Restrained ion population transfer: a novel ion transfer method for mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometers function such that the ion accumulation event takes place in a region of higher pressure outside the magnetic field which allows ions to be thermally cooled before being accelerated toward the ICR cell where they are decelerated and re-trapped. This transfer process suffers from mass discrimination due to time-of-flight effects. Also, trapping ions with substantial axial kinetic energy can decrease the performance of the FTICR instrument compared with the analysis of thermally cooled ions located at the trap center. Therefore, it is desirable to limit the energy imparted to the ions which results in lower applied trap plate potentials and reduces the spread in axial kinetic energy. The approach presented here for ion transfer, called restrained ion population transfer or RIPT, is designed to provide complete axial and radial containment of an ion population throughout the entire transfer process from the accumulation region to the ICR cell, eliminating mass discrimination associated with time-of-flight separation. This was accomplished by use of a number of quadrupole segments arranged in series with independent control of the direct current (DC) bias voltage applied to each segment of the quadrupole ion guide. The DC bias voltage is applied in such a way as to minimize the energy imparted to the ions allowing transfer of ions with low kinetic energy from the ion accumulation region to the ICR cell. Initial FTICR mass spectral data are presented that illustrate the feasibility of RIPT. A larger m/z range for a mixture of peptides is demonstrated compared with gated trapping. The increase in ion transfer time (3 ms to 130 ms) resulted in an approximately 11% decrease in the duty cycle; however this can be improved by simultaneously transferring multiple ion populations with RIPT. The technique was also modeled with SIMION 7.0 and simulation results that support our feasibility studies of the ion transfer process are presented.     

High performance fourier transform ion cyclotron resonance mass spectrometry via a single trap electrode

An open-ended cylindrical cell with a single annular trap electrode located at the center of the excitation and detection region is demonstrated for Fourier transform ion cyclotron resonance mass spectrometry. A trapping well is created by applying a static potential to the trap electrode of polarity opposite the charge of the ion to be trapped, after which conventional dipolar excitation and detection are performed. The annular trap electrode is axially narrow to allow the creation of a potential well without excessively shielding excitation and detection. Trapping is limited to the region of homogeneous excitation at the cell centerline without the use of capacitive coupling. Perfluorotributylamine excitation profiles demonstrate negligible axial ejection throughout the entire excitation voltage range even at an effective centerline potential of only ?0.009 V. High mass resolving power in the single-trap electrode cell is demonstrated by achievement of mass resolving power of 1.45 × 10⁶ for benzene during an experiment in which ions created in a high pressure source cubic cell are transferred to the low pressure analyzer single-trap electrode cell for detection. Such high performance is attributed to the negligible radius dependent radial electric field for ions cooled to the center of the potential well and accelerated to less than 60% of the cell radius. An important distinction of the single-trap electrode geometry from all previous open and closed cell arrangements is exhibition of combined gated and accumulated trapping. Because there is no potential barrier, all ions penetrate into the trapping region regardless of their translational energy as in gated trapping, but additional ions may accumulate over time, as in accumulated trapping. Ions of low translational kinetic energy are demonstrated to be preferentially trapped in the single-trap electrode cell. In a further demonstration of the minimal radial electric field of the single-trap electrode cell, positive voltages can be applied to the annular trap electrode as well as the source cell trap electrode to achieve highly efficient transfer of ions between cells.     

Improved ion optics for introduction of ions into a 9.4‐T Fourier transform ion cyclotron resonance mass spectrometer

Enhancements to the ion source and transfer optics of our 9.4 T Fourier transform ion cyclotron resonance (ICR) mass spectrometer have resulted in improved ion transmission efficiency for more sensitive mass measurement of complex mixtures at the MS and MS/MS levels. The tube lens/skimmer has been replaced by a dual ion funnel and the following octopole by a quadrupole for reduced ion cloud radial expansion before transmission into a mass‐selective quadrupole. The number of ions that reach the ICR cell is increased by an order of magnitude for the funnel/quadrupole relative to the tube lens/skimmer/octopole. Copyright © 2015 John Wiley & Sons, Ltd.     

Matrix-assisted laser desorption/ionization fourier transform ion cyclotron resonance mass spectrometry with pulsed in-source collision gas and in-source ion accumulation

A new matrix-assisted laser desorption/ionization (MALDI) source for Fourier transform ion cyclotron resonance mass spectrometry (FTMS) has been developed. The new source is equipped with a hexapole ion guide. The sample on the laser target is one millimeter from the hexapole ion guide, so that ions are desorbed directly into the guide. A device for pulsing collision gas in direct proximity to the laser target makes it possible to cool the ions, which have a kinetic energy spread of several electron volts when produced by the MALDI process. These ions are trapped in the hexapole where positive potentials at the laser target and at an extraction plate help trap ions along the longitudinal axis. After a pre-defined trapping time the voltage of the extraction plate is reversed and the trapped ions are extracted for transmission to the ion cyclotron resonance cell. Accumulation of ions from multiple laser shots in the hexapole before mass spectrometric analysis increases sensitivity. Preliminary sensitivity studies with substance P show that 10 attomoles of analyte applied on the target can be detected with a signal-to-noise (S/N) ratio >15.     

Reduction of axial kinetic energy induced perturbations on observed cyclotron frequency

With Fourier transform ion cyclotron resonance (FTICR) mass spectrometry one determines the mass-to-charge ratio of an ion by measuring its cyclotron frequency. However, the need to confine ions to the trapping region of the ion cyclotron resonance (ICR) cell with electric fields induces deviations from the unperturbed cyclotron frequency. Additional perturbations to the observed cyclotron frequency are often attributed to changes in space charge conditions. This study presents a detailed investigation of the observed ion cyclotron frequency as a function of ion z-axis kinetic energy. In a perfect three-dimensional quadrupolar field, cyclotron frequency is independent of position within the trap. However, in most ICR cell designs, this ideality is approximated only near the trap center and deviations arise from this ideal quadrupolar field as the ion moves both radially and axially from the center of the trap. To allow differentiation between deviations in observed cyclotron frequency caused from changes in space charge conditions or differences in oscillation amplitude, ions with identical molecular weights but different axial kinetic energy, and thus amplitude of z-axis motion, were simultaneously trapped within the ICR cell. This allows one to attribute deviations in observed cyclotron frequency to differences in the average force from the radial electric field experienced by ions of different axial amplitude. Experimentally derived magnetron frequency is compared with the magnetron frequency calculated using SIMION 7.0 for ions of different axial amplitude. Electron promoted ion coherence, or EPIC, is used to reduce the differences in radial electric fields at different axial positions. Thus with the application of EPIC, the differences in observed cyclotron frequencies are minimized for ions of different axial oscillation amplitudes.     

Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps

We present a numerical method for computation of electrostatic (trapping) and time-varying (excitation) electric fields and the resulting ion trajectory and detected time-domain-induced voltage signal in a rectangular (or cubic) ion cyclotron resonance (ICR) ion trap. The electric potential is calculated by use of the superposition principle and relaxation method with a large number of grid points (e.g., 100 × 100 × 100 for a cubic trap). Complex ICR experiments and spectra may now be simulated with high accuracy. Ion trajectories may be obtained for any combination of trapping and excitation modes, including quadrupolar or cubic trapping in static or dynamic mode; and dipolar, quadrupolar, or parametric excitation with single-frequency, frequency-sweep (chirp), or stored waveform inverse Fourier transform waveforms. The resulting ion trajectory may be represented either as its three dimensional spatial path or as two-dimensional plots of x-, y-, or z-position, velocity, or kinetic energy versus time in the absence or presence of excitation. Induced current is calculated by use of the reciprocity principle, and simulated ICR mass spectra are generated by Fourier transform of the corresponding time-domain voltage signal.     

Ion trajectories in an electrostatic ion guide for external ion source fourier transform ion cyclotron resonance mass spectrometry

An electrostatic ion guide (EIG) that consists of concentric cylinder and central wire electrodes can transport ions efficiently from an external ion source to an ion cyclotron resonance (ICR) ion trap for mass analysis, with several advantages over current injection methods. Because the electrostatic force of the EIG captures ions in a stable orbit about the wire electrode, ions with initially divergent trajectories may be redirected toward the ICR ion trap for improved ion transmission efficiency. SIMION trajectory calculations (ion kinetic energy, 1–200 eV; elevation angle, 0.30 °; azimuthal angle, 0.360°) predict that ions of m/z 1000 may be transmitted through a strong (0.01 → 3.0-T) magnetic field gradient. Judicious choice of ion source position and EIG potential minimizes the spread in ion axial kinetic energy at the ICR ion trap. Advantages of the EIG include large acceptance angle, even for ions that have large initial kinetic energy and large radial displacement with respect to the central z-axis, low ion extraction voltage (5–20 V), and efficient trapping because ions need not be accelerated to high velocity to pass through the magnetic field gradient.     

An automated high performance capillary liquid chromatography-Fourier transform ion cyclotron resonance mass spectrometer for high-throughput proteomics

We describe a fully automated high performance liquid chromatography 9.4 tesla Fourier transform ion resonance cyclotron (FTICR) mass spectrometer system designed for proteomics research. A synergistic suite of ion introduction and manipulation technologies were developed and integrated as a high-performance front-end to a commercial Bruker Daltonics FTICR instrument. The developments incorporated included a dual-ESI-emitter ion source; a dual-channel electrodynamic ion funnel; tandem quadrupoles for collisional cooling and focusing, ion selection, and ion accumulation, and served to significantly improve the sensitivity, dynamic range, and mass measurement accuracy of the mass spectrometer. In addition, a novel technique for accumulating ions in the ICR cell was developed that improved both resolution and mass measurement accuracy. A new calibration methodology is also described where calibrant ions are introduced and controlled via a separate channel of the dual-channel ion funnel, allowing calibrant species to be introduced to sample spectra on a real-time basis, if needed. We also report on overall instrument automation developments that facilitate high-throughput and unattended operation. These included an automated version of the previously reported very high resolution, high pressure reversed phase gradient capillary liquid chromatography (LC) system as the separations component. A commercial autosampler was integrated to facilitate 24 h/day operation. Unattended operation of the instrument revealed exceptional overall performance: Reproducibility (1-5% deviation in uncorrected elution times), repeatability (<20% deviation in detected abundances for more abundant peptides from the same aliquot analyzed a few weeks apart), and robustness (high-throughput operation for 5 months without significant downtime). When combined with modulated-ion-energy gated trapping, the dynamic calibration of FTICR mass spectra provided decreased mass measurement errors for peptide identifications in conjunction with high resolution capillary LC separations over a dynamic range of peptide peak intensities for each spectrum of 10(3), and >10(5) for peptide abundances in the overall separation.     

Improved ion extraction from a linear octopole ion trap: SIMION analysis and experimental demonstration

Externally generated ions are accumulated in a linear octopole ion trap before injection into our 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer. Such instrumental configuration has previously been shown to provide improved sensitivity, scan rate, and duty cycle relative to accumulated trapping in the ICR cell. However, inefficient ion ejection from the octopole currently limits both detection limit and scan rate. SIMION 7.0 analysis predicts that a dc axial electric field inside the linear octopole ion trap expedites and synchronizes the efficient extraction of the octopole-accumulated ions. Further SIMION analysis optimizes the ion ejection properties of each of three electrode configurations designed to produce a near-linear axial potential gradient. More efficient extraction and transfer of accumulated ions spanning a wide m/z range promises to reduce detection limit and increase front-end sampling rate (e.g., to increase front-end resolution for separation techniques coupled with FT-ICR mass analysis). Addition of the axial field improves experimental signal-to-noise ratio by more than an order of magnitude.     

Electron beam potential depression as an ion trap in Fourier transform ion cyclotron resonance mass spectrometry

Trapping of ions in the electron beam of a FTICR mass spectrometer is investigated and a simple model describing the confinement process is presented. Detection of resistive-wall destabilization of the magnetron motion of ions in the trapped-ion cell is used to determine conditions for ion trapping within and escape from the electron beam. The model predicts a potential well that is dependent on electron beam current, energy, and dimension in defining its capacity for low energy ions. Plots of ion retention time versus ion number are consistent with a model in which ions are initially trapped in the electron beam but with increasing ion formation will eventually overcome the potential depression in the electron beam and escape into magnetron orbits. Based upon this model, expressions are derived for ion retention time which are then fit to the experimental data. The model is used to estimate ion number, initial magnetron radius and ion cloud shape and density. One example in which electron trapping is important in the FTICR experiment is in the efficient transfer of ions between dual trapped-ion cells. Ion transfer within the potential depression of the electron beam environment is shown to be virtually 100% efficient over a 10 ms interval whereas all ions are lost to collisions with the conductance limit after 2 ms when transferring without the confining aid of the electron beam. Several analytical applications of electron traps in the ICR cell are now being investigated.     

Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps. Effect of ion initial axial position on ion coherence,induced signal,and radial or z ejection in a cubic trap

The effects of ion initial axial position on coherence of ion motion, induced ion cyclotron resonance (ICR) signal. and radial and z ejection have been evaluated by numerical simulation for a cubic Fourier transform-ion cyclotron resonance ion trap. For a given initial ion cyclotron phase and radius, ions of different initial z position are shown to be excited to significantly different ion cyclotron radii (and ultimately radially ejected at significantly different excitation amplitude-duration products). Ion initial z displacement from the trap midplane affects observed ICR signal magnitude in two ways: (1) for the same postexcitation cyclotron radius, an ion with larger initial z displacement induces a smaller ICR signal and (2) an ion with larger initial z displacement is excited to a smaller cyclotron radius. We also evaluate the induced ICR signal as a function of excitation amplitude-duration product for spatially uniform or Gaussian ion initial z distributions. In general, if the excitation waveform contains components at frequency, 2 ω_z or (ω₊ + 2 ω_z, in which ω_z is the axial C“trapping”) oscillation frequency, then ejection occurs axially. However, the resulting excitation amplitude-duration product for such axial ejection is significantly higher (factor of, ～ 4) than that required for radial ejection (at ω₊) for ions of small initial radius. The present results offer the first explanation of how, even if the ion is initially at rest on the z axis (i.e., zero excitation electric field amplitude on the z axis), z ejection (axial ejection) may nevertheless occur if the excitation waveform contains frequency components at ω₊ + 2ω_z and/or 2_{w z} Namely, our simulations reveal that off-resonant excitation pushes ions away from the z axis, after which the ions are exposed to z excitation and eventual z ejection.     

Electron capture dissociation Fourier transform ion cyclotron resonance mass spectrometry in the electron energy range 0-50 eV

Electron capture dissociation (ECD) of polypeptide cations was obtained with pencil and hollow electron beams for both sidekick and gas-assisted dynamic ion trapping (GADT) using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) with an electrostatic ion transfer line. Increasing the number of trapped ions by multiple ICR trap loads using GADT improved the ECD sensitivity in comparison with sidekick ion trapping and ECD efficiency in comparison with single ion trap load by GADT. Furthermore, enhanced sensitivity made it possible to observe ECD in a wide range of electron energies (0-50 eV). The degree, rate and fragmentation characteristics of ECD FTICR-MS were investigated as functions of electron energy, electron irradiation time, electron flux and ion trapping parameters for this broad energy range. The results obtained show that the rate of ECD is higher for more energetic (>1 eV) electrons. Long electron irradiation time with energetic electrons reduces average fragment ion mass and decreases efficiency of formation of c- and z-type ions. The obtained dependencies suggest that the average fragment ion mass and the ECD efficiency are functions of the total fluence of the electron beam (electron energy multiplied by irradiation time). The measured electron energy distributions in low-energy ECD and hot ECD regimes are about 1 eV at full width half maximum in employed experimental configurations.     

In-source H/D exchange and ion-molecule reactions using matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry with pulsed collision and reaction gases

Controlled in-source ion-molecule reactions are performed for the first time in an external matrix assisted laser desorption ionization (MALDI) source of a Fourier transform ion cyclotron resonance mass spectrometer. The MALDI source with a hexapole ion guide that was originally designed to incorporate pulsed gas to collisionally cool ions (Baykut, G.; Jertz, R.; Witt, M. Rapid Commun. Mass Spectrom. 2000, 14, 1238-1247) has been modified to allow the study of in-source ion-molecule reactions. Upon laser desorption, a reaction gas was introduced through a second inlet and allowed to interact with the MALDI-generated ions trapped in the hexapole ion guide. Performing ion-molecule reactions in the high pressure range of the ion source prior to analysis in the ion cyclotron resonance (ICR) cell allows to maintain the ultra high vacuum in the cell which is crucial for high mass resolution measurements. In addition, due to the reaction gas pressure in the hexapole product ion formation is much faster than would be otherwise possible in the ICR cell. H/D exchange reactions with different peptides are investigated, as are proton-bound complex formations. A typical experimental sequence would be ion accumulation in the hexapole ion guide from multiple laser shots, addition of cooling gas during ion formation, addition of reaction gas, varied time delays for the ion-molecule reactions, and transmission of the product ions into the ICR cell for mass analysis. In this MALDI source H/D exchange reactions for different protonated peptides are investigated, as well as proton-bound complex formations with the reaction gas triethylamine. Amino acid sequence, structural flexibility and folding state of the peptides can be seen to play a part in the reactivity of such ions.     

Laser-induced fluorescence for ion tomography in a Penning trap

Laser-induced ion fluorescence of laser-desorbed Ba⁺ ions provides a measure of the relative number of ions near the center of the Penning trap of a Fourier transform ion cyclotron resonance mass spectrometer. Here, we report the detection of Penning-trapped ions by ion fluorescence, subject to radially outward ion cloud expansion (because of ion-neutral collisions), radially inward ion cloud compression (because of quadrupolar axialization), and the effects of buffer gas pressure and electrostatic trapping potential on those processes. At high pressure and high trapping voltage, radial ejection is far more rapid than axial ejection; quadrupolar axialization increases the number of ions near the center of the trap as well as the length of time that ions may be trapped; higher pressure results in faster magnetron radial expansion; and the choice of azimuthal quadrupolar excitation waveform significantly affects the efficacy of axialization. Based on these results, we suggest that directly detected laser-induced ion fluorescence provides a general new tool for mapping the ion distribution and its time evolution in response to various excitatory and damping effects.     

Electrospray ionization-Fourier transform ion cyclotron mass spectrometry using ion preselection and external accumulation for ultrahigh sensitivity

The dynamic range of Fourier transform ion cyclotron mass spectrometry (FTICR) is typically limited by the useful charge capacity of an FTICR cell (to approximately 10(6) to 10(7) elementary charges) and the minimum number of ions required to produce a useful signal (approximately 10(2) elementary charges). We show that the expansion of the dynamic range by 2 orders of magnitude can be achieved by preselecting lower abundance species in a quadrupole interface to an electrospray ionization (ESI) source. Ion preselection is then followed by ion accumulation in external to the FTICR cell a linear (2-D) quadrupole trap and subsequent transfer to the region of high magnetic field for gated trapping in the FTICR cell. Two modes of ion preselection, using either the quadrupole filtering mode or rf-only dipolar excitation, were studied and mass resolutions of 30 to 100 were achieved for selective external ion accumulation of peptides and proteins with molecular weights ranging from 500 to 17,000 Da. The ability to selectively eject the most abundant species before trapping in the FTICR has enormous practical benefits for increasing the sensitivity and dynamic range of measurements.     

Dynamically assisted gated trapping for Fourier transform ion cyclotron mass spectrometry.

An efficient approach for trapping ions and enhancing signal based on 'adiabatic amplitude reduction' for Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is described and evaluated. This method is a modification to the widely used gated trapping technique in which the trapping potential is raised adiabatically rather than instantaneously (non-adiabatically). Compared with non-adiabatic gated trapping, the final amplitudes of ion axial oscillations and energies are lower in the proposed method. All performance aspects of the FTICR spectrum (e.g., peak intensities, mass resolution, and mass accuracy) improve significantly compared to the conventional gated trapping technique.     

In-cell matrix-assisted laser desorption-ionization fourier transform ion cyclotron resonance mass spectrometry

A new internal matrix-assisted laser desorption-ionization (MALDI) Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) method is introduced. The target is directly positioned at one trapping electrode of a single cylindrical ion cyclotron resonance (ICR) cell and becomes a part of it. The ionization occurs inside the ICR cell in contrast to external or near-cell MALDI-FTICR-MS techniques. Very efficient trapping and mass resolving power better than unit resolution of singly charged peptides and proteins ions up to 2000 u is possible by using only basic FTICR-MS techniques. The sole application of a pulsed retarding potential increases the mass range to 6000 u. No collisional cooling and quadrupolar excitation was done. Sensitivities below 1 fmol, and ion storage times of more than 15 s are shown. High resolving powers of 16,000 and 56,000 are obtained on bovine insulin (5.7 ku) and gramicidin D (1.9 ku), respectively.     

Utilizing artificial neural networks in matlab to achieve parts-per-billion mass measurement accuracy with a fourier transform ion cyclotron resonance mass spectrometer

Fourier transform ion cyclotron resonance mass spectrometry has the ability to realize exceptional mass measurement accuracy (MMA); MMA is one of the most significant attributes of mass spectrometric measurements as it affords extraordinary molecular specificity. However, due to space-charge effects, the achievable MMA significantly depends on the total number of ions trapped in the ICR cell for a particular measurement, as well as relative ion abundance of a given species. Artificial neural network calibration in conjunction with automatic gain control (AGC) is utilized in these experiments to formally account for the differences in total ion population in the ICR cell between the external calibration spectra and experimental spectra. In addition, artificial neural network calibration is used to account for both differences in total ion population in the ICR cell as well as relative ion abundance of a given species, which also affords mean MMA values at the parts-per-billion level.     

Application of sustained off-resonance irradiation infrared multiphoton dissociation tandem mass spectrometry to the analysis of biomolecules

A modified pulse sequence for infrared multiphoton dissociation (IRMPD) experiments on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer in conjunction with sidekick trapping is presented. For IRMPD tandem mass spectrometry experiments gated trapping is normally applied. It ensures that the ions remain on-axis and, thus, cross the laser beam which is aligned on-axis in commercially available instruments. Sidekick trapping is used to capture more ions in the ICR cell in order to increase the signal intensity. However, it may lead to off-axis ion motion, which reduces or even excludes interaction with the laser beam. In this contribution sustained off-resonance irradiation (SORI) was applied to overcome this disadvantage of sidekick trapping. SORI is normally used in conjunction with collision-induced dissociation (CID) experiments to increase the kinetic energy of the ions. Here, SORI is used to influence the cyclotron motion during the laser irradiation time, which leads to temporary intersection of the ion trajectory with the laser beam. With this easy-to-handle experimental setup, IRMPD of ions captured with sidekick trapping leads again to the generation of fragment ions as is demonstrated with several biologically relevant samples like peptides, lipids and glycolipids.     

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.