Observation of increased ion cyclotron resonance signal duration through electric field perturbations

Ion motion in Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) is complex and the subject of ongoing theoretical and experimental studies. Two predominant pathways for the loss of ICR signals are thought to include damping of cyclotron motion, in which ions lose kinetic energy and radially damp toward the center of the ICR cell, and dephasing of ion coherence, in which ions of like cyclotron frequency become distributed out of phase at similar cyclotron radii. Both mechanisms result in the loss of induced ion image current in FTICR-MS measurements and are normally inseparable during time-domain signal analysis. For conventional ICR measurements which take advantage of ion ensembles, maximization of the ion population size and density can produce the desired effect of increasing phase coherence of ions during cyclotron motion. However, this approach also presents the risk of coalescence of ion packets of similar frequencies. In general, ICR researchers in the past have lacked the tools necessary to distinguish or independently control dephasing and damping mechanisms for ICR signal loss. Nonetheless, the ability to impart greater phase coherence of ions in ICR measurements will allow significant advances in FTICR-MS research by improving the current understanding of ICR signal loss contributions of dephasing and damping of ion ensembles, increasing overall time-domain signal length, and possibly, resulting in more routine ultrahigh resolution measurements. The results presented here demonstrate the ability to employ a high density electron beam to perturb electric fields within the ICR cell during detection of cyclotron motion, in an approach we call electron-promoted ion coherence (EPIC). As such, EPIC reduces ICR signal degradation through loss of phase coherence, and much longer time-domain signals can be obtained. Our results demonstrate that time-domain signals can be extended by more than a factor of 4 with the implementation of EPIC, as compared to conventional experiments with otherwise identical conditions. The application of EPIC has also been observed to reduce the appearance of peak coalescence. These capabilities are not yet fully optimized nor fully understood in terms of the complex physics that underlies the enhancement. However, the enhanced time-domain signals can result in improved resolution in frequency-domain signals, and as such, this result is important for more efficient utilization of FTICR-MS. High resolution and accurate mass analysis are prime motivating factors in the application of advanced FTICR technology. We believe the approach presented here and derivatives from it may have significant benefit in future applications of advanced FTICR technology.     

Combination of sustained off-resonance irradiation and on-resonance excitation in FT-ICR

Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is becoming more widely used among the mass spectrometric techniques and has excellent figures of merit. Ion activation and fragmentation via sustained off-resonance irradiation (SORI) collision-induced dissociation (CID) is commonly used in FT-ICR. However, one of the limitations of SORI-CID is that only low-energy processes are typically observed in the product ion spectra. Here we present another option for performing CID in FT-ICR, a combination of SORI and on-resonance excitation (RE), termed SORI-RE. In comparison to SORI, this method produces more abundant ions resulting from higher energy fragmentation pathways. The result is the observation of a significant abundance of both higher and lower energy fragmentation pathways in the same mass spectrum. The comparison of SORI, RE, and SORI-RE spectra may lead to mechanistic insights as the relative abundances of certain fragment ions change as a function of internal energy deposition. This technique is simple to incorporate in existing instruments, does not require hardware or software modification, and requires only an additional 20-40 ms acquisition time. The technique is illustrated for a peptide (YGGFL), two disaccharides differing in the position of the glycosidic linkage (2alpha-mannobiose, 3alpha-mannobiose), an oligosaccharide (Alditol XT), a small protein (ubiquitin), and an inorganic cation (UO2+). Examples of higher energy fragmentation pathways enhanced by SORI-RE include the formation of immonium ions and oligosaccharide cross-ring cleavages.     

Centering of the ion trajectory in the cyclotron

In this paper we present the theory of the horizontal motion ions through the acceleration gap in the case of a static magnetic field and time-varying electric field. It also describes the motion of the center of the ion trajectory through the acceleration gap and also during the acceleration process. The result also describes analytically the angle through which the ion passed (so called “flying angle”) between two acceleration gaps. That angle can be measured from the center of the ion trajectory or from the center of the cyclotron. The analysis shows that the “flying angle” of the ions measured in these two reference systems is not same. Namely, the “flying angle” of the ion measured from the center of its trajectory is bigger than in the case when the “flying angle” is measured from the center of the cyclotron. It shows that difference between “flying angle” in these two reference systems becomes less and less during acceleration process.     

A precision gas feeding system for a PIG heavy ion source

A precision gas feeding system aiming at stable and efficient operation of a small, cold-cathode PIG heavy ion source for the K = 50 MeV AVF cyclotron is described in detail. The system can regulate the supply gas pressure with a precision of 0.6 Torr and keep a constant gas flow rate with a stability of 0.010 ± 0.003 cm³/min. The ion source operating parameters such as the arc current, the arc voltage and the gas flow rate remain fairly constant through the lifetime of the source. By adopting this system in the routine accelerations of carbon or oxygen ions, the average beam current was increased by a factor of 2–3.     

Design of the central region for axial injection in the VINCY cyclotron

This paper describes the design of the central region for h = 1, h = 2 and h = 4 modes of acceleration in the VINCY cyclotron. The result which is worth reported in that the central region is unique and compatible with the three above mentioned harmonic modes of operation. Only one spiral type inflector will be used. The central region is designed to operate with two external ion sources: (a) an ECR ion source with the maximum extraction voltage of 25 kV for heavy ions, and (b) a multicusp ion source with the maximum extraction voltage of 30 kV for H⁻ and D⁻ ions. Heavy ions will be accelerated by the second and fourth harmonics, D⁻ ions by the second harmonic and H⁻ ions by the first harmonic of the RF field. The central region is equipped with an axial injection system. The electric field distribution in the inflector and in the four acceleration gaps has been numerically calculated from an electric potential map produced by the program RELAX3D. The geometry of the central region has been tested with the computations of orbits carried out by means of the computer code CYCLONE. The optical properties of the spiral inflector and the central region were studied by using the programs CASINO and CYCLONE respectively. We have also made an effort to minimize the inflector fringe field using the RELAX3D program.     

Influence of charge state on product ion mass spectra and the determination of 4S/6S sulfation sequence of chondroitin sulfate oligosaccharides

Electrospray ionization-Fourier transform ion cyclotron resonance tandem mass spectrometry is used to study the influence of charge state on the product ion spectra of chondroitin sulfate oligosaccharides for determination of the sulfate position on N-acetylgalactosamine residues. Sustained off-resonance irradiation collision-induced dissociation and infrared multiphoton dissociation are investigated for tandem mass spectrometry of chondroitin sulfate. Product ion spectra were obtained for ions of varying charge states from (4,5)-unsaturated (delta-unsaturated), reduced delta-unsaturated, and saturated oligosaccharides from chondroitin sulfate A and chondroitin sulfate C, separately. It was observed that ions in which the charge (z) is less than the number of sulfates dissociate to produce predominantly even-numbered B(n), C(n), Y(n), and Z(n) ions, and that odd-numbered fragment ions are observed for ions that have z equal to the number of sulfates. Sulfate adducted ions were observed in the product ion spectra of singly charged tetramer and hexamer oligosaccharides. This sulfate adduction was determined to result from migration of neutral sulfate during excitation.     

Ion energy distribution of an inductively coupled radiofrequency discharge in argon and oxygen

We report an experimental investigation of the ion energy distribution in an inductively coupled electron cyclotron wave resonance (ECWR) discharge with a superimposed static magnetic field. The inductively coupled discharge is sustained by applying a 13.56 MHz radiofrequency (RF) power to an aluminium single-turn coil located inside the vacuum chamber. The source region was separated by a grid from the diffusion region. Ion energy distribution (IEDF) measurements employing an energy-dispersive mass spectrometer or plasma process monitor (PPM) whose entrance opening was 15 cm away from the grid were performed in the diffusion region. The IEDF is composed of two peaks; a low-energy peak due thermalized ions and a high-energy peak due to ions coming directly from the source region without undergoing thermalization. The energetic difference between the groups thus reflects the plasma potential difference between the source region and the diffusion region. The pronounced intensity variation of the high-energy peak with increasing pressure is caused by charge-changing collisions yielding a depletion of the high-energy ions with increasing effective path length.     

Bubble‐Sheet‐Like Interface Design with an Ultrastable Solid Electrolyte Layer for High‐Performance Dual‐Ion Batteries

In this work, a bubble‐sheet‐like hollow interface design on Al foil anode to improve the cycling stability and rate performance of aluminum anode based dual‐ion battery is reported, in which, a carbon‐coated hollow aluminum anode is used as both anode materials and current collector. This anode structure can guide the alloying position inside the hollow nanospheres, and also confine the alloy sizes within the hollow nanospheres, resulting in significantly restricted volumetric expansion and ultrastable solid electrolyte interface (SEI). As a result, the battery demonstrates an excellent long‐term cycling stability within 1500 cycles with ≈99% capacity retention at 2 C. Moreover, this cell displays an energy density of 169 Wh kg^?1 even at high power density of 2113 W kg^?1 (10 C, charge and discharge within 6 min), which is much higher than most of conventional lithium ion batteries. The interfacial engineering strategy shown in this work to stabilize SEI layer and control the alloy forming position could be generalized to promote the research development of metal anodes based battery systems.     

Direct recovery of ion beam energy using magnetic electron suppression: I. Analysis of ORNL energy recovery experiments

The charge-exchange neutralization efficiency of positive ion-based neutral beams used in plasma heating applications decreases as the beam energy increases. Direct energy recovery from the remaining charged particles can be accomplished by electrostatically decelerating the positive ions; the space-charge neutralizing electrons are constrained from being accelerated by the application of a transverse magnetic field. A finite difference nonlinear sheath analysis is used to analyze the transverse magnetic field electron suppression experiments carried out at Oak Ridge National Laboratory in the early 1980s. A double plasma model, which assumes an equilibrium Boltzmann distribution of electrons at both the neutralizer potential and the ion collector potential, is used to study the experimental data obtained from operating 40 keV, 10 A ion beam energy recovery experiments. The effects of the magnetic field strength, ion “boost” energy, and ion beam current density are examined in detail.     

Effective temperature of ions in traveling wave ion mobility spectrometry

Traveling wave ion mobility spectrometers (TW IMS) operate at significantly higher fields than drift tube ion mobility spectrometers. Here we measured the fragmentation of the fragile p-methoxybenzylpyridinium ion inside the TW ion mobility cell of the first-generation SYNAPT HDMS spectrometer. The ion's vibrational internal energy was quantified by a vibrational effective temperature T(eff,vib), which is the mean temperature of the ions inside the cell that would result in the same fragmentation yield as observed experimentally. Significant fragmentation of the probe ion inside the TW IMS cell was detected, indicating that field heating of the ions takes place in TW IMS. For typical small molecule IMS conditions, T(eff,vib) = 555 ± 2 K. The variations of the effective temperature were studied as a function of the IMS parameters, and we found that T(eff,vib) decreases when the wave height decreases, when the pressure increases, or when the wave speed increases. The energy transfer efficiency of argon is higher than for He, N(2), or CO(2). With T(eff,vib) being directly related to the ion speed inside the TW IMS, our results also provide new insight on the ion movement in TW IMS. We also discuss the influence of field heating of ions for calibration and structural studies in TW IMS.     

Ion detection by Fourier transform ion cyclotron resonance: the effect of initial radial velocity on the coherent ion packet

Ion detection by Fourier transform ion cyclotron resonance (FT-ICR) is accomplished by observing a coherent ion packet produced from an initially random ensemble of ions. The coherent packet is formed by excitation with a resonant oscillating electric field. Ions that are out of phase with the applied radio frequency (rf) electric field experience a continuous misalignment of the electric field vector. The misalignment creates a net force of the electric field perpendicular to ion motion. The perpendicular component of the rf electric field creates a frequency shift resulting in phase synchronization of the ion ensemble. The phase coherence of the ion packet affects both the sensitivity and the resolution of FT-ICR.     

Differentiation of isomers by wavelength-tunable infrared multiple-photon dissociation-mass spectrometry: application to glucose-containing disaccharides

Variation in the wavelength of irradiation in infrared multiple-photon dissociation (IR-MPD) of lithium-tagged glucose-containing disaccharide ions (1-2-, 1-3-, 1-4-, and 1-6-linked isomers of both anomeric configurations) resulted in marked differences in their mass spectral fragmentation patterns. Two-dimensional plots of the fragment yield versus infrared wavelength for each mass spectral product ion were unique for each isomer and can be considered a spectral fingerprint. Individual product ions or diagnostic ratios of key product ions can be optimized at specific IR wavelengths. The technique permits both linkage position and anomeric configuration to be assigned. The ratio of the fragments derived by cleavage at the glycosidic bond (m/z 169/187) is much enhanced for beta-anomers compared to alpha-anomers. Differences in the diagnostic product ions 169 and 187 were largest in the range of 9.0-9.4 microm, where the maximum dissociation yield was observed. Conversely, at 10.6 microm, the wavelength of nontunable CO2 lasers that accompany commercial Fourier transform ion cyclotron resonance mass spectrometers, the dissociation yield was poor and anomeric differentiation was not possible. In contrast to previous studies by collision-induced dissociation, the trends in dissociation behavior between anomers using IR-MPD are significant and allow simple diagnostic rules to be established. By depositing energy into these isobaric ions via narrow-band IR excitation, the resulting internal energy can be finely controlled, thereby obtaining high reproducibility in dissociation patterns. Given the multidimensionality of variable-wavelength IR-MPD of lithiated disaccharides, it is expected that this approach can overcome some of the current limitations in isomer differentiation.     

Changes in silicon samples bombarded by 900–2300 eV hydrogen ions

We have performed a series of statistically designed experiments to evaluate the changes which occur in silicon samples as a result of hydrogen ion bombardment. Single-crystal silicon and polycrystalline silicon, and also edge-defined film-fed growth (EFG) solar cell devices, were exposed to a hydrogen ion beam produced by a Kaufman ion source. The experimental parameters which were systematically varied include the maximum hydrogen ion energy (900, 1600 and 2300 eV), the energy spread of the ions in the beam, the ion current density (0.8, 1.4 and 2.0 mA cm^?2), the total dose (1 × 10¹⁸, 2 × 10¹⁸ and 4 × 10¹⁸ ions cm^?2) and the bulk sample temperature during bombardment (200, 275 and 350 °C). We observed the changes in the short-circuit current, the open-circuit voltage, the photovoltaic conversion efficiency and the fill factor associated with the EFG devices, the change in the spectral reflectivity of the single-crystal silicon samples and the ratio of the numbers of SiH to SiH₂ groups present in the polycrystalline silicon samples. The results of our study indicate that the properties of hydrogen-ion-bombarded silicon change with both the maximum ion energy and the ion current density with a significant parametric interaction between the current density and the bulk sample temperature.The energy distribution of the ions present in the hydrogen ion beam affects the spectral reflectivity of single-crystal silicon. The changes associated with hydrogen ion bombardment are independent of dose for the conditions studied. The average absolute air mass 1 efficiency of EFG devices without intentionally applied antireflective coatings increased from 8.7% before exposure to 10.2% after exposure to the optimal conditions determined by this study.     

An improved form of the oscillating electron electrostatic ion source for ion etching

The performance of the oscillating electron electrostatic ion source has been improved by incorporating a small chimney into the ion exit aperture to reduce the field distortion at the aperture. A focussing electrode placed near the aperture, and at a negative potential with respect to the cathode, has also been added. This increases the ion beam intensity and also increases the total ion beam current by extracting more low energy ions from the source. When the source is used for ion etching, the specimen can be placed at some distance from the source to reduce the heating of the specimen by radiation from the source. Using argon ions, etching rates of copper film of more than 10 μm^?1 h have been observed with this improved source. It has also been found that the ion beam contains electrons which are suppressed with the focusing electrode at more than 100 V. Thus it was found that to satisfactorily etch a glass specimen it was necessary to operate with the focussing electrode at cathode potential to avoid the build up of charges on the glass.     

Phase synchronization of an ion ensemble by frequency sweep excitation in Fourier transform ion cyclotron resonance

The Fourier transform ion cyclotron resonance (FT-ICR) signal is produced by the coherent motion of a population of ions. The ability to produce a well-defined ion packet by excitation of an initially random ion ensemble is a major limiting factor of high mass FT-ICR. Ions must be both resonant and in phase with the applied radio frequency excitation field to be accelerated to radii suitable for detection by FT-ICR. Synchronization of the phase angles of an ensemble of ions occurs by off-resonant acceleration during frequency swept excitation. Results from computer-simulated ion trajectories suggest that phase synchronization of the ion packet prior to resonant excitation results in better spatial definition of the ion ensemble.     

Boosting Zn‐Ion Energy Storage Capability of Hierarchically Porous Carbon by Promoting Chemical Adsorption

The construction of advanced Zn‐ion hybrid supercapacitors (ZHSCs) with high energy density is promising but still challenging, especially at high current densities. In this work, a high‐energy and ultrastable aqueous ZHSC is demonstrated by introducing N dopants into a hierarchically porous carbon cathode for the purpose of enhancing its chemical adsorption of Zn ions. Experimental results and theoretical simulations reveal that N doping not only significantly facilitates the chemical adsorption process of Zn ions, but also greatly increases its conductivity, surface wettability, and active sites. Consequently, the as‐fabricated aqueous ZHSC based on this N‐doped porous carbon cathode displays an exceptionally high energy density of 107.3 Wh kg^?1 at a high current density of 4.2 A g^?1, a superb power density of 24.9 kW kg^?1, and an ultralong‐term lifespan (99.7% retention after 20 000 cycles), substantially superior to state‐of‐the‐art ZHSCs. Particularly, such a cathode also leads to a quasi‐solid‐state device with satisfactory energy storage performance, delivering a remarkable energy density of 91.8 Wh kg^?1. The boosted energy storage strategy by tuning the chemical adsorption capability is also applicable to other carbon materials.     

Automated gain control and internal calibration with external ion accumulation capillary liquid chromatography-electrospray ionization Fourier transform ion cyclotron resonance

When combined with capillary LC separations, electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS) has demonstrated capabilities for advanced characterization of proteomes based upon analyses of proteolytic digests. Incorporation of external (to the ICR cell) multipole devices with FTICR for ion selection and ion accumulation has enhanced the dynamic range, sensitivity, and duty cycle of measurements. However, the highly variable ion production rate from an LC separation can result in "overfilling" of the external trap during the elution of major peaks and result in m/z discrimination and fragmentation of peptide ions. Excessive space charge trapped in the ICR cell also causes significant shifts in the detected ion cyclotron frequencies, reducing the achievable mass measurement accuracy (MMA) and making protein identification less effective. To eliminate m/z discrimination in the external ion trap, further increase duty cycle, and improve MMA, we have developed the capability for data-dependent adjustment of ion accumulation times in the course of an LC separation, referred to as automated gain control (AGC). This development has been implemented in combination with low kinetic energy gated ion trapping and internal calibration using a dual-channel electrodynamic ion funnel. The overall system was initially evaluated in the analysis of a tryptic digest of bovine serum albumin. In conjunction with internal calibration, the capillary LC-ESI-AGC-FTICR instrumentation provided a approximately 10-fold increase in the number of identified tryptic peptides compared to that obtained using a fixed ion accumulation time and external calibration methods.     

Ion beam effects on dust grains: 2—Influence of charging history

Dust grains in space are charged by various processes. Impacts of energetic ions lead to deposition of positive charge on the grain, increasing the grain potential and, as a consequence, the electric field at its surface. The accumulated charge is spontaneously released as an emission current when the electric field reaches a threshold. This discharging current is usually attributed to field ionization of any gas surrounding the grain or to ion field emission and would thus be predominantly a function of the surface potential. However, preliminary studies [Velyhan A, Z?ilavý P, Pavl? J, S?afránková J, Něme?ek Z. Ion beam effects on dust grains. Vacuum 2004;76:447-55] using melamine formaldehyde spheres have shown that the discharging current depends strongly on the energy of primary ions. The present paper continues these investigations with the motivation to understand the whole charging/discharging process. The experiment is based on the capture of a single dust grain in an electrodynamic quadrupole. The trapped grain is exposed to an ion beam with different energies up to 5 keV and its charge and surface potential are estimated from the frequency of its oscillations in the quadrupole. The charging/discharging currents are determined from temporal changes of the grain charge. Our results suggest that the grain charge is accumulated in a thick surface layer of non-conducting samples. The thickness of this layer depends on the mass and energy of primary ions. On the other hand, the beam ions probably recombine on the metallic surfaces and create an adsorbed layer there. We believe that the main discharging process is field desorption complemented in this particular case with post-ionization.     

High-performance mass spectrometry: Fourier transform ion cyclotron resonance at 14.5 Tesla

We describe the design and current performance of a 14.5 T hybrid linear quadrupole ion trap Fourier transform ion cyclotron resonance mass spectrometer. Ion masses are routinely determined at 4-fold better mass accuracy and 2-fold higher resolving power than similar 7 T systems at the same scan rate. The combination of high magnetic field and strict control of the number of trapped ions results in external calibration broadband mass accuracy typically less than 300 ppb rms, and a resolving power of 200,000 (m/Delta m50% at m/z 400) is achieved at greater than 1 mass spectrum per second. Novel ion storage optics and methodology increase the maximum number of ions that can be delivered to the FTICR cell, thereby improving dynamic range for tandem mass spectrometry and complex mixture applications.     

Routine Part-per-Million Mass Accuracy for High- Mass Ions: Space-Charge Effects in MALDI FT-ICR

The effect of ion space-charge on mass accuracy in Fourier transform ion cyclotron resonance mass spectrometry is examined. Matrix-assisted laser desorption/ionization is used to form a population of high-molecular-weight polymer ions with a wide mass distribution. The density of the ions in the analyzer cell is varied using ion remeasurement and suspended trapping techniques to allow the effect of ion space charge to be examined independently of other experimental influences. Observed cyclotron frequency exhibits a linear correlation with ion population. Mass errors of 100 ppm or more in externally calibrated mass spectra result when ion number is not taken into account. By matching the total ion intensities of calibrant and analyte mass spectra, the protonated ion of insulin B-chain, 3494.6513 Da, is measured with an accuracy of 0.07 ppm (average of 10 measurements, σ = 2.3 ppm, average absolute error 1.6 ppm) using a polymer sample as an external calibrant. Alternatively, the correction for space charge can be made by using a calibration equation that accounts for the total ion intensity of the mass spectrum. A calibration procedure is proposed and is tested with the measurement of the mass of insulin B-chain. A mass accuracy of 2.0 ppm (average of 20 measurements, σ = 4.2 ppm, average absolute error 3.5 ppm) is achieved. Space-charge-induced mass errors are more significant for samples with many components, such as a polymer, than for single-component samples such as purified peptides or proteins.