Radio-frequency Quadrupole Ion Trap Research Articles

In-source and in-trap formation of molecular ions in the actinide mass range at CERN-ISOLDE

The use of radioactive molecules for fundamental physics research is a developing interdisciplinary field limited dominantly by their scarce availability. In this work, radioactive molecular ion beams containing actinide nuclei extracted from uranium carbide targets are produced via the Isotope Separation On-Line technique at the CERN-ISOLDE facility. Two methods of molecular beam production are studied: extraction of molecular ion beams from the ion source, and formation of molecular ions from the mass-separated ion beam in a gas-filled radio-frequency quadrupole ion trap. Ion currents of U+, UO1−3+, UC1−3+, UF1−4+, UF1,2O1,2+ are reported. Metastable tantalum and uranium fluoride molecular ions are identified. Formation of UO1−3+, U(OH)1−3+, UC1−3+, UF1,2O1,2+ from mass-separated beams of U+, UF1,2+ with residual gas is observed in the ion trap. The effect of trapping time on molecular formation is presented.

Developments at CERN-ISOLDE’s OFFLINE 2 mass separator facility for studies of molecular ion beams

ISOLDE’s Offline 2 laboratory has been upgraded to facilitate development for the production and study of molecular ion beams. New gas injection systems have been implemented for both molecular formation in the ion source and in the radio-frequency quadrupole ion trap used for beam preparation. MagneToF detectors and time-resolved single ion counting data acquisition have been implemented for low intensity beams and studies of laser-atom or laser-molecule interactions. We present a study of the formation and ionization of BaF+ using the upgraded facility.

Applications of the fractional calculus to study the physical theory of ion motion in a quadrupole ion trap.

In this article, fractional calculus has been applied to study the motion of ions in a three-dimensional radio frequency quadrupole ion trap; we have called this arrangement a fractional quadrupole ion trap. The main purpose of the article is to show that by controlling the fractional parameter of a trapped ion, one can gain a more efficient mass separation. In what follows, we will see that with decreasing the fractional parameter, we can achieve a smaller first stability region. Note that a small stability diagram will result in a good and acceptable mass separation. Various methods can be proposed to obtain a desired ion acceleration with a sufficient accuracy for good mass separation, which is similar to the one obtained by a fractional ion trap. Some of these methods are using the effects of a damping force, a magnetic field or both on the confinement of particles in the quadrupole ion trap. The first stability regions are plotted for all of the aforementioned methods, and simulation results are provided to compare them with those for the fractional case.

Truncated series solution of the equation of motion of ions trapped in a radiofrequency quadrupole trap with superimposed octopolar potential

The power series method is used to solve numerically the equation of motion of ions in a radiofrequency quadrupole ion trap when a small octopolar potential is present. Every time step, the coordinates of the ion are represented by a truncated series of degree 15. The stability diagram of the trap is obtained by using the method to simulate the behavior of hundreds of ions up to a relatively large time. The nonlinear resonance lines are observed. The ion's oscillation frequencies are then studied. Their theoretical expression is derived and compared to the results of the truncated series solution.

Novel radio-frequency quadrupole ion trap with spherical geometry

Confinement of single ions in a novel radio-frequency (RF) quadrupole ion trap with spherical shape is investigated. An optimization of this spherical ion trap (SIT) is carried out in order to suppress its nonlinearity substantially by eliminating the electric octupole moment. Hence, a trapping potential and consequently an electric field very similar to the ideal quadrupole ion trap (QIT) are obtained. Afterwards, three stability regions for the optimized SIT are numerically computed. The regions coincide well with those reported in the literature for the ideal QIT. The reason is attributed to the zero electric octupole moment of our proposed trap. The simple geometry of our optimized SIT and relative ease of fabrication along with its increased trapping volume in comparison with the conventional hyperbolic quadrupole ion trap, make it an appropriate choice for miniaturization.

Elucidating the sequence of intact bioactive peptides by using electron capture dissociation and hot electron capture dissociation in a linear radio‐frequency quadrupole ion trap

Electron capture dissociation (ECD) is useful tool for sequencing of peptides and proteins with post-translational modifications. To increase the sequence coverage for peptides and proteins, it is important to develop ECD device with high fragmentation efficiency. Sequence analysis of intact undigested bioactive peptides (3000-5000 Da) was performed by use of electron capture dissociation (rf-ECD) and collision-induced dissociation (CID) in a linear radio-frequency quadrupole ion trap that was coupled to a time-of-flight mass spectrometer. We applied rf-ECD, hot rf-ECD (rf-ECD with high electron energy), and CID for intact bioactive peptide ions of various charge states and evaluated the sequence coverage of their fragment spectra. Hot rf-ECD produced a higher number of c- and z-type fragment ions of modified peptide ions as electron energy increased in lower charged peptide ions, and sequence coverage greater than 80% was obtained compared with the CID case (40-80%). The result indicates that intact bioactive modified peptides (Ghrelin, ANP) were correctly identified by use of hot rf-ECD.

A novel ion cooling trap for multi-reflection time-of-flight mass spectrograph

A radiofrequency quadrupole ion trap system for use with a multi-reflection time-of-flight mass spectrograph (MRTOF) for short-lived nuclei has been developed. The trap system consists of two different parts, an asymmetric taper trap and a flat trap. The ions are cooled to a sufficient small bunch for precise mass measurement with MRTOF in only 2ms cooling time in the flat trap, then orthogonally ejected to the MRTOF for mass analysis. A trapping efficiency of ≈27% for 23Na+ and ≈5.1% for 7Li+ has been achieved.

An Efficient Approach for the Characterization of Mucin‐Type Glycopeptides: The Effect of O‐Glycosylation on the Conformation of Synthetic Mucin Peptides

Despite the growing importance of mucin core O-glycosylation in many biological processes including the protection of epithelial cell surfaces, the immune response, cell adhesion, inflammation, and tumorigenesis/metastasis, the regulation mechanism and conformational significance of the multiple introduction of α-GalNAc residues by UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferases (ppGalNAcTs) remains unclear. Here we report an efficient approach by combining MS and NMR spectroscopy that allows for the identification of O-glycosylation site(s) and the effect of O-glycosylation on the peptide backbone structures during enzymatic mucin domain assembly by using an isoform UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase-T2 (ppGalNAcT2) in vitro. An electron-capture dissociation device in a linear radio-frequency quadrupole ion trap (RFQ-ECD) combined with a time-of-flight (TOF) mass spectrometer was employed for the identification of Thr/Ser residues occupied by α-GalNAc branching among multiple and potential O-glycosylation sites in the tandem repeats of human mucin glycoproteins MUC4 (Thr-Ser-Ser-Ala-Ser-Thr-Gly-His-Ala-Thr-Pro-Leu-Pro-Val-Thr-Asp) and MUC5AC (Pro-Thr-Thr-Val-Gly-Ser-Thr-Thr-Val-Gly). In the present study, O-glycosylation was initiated specifically at Thr10 in naked MUC4 peptide and additional introduction of α-GalNAc proceeded preferentially but randomly at three other Thr residues to afford densely glycosylated MUC4 containing six α-GalNAc residues at Thr1, Ser2, Ser5, Thr6, Thr10, and Thr15. On the contrary, O-glycosylation of naked MUC5AC peptide occurred predominantly at consecutive Thr residues and led to MUC5AC with four α-GalNAc residues at Thr2, Thr3, Thr7, and Thr8. The solution structures determined by NMR spectroscopic studies elicited that the preferential introduction of α-GalNAc at Thr10 of MUC4 stabilizes specifically a β-like extended backbone structure at this area, whereas other synthetic models with a single α-GalNAc residue at Thr1, Thr6, or Thr15 did not exhibit any converged three-dimensional structure at the proximal peptide moiety. Such conformational impact on the underlying peptides was proved to be remarkable in the glycosylation at the consecutive Thr residues of MUC5AC.

Quadrupole ion traps

The extraordinary story of the three-dimensional radiofrequency quadrupole ion trap, accompanied by a seemingly unintelligible theoretical treatment, is told in some detail because of the quite considerable degree of commercial success that quadrupole technology has achieved. The quadrupole ion trap, often used in conjunction with a quadrupole mass filter, remained a laboratory curiosity until 1979 when, at the American Society for Mass Spectrometry Conference in Seattle, George Stafford, Jr., of Finnigan Corp., learned of the Masters' study of Allison Armitage of a combined quadrupole ion trap/quadrupole mass filter instrument for the observation of electron impact and chemical ionization mass spectra of simple compounds eluting from a gas chromatograph. Stafford developed subsequently the mass-selective axial instability method for obtaining mass spectra from the quadrupole ion trap alone and, in 1983, Finnigan Corp. announced the first commercial quadrupole ion trap instrument as a detector for a gas chromatograph. In 1987, confinement of ions generated externally to the ion trap was demonstrated and, soon after, the new technique of electrospray ionization was shown to be compatible with the ion trap.

Dipole excitation of ions in linear radio frequency quadrupole ion traps with added multipole fields

Ion motion with auxiliary dipole excitation and collisional damping in a linear radiofrequency quadrupole ion trap incorporating small amounts of even higher order multipoles is studied analytically. The ion motion is modeled in a pseudopotential that is mostly quadratic with small amounts of higher spatial harmonics. Ion motion along x and y axes is characterized by two uncoupled forced and damped anharmonic oscillator equations. A multiple time scales method is used to solve the equations of motion of ions with a first order perturbation correction. Analytical relations between the oscillation amplitudes at steady state (the stationary amplitudes) and excitation frequency are calculated. The frequency response curves show that in some cases bistable behavior might be obtained, i.e., there are two stable stationary amplitudes for a given excitation frequency.

Electron transfer dissociation of N‐glycopeptides: loss of the entire N‐glycosylated asparagine side chain

The recently introduced electron transfer dissociation (ETD) technique opens new possibilities for the structural characterization of glycoproteins at the glycopeptide level. In this report, we investigate the ETD mass spectra of tryptic N-glycopeptides of the model glycoprotein horseradish peroxidase (HRP). Multiply protonated N-glycopeptides obtained by electrospray ionization were subjected to ETD. Fragment ions obtained by ETD were further analyzed by collision-induced dissociation (CID) (MS(3)) for their unambiguous structural assignment. The following fragmentation features were revealed: (1) c- and z-type peptide backbone cleavages were observed with retention of the intact glycan moiety revealing peptide sequence, glycan attachment site, and glycan mass; (2) to a lesser extent, glycosidic bond cleavages were registered with retention of the intact peptide sequence; and (3) a range of amino acid side chain losses did occur. Remarkably, the loss of the complete N-glycosylated asparagine side chain was observed. This loss of the glycan-modified side chain helps with the structural characterization of glycopeptides by allowing the facile deduction and verification of the glycan mass and the nature of the amino acid residue at the glycan attachment site. Importantly, informative ETD spectra were obtained in this study by reversed-phase nano-liquid chromatography (LC) coupled online to a radio-frequency (rf) quadrupole ion trap (QIT) mass spectrometer with alternating acquisition of CID and ETD mass spectra from an automatically selected set of precursors (data-dependent mode). Thus, our study brings nano-LC/QIT-MS(n) with CID and ETD to the fore as a powerful technique for glycoproteomics at the glycopeptide level.

A highly selective laser ion source for bunched, low emittance beam release

A novel type of resonance ionization laser ion source (RILIS) is under development, which combines the advantages of laser ionization with those of a source-implemented ion trap. This laser ion source trap (LIST) system, based on a gas-filled linear radio-frequency quadrupole ion trap, decouples the evaporation and ionization process by introduction of a primary surface ion repeller. Apart from significantly enhancing the selectivity in radioactive ion beam production, optimum control on the temporal pulse structure and the emittance of the generated laser ion bunch is obtained. A variety of operational modes from quasi-dc to microseconds-bunched ion beams with variable repetition rate can be set. Principles and layout of individual components of the LIST are presented on the basis of atom and ion trajectory simulations and resulting performance parameters are discussed.

Kinetic theory of radio frequency quadrupole ion traps. I. Trapping of atomic ions in a pure atomic gas.

A kinetic theory based on the Boltzmann equation is developed for the trapping of atomic ions in a radio-frequency quadrupole ion trap containing enough neutral atoms that ion-neutral collisions cannot be ignored. The collisions are treated at the same level of sophistication and detail as is used to deal with the time- and space-dependent electric fields in the trap. As a result, microscopic definitions are obtained for the damping and stochastic forces that originate from such collisions. These definitions contrast with corresponding phenomenological terms added ad hoc in previous treatments to create damped Mathieu and Langevin equations, respectively. Furthermore, the theory indicates that either collisional cooling or heating of the ions is possible, depending upon details of the ion-neutral mass ratios and interaction potential. The kinetic theory is not dependent on any special assumptions about the electric field strengths, the ion-neutral interaction potentials, or the ion-neutral mass ratio. It also provides an ab initio way to describe the ion kinetic energies, temperatures, and other properties by a series of successive approximations.

A novel scheme for a highly selective laser ion source

A new type of resonance ionization laser ion source, which shall combine the advantages of a laser ion source with those of an ion trap, is proposed. The primary purpose of such a laser ion source trap, which is based on a gas-filled linear radio-frequency quadrupole ion trap system, is the decoupling of evaporation and ionization processes. Furthermore optimum temporal control on the generated ion bunch is obtained. Both effects will lead to a significantly increased isobaric selectivity and ion beams of low emittance. A large variety of operational modes, ranging from quasi-dc to microseconds-bunched radioactive ion beams with variable pulse width and repetition rate, can be chosen freely and beam energies can easily be altered. The principles and the layout of LIST will be discussed on the basis of atom and ion trajectory simulations and resulting performance parameters.

Chemical reaction of sympathetically laser-cooled molecular ions

We report an ion/neutral reaction of sympathetically laser-cooled molecular ions. Using laser-cooled fluorescence mass spectrometry, a reaction of about 100 sympathetically cooled H3O+ ions with NH3 were traced at 4 min interval as the reacting ions were kept within a radiofrequency-quadrupole ion trap. To obtain the reaction rate of the reaction molecular dynamical calculation was used to relate the fluorescence mass spectra to the quantity of the molecular ions.

A linear radiofrequency quadrupole ion trap for the cooling and bunching of radioactive ion beams

A linear radio-frequency quadrupole ion guide and beam buncher has been installed at the ISOLTRAP mass spectrometry experiment at the ISOLDE facility at CERN. The apparatus is being used as a beam cooling, accumulation, and bunching system. It operates with a buffer gas that cools the injected ions and converts the quasicontinuous 60-keV beam from the ISOLDE facility to 2.5-keV beam pulses with improved normalized transverse emittance. Recent measurements suggest a capture efficiency of the ion guide of up to 40% and a cooling and bunching efficiency of at least 12% which is expected to still be increased. The improved ISOLTRAP setup has so far been used very successfully in three on-line experiments.

Confinement of ions in a radio frequency quadrupole ion trap supplied with a periodic impulsional potential

This article explains the confinement of the ion in the first stability region of the three-dimensional radio frequency quadrupole ion trap using a periodic impulsional potential of the form V 0 cos Ω t/(1 − k cos 2Ω t) with 0 ≤ k < 1. Numerical computations have been used to study the different aspects of impulsional potential when k = 0.8, and compared with a sinusoidal potential k = 0 for some value of equivalent points: two operating points located in their corresponding stability diagram having the some β z .

Hyperfine structure and isotope shifts in the 5d6s 2 a 2 D 3/2 --5d6s(a 3 D)6p z 4 F 5/2 ° transition of Hf II

The hyperfine structure and isotope shifts of the transition between the 5d6s2 a2D3/2 ground state and the 5d6s(a3D)6p z4F5/2 ° excited state of singly ionized hafnium at \lambda=340 nm have been investigated by laser spectroscopy using a radio-frequency quadrupole ion trap. The magnetic dipole coupling constant A and electric quadrupole coupling constant B of the two atomic levels for both stable isotopes 177Hf and 179Hf are determined. The changes of mean square nuclear charge radii \delta[ r2] of the stable Hf isotopes and the radioactive isotope 172Hf (T1/2=1.87a) have been extracted from the data.

Multiparticle Simulation of Ion Injection into the Quadrupole Ion Trap Under the Influence of Helium Buffer Gas Using Short Injection Times and DC Pulse Potentials

The motion of an ensemble of ions during their gated injection into the three-dimensional radio-frequency quadrupole ion trap was simulated considering helium buffer gas collisions, injection at certain RF phase angles and using DC impulsive fields to optimize the trapping efficiency. Simulations using a simple model of ion–neutral collisions show that buffer gas alone, even at 1–10 mTorr pressure, is not able to remove sufficient kinetic energy from the injected ions through ion–neutral collisions to prevent their loss and does not solve the problem of low efficiency of ion retention in the trap, even under qz-optimization. Accumulation of ions over long periods is not very effective, in such cases trapping efficiencies are less than 5%. It is shown that under RF phase angles between 110° and 220° the injected ions can be trapped, although only temporarily, with very high efficiencies. The simulations lead to the conclusion that all of the desired injected ions covering a wide mass/charge range can be trapped for indefinite times when short injection pulses are used together with dipolar DC pulses applied during injection. The additional damping achieved through these dipolar DC impulsive fields and the resulting cooling of the ion cloud is shown to give efficient trapping.

Ion storage and laser-induced fluorescence measurements in an RFQ trap

A radio-frequency quadrupole (RFQ) ion trap has been constructed for laser spectroscopic measurements of isotope shifts (IS) and hyperfine structures (HPS) with a small number of ions produced from a very dilute hafnium sample placed inside the RFQ trap. The target atoms are evaporated by a Nd:YAG heating laser pulse, and then selectively ionized by resonance ionization. Through repetitive application of this procedure, hafnium ions are accumulated in the trap, and a storage time of hours has been achieved with H 2 as a buffer gas. Laser-induced fluorescence (LIF) with the 5d 6s 2 ( a 2D) ( J = 3/2) — 5d6s ( a 3D)6p ( z 4F o) ( J = 5/2) transition is observed. Through an RF phase timing coincident technique, a resolution of about 1 GHz car. Le obtained, sufficient to resolve most of the hyperfine components of the odd A isotopes Also, by continuously monitoring the LIF signal, the ion cooling time in the RFQ trap is measured as a function of buffer gas pressure, and the ion-molecule collision cross section deduced is consistent with theoretical expectations. The sensitivity of this method was tested with the IS measurement of a mass separated 173Hf source.