An experimental study of ion motions in a double cell FT/ICR instrument

Concentric tube vacuum chamber for high magnetic field, high-pressure ionization in a fourier transform ion cyclotron resonance mass spectrometer

A new differential pumping design for external source Fourier transform ion cyclotron resonance mass spectrometry is described. A network of concentric tubes of increasing diameter terminates at a series of conductance limits across which a pressure from atmosphere to low-10(-8) torr is achieved. This design permits high-pressure sources to be positioned within the solenoidal superconducting magnet less than 20 cm from the analyzer trapped ion cell. Ionization at high magnetic field offers the advantage of radial ion confinement and consequently delivers enhanced ion current to the trapped ion cell. Ion injection utilizing this vacuum chamber design is simpler than previously reported serial pumping stage designs because elaborate focusing optics to overcome the magnetic mirror effect are unnecessary. Two probe-mounted atmospheric pressure sources are described as evidence of the general applicability of the concentric tube vacuum chamber. An electrospray source that delivers several hundred picoamperes of ion current to the cell yields high-sensitivity spectra of proteins beyond 100 kDa. Improved pumping compared with a prototype concentric tube network configuration now permits mass resolution in excess of 20,000 for the [M + 4H](4+) ion of melittin. The resolution is sufficient to distinguish isotope peaks within a single charge state. A probe-mounted, pulsed-laser ablation source that permits cluster formation in the strong magnetic field is also demonstrated.

Direct observation of trapping motion in elongated fourier-transform mass spectrometry trapped ion cells

Measurements by Fourier-transform mass spectrometry (FTMS) have been used to measure trapping oscillation profiles in elongated trapped ion cells of length 10-43 cm. Trapping periods extracted from these profiles are found to vary linearly with cell length for elongated cells. This is in contrast with the prediction based on a quadrupolar approximation of the electric field that trapping period should increase exponentially with increased cell length. An alternate analytical expression for trapping motion is derived that better accounts for the motion of ions with sufficient energy to approach the trap plates. Calculated trapping frequencies are within a few percent of values determined from ion trajectory simulations for any combination of cell length, trap potential, ion mass, and ion kinetic energy. The new expression also explains the experimentally determined trapping data obtained in elongated cells. This expression predicts an average axial energy near 0.6 eV for the ions that are preferentially detected by FTMS with the specific pulse sequence employed. Department of Chemistry.