Dynamically Harmonized FT-ICR Cell with Specially Shaped Electrodes for Compensation of Inhomogeneity of the Magnetic Field. Computer Simulations of the Electric Field and Ion Motion Dynamics

Abstract

The recently introduced ion trap for FT-ICR mass spectrometers with dynamic harmonization showed the highest resolving power ever achieved both for ions with moderate masses 500-1000 Da (peptides) as well as ions with very high masses of up to 200 kDa (proteins). Such results were obtained for superconducting magnets of very high homogeneity of the magnetic field. For magnets with lower homogeneity, the time of transient duration would be smaller. In superconducting magnets used in FT-ICR mass spectrometry the inhomogeneity of the magnetic field in its axial direction prevails over the inhomogeneity in other directions and should be considered as the main factor influencing the synchronic motion of the ion cloud. The inhomogeneity leads to a dependence of the cyclotron frequency from the amplitude of axial oscillation in the potential well of the ion trap. As a consequence, ions in an ion cloud become dephased, which leads to signal attenuation and decrease in the resolving power. Ion cyclotron frequency is also affected by the radial component of the electric field. Hence, by appropriately adjusting the electric field one can compensate the inhomogeneity of the magnetic field and align the cyclotron frequency in the whole range of amplitudes of z-oscillations. A method of magnetic field inhomogeneity compensation in a dynamically harmonized FT-ICR cell is presented, based on adding of extra electrodes into the cell shaped in such a way that the averaged electric field created by these electrodes produces a counter force to the forces caused by the inhomogeneous magnetic field.

Influences of non-neutral plasma effects on analytical characteristics of the top instruments in mass spectrometry for biological research

“…This may be achieved by using hyperbolic electrodes in ion traps or by dynamic harmonization [20]. Limitations of the dynamic range caused by an inhomogeneous magnetic field can be reduced in a certain mass range interval by using special electrodes for compensation of inhomogeneity of the magnetic field by the electric field [21]. For most magnets the limitation of resolving power caused by the inhomogeneity of the magnetic field is much weaker than the limitation caused by the inharmonic electric field.…”

“…We have proposed, constructed and experimentally tested a new FT-ICR cell based on completely new principles of formation of the effective electric potential distribution in Penning type traps [20,21,24] (see fig. 3).…”

From Supercomputer Modeling to Highest Mass Resolution in FT-ICR

“…19) It was demonstrated on reserpine that it is possible to achieve a resolving power 39,000,000 (Fig. 5).…”

“…18) The limitation of dynamic range caused by inhomogeneous magnetic field can be reduced in some mass range interval by using compensation of inhomogeneity of the magnetic field by electric field created from special electrodes. 19) For most magnets the limitation of resolving power caused by inhomogeneity of the magnetic field is much weaker than limitation caused by inharmonic electric field. Simulations show that the faster is dephasing caused by inharmonic electric or inhomogeneous magnetic field, the more ions are needed to initialize "condensation" which stabilizes ion clouds and prevent dephasing.…”

“…We developed a method of compensation the inhomogeneity of the magnetic field by adding additional segments shaped by curve of 4-th order inside trapping electrodes it is possible to introduce a 4-th order correction to the field which can be used to compensate quadratic inhomogeneity of magnetic field. 19) …”