Mass measurements of very high accuracy by time-of-flight ion cyclotron resonance of ions injected into a penning trap

Precision Mass Measurement of the Proton Dripline Halo Candidate ^{22}Al

We report the first mass measurement of the proton-halo candidate ^{22}Al performed with the low energy beam ion trap facility's 9.4 T Penning trap mass spectrometer at facility for rare isotope beams. This measurement completes the mass information for the lightest remaining proton-dripline nucleus achievable with Penning traps. ^{22}Al has been the subject of recent interest regarding a possible halo structure from the observation of an exceptionally large isospin asymmetry [J. Lee et al., Large isospin asymmetry in Si22/O22 Mirror Gamow-Teller transitions reveals the halo structure of ^{22}Al, Phys. Rev. Lett. 125, 192503 (2020).PRLTAO0031-900710.1103/PhysRevLett.125.192503]. The measured mass excess value of ME=18 092.5(3)  keV, corresponding to an exceptionally small proton separation energy of S_{p}=100.4(8)  keV, is compatible with the suggested halo structure. Our result agrees well with predictions from sd-shell USD Hamiltonians. While USD Hamiltonians predict deformation in the ^{22}Al ground state with minimal 1s_{1/2} occupation in the proton shell, a particle-plus-rotor model in the continuum suggests that a proton halo could form at large quadrupole deformation. These results emphasize the need for a charge radius measurement to conclusively determine the halo nature.

Excitation of radial ion motion in an rf-only multipole ion guide immersed in a strong magnetic field gradient

Radiofrequency (rf) multipole ion guides are widely used to transfer ions through the strong magnetic field gradient between source and analyzer regions of external source Fourier transform ion cyclotron resonance mass spectrometers. Although ion transfer as determined solely by the electric field in a multipole ion guide has been thoroughly studied, transfer influenced by immersion in a strong magnetic field gradient has not been as well characterized. Recent work has indicated that the added magnetic field can have profound effects on ion transfer, ultimately resulting in loss of ions initially contained within the multipole. Those losses result from radial ejection of ions due to transient cyclotron resonance that occurs when ions traverse a region in which the magnetic field results in an effective cyclotron frequency equal to the multipole rf drive frequency divided by the multipole order (multipole order is equal to one-half the number of poles). In this work, we describe the analytical basis for ion resonance in a rf multipole ion guide with superposed static magnetic field and compare with results of numerical trajectory simulations.

Three-dimensional motional stabilization in the trapping field of an open-ended trapped-ion cell: application to the remeasurement experiment in Fourier transform ion cyclotron resonance mass spectrometry

Three-dimensional motional stabilization of radial trajectories of low-mass organic ions in an open cell using only a dc trapping field is applied to the FT-ICR remeasurement experiment. More than 300 remeasurement cycles are observed with 99.59% remeasurement efficiency for benzene (m/z 78) using a high-pressure helium buffer gas. The enhancement in remeasurement efficiency is due to collisional stabilization of the guiding center of ion motion by dynamic motional averaging in the axial position-dependent radial electric field. Such dc-induced radial stabilization is in contrast to the stability produced by application of radio frequency fields characteristic of quadrupolar axialization or rf-only mode operation. The same effect is produced because ions experience a radial "pseudopotential" during axial oscillation as in time-varying fields. Trajectory simulations for ions oscillating in an open cell trapping well above a z-amplitude critical threshold energy of 0.60 eV (in a potential well of 0.84 V) indicate that radially stabilized trapping motion is achieved because the outward-directed radial electric field existing near the cell center line is compensated by an opposing inward-directed radial electric field at extended z-amplitude. Sufficient axial kinetic energy permits ion penetration into the inward-directed radial electric field regions, enabling > 50% residence time of each trapping oscillation period in regions inducing radial stability, thereby inhibiting magnetron radius growth. A high-pressure, low-mass buffer gas such as helium provides the requisite increase in the axial amplitude of the ion cloud, similar to the mechanism observed for axial excitation of low-mass ions observed in collision-induced dissociation. The result is radial stability at high pressure, even after multiple remeasurement cycles. An optimized excitation radius of 12.5% of the cell radius yields maximum remeasurement efficiency with a 500 ms relaxation delay between excitation events. Summed signal intensity decreases with increased trap potential due to the greater radial electric field and reduced axial expansion of the ion cloud and also decreases with buffer gas mass in response to greater radial scattering.

Simplified application of quadrupolar excitation in Fourier transform ion cyclotron resonance mass spectrometry

A new method for application of quadrupolar excitation to the trapped ion cell of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer is presented. Quadrupolar excitation is conventionally applied to the two pairs of opposed electrodes that normally perform the excitation and detection functions in the FTICR experiment. Symmetry arguments and numerically calculated isopotential contours within the trapped ion cell lead to the conclusion that quadrupolar excitation can be applied to a single pair of opposed side electrodes. Examples of effective quadrupolar axialization via this method include a sevenfold signal-to-noise enhancement derived from 50 remeasurements of a single population of trapped bovine insulin ions and the selective isolation of a single charge state of horse heart myoglobin after an initial measurement that revealed the presence of 14 charge states.

Excitation modes for fourier transform-ion cyclotron resonance mass spectrometry

Various geometric configurations for the excitation of coherent ion motion in Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR/MS) are analyzed (in some cases for the first time) with unified notation. The instantaneous power absorption, F v, in which v is ion velocity and F the force produced by the applied excitation electric field (harmonic, single frequency, on-resonance, in-phase), is time averaged and then set equal to the time rate of change of ion total (cyclotron + magnetron + trapping) energy, to yield a differential equation that is readily solved for the (time-dependent) amplitude of each of the various ion motions. The standard FT-ICR excitation (namely, radial dipolar) is reviewed. The effects of quadrature and radial quadrupolar excitation on ion radial (cyclotron and magnetron) motions are also reviewed. Frictional damping is shown to decrease the ion cyclotron orbital radius and trapping amplitude but increase the magnetron radius. Feedback excitation (i.e., excitation at the simultaneously detected ion cyclotron orbital frequency of the same ion packet) is introduced and analyzed as a means for exciting ions whose cyclotron frequency changes during excitation (as for relativistically shifted low-mass ions). In contrast to conventional radial dipolar excitation, axial dipolar excitation of the trapping motion leads to a mass-dependent ion motional amplitude. Parametric (i.e., axial quadrupolar) excitation is shown to produce an exponential increase in the ion motional amplitudes (hyperbolic sine and hyperbolic cosine amplitude for cyclotron and magnetron radii, respectively). More detailed consideration of parametric excitation leads to an optimal ion initial radial position in parametric-mode FT-ICRjMS.

Experimental evaluation of a hyperbolic ion trap for fourier transform ion cyclotron resonance mass spectrometry

The Penning ion trap, consisting of hyperbolically curved electrodes arranged as an unbroken ring electrode capped by two end electrodes whose interelectrode axis lies along the direction of an applied static magnetic field, has long been used for single-ion trapping. More recently, it has been used in "parametric" mode for ion cyclotron resonance (lCR) detection of off-axis ions. In this article, we describe and test a Penning trap whose ring electrode has been cut into four equal quadrants for conventional dipolar ICR excitation (on one pair of opposed ring quadrants) and dipolar ICR detection (on the other pair). In direct comparisons to a cubic trap, the present hyperbolic trap offers somewhat improved ICR mass spectral peak shape, higher mass resolving power, and comparable frequency shift as a function of trapping voltage. Mass measurement accuracy over a wide mass range is improved twofold and mass discrimination is somewhat worse than for a cubic trap. The relative advantages of parametric, dipolar, and quadrupole modes are briefly discussed in comparison to screened and unscreened cubic traps.