OPTIMIZATION OF A FIXED-VOLUME OPEN GEOMETRY TRAPPED ION CELL FOR FOURIER-TRANSFORM ION-CYCLOTRON MASS-SPECTROMETRY

Several open cylindrical trapped ion cells of fixed volume but varying electrode dimension are evaluated for the Fourier transform ion cyclo tron resonance mass spectrometry experiment. In closed geometry cells the z-axis dimension is increased to form elongated cells with reduced radial electric field. In contrast, the aspect ratio for open cells c an be increased without increasing the overall cell volume, an importa nt consideration when magnetic field homogeneity is a concern. The inc rease in aspect ratio is achieved by reducing the relative dimensions of the trap electrodes with respect to the excitation and detection el ectrodes. The radial electric field magnitude for a fixed-length open cell with aspect ratios ranging from 0.84 to 1.60 is evaluated both th eoretically and experimentally. Experimental frequency shifts for fixe d-volume cells are reduced from -74.3 to -18.1 Hz V-1, a 411% reductio n, by increasing the aspect ratio of an open cell from 1.20 to 1.60 re spectively. As an alternative to the present standard open cell with e lectrodes of equal length, an elongated open cell geometry is optimize d for maximized well depth (ion capacity) and minimized radial electri c field..