Open trapped ion cell geometries for Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance mass spectrometry is evaluated in trapped ion cells which substitute open rectangular trapping electrodes for trapping plates positioned perpendicular to the magnetic field. Compared with closed cells, the open cell geometry offers improved access to the cell interior with obvious benefits for external ion injection. The well depth along the z-axis centerline of the open cell is about 37% of the applied potential for cubic geometry but increases to 81% for an elongated cell with aspect ratio two. Isopotential field maps are created to compare axial and radial components of trapping and excitation fields for open and closed orthorhombic cells. Radial trapping electric fields are somewhat larger in the center of the open cells but axial excitation fields are a factor of two smaller compared with the closed cell. An open elongated cell with an aspect ratio of two is constructed and compared with a closed cell of similar aspect ratio. Experimentally, a −57 Hz V−1 shift in cyclotron frequency owing to the radial trapping field is obtained for the open cell compared with a −32 Hz V−1 shift for the closed cell. However, axial ejection is significantly reduced in the open cell compared with the closed cell. Suggestions are given for generalizing the open cell to a mutisegmented electrode assembly in which excitation, detection and trapping are performed interchangeably with the same electrodes. For example, capacitive coupling of the excitation potential to the trap electrodes is suggested as a simple means of eliminating axial ejection during excitation.