excited ions [17]. André and coworkers [18] have carried out simulation studies of the axial/radial secular motion in a QIT operating in a Fourier transform mode [19] and consequently enhanced the performance of a field mass spectrometer in analyses of atmospheric pollutants [20].…”

Radio frequency quadrupole technology: Evolution and contributions to mass spectrometry

“…The distribution of initial axial positions is narrow around an adjustable off-centre position; consequently, the motion of the cloud of confined ions is coherent. With such initial conditions at confinement, the amplitude of the detected image signal is optimal and depends only on the initial axial positions of the ions and on the number of ions [82].…”

“…In addition, the authors have combined the Fourier transform operating mode with an injection mode denoted as steady ion flow injection mode (SIFIM) [82]. The steady ion flow injection mode involves two switched DC potentials applied to the two end-cap electrodes of the 3D-QIT.…”