Geometry and development

The ICR cell is the heart of the FT-ICR mass spectrometer, it is the mass analyser required in all FT- ICR MS based experiments, yet ICR cells appear in many varieties, each designed and optimised to perform particularly well in certain situations and/or combat one or more challenges during an FT- ICR MS experiment (trapping field effects, z-axis ejection, peak harmonics, etc).

The first FT-ICR MS cells were based on the cubic geometry (Figure 1.19, below), with straight forward fabrication, able to fit effectively between the poles of permanent magnets, and to a lesser degree in superconducting magnets, cubic ICR cells were used for many years on extremely

successful platforms.26,28,29 Cubic cells had typical electrode configurations – two excitation electrodes, two detection electrodes, and two trapping electrodes, all equal in size. Multiple-cell instruments were also constructed using Cubic cells for unique experiments,5 _{more recently this}

approach has been revisited and utilised for fast acquisition using 3 linked cubic cells in a parallel acquisition FT-ICR MS experiment.30 However Cubic cells suffered from low ion capacity and were not optimised for superconducting magnet geometries now in more common use.

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Figure 1.19: examples of various ICR cells which have been effectively used in FT-ICR MS, including the (a) Cubic (b) cylindrical (c) segmented end cap cylindrical cell aka the “infinity cell” (d) open cylindrical cell (e) open capacitively coupled cylindrical cell (f) dual cubic cell (g) “matrix shimmed”

cell. E = excitation D = detection T = trapping plates. Reproduced from Marshall et. al.5

Cubic cells were succeed by cylindrical ICR cells, using rounded, quarter-circumference

excitation/detection plates, with either circular end cap trapping electrodes (for closed cylindrical cells) or cylindrical trapping plates (for open cylindrical cells). The curved shape of the detection electrodes used in cylindrical cells enabled a more prolonged interaction with ion-packets as they passed the surface during detection, increasing the interaction between ions and available electrons, improving sensitivity. End-capped cylindrical cells suffer from electric field permeation throughout the ICR cell and disruption of the ion orbit away from the centre of the cell, as discussed previously.31

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While open cylindrical cells suffer more from z-axis ejection of ions during excitation, due to resonant energy transfer into this frequency mode.32 In order to combat these issues ever- improving ICR cell designs have been released; the capacitively coupled open-cylindrical cell was introduced by Beu et. al.32 in order to capacitively couple the excitation and detection electrodes during ion excitation to eliminate z-axis ejection of ions along the magnetic field axis. In another approach to avoid the problems caused by using end cap electrodes and a cylindrical cell of finite dimensions, Caravatti et. al.33 introduced the aptly named “Infinity cell”, a closed-cylindrical cell with special resistively-coupled, surface-mounted trapping electrode wires which are positioned and designed to mimic an infinitely long trapping electrode geometry, and thus minimise/eliminate some axial excitation and ejection during the ion excitation event.

The Infinity cell and the capacitively coupled open cylindrical cell are both shown above in Figure 1.19. The Infinity cell has for many years been the proprietary cell of Bruker Daltonics FT-ICR MS instruments, measuring just 6cm in diameter and 6cm in length, the infinity cell has been shown (with careful tuning and operation) to achieve performance into the multiple millions of resolving power34 and low part-per-million (ppm) to part-per-billion (ppb) accuracy in mass measurement.34 The majority of the work presented herein was conducted on a Bruker Daltonics FT-ICR MS

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Figure 1.20: Schematic representation of the Infinity ICR cell, a closed cylindrical cell equipped with side-kick electrodes, two excitation plates, to detection plates, and two trapping plates. Courtesy of Bruker Daltonics, Bremen, Germany.

Figure 1.21: Left: Schematic of the Infinity cell trapping electrodes (a), showing the resistor chains and circuitry associated with the Infinity cell design (b) which attempts to mimic an infinitely long cell in order to avoid unwanted ejection of ions along the z-axis during excitation. Reproduced

from Caravatti et.al.31 Right: Image of the Infinity cell mounted on a standard 6” stainless steel

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The Infinity cell has the benefit of virtually eliminating the unwanted ejection of ions along the z-axis during ion excitation by using the capacitor-functionalised trapping plates to mimic an infinitely long cylindrical cell. This enables higher sensitivity than the corresponding open-cylindrical cell as fewer ions are lost during excitation. The Infinity cell has been shown to require slightly more tuning than open-cylindrical cell geometries, but can achieve extremely long transients and thus resolving power performance. The main disadvantages of the Infinity cell are the need for more tuning at high performances, the closed cell geometry producing challenges for application/alignment of lasers/electron beams for further experiments, and its small size (6cmx6cm) causing a lower ion capacity than a corresponding cylindrical cell which could fit in the same magnet bore. Though the Infinity cell suffers from less ion loss and thus increased sensitivity, which reduces the latter point.