A different symmetry is required to optimize each of the three most common Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) electric potentials in a Penning (ICR) ion trap: one-dimensional dipolar ac for excitation (or detection), two-dimensional azimuthal quadrupolar ac excitation for ion axialization, and three-dimensional axial quadrupolar dc potential for ion axial confinement (trapping). Since no single trap shape simultaneously optimizes all three potentials, many trap configurations have been proposed to optimize the tradeoffs between the three requirements for a particular experiment. A more general approach is to divide each electrode into small segments and then apply the appropriate potential to each segment. Here, we extend segmentation to its logical extreme, by constructing a “matrix-shimmed” trap consisting of a cubic trap, with each side divided into a 5 × 5 grid of electrodes for a total of 150 electrodes. Theoretically, only 48 independent voltages need be applied to these 150 electrodes to generate all three desired electric potential fields simultaneously. In practice, it is more convenient to employ 63 independent voltages due to construction constraints. Resistive networks generate the potentials required for optimal quadrupolar trapping and quadrupolar excitation. To avoid resistive loss of excitation amplitude and detected signal, dipolar excitation/detection voltages are generated with a capacitive network. Theoretical Simion 6. 0 simulations confirm the achievement of near-ideal potentials of all three types simultaneously. From a proof-of-principle working model, several experimental benefits are demonstrated, and proposed future improvements are discussed.
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Jackson et al. (1999) studied this question.