Possible Applications of an External Resonant Circuit in Fourier Transform Ion Cyclotron Resonance

The narrow-band cooling of ion motion in the trap of a Fourier transform ion cyclotron resonance mass spectrometer (FTICR) using an external parallel resonant circuit is considered. The parameters of the circuit necessary for effective cooling of both axial and cyclotron ion motion are estimated. It is shown that the use of a resonant circuit may be helpful in studying the highly charged heavy ions produced in experiments with electrospray ionization combined with FTICR. Once ionized, ions oscillate quasi-harmonically between the trapping electrodes of a Fourier transform ion cyclotron resonance (FTICR) trap', with the trapping frequency, w,,, defined by the well-known equation, w,,=~2yqV,,lmd2, where y is a geometrical factor for the trap (y=2.77 for a cubic c~nfiguration),~ -~ m and 4 are the mass and charge of the ion, d is the trap size, and V,, is the trap electrostatic potential between the end and side plates. The energy of this oscillator will be defined by the initial ion position along the z-coordinate (the direction of the magnetic field) at the moment of ionization or by the ion energy upon external injection into the trap. Because of the distortion of the electric trapping potential from the ideal quadrupolar form, the trapping frequency depends on the oscillation amplitude. This decreases the observed resolving power of the FTICR trap because of the ion frequency distribution and leads to systematic errors in precise mass measurements. The problem of effective ion cooling in FTICR exists not only for cooling the ion's azimuthal motion, but also for cooling the ion's motion in the cyclotron plane (the x-y plane). In this plane, ions will exhibit two fundamental types of motions: cyclotron rotation in the magnetic field (B) with cyclotron frequency, wc= qBlm, and magnetron motion, which is the drift along equipotential surfaces of the trapping electric field with magnetron frequency, om = f(r)/B, where f(r) is defined by the radial component of the trapping electric field, E, = f(r)r. Because of the radial potential hill, the radius of the ion magnetron motion increases due to any energy dissipative process (e.g. ion collisions with neutrals). This effect produces an obstacle for ion transfer from the source trap into the analyzer trap in a dual trap system, as well as other ion-loss mechanisms. To solve this problem, a quadrupolar cooling technique is used',' where the ion magnetron motion is converted into cyclotron motion by applying a quadrupole coupling electric field in the cyclotron plane. Cyclotron motion is then cooled via ion collisions with neutrals. The net effect is thar the magnetron radius is thereby reduced. A comprehensive theory for the method of both narrow-band and broad-band ion axialization in FTICR via azimuthal quadrupolar excitation has been presented recently.' This technique becomes inefficient when the mass of ions is high, such as in electrospray ionization (ES1)-FTICR experiments in which ions of extremely high masses have been trapped."-'* Quadrupolar ion cooling does not work in this case because the mass of the neutrals in the collision gas is much lower (e.g., 130 u for xenon) than the ion mass. In most FTICR applications, ions are produced outside the trap, either in an external ion so~rce'~-'~ or in the source trap,I6 and introduced into the analyzer trap. In these cases, ions in an analyzer trap will have a large z-oscillation amplitude and a large kinetic energy. This restricts ion lifetime in a trap, decreases the resolving power of the instrument, and limits the energy region in thermochemical studies. As an alternative, ions may be cooled by using their interaction with an external parallel resonant circuit of high quality factor Qf, adjusted to the same frequency as the ion trapping frequency. In this case, the ions will exchange energy with the circuit, and this energy will dissipate in the resistance of the circuit. Figure l(b) shows the equivalent circuits necessary to realize this cooling process for ion axial and cyclotron motions. An external, high Q,, resonant circuit has been used in Penning-trap mass spectrometry to cool trapped electrons. j7, I' The same technique has been applied to protons in a Penning trap." Previously, a resonant circuit was used in precise mass measurements with FTICR for the purpose of increasing the sensitivity.20,21 We consider the possibility of using an external resonant circuit for ion cooling in FTICR.