Resonance Ejection Research Articles

Theory, simulation and measurement of chemical mass shifts in RF quadrupole ion traps

A comprehensive description of chemical mass shifts in the RF quadrupole ion trap (Paul trap) is given and their origin is explained. Extending a previously proposed model, it is shown that chemical mass shifts are a result of an ejection delay caused by field imperfections in RF ion traps with non-optimized geometry, in particular, field imperfections resulting from holes in the end-cap electrodes, and of compound-dependent modifications of this ejection delay by collisions of the ions with the buffer gas. Elastic collisions change peak shapes and peak widths, but give rise to only very small chemical mass shifts. Inelastic collisions, which result in dissociation, can lead to large chemical mass shifts. The field imperfections present in real ion traps are examined, and differences in the ion ejection behavior and in the chemical mass shifts occurring in boundary ejection and resonance ejection scans are investigated. The dependence of the shifts on the trap geometry, on the buffer gas pressure, on the RF amplitude scan rate, on ion mass, and on ion chemical structure, are explained. The proposed model for chemical mass shifts is validated through experimental measurements and quantitative simulations using the ion trap simulation program ITSIM. It is shown that peak shapes in mass spectra can be reproduced or predicted by simulations, for ion traps with a variety of geometries and operating conditions. Very good agreement between simulations and experiments is found, provided experimental and simulated geometries differ slightly in the end-cap separation, indicating that additional previously neglected field imperfections may be present in common RF ion traps.

Termolecular ion-molecule reactions in Titan’s atmosphere. IV. A search made at up to 1 micron in pure hydrocarbons

The results of a study of ion-molecule reactions occurring in pure methane, acetylene, ethylene, ethane, propyne, propene, propane, and diacetylene at pressures up to 40 microns of pressure are reported. A variety of experimental methods are used: The standard double resonance in an ICR, for determination of the precursor ions and the modulated double resonance ejection in an ICR, for the determination of the daughter ions. The FA-SIFT technique was used for validation and examination of termolecular reactions with rate coefficients that are less than 10 −26 cm 6 s −1. An extensive database of reaction kinetics already exists for many of these reactions. The main point of this study was the determination of the accuracy of this database and to search for any missing reactions and reaction channels that may have been omitted from earlier investigations. A specific objective of this work was to extend the study to the highest pressures possible to find out if there were any important termolecular reaction channels occurring. A new approach was used here. In the pure hydrocarbon gases the mass spectra were followed as a function of the pressure changes of the gas. An initial guess was first made using the current literature as a source of the reaction kinetics that were expected. A model of the ion abundances was produced from the solution of the partial differential equations in terms of reaction rate coefficients and initial abundances. The experimental data was fitted to the model for all of the pressures by a least squares minimization to the reaction rate coefficients and initial abundances. The reaction rate coefficients obtained from the model were then compared to the literature values. Several new channels and reactions were discovered when the modeled fits were compared to the actual data. This is all explained in the text and the implications of these results are discussed for the Titan atmosphere.

Investigations into the use of a reverse scan in a quadrupole ion trap mass spectrometer.

The forward scan (i.e. an increasing RF voltage ramp for the mass-selective instability scan) is commonly used as an analytical scan for ion detection with quadrupole ion trap instruments. A number of phenomena have been observed while using this scan technique. These include space charge effects resulting in the delayed ejection of ions from the ion trap, and the fragmentation of fragile ions producing very broad peaks. Here the use of a reverse scan (i.e. a decreasing RF voltage ramp) is examined to determine the effect of the above phenomena in this acquisition method. With regard to space charge effects, the apparent reduction of the carbon isotope spacing below one Thomson (for singly charged ions) that is observed with the forward scan is now replaced by an apparent increase in this spacing. The reverse scan, which optimizes at lower axial modulation ejection voltages than the forward scan, allows for the intact ejection of fragile ions under its typical operating conditions whereas the forward scan results in fragmentation. Reducing the axial modulation voltage for the ejection of ions in the forward scan results in less dissociation of the fragile ions during ion ejection, but with the observation of ghost peaks due to incomplete ejection of all of the ions at the resonance ejection condition. While performing the reverse scan experiment, the formation of product ions from dissociation of the MH(+) ion has also been observed.

Chemical mass shifts in quadrupole ion traps as analytical characteristics of nitro-aromatic compounds

Chemical mass shifts of nitro-aromatic compounds were measured in a custom-modified GCQ ion trap instrument. The average normalized chemical mass shift of 0.91 (Δ m/ m, %) for the molecular ions of 24 nitro-aromatic compounds examined is much larger than the average value of 0.41 (Δ m/ m, %) measured for a group of aromatic compounds without the nitro substituent. The larger chemical mass shifts associated with the nitro group could serve as a diagnostic for identifying this class of compounds in mixtures and a simple method of implementing this test is described. Samples were examined at two different ac resonance ejection frequencies, one corresponding to the non-linear resonance frequency at q z=0.80 where chemical mass shifts are small for all compounds, the other corresponding to q z=0.75 where chemical mass shifts are in full operation. The difference in apparent mass under these two conditions is used as an easily measured criterion for recognizing compounds that satisfy the mass shift criterion for nitro-aromatic compounds.

Chemical mass shifts in resonance ejection experiments in the quadrupole ion trap.

Chemical mass shifts were measured in a Paul ion trap operated in the mass-selective instability scan with resonance ejection using a custom-built instrument. These shifts, which can be as much as 2%, decrease with increasing endcap electrode separation owing to changes in the higher order contributions to the electric field. They also decrease with decreasing helium buffer gas pressure. Both of these effects are analogous to those found with boundary ejection. This suggests that the previously proposed chemical mass shift mechanism based on compound-dependent collisional modification of the ejection delay produced by field faults near the endcap electrode apertures holds true also for resonance ejection. The influence of the resonance frequency on chemical mass shifts was also investigated and it is shown that at certain working points (values of the Mathieu parameter q(z) and a(z)) non-linear resonances greatly reduce the ejection delay for all ions, regardless of their chemical structures, and thus reduce the magnitude of the chemical mass shift. Energetic collisions leading to dissociation can take place at an earlier stage during the ejection process in the mass analysis scan when using resonance ejection compared with boundary ejection. This leads to even larger chemical mass shifts of fragile ions in resonance ejection. Increasing the resonance voltage amplitude can enhance this effect. The chemical mass shifts of fragile ions increase with increase in the resonance voltage amplitude, whereas negligible changes occur for structurally stable ions.

A secondary ion microprobe ion trap mass spectrometer

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.

A two-dimensional quadrupole ion trap mass spectrometer

The use of a linear or two-dimensional (2-D) quadrupole ion trap as a high performance mass spectrometer is demonstrated. Mass analysis is performed by ejecting ions out a slot in one of the rods using the mass selective instability mode of operation. Resonance ejection and excitation are utilized to enhance mass analysis and to allow isolation and activation of ions for MS n capability. Improved trapping efficiency and increased ion capacity are observed relative to a three-dimensional (3-D) ion trap with similar mass range. Mass resolution comparable to 3-D traps is readily achieved, including high resolution at slower scan rates, although adequate mechanical tolerance of the trap structure is a requirement. Additional advantages of 2-D over 3-D ion traps are also discussed and demonstrated.

Effects of fragile ions on mass resolution and on isolation for tandem mass spectrometry in the quadrupole ion trap mass spectrometer.

In the quadrupole ion trap, it has been noted that factors other than an ion's mass and charge may affect its measured m/z, resulting in compound-dependent, or "chemical", mass shifts. We propose that ions can exhibit a chemical mass shift because they are "fragile" and may fragment during the application of resonance ejection during mass analysis; these effects were studied using ions that include protonated, deprotonated, and adduct ions of explosives, acylcarnitines, and macrolide antibiotics. Fragile ions affect mass resolution by causing broader peaks than nonfragile ions, especially at slower scan speeds, as the result of the application of resonance ejection. Fragile ions may also be fragmented by the application of the isolation waveform during selection of the parent ion for tandem mass spectrometry experiments, making it impossible to achieve unit isolation of a fragile ion. To obtain adequate isolation intensity, the isolation waveform notch width must be increased and the time period of isolation must be decreased. Fragile ions also require lower optimum collision energy to achieve efficient collision-induced dissociation. We have developed criteria for the determination of the degree of ion fragility based upon experimental results.

Control of chemical mass shifts in the quadrupole ion trap through selection of resonance ejection working point and rf scan direction

Compound-dependent chemical mass shifts are observed and their origin is elucidated in a modified Finnigan GCQ quadrupole ion trap mass spectrometer. The dependence of chemical mass shifts on ion trap geometry, specifically the center to end-cap spacing, z0, and the size of the apertures in the end caps, is demonstrated. The effects of the working point (qeject value) used for resonance ejection and the direction of the rf mass analysis scan are also studied, and the results are found to be in agreement with a previously proposed model for the chemical mass shift mechanism. It is shown that chemical mass shifts are present when resonance ejection is used, unless the qeject is chosen to correspond to a nonlinear resonance point, where the shifts are removed. The shifts are also removed by performing the mass analysis scan in the reverse direction, i.e., from high mass to low mass.

Origin of mass shifts in the quadrupole ion trap: dissociation of fragile ions observed with a hybrid ion trap/mass filter instrument

A novel hybrid tandem mass analyzer, coupling a quadrupole ion trap with a quadrupole mass filter, has been constructed to permit mass analysis of ions ejected from the ion trap. The initial application of this instrument is the investigation of the origin of mass shifts in the ion trap due to ion fragility. We hypothesize that fragile ions undergo mass shifts, characterized by peak fronting, due to early ejection from the quadrupole ion trap. As these ions come into resonance with the ejection frequency, they gain kinetic energy, collide with buffer gas molecules and thus can dissociate to produce fragment ions. These fragment ions will not be stable within the ion trap as they are situated past the stability boundary at q(z) = 0. 908. Consequently the fragment ions are ejected prematurely. This results in an apparent mass shift due to peak fronting. The experiments reported here clearly document the production of fragment ions as the origin of mass shifts during the resonant ejection of fragile ions. Copyright 2000 John Wiley & Sons, Ltd.

Analysis of non-covalent protein complexes up to 290 kDa using electrospray ionization and ion trap mass spectrometry.

Non-covalently-bound subunit complexes of proteins have been measured by an ion trap mass spectrometer equipped with an orthogonal electrospray ionization source. For the analysis of the generated molecular ions with high mass/charge ratios, the mass/charge range of the ion trap was extended by increasing its radio frequency (rf) voltage to 15 kV (V(0-p)) and by resonant ion ejection. Ions of the non-covalent dimer of bovine serum albumin (BSA), as well as of subunit complexes of alcohol dehydrogenase (ADH) from bakers' yeast and from horse liver, have been detected at mass/charge values between 3000-9000 Th. The maximum observed molecular weight was that of a non-covalently-bound subunit-octamer of bakers' yeast ADH (two non-covalently-bound subunit-tetramers) at ca. 290 kDa.

A comparison of overtone and fundamental resonances for mass range extension by resonance ejection in a quadrupole ion trap mass spectrometer

Mass range extension in a quadrupole ion trap has been investigated using resonant ejection of ions at the fundamental secular frequency (ω z ) and at the Ω-ω z overtone resonance frequency, where Ω is the drive rf angular frequency. The procedure is demonstrated for two commercial ion trap spectrometers operated at different drive rf frequencies. Higher rf amplitudes were required for ion ejection at low q z values using the Ω-ω z resonance than at the fundamental secular frequency, resulting in lower observed mass range enhancements for the Ω-ω z resonance at the same rf amplitude. Resonance excitation tandem mass spectrometric experiments also show lower energy deposition for the Ω-ω z resonance, although the effect is less marked at higher q z values.

Paul trap: axial ejection by auxiliary radio frequency voltage connection to the end-caps or by pure dipole field

Abstract The ion motion during resonant ejection from Paul traps has been described by simple expressions. They were derived for a supplementary radio-frequency voltage applied as a dipole field or connected to the trap end-caps. These expressions were derived for the ion axial motion by using Mathieu functions of fractionary order and perform well in the operating parameter range known as ensuring good mass resolutions. For ions uniformly distributed in the phase space, peak shapes were calculated by a computer program.

Mean Motion Resonances in The Asteroid Belt

The Kirkwood gaps in the main asteroidal belt (2 – 3.5 AU) coincide with the mean motion resonances with Jupiter (4/1, 3/1, 5/2, 7/3, 2/1). Similarly, several narrower gaps are observed in the outer asteroid belt (3.5 – 4 AU) at places of 11/6, 9/5, 7/4 and 5/3 Jovian resonances (Holman and Murray 1996). As it is now generally accepted, the formation and preservation of these gaps is due to the chaos of the resonant space and efficient ejection of the primordial and collisionaly injected bodies towards high eccentricities and planet-crossing orbits.The Jovian mean motion resonances are not the most important in what concerns the chaos of the observed (i.e. remaining) asteroid population. It was estimated by Šidlichovský and Nesvorný (1998) that about 40% of known objects have the Lyapunov time less than 105 years. It was later found (Nesvorný and Morbidelli 1998, 1999; Morbidelli and Nesvorný 1999) that the resonances responsible for this chaos are, in decreasing order of importance: 1) three-body resonances with Jupiter and Saturn, 2) exterior resonances with Mars, 3) moderate order Jovian resonances, and 4) three-body resonances with Mars and Jupiter.

Relation between the number of image planes and the accuracy of three-dimensional echocardiography for measuring left ventricular volumes and ejection fraction

The relation between accuracy of 3-dimensional echocardiography (3DE) in determining left ventricular end-diastolic volume, end-systolic volume, and ejection fraction (compared with magnetic resonance imaging) and the number of component planes used for 3DE ventricular reconstruction was evaluated in 41 adult subjects with normal (n = 24) and abnormal (n = 17) left ventricles. Accuracy and confidence of 3DE gradually increased with use of additional component planes, so that > or = 10 planes from both parasternal and apical windows provided 3DE reconstructions that accurately predict magnetic resonance imaging-measured left ventricular volumes and ejection fraction with confidence.

Losses of ions during forward and reverse scans in a quadrupole ion trap mass spectrometer and how to reduce them

The higher order fields present in the quadrupole ion trap may have beneficial effects such as increases in mass resolution in the mass-selective instability or resonance ejection modes of operation, but may also result in losses of ions due to nonlinear resonances. In this work, the reduction in ion intensities observed in the mass spectra of polyethylene glycol (PEG 1000) has been utilized to monitor the ion losses resulting from these higher order fields during the rf voltage scans in both the forward and reverse directions. Extensive ion losses were observed in reverse rf voltage scans at qz=0.64 (az=0), which corresponds to octopole resonance at βz=1/2. The losses depended upon rf voltage scan rate and ion mass being greater for lower scan rates and lower masses. For ions of m/z 877, losses of up to 60% of the stored ions were observed at low scan rates (<1×104 Da/s), but were minimal at higher scan rates. Thus, it is possible to avoid such losses during reverse scans by scanning the region qz=0.64 at rates in excess of 4×104 Da/s. In forward rf voltage scans, ion storage was considerably more reliable, with significant losses observed only at very high scan rates near the region qz=0.78 (hexapole resonance at βz=2/3).

Anion Effects on Storage and Resonance Ejection of High Mass-to-Charge Cations in Quadrupole Ion Trap Mass Spectrometry

The presence of negative charge created by an anion cloud in the center of a quadrupole ion trap is shown to be capable of affecting both the storage and the resonance ejection of high mass-to-charge cations. The attractive potential created by the presence of the anion cloud in the center of the ion trap can facilitate storage of high mass-to-charge cations under conditions that otherwise would allow the cations to escape. However, the presence of the anion cloud can interfere with the resonance ejection method for mass analysis due to the fact that the electric field created by the anion cloud can rival or exceed the influence of the quadrupole field on cation motion. Recognizing these phenomena allows for optimum conditions to be established for both ion storage and resonance ejection of high mass-to-charge ions formed via ion−ion reactions. The effect of high numbers of anions on the storage and resonance ejection of high mass-to-charge cations is illustrated with low charge state cations derived from ovalbumin and lysozyme formed via proton transfer reactions to anions derived from perfluoro-1,3-dimethylcyclohexane.

Resonant Excitation and/or Ejection of Ions Subjected to DC and RF fields in a Commercial Quadrupole Ion Trap

Resonant Excitation and/or Ejection of Ions Subjected to DC and RF fields in a Commercial Quadrupole Ion Trap

Resonant Excitation and/or Ejection of Ions Subjected to DC and RF fields in a Commercial Quadrupole Ion Trap

An empirical method has been proposed for the prediction of the frequency at which optimum resonant excitation can be achieved for ions confined in a quadrupole ion trap. The method, which can be used for both ion excitation and ion ejection, is based upon the changes which can occur in ion axial and radial secular frequencies as the excursions of ion trajectories increase from the centre of the ion trap. The changes in ion axial and radial secular frequencies wrought by optimum ion excitation can be expressed in terms of an equivalent direct current (DC) potential applied to the ion trap ring electrode; the method has been tested in three experiments in which DC potentials have been applied. © 1997 John Wiley & Sons, Ltd.

Resonance Ejection from the Paul Trap: A Theoretical Treatment Incorporating a Weak Octapole Field

In response to the growing experimental evidence of the importance of nonlinear phenomena in ion trap operation, a new theoretical model of ion ejection is developed. The pseudopotential well approximation for forced ion oscillations in an ion trap under the conditions of ion-molecule collisions is modified to include octapole perturbations on the quadrupole field. Ion ejection is investigated using the first-order Mitropol'skii asymptotic method for both infinitesimal and finite scan rates. It is shown that the combined action of collisional damping and nonlinearity distorts the resonance curve in such a way that "quenching" of oscillations takes place. As a result, with appropriate excitation and direction of scanning, the amplitude increases as if no damping exists! The main characteristics of the jump are derived as functions of scan rate and used for analytical estimation of mass resolution, mass peak width, and excitation voltage. Satisfactory agreement between calculated and experimental peak widths is demonstrated for the range of scanning rates in excess of 6 orders of magnitude.