Open cell analog of the screened trapped-ion cell using compensation electrodes for Fourier transform ion cyclotron resonance mass spectrometry

Abstract

The open cell collinear electrode geometry permits the inclusion of an auxiliary set of electrodes adjacent to the trap electrodes to perform several functions. Here they are used as compensation electrodes that virtually eliminate the radial electric field at the z = 0 midplane of the cell. The function of the compensation electrodes in reducing the radial electric field is analogous to the grounded transmissive screen inserted between the trap electrodes and the trapping volume in the closed screened cell whereby the interior of the cell is shielded from the trapping field. Segmenting the trap electrodes and applying oppositely biased potentials to each set of segments reduces the radial electric field at the z = 0 midplane of the cell by superposition of the opposing electric fields. In addition, dynamic adjustment of the potential well contour is performed by adjusting the relative potentials on each set of electrodes. The supplementary voltage applied to the inner set of compensation electrodes reduces the radial electric field by nearly two orders of magnitude and increases the potential well depth for greater ion capacity. In addition, the potential well assumes increased particle-in-a-box character without increasing the physical size of the cell, thereby reducing the effect of space charge. A FORTRAN program is developed that models the cell geometry and predicts the relationship between trap and compensation electrode voltage required to minimize the radial electric field throughout a specified cell volume. A theoretically optimum ratio of -0.33 V applied to the compensation electrodes for 1.0 V applied to the trap electrodes is predicted and is in close agreement with an experimentally determined optimum ratio of -0.36 V applied to the compensation electrodes for 1.0 V applied to the trap electrodes. This ratio reduces the cyclotron frequency shift from -70.8 Hz V -1 in an uncompensated open cell to -0.50 Hz V -1, a reduction of more than 99%. The radial electric field at the z = 0 midplane of the cell and 90% of the cell radius is reduced 97%, from 0.0139 V mm -1 to 0.0004 V mm -1. The reduction in frequency shift is accomplished without compromising mass accuracy. By collisionally damping ions to the center of the cell, mass accuracy over a one-decade range (60–600 u) approaches the mass accuracy of the hyperbolic cell geometry.