Resonance Ejection Research Articles

Development of an ion trap with a metastable atom bombardment ionization source to observe ion-molecule reactions

An ion trap/time-of-flight mass spectrometer equipped with a metastable atom bombardment (MAB) ionization source was constructed in order to observe ion-molecule reactions controlled by the internal energy of ions. In the MAB source, the internal energy of the generated ions was controlled by using different rare gases. Moreover, the constructed device was used to analyze ion-molecule reactions of the p-xylene cation and its dissociation ions with p-xylene. It was found that the reaction afforded a single product without any structural isomers, which could then be easily analyzed. An abundant amount of [M]+ (m/z 106, C8H10+) and trace levels of dissociation products were observed in the Kr MAB ionization, while in the Ar MAB ionization, [M−CH3]+, [M−H]+ (m/z 105, C8H9+), and [M+CH]+ (m/z 119, C9H11+) ions were generated in addition to the molecular ion. Moreover, investigating the production routes to [M−H]+ and [M+CH]+ ions using a resonance ejection method in the ion trap indicating that generation of these ions was related to the hydride transfer reaction of C7H7+ with p-xylene. Results from the calculations of the reaction profile of this transfer using the density functional theory (DFT) method supported the experimental results.

Optimal fabrication methods for miniature coplanar ion traps.

Ion trap mass spectrometers are beneficial due to their intrinsic sensitivity and specificity. Therefore, a portable version for in situ analysis of various compounds is very attractive. Miniaturization of ion traps is paramount for the portability of such mass spectrometers. We developed an optimized design for a planar linear ion trap mass spectrometer, consisting of two trapping plates with photolithographically patterned electrodes. Each plate is constructed using a machined glass substrate and standard microfabrication procedures. The plates are attached to a patterned circuit board via wire bonds then positioned approximately 5mm apart. Trapped ions are detected by ejecting them through tapered slits, which alleviate charge buildup. Mass analysis can be performed through either boundary or resonant ion ejection. Better than unit mass resolution is demonstrated with resonant ejection. The optimized planar linear ion trap provides good resolution and the potential for further miniaturization. This was accomplished by vigorously testing variables associated with ion trap design including electrical connections, substrate materials, and electrode designs.

Development of a low power miniature linear ion trap mass spectrometer with extended mass range

A miniaturized ion trap mass spectrometer with continuous atmospheric pressure interface was built, which could be used in conjunction with internal ionization (in-vacuum plasma ionization) and external ionization (electrospray ionization). To improve its mass range, a supplemental AC signal frequency scan mode was performed for this miniature mass spectrometer, in which the fundamental RF signal was remained constant with an amplitude as low as several hundreds volts. Experiments showed that the upper limit of the mass range can be extended to 2500 Da for the miniature mass spectrometer developed in this work, and the power consumption can be reduced by 2/5, while maintaining good balance with performance parameters such as stability, sensitivity, and resolution. Due to the improvement in mass range and significant reduction in energy consumption compared with the instrument using the traditional resonant ejection mode, the scan method developed in this work is helpful to promote the miniature design of mass spectrometers for field analysis and space exploration.

Sex hormone levels and change in left ventricular structure among men and post-menopausal women: The Multi-Ethnic Study of Atherosclerosis (MESA)

ObjectiveSex hormone (SH) levels may contribute to sex differences in the risk of heart failure with preserved ejection fraction (HFpEF). We examined the associations of SH levels with left ventricular mass (LVM) and mass (M):volume (V) ratio, which are risk markers for HFpEF. Study designWe studied 1941 post-menopausal women and 2221 men, aged 45–84 years, participating in the Multi-Ethnic Study of Atherosclerosis (MESA). Serum SH levels, cardiac magnetic resonance imaging (MRI) and ejection fraction (EF) ≥50% had been recorded at baseline (2000–2002). Of these participants, 2810 underwent repeat MRI at Exam 5 (2010–2012). Stratified by sex, linear mixed-effect models were used to test associations between SH and sex hormone binding globulin (SHBG) level [per 1 SD greater log-transformed (SH)] with baseline and change in LV structure. Models were adjusted for age, race/ethnicity, center, height, weight, education, physical activity and smoking, and, in women, for hormone therapy and years since menopause. Main outcome measuresLVM and M:V ratio. ResultsAfter a median of 9.1 years, higher free testosterone levels were independently associated with a modest increase in LVM (g/yr) in women [0.05 (95% CI 0.01, 0.10)] and men [0.16 (0.03, 0.28)], while higher SHBG levels were associated with less LVM change (g/yr) in women [−0.07 (−0.13, −0.01)] and men [−0.15 (−0.27, −0.02)]. In men, higher dehydroepiandrosterone and estradiol levels were associated with increased LVM. Among women, free testosterone levels were positively and SHBG levels inversely associated with change in M:V ratio. ConclusionA more androgenic profile (higher free testosterone and lower SHBG levels) is associated with a greater increase in LVM in men and women and greater increase in M:V ratio in women over the course of 9 years.

Top-Down Charge Transfer Dissociation (CTD) of Gas-Phase Insulin: Evidence of a One-Step, Two-Electron Oxidation Mechanism.

Top-down analyses of protonated insulin cations of charge states of 4+, 5+, or 6+ were performed by exposing the isolated precursor ions to a beam of helium cations with kinetic energy of more than 6 keV, in a technique termed charge transfer dissociation (CTD). The ~100 ms charge transfer reaction resulted in approximately 20% conversion efficiency to other intact charge exchange products (CTnoD), and a range of low abundance fragment ions. To increase backbone and sulfide cleavages, and to provide better structural information than straightforward MS2 CTD, the CTnoD oxidized products were isolated and subjected to collisional activation at the MS3 level. The MS3 CTD/CID reaction effectively broke the disulfide linkages, separated the two chains, and yielded more structurally informative fragment ions within the inter-chain cyclic region. CTD also provided doubly oxidized intact product ions at the MS2 level, and resonance ejection of the singly oxidized product ion revealed that the doubly oxidized product originates directly from the isolated precursor ion and not from consecutive CTD reactions of a singly oxidized intermediate. MS4 experiments were employed to help identify potential radical cations and diradical cations, but the results were negative or inconclusive. Nonetheless, the two-electron oxidation process is a demonstration of the very large potential energy (>20 eV) available through CTD, and is a notable capability for a 3D ion trap platform. Graphical Abstract ᅟ.

Improving mass assignments in quadrupole ion traps operated using ac scans: Theory and experimental validation

Two theoretically equivalent methods of improving mass assignments in quadrupole ion traps operated using ac frequency scans are described side-by-side. Both use the inverse Mathieu q scan under constant rf conditions. In a first method, the ac amplitude is varied quadratically as a function of the Mathieu q parameter so as to force mass shifts to be constant with respect to ion m/z, thereby giving a precisely linear relationship between m/z and time. The average mass error improves by a factor of four when using the quadratic regression of ac amplitude vs. Mathieu q compared to constant ac amplitude conditions, assuming a linear mass calibration procedure. The quadratic relationship is also predicted by theory. In a second, simpler method, the ac signal amplitude is kept constant during the inverse Mathieu q scan and a quadratic relationship between m/z and time is used for mass calibration. Although the mass calibration is not linear in this case, the complicated amplitude optimization process is bypassed while retaining the same degree of mass accuracy. Importantly, in this case no parameter aside from the overall ac amplitude needs to be optimized, which is an advantage over the standard resonance ejection method in which the ac amplitude must be optimized for a given rf amplitude scan. Average mass error as low as 0.1Da is shown to be possible in the two ac scan methods, which compares reasonably well with commercial resonance ejection data. This performance is adequate for ion traps used on planetary exploration missions and other in situ measurements, for which the ac frequency scanning method is particularly applicable.

Reducing mass peak instability caused by the phase changes of RF and AC signals in a rectilinear ion-trap analyzer.

For an ion trap with resonance ejection, peak intensity and peak position of the acquired mass spectra are affected by the phase difference between the radio frequency (RF) and auxiliary alternating current (AC) potentials. To ensure measurement stability, RF and AC phase-locking is commonly used in commercial ion trap mass spectrometers. In this study, a compact electronic control system was developed to accurately regulate the RF and AC phases and was employed in a photoionization rectilinear ion trap (RIT) mass spectrometer. We found that the phase-locking method was defective in multicomponent analysis because the optimal RF and AC phase difference was usually different for different m/z peaks. After studying and characterizing the relationship between the peaks and the RF and AC phases, a correction method based on data processing was used to improve the peaks' stability and accuracy. The results show that the fluctuations of both peak intensity and peak position were significantly reduced and that the instrument presented satisfying reproducibility and quantitative ability.

Improving Performance Metrics of Ultraviolet Photodissociation Mass Spectrometry by Selective Precursor Ejection.

Confident protein identifications derived from high-throughput bottom-up and top-down proteomics workflows depend on acquisition of thousands of tandem mass spectrometry (MS/MS) spectra with adequate signal-to-noise and accurate mass assignments of the fragment ions. Ultraviolet photodissociation (UVPD) using 193 nm photons has proven to be well-suited for activation and fragmentation of peptides and proteins in ion trap mass spectrometers, but the spectral signal-to-noise ratio (S/N) is typically lower than that obtained from collisional activation methods. The lower S/N is attributed to the dispersion of ion current among numerous fragment ion channels (a,b,c,x,y,z ions). In addition, frequently UVPD is performed such that a relatively large population of precursor ions remains undissociated after the UV photoactivation period in order to prevent overdissociation into small uninformative or internal fragment ions. Here we report a method to improve spectral S/N and increase the accuracy of mass assignments of UVPD mass spectra via resonance ejection of undissociated precursor ions after photoactivation. This strategy, termed precursor ejection UVPD or PE-UVPD, allows the ion trap to be filled with more ions prior to UVPD while at the same time alleviating the space charge problems that would otherwise contribute to the skewing of mass assignments and reduction of S/N. Here we report the performance gains by implementation of PE-UVPD for peptide analysis in an ion trap mass spectrometer.

Extending the mass range of a miniature ion trap mass spectrometer using the inverse Mathieu q scan

The mass/charge range of a mass spectrometer operated in either the boundary or resonance ejection mode is usually limited by the highest radiofrequency (rf) voltage that can be attained, although lowering the resonance ejection Mathieu q value (qeject) can increase this range at the expense of resolution and unintended boundary ejection can result in spectral complexity. High voltage requirements are particularly troublesome for miniature instruments, which have tight electronic constraints and closely-spaced electrodes prone to discharging. Here we demonstrate an alternative approach to mass range extension based on a method of scanning the resonance ejection frequency nonlinearly in the form of an inverse Mathieu q scan. The results show an increase in mass range of up to 3.5 times on both a benchtop LTQ linear ion trap and the Mini 12 miniature linear ion trap mass spectrometer without instrumental modifications, and unit resolution is observed on the benchtop instrument by controlling the scan rate and minimizing space charge effects.

Linear mass scans in quadrupole ion traps using the inverse Mathieu q scan.

Secular frequency scanning is a method of mass selectively scanning ions out of a quadrupole ion trap by linearly ramping the frequency of the resonance ejection signal through ion secular frequencies at constant rf amplitude and frequency. The method is electronically much simpler than resonance ejection but it requires a complex nonlinear calibration procedure to correlate mass-to-charge with time. A method of secular frequency scanning in quadrupole ion traps is described in which mass-to-charge is linear with time. This method, termed an "inverse Mathieu q scan", contrasts with linear frequency sweeping which requires a complex nonlinear mass calibration procedure. In the current method, mass scans are forced to be linear with time by scanning the frequency of the supplementary ac so that there is an inverse relationship between the ejected ion's Mathieu q parameter and time. In all cases, excellent mass spectral linearity is observed. The rf amplitude is shown to control both the scan range and the scan rate, whereas the ac amplitude and scan rate influence the mass resolution. The scan rate depends linearly on the rf amplitude, a unique feature of this scan. Although changes in either rf or ac amplitude affect the positions of peaks in time, they do not change the mass calibration procedure since this only requires a simple linear fit of m/z vs time. Space charge effects are shown to give rise to significant changes in resolution as well as to mass shifts. A method of secular frequency scanning which provides a linear mass scale has been demonstrated. The inverse Mathieu q scan offers a significant increase in mass range and power savings while maintaining access to linearity, paving the way for a mass spectrometer based completely on ac waveforms for ion isolation, ion activation, and ion ejection. Copyright © 2016 John Wiley & Sons, Ltd.

Where Does the Electron Go? Stable and Metastable Peptide Cation Radicals Formed by Electron Transfer.

Electron transfer to doubly and triply charged heptapeptide ions containing polar residues Arg, Lys, and Asp in combination with nonpolar Gly, Ala, and Pro or Leu generates stable and metastable charge-reduced ions, (M + 2H)+●, in addition to standard electron-transfer dissociation (ETD) fragment ions. The metastable (M + 2H)+● ions spontaneously dissociate upon resonant ejection from the linear ion trap, giving irregularly shaped peaks with offset m/z values. The fractions of stable and metastable (M + 2H)+● ions and their mass shifts depend on the presence of Pro-4 and Leu-4 residues in the peptides, with the Pro-4 sequences giving larger fractions of the stable ions while showing smaller mass shifts for the metastables. Conversion of the Asp and C-terminal carboxyl groups to methyl esters further lowers the charge-reduced ion stability. Collisional activation and photodissociation at 355 nm of mass-selected (M + 2H)+● results in different dissociations that give sequence specific MS3 spectra. With a single exception of charge-reduced (LKGLADR + 2H)+●, the MS3 spectra do not produce ETD sequence fragments of the c and z type. Hence, these (M + 2H)+● ions are covalent radicals, not ion-molecule complexes, undergoing dramatically different dissociations in the ground and excited electronic states. The increased stability of the Pro-4 containing (M + 2H)+● ions is attributed to radicals formed by opening of the Pro ring and undergoing further stabilization by hydrogen atom migrations. UV-VIS photodissociation action spectroscopy and time-dependent density functional theory calculations are used in a case in point study of the stable (LKGPADR + 2H)+● ion produced by ETD. In contrast to singly-reduced peptide ions, doubly reduced (M + 3H)+ ions are stable only when formed from the Pro-4 precursors and show all characteristics of even electron ions regarding no photon absorption at 355 nm or ion-molecule reactions, and exhibiting proton driven collision induced dissociations. Graphical Abstract ᅟ.

Successive Resonances for Ion Ejection at Arbitrary Frequencies in an Ion Trap.

The use of successive resonances for ion ejection is demonstrated here as a method of scanning quadrupole ion traps with improvement in both resolution and sensitivity compared with single frequency resonance ejection. The conventional single frequency resonance ejection waveform is replaced with a dual-frequency waveform. The two included frequencies are spaced very closely and their relative amplitudes are adjusted so that the first frequency that ions encounter excites them to higher amplitudes where space charge effects are less prominent, thereby giving faster and more efficient ejection when the ions come into resonance with the second frequency. The method is applicable at any arbitrary frequency, unlike double and triple resonance methods. However, like double and triple resonance ejection, ejection using successive resonances requires the rf and AC waveforms to be phase-locked in order to retain mass accuracy and mass precision. The improved performance is seen in mass spectra acquired by rf amplitude scans (resonance ejection) as well as by secular frequency scans. Graphical Abstract ᅟ.

Experimental Characterization of Secular Frequency Scanning in Ion Trap Mass Spectrometers.

Secular frequency scanning is implemented and characterized using both a benchtop linear ion trap and a miniature rectilinear ion trap mass spectrometer. Separation of tetraalkylammonium ions and those from a mass calibration mixture and from a pesticide mixture is demonstrated with peak widths approaching unit resolution for optimized conditions using the benchtop ion trap. The effects on the spectra of ion trap operating parameters, including waveform amplitude, scan direction, scan rate, and pressure are explored, and peaks at black holes corresponding to nonlinear (higher-order field) resonance points are investigated. Reverse frequency sweeps (increasing mass) on the Mini 12 are shown to result in significantly higher ion ejection efficiency and superior resolution than forward frequency sweeps that decrement mass. This result is accounted for by the asymmetry in ion energy absorption profiles as a function of AC frequency and the shift in ion secular frequency at higher amplitudes in the trap due to higher order fields. We also found that use of higher AC amplitudes in forward frequency sweeps biases ions toward ejection at points of higher order parametric resonance, despite using only dipolar excitation. Higher AC amplitudes also increase peak width and decrease sensitivity in both forward and reverse frequency sweeps. Higher sensitivity and resolution were obtained at higher trap pressures in the secular frequency scan, in contrast to conventional resonance ejection scans, which showed the opposite trend in resolution on the Mini 12. Mass range is shown to be naturally extended in secular frequency scanning when ejecting ions by sweeping the AC waveform through low frequencies, a method which is similar, but arguably superior, to the more usual method of mass range extension using low q resonance ejection. Graphical Abstract ᅟ.

Effect of basic residue on the kinetics of peptide fragmentation examined using surface-induced dissociation combined with resonant ejection

In this work, resonant ejection coupled with surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer is used to examine fragmentation kinetics of two singly protonated hexapeptides, RYGGFL and KYGGFL, containing the basic arginine residue and less basic lysine residue at the N-terminus. The kinetics of individual reaction channels at different collision energies are probed by applying a short ejection pulse (1ms) in resonance with the cyclotron frequency of a selected fragment ion and varying the delay time between ion-surface collision and resonant ejection while keeping total reaction delay time constant. Rice-Ramsperger-Kassel-Marcus (RRKM) modeling of the experimental data provides accurate threshold energies and activation entropies of individual reaction channels. Substitution of arginine with less basic lysine has a pronounced effect on the observed fragmentation kinetics of several pathways, including the b2 ion formation, but has little or no effect on formation of the b5+H2O fragment ion. The combination of resonant ejection SID, time- and collision energy-resolved SID, and RRKM modeling of both types of experimental data provides a detailed mechanistic understanding of the primary dissociation pathways of complex gaseous ions.

A modified auxiliary excitation signal for achieving unidirectional ion ejection in quadrupole ion trap mass spectrometers operating in the resonance ejection mode

This paper proposes a technique to cause unidirectional ion ejection in a quadrupole ion trap mass spectrometer operated in the resonance ejection mode. In this technique a modified auxiliary dipolar excitation signal is applied to the endcap electrodes. This modified signal is a linear combination of two signals. The first signal is the nominal dipolar excitation signal which is applied across the endcap electrodes and the second signal is the second harmonic of the first signal, the amplitude of the second harmonic being larger than that of the fundamental.We have investigated the effect of the following parameters on achieving unidirectional ion ejection: primary signal amplitude, ratio of amplitude of second harmonic to that of primary signal amplitude, different operating points, different scan rates, different mass to charge ratios and different damping constants.In all these simulations unidirectional ejection of destabilized ions has been successfully achieved.

Surface-induced dissociation: a unique tool for studying energetics and kinetics of the gas-phase fragmentation of large ions.

Surface-induced dissociation (SID) is a valuable tool for investigating the activation and dissociation of large ions in tandem mass spectrometry. This account summarizes key findings from studies of the energetics and mechanisms of complex ion dissociation in which SID experiments were combined with Rice-Ramsperger-Kassel-Marcus modeling of the experimental data. These studies used time- and collision-energy-resolved SID experiments and SID combined with resonant ejection of selected fragment ions on a specially designed Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Fast-ion activation by collision with a surface combined with the long and variable timescale of FT-ICR mass spectrometry is perfectly suited to studying the energetics and dynamics of complex ion dissociation in the gas phase. Modeling of time- and collision-energy-resolved SID enables the accurate determination of energy and entropy effects in the dissociation process. It has been demonstrated that entropy effects play an important role in determining the dissociation rates of both covalent and noncovalent bonds in large gaseous ions. SID studies have provided important insights on the competition between charge-directed and charge-remote fragmentation in even-electron peptide ions and the role of the charge and radical site on the energetics of the dissociation of odd-electron peptide ions. Furthermore, this work examined factors that affect the strength of noncovalent binding, as well as the competition between covalent and noncovalent bond cleavages and between proton and electron transfer in model systems. Finally, SID studies have been used to understand the factors affecting nucleation and growth of clusters in solution and in the gas phase.

Rapid Mass Analysis of Reserpine and Arginine by Ion Trap Mass Spectrometer

Abstract A fast mass analysis method using digital ion trap technology was reported. The mass analysis was realized by scanning the frequency of digital RF voltage and dipole ion resonance voltage during ion injection period without the conventional ion cooling, ion resonance ejection or ion cleaning process. It can decrease about three fourth of traditional analysis time using ion trap mass spectrometry. The ion signal was optimized through the modification of frequency scanning rate, ion gate voltage and other experimental parameters. For example, an optimal mass spectroscopy of reserpine ( m / z 609) and arginine ( m / z 174) were obtained when the ion gate voltage was 9 V and the digital RF frequency was scanned from 1 MHz to 400 kHz at rate of 2385 Th s −1 . The results indicated that the obtained mass spectra were in very good agreement with conventional analytical method using ion trap mass analyzer.

Evaluation of right ventricular function in patients with pulmonary arterial hypertension by real-time three-dimensional echocardiography

Objective To evaluate right ventricular systolic function in patients with pulmonary hypertension (PH) by real-time three-dimensional echocardiography (RT-3DE), and compared with cardiac magnetic resonance. Methods A total of 23 patients with PH who underwent MRI and echocardiography in the study. Right ventricular index of myocardial performance (RIMP), fractional area change (RVFAC), tricuspid annular plane systolic excursion (TAPSE) and tissue Doppler-derived tricuspid lateral annular systolic velocity (S') were measured by echocardiography. RV end-diastolic volume (RVEDV) and end-systolic volume (RVESV) were measured by RT-3DE and cardiac magnetic resonance and RV ejection fraction (RVEF) was calculated. Results A positive correlation was found between RVFAC and RVEF (r=0.595, P=0.003), there was a negative correlation between RIMP and RVEF (r=-0.745, P=0.000), has no correlation between TAPSE and RVEF (r=-0.029, P=0.896), a positive correlation was found between S' and RVEF (r=0.489, P=0.018). There were close correlation between RVEDV, RVESV and RVEF measured by RT-3DE and MRI (P<0.001, respectively); Bland-Altman analyses showed good agreement between them. Conclusions RT-3DE can noninvasive, accurate assessment right ventricular systolic function in patients with PH, and provide prognosis and treatment choice for clinical demands. Key words: Echocardiography, real-time three-dimensional; Hypertension, pulmonary; Ventricular function, right

Re-print of “Insight into the Resonance Ejection Process during Mass Analysis through Simulations for Improved Linear Quadrupole Ion Trap Mass Spectrometer Performance”

A methodology for the simulation of mass analysis of ions in RF ion traps is described and demonstrated. This methodology utilizes the very high computational throughput capabilities of a GPU to accelerate ion trajectory modeling by ∼10,000 fold over simple CPU based systems. This capability enables modeling of ion trajectories for realistic numbers of ions in realistic ion trap devices (defined by electrode structures and applied electrode voltages) and provides simulated mass spectral peaks as an end product. Comprehensive characterization of the effects on peak shape and resolution of electrode features such as slots or particular profiles, as well as critical operating parameters such as the amplitude and phase of the auxiliary voltages, is demonstrated. The simulated dependence of observed m/z peak resolution on auxiliary voltage phase and amplitude corresponds well with the results from a real commercial linear ion trap instrument. It is further confirmed via simulation that the extent of radial electrode displacement (stretch) for our commercial device corresponds to the minimum electrode displacement necessary to optimize m/z resolution. The results of a simulation study involving a set of 15 different ion trap configurations incorporating various trapping field compositions, electrode geometries, and applied voltages are presented. Several of the modeled trap configurations indicate the promise for improved m/z resolution.

New approach for studying slow fragmentation kinetics in FT-ICR: Surface-induced dissociation combined with resonant ejection

We introduce a new approach for studying the kinetics of large ion fragmentation in the gas phase by coupling surface-induced dissociation (SID) in a Fourier transform ion cyclotron resonance mass spectrometer with resonant ejection of selected fragment ions using a relatively short (5ms) ejection pulse. The approach is demonstrated for singly protonated angiotensin III ions excited by collisions with a self-assembled monolayer of alkylthiol on gold (HSAM). The overall decomposition rate and rate constants of individual reaction channels are controlled by varying the kinetic energy of the precursor ion in a range of 65–95eV. The kinetics of peptide fragmentation are probed by varying the delay time between ion activation by collision and short (5ms) resonant ejection of selected fragment ions at a constant total reaction time of 150s. RRKM modeling indicates that the shape of the kinetics plots is strongly affected by the shape and position of the energy deposition function (EDF) describing the internal energy distribution of the ion following ion-surface collision. Modeling of the kinetics data provides detailed information on the shape of the EDF and energy and entropy effects of individual reaction channels.