Control of ion trapping and ejection in external source Fourier transform mass spectrometry

The behavior of ions introduced to the trapped-ion cell of a Fourier transform mass spectrometer (FTMS) from an external ionization event is examined. Instrumentation optimized for the manipulation and observation of such ions in described and fundamental ion trapping and loss mechanisms are evaluated. An external source tandem time-of-flight (TOF)/FTMS that provides programmed control of ion injection and trapping, and sensitive TOF analysis of ions ejected from the trapped-ion cell is described. A theoretical treatment of ion propagation is presented along with new trapping procedures that provide control over the mass range of ions acquired. A modular data system that provides arbitrary waveform excitation and selective control of ion radial and trapping motion is described, and a novel trapped-ion cell design that substitutes open trapping electrodes for trapping plates positioned perpendicular to the magnetic field is evaluated. This design allows a simple coupling of the excitation waveforms to the trapping electrodes that is demonstrated to eliminate axial ejection of ions during excitation.; Mechanisms for the trapping of externally generated ions by static trapping potentials are examined and attributed to ion/molecule reactions occurring in the cell for the case of an external electron ionization (EI) source, and to Debye shielding of low energy ions from the cell trapping fields due to quasineutral plasma behavior in the case of laser desorption ionization (LDI) of metals. Spontaneous coherent magnetron motion of ions acquired from LDI is described and found to parallel that of magnetron transients acquired by EI. Initiation of coherent motion is attributed to radial ion cloud displacement resulting from off-axis ion formation or injection, and to forces on the ion cloud that result from asymmetrically distributed image potentials developed on cell electrodes when large ion populations are acquired. Destabilization of coherent magnetron motion due to loss of ion energy in resistive external circuit elements is found to result in radial ion transport and ultimately ion loss from the trapped-ion cell. A simple model describing the transport effect is derived and found to relate the rate of transport to ion number, external circuit resistance, and applied trapping potential.