Optimization of Radially Segmented Ion Mirrors for High Resolution Charge Detection Mass Spectrometry

Charge detection mass spectrometry (CD-MS) enables mass measurements to be made for heterogeneous samples into the gigadalton regime. In CD-MS, ions are trapped in an electrostatic linear ion trap (ELIT) where they oscillate back and forth through a detection cylinder. The m/z is determined from the oscillation frequency, and the charge is obtained from the signal amplitude. The charge can be measured with a precision of better than 0.2 e (elementary charges), where ions can be assigned to the correct charge state with a low error rate. Thus, the main factor limiting mass resolution in CD-MS measurements is the imprecision in the m/z determination for individual ions. In prior work, it was shown that m/z resolving powers >300,000 could be achieved by optimizing the ELIT design to minimize the dependence of the ion's oscillation frequency on the ion's kinetic energy and trajectory. However, the high-resolution ELIT designs that we found were intolerant to small misalignments of the trap electrodes that result from manufacturing imprecision. A misalignment of less than 20 μm caused the trapping efficiency to drop to zero. The best resolving power achieved with a more tolerant ELIT design (where manufacturing imprecision does not catastrophically reduce the trapping efficiency) is 14,000-15,000. Here, we explore a solution to the intolerant ELIT designs where some of the ELIT mirror electrodes are segmented to allow small trim potentials to correct for mechanical misalignments. The trim potentials can be optimized under computer control to maximize trapping efficiency and m/z resolution. Trajectory simulations indicate that a high trapping efficiency can be recovered (>90%) while retaining high resolving powers (>200,000).