Resonance excitation of ions stored in a quadrupole ion trap. Part 1. A simulation study

A simple numerical simulation model can elucidate the key factors for designing a miniaturized ion trap mass spectrometer

BACKGROUND

Miniature ion trap mass spectrometer enables mass-to-charge ratio analysis of ions via quadrupole field in a low vacuum environment. It plays an important role in on-site detection due to its portability and specificity. In order to gain a deeper understanding of the analysis mechanism of miniature ion trap mass spectrometers, a quadrupole MS ion trajectory numerical simulation model (QITNS) is established in this paper for ions trajectory calculation under the action of quadrupole field, exciting field and neutral gas molecule collision. Compared with the existing methods, the model in this paper is simpler and more direct, which effectively explored the effects of dipole excitation and quadrupole excitation on ion manipulation under high background pressure.

RESULTS

The simulation results demonstrate that high RF amplitude, low auxiliary AC amplitude and quadrupole excitation can effectively improve the isolation resolution. Besides, it clarified the difference between the analysis mechanism of ion trap mass spectrometers under high background pressure (above 13.332 Pa) and absolute vacuum conditions. The relevant results are consistent with the conclusions of previous experiments and other theories, proving the applicability and accuracy of the proposed calculation model and solution method.

SIGNIFICANCE

This research bears the guiding significance for further understanding the mechanism of quadrupole mass spectrometry as well as designing and developing miniature mass spectrometers.

Energy-Resolved Mass Spectrometry as a Tool for Identification of Lignin Depolymerization Products

Lignin is the largest source of bio-based aromatic compounds in nature, and its valorization is essential to the sustainability of lignocellulosic biorefining. Characterizing lignin-derived compounds remains challenging due to the heterogeneity of this biopolymer. Tandem mass spectrometry is a promising tool for lignin structural analytics, as fragmentation patterns of model compounds can be extrapolated to identify characteristic moieties in complex samples. This work extended previous resonance excitation-type collision-induced dissociation (CID) methods that identified lignin oligomers containing β-O-4, β-5, and β-β bonds, to also identify characteristics of 5-5, β-1, and 4-O-5 dimers, enabled by quadrupole time-of-flight (QTOF) CID with energy-resolved mass spectrometry (ERMS). Overall, QTOF-ERMS offers in-depth structural information and could ultimately contribute to tools for high-throughput lignin dimer identification.

Using isotopologues to probe the potential energy surface of reactions of C2H2 ++C3H4

Investigations into bimolecular reaction kinetics probe the details of the underlying potential energy surface (PES), which can help to validate high-level quantum chemical calculations. We utilize a combined linear Paul ion trap with a time-of-flight mass spectrometer to study isotopologue reactions between acetylene cations (C₂H₂ ⁺) and two isomers of C₃H₄: propyne (HC₃H₃) and allene (H₂C₃H₂). In a previous study [Schmid et al., Phys. Chem. Chem. Phys. 22, 20303 (2020)],¹ we showed that the two isomers of C₃H₄ have fundamentally different reaction mechanisms. Here, we further explore the calculated PES by isotope substitution. While isotopic substitution of reactants is a standard experimental tool in the investigation of molecular reaction kinetics, the controlled environment of co-trapped, laser-cooled Ca⁺ ions allows the different isotopic reaction pathways to be followed in greater detail. We report branching ratios for all of the primary products of the different isotopic species. The results validate the previously proposed mechanism: propyne forms a bound reaction complex with C₂H₂ ⁺, while allene and C₂H₂ ⁺ perform long-range charge exchange only.

Emerging Technologies in Mass Spectrometry-Based DNA Adductomics

The measurement of DNA adducts, the covalent modifications of DNA upon the exposure to the environmental and dietary genotoxicants and endogenously produced electrophiles, provides molecular evidence for DNA damage. With the recent improvements in the sensitivity and scanning speed of mass spectrometry (MS) instrumentation, particularly high-resolution MS, it is now feasible to screen for the totality of DNA damage in the human genome through DNA adductomics approaches. Several MS platforms have been used in DNA adductomic analysis, each of which has its strengths and limitations. The loss of 2′-deoxyribose from the modified nucleoside upon collision-induced dissociation is the main transition feature utilized in the screening of DNA adducts. Several advanced data-dependent and data-independent scanning techniques originated from proteomics and metabolomics have been tailored for DNA adductomics. The field of DNA adductomics is an emerging technology in human exposure assessment. As the analytical technology matures and bioinformatics tools become available for analysis of the MS data, DNA adductomics can advance our understanding about the role of chemical exposures in DNA damage and disease risk.

Multigenerational Broadband Collision-Induced Dissociation of Precursor Ions in a Linear Quadrupole Ion Trap

A method of fragmenting ions over a wide range of m/z values while balancing energy deposition into the precursor ion and available product ion mass range is demonstrated. In the method, which we refer to as "multigenerational collision-induced dissociation", the radiofrequency (rf) amplitude is first increased to bring the lowest m/z of the precursor ion of interest to just below the boundary of the Mathieu stability diagram (q = 0.908). A supplementary AC signal at a fixed Mathieu q in the range 0.2-0.35 (chosen to balance precursor ion potential well depth with available product ion mass range) is then used for ion excitation as the rf amplitude is scanned downward, thus fragmenting the precursor ion population from high to low m/z. The method is shown to generate high intensities of product ions compared with other broadband CID methods while retaining low mass ions during the fragmentation step, resulting in extensive fragment ion coverage for various components of complex mixtures. Because ions are fragmented from high to low m/z, space charge effects are minimized and multiple discrete generations of product ions are produced, thereby giving rise to "multigenerational" product ion mass spectra. Graphical Abstract ᅟ.

Ion Isolation in a Linear Ion Trap Using Dual Resonance Frequencies

Ion isolation in a linear ion trap is demonstrated using dual resonance frequencies, which are applied simultaneously. One frequency is used to eject ions of a broad m/z range higher in m/z than the target ion, and the second frequency is set to eject a range of ions lower in m/z. The combination of the two thus results in ion isolation. Despite the simplicity of the method, even ions of low intensity may be isolated since signal attenuation is less than an order of magnitude in most cases. The performance of dual frequency isolation is demonstrated by isolating individual isotopes of brominated compounds. Graphical Abstract ᅟ.

Discovery and characterization of hydroxylysine in recombinant monoclonal antibodies

Tryptic peptide mapping analysis of a Chinese hamster ovary (CHO)-expressed, recombinant IgG1 monoclonal antibody revealed a previously unreported +16 Da modification. Through a combination of MS(n) experiments, and preparation and analysis of known synthetic peptides, the possibility of a sequence variant (Ala to Ser) was ruled out and the presence of hydroxylysine was confirmed. Post-translational hydroxylation of lysine was found in a consensus sequence (XKG) known to be the site of modification in other proteins such as collagen, and was therefore presumed to result from the activity of the CHO homolog of the lysyl hydroxylase complex. Although this consensus sequence was present in several locations in the antibody sequence, only a single site on the heavy-chain Fab was found to be modified.

Computer Modeling of an Ion Trap Mass Analyzer, Part I: Low Pressure Regime

We present the multi-particle simulation program suite Computational Ion Trap Analyzer (CITA) designed to calculate the ion trajectories within a Paul quadrupole ion trap developed by the Jet Propulsion Laboratory (JPL). CITA uses an analytical expression of the electrodynamic field, employing up to six terms in multipole expansion and a modified velocity-Verlet method to numerically calculate ion trajectories. The computer code is multithreaded and designed to run on shared-memory architectures. CITA yields near real-time simulations with full propagation of 26 particles per second per core. As a consequence, a realistic numbers of trapped ions (100+ million) can be used and their trajectories modeled, yielding a representative prediction of mass spectrometer analysis of trace gas species. When the model is compared with experimental results conducted at low pressures using the conventional quadrupole and dipole excitation modes, there is an excellent agreement with the observed peak shapes. Owing to the program's efficiency, CITA has been used to explore regions of trapping stability that are of interest to experimental research. These results are expected to facilitate a fast and reliable modeling of ion dynamics in miniature quadrupole ion trap and improve the interpretation of observed mass spectra. Graphical Abstract ᅟ.

Alternative representation for the stability diagram of quadrupole ion traps upon additional quadrupolar excitation

We present a combined theoretical and experimental study of the stability of ions in a linear ion trap under the application of one or two auxiliary radiofrequency (RF) fields, in order to perform simultaneous resonant excitation/ejection of several different ions. The influence of the amplitude and frequency of the auxiliary field is addressed through the construction of experimental and theoretical stability diagrams. Theoretical diagrams are constructed using the method developed by Konenkov et al. [J. Am. Soc. Mass Spectrom. 13, 597 (2002)]. We propose a new representation of stability diagrams more adapted to the study of auxiliary excitations than the canonical one. Stability regions are represented as a function of the fundamental RF amplitude and of the relative intensity of the excitation. This representation facilitates the monitoring of the evolution of the mass-selectivity of first- and higher-order resonant excitations in the trap, for which an empirical law is derived. We also show that the relative phase shift between the excitation field and the main driving field has a strong influence on the shape of the diagrams.

Accelerated simulation study of space charge effects in quadrupole ion traps using GPU techniques

Space charge effects play important roles in the performance of various types of mass analyzers. Simulation of space charge effects is often limited by the computation capability. In this study, we evaluate the method of using graphics processing unit (GPU) to accelerate ion trajectory simulation. Simulation using GPU has been compared with multi-core central processing unit (CPU), and an acceleration of about 390 times have been obtained using a single computer for simulation of up to 10(5) ions in quadrupole ion traps. Characteristics of trapped ions can be investigated at detailed levels within a reasonable simulation time. Space charge effects on the trapping capacities of linear and 3D ion traps, ion cloud shapes, ion motion frequency shift, mass spectrum peak coalescence effects between two ion clouds of close m/z are studied with the ion trajectory simulation using GPU.

Calculation of coupled secular oscillation frequencies and axial secular frequency in a nonlinear ion trap by a homotopy method

In this paper the homotopy perturbation method is used for calculation of the frequencies of the coupled secular oscillations and axial secular frequencies of a nonlinear ion trap. The motion of the ion in a rapidly oscillating field is transformed to the motion in an effective potential. The equations of ion motion in the effective potential are in the form of a Duffing-like equation. The homotopy perturbation method is used for solving the resulted system of coupled nonlinear differential equations and the resulted axial equation for obtaining the expressions for ion secular frequencies as a function of nonlinear field parameters and amplitudes of oscillations. The calculated axial secular frequencies are compared with the results of Lindstedt-Poincare method and the exact results.

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.

Mapping the stability diagram of a digital ion trap (DIT) mass spectrometer varying the duty cycle of the trapping rectangular waveform

In a digital ion trap the beta(r) and beta(z) boundary lines of the stability diagram are determined experimentally using an innovative approach. In the rectangular waveform-driven digital ion trap (DIT) manipulation of the waveform duty cycle allows introduction of a precisely defined DC quadrupole component into the main trapping field. Variation of the duty cycle can be controlled at software level without any hardware modification. The data generated use peptide ions, which produce stability diagrams in good agreement with the theoretical stability diagrams previously determined by simulation studies.

Dynamic collision-induced dissociation (DCID) in a quadrupole ion trap using a two-frequency excitation waveform: II. Effects of frequency spacing and scan rate

Dynamic CID of selected precursor ions is achieved by the application of a two-frequency excitation waveform to the end-cap electrodes during the mass instability scan of a quadrupole ion trap (QIT) mass spectrometer. This new method permits a shorter scanning time when compared with conventional on-resonance CID. When the excitation waveform consists of two closely-spaced frequencies, the relative phase-relationship of the two frequencies plays a critical role in the fragmentation dynamics. However, at wider frequency spacings (>10 kHz), these phase effects are diminished, while maintaining the efficacy of closely-spaced excitation frequencies. The fragmentation efficiencies and energetics of n-butylbenzene and tetra-alanine are studied under different experimental conditions and the results are compared at various scan rate parameters between 0.1 and 1.0 ms/Th. Although faster scan rates reduce the analysis time, the maximum observed fragmentation efficiencies rarely exceed 30%, compared with values in excess of 50% achieved at slower scan rates. The internal energies calculated from the simulations of n-butylbenzene at fast scan rates are approximately 4 eV for most experimental conditions, while at slow scan rates, internal energies above 5.5 eV are observed for a wide range of conditions. Extensive ITSIM simulations support the observation that slowing the scan rate has a similar effect on fragmentation as widening the frequency spacing between the two excitation frequencies. Both approaches generally enhance CID efficiencies and make fragmentation less dependent upon the relative phase angle between the excitation waveform and the ion motion.

Study of the enhancement of dipolar resonant excitation by linear ion trap simulations

Resolution improvements in dipolar resonant excitation have been examined in a round-rod quadrupolar collision cell for values of the Mathieu characteristic exponent beta equal to n/p, where n and m are small integers (prime beta values) versus other beta values where n and p are not small (ordinary beta values). The trajectories of ions moving in the time-varying electric fields of a quadrupole with and without buffer-gas molecules were calculated to determine the relationship of prime and ordinary beta values to frequency resolution for resonant ion excitation and ejection. For prime beta values, the ion trajectory in the hyperbolic quadrupole field will be exactly periodic with a period of at most 4 pi p/Omega, where Omega is the angular frequency of the main drive radio-frequency (RF) potential. Ion trajectory simulations with prime beta versus ordinary beta values show that the motion of ions with prime beta values have simpler trajectories of shorter periods. Frequency response profiles (FRPs) for round-rod quadrupoles at zero pressure show that dipolar resonant excitations with prime beta values exhibit significantly narrower bandwidths than those with ordinary beta values. Simulations show that at 0.05 to 0.8 mTorr of nitrogen, it is possible to reduce the FRP bandwidth by 20% (measured at 50% depth).

Supplemental activation method for high-efficiency electron-transfer dissociation of doubly protonated peptide precursors

Electron-transfer dissociation (ETD) delivers the unique attributes of electron capture dissociation to mass spectrometers that utilize radio frequency trapping-type devices (e.g., quadrupole ion traps). The method has generated significant interest because of its compatibility with chromatography and its ability to: (1) preserve traditionally labile post-translational modifications (PTMs) and (2) randomly cleave the backbone bonds of highly charged peptide and protein precursor ions. ETD, however, has shown limited applicability to doubly protonated peptide precursors, [M + 2H]2+, the charge and type of peptide most frequently encountered in "bottom-up" proteomics. Here we describe a supplemental collisional activation (CAD) method that targets the nondissociated (intact) electron-transfer (ET) product species ([M + 2H]+*) to improve ETD efficiency for doubly protonated peptides (ETcaD). A systematic study of supplementary activation conditions revealed that low-energy CAD of the ET product population leads to the near-exclusive generation of c- and z-type fragment ions with relatively high efficiency (77 +/- 8%). Compared to those formed directly via ETD, the fragment ions were found to comprise increased relative amounts of the odd-electron c-type ions (c+*) and the even-electron z-type ions (z+). A large-scale analysis of 755 doubly charged tryptic peptides was conducted to compare the method (ETcaD) to ion trap CAD and ETD. ETcaD produced a median sequence coverage of 89%-a significant improvement over ETD (63%) and ion trap CAD (77%).

Resonant ac Dipolar Excitation for Ion Motion Control in the Orbitrap Mass Analyzer

A dipolar ac signal applied to the split outer electrode of an Orbitrap mass spectrometer at the axial resonance frequency causes excitation of ion axial motion and either eventual ion ejection from the trap, if applied in phase with ion motion, or de-excitation, if applied 180 degrees out of phase. Both de-excitation and excitation may be achieved mass-selectively. The extent of ion axial de-excitation depends on the ac amplitude and on the number of cycles applied; sufficient de-excitation can be accomplished such that the ion signal cannot be observed above baseline noise. After de-excitation, the ions remain trapped and in rapid orbital (but not axial) motion, which allows them to be re-excited coherently by application of a second ac waveform allowing the signal again to be observed. Both broad-band and narrow-band waveforms have been used to de-excite and to re-excite ion motion. Using narrow-band waveforms, selective de-excitation and re-excitation can be performed with unit mass selection, leaving an adjacent 13C isotopic peak unaffected. The origin and potential applications of these new capabilities is delineated.

Hybrid triple quadrupole-linear ion trap mass spectrometry in fragmentation mechanism studies: application to structure elucidation of buspirone and one of its metabolites

The use of a hybrid triple quadrupole-linear ion trap (QqQ(LIT)) mass spectrometer system for a comprehensive study of fragmentation mechanisms is described. The anxiolytic drug, buspirone, was chosen as a model compound for this study. With the advent of a QqQ(LIT) instrument, both the traditional quadrupole and the new linear ion trap scans (LIT) could be performed in a single LC run. In the past, a sample had to be run on two different instruments, namely, a triple quadrupole instrument (QqQ) and a 3D ion trap (3D IT) to obtain similar information. With the new QqQ(LIT) technology, collision-induced dissociation (CID) occur in a quadrupole collision cell, q2, and fragment ions are trapped and analyzed in Q3 operated in LIT mode. In this work, high-sensitivity product ion spectra of buspirone were obtained from the one-stage 'Enhanced Product Ion' scan (EPI) with rich product ions and no low mass cut-off. Furthermore, detailed fragmentation pathways were elucidated by further dissociation of each of the fragment ions in the EPI spectrum using MS(3) mode in the same run. The MS(3) scan was performed by incorporating CID in q2, and trapping, cooling, isolation, and resonance-excitation in Q3 when operating in LIT mode. This approach allowed unambiguous assignment of all fragment ions quickly with fewer experiments and easier interpretation than the previous approach. The overall sensitivity for obtaining complete fragment ion data was significantly improved for QqQ(LIT) as compared with that of QqQ and 3D IT mass spectrometers. This is beneficial for structure determination of unknown trace components. The method allowed structure determination of metabolites of buspirone in rat microsomes at 1 microM concentration, which was a 10-fold lower concentration than was needed for QqQ or 3D IT instruments. The QqQ(LIT) instrument provided a simple, rapid, sensitive and powerful approach for structure elucidation of trace components.

Observation of higher order quadrupole excitation frequencies in a linear ion trap

The resonant frequencies for quadrupole excitation of ions confined with a buffer gas in a linear quadrupole ion trap with Mathieu parameters a = 0 and q approximately 0.36 have been measured. The resonances are predicted to occur at angular frequencies omega(K)n given by omega(K)n = (omega/K)n + beta without the presence of a buffer gas where omega is the angular frequency of the trapping radio frequency, K = 1, 2, 3... is the order of the resonance calculated with perturbation theory, and n = 0, +/-1, +/-2, +/-3.... The resonances are measured through fragmentation of protonated reserpine. The observed frequencies agree closely with the theoretical values but there are small differences which vary from +0.6% at K = 2 to -2.7% at K = 6. This is believed to be the result of the dependence of the resonant frequencies upon the buffer gas density and/or the excitation amplitude. The resolution of the resonances (measured from the depletion of precursor and formation of fragment ions) increased by a factor of 2 as K increased from 1 to 6. This increase in resolution warrants further investigation into the use of higher order resonances for isolation and excitation of trapped ions.

An empirical approach to estimation of critical energies by using a quadrupole ion trap

A simple energy-resolved mass spectrometric technique is described for the estimation of critical energies for dissociation of ions via threshold collisional activation measurements in a quadrupole ion trap. The method is calibrated by using compounds with well-defined dissociation energies, and separate calibration curves must be constructed for radical ions that are bound by covalent bonds versus hydrogen-bonded complexes. For these sets of experiments, the threshold point is defined as the activation voltage required for the fragment ion intensity to be 10% of the total ion intensity. A plot of threshold activation voltage of the calibrant versus literature critical energies shows a near-linear function, and accuracies are estimated as better than ± 6 kcal/mol. The q z value during activation seems to have little effect on the threshold voltages as long as very low q z values that cause ion ejection are avoided. Activation periods that are substantially longer than 10-ms result in nonlinear behavior in the calibration curves for ions that have critical energies above 30 kcal/mol. This energy-resolved method was also useful for the estimation of critical energies of complexes bound by electrostatic forces, such as hydrogen-bonding interactions. A quantitative evaluation of proton-bound polyether-amine complexes showed that the number of available hydrogen-binding sites, the gas-phase basicities of the polyether and amine components, and the ability of the complex to attain the most favorable near-linear hydrogen bonds correlate with the threshold values.

Plasma source ion trap mass spectrometry: Enhanced abundance sensitivity by resonant ejection of atomic ions

An experimental study of resonant ion excitation in an rf quadrupole ion trap is reported. Atomic ions are generated in an inductively coupled plasma and injected into the ion trap where, after collisional cooling, they are irradiated by a low-voltage, dipole coupled waveform. Single frequency, narrowband, and broadband excitation pulses have been used. Absorption lineshapes (plots of observed ion signal versus excitation frequency) are shown for variations in buffer gas pressure and the amplitude and duration of the single frequency pulses. The absorption lineshapes are usually asymmetric and tail toward lower frequencies. At sufficiently low buffer gas pressure or potential well depth, the lineshapes broaden and become more asymmetric to the point that absorption by ions with adjacent mass-to-charge ratios overlaps. This overlapping absorption reduces the selectivity with which a single mass-to-charge ratio ion can be excited and ejected relative to nearby mass-to-charge ratio ions. The rate of ion ejection is different on the low versus high frequency edges of the absorption lines. This difference in ejection rates provides an important key to understanding the shape of the absorption lines. All of these observations are explained in terms of the known kinematic behavior of ions in real traps, that is, traps with substantial higher order symmetry components in the trapping field ("nonlinear" fields). The importance of the nonlinearity of the trapping field in understanding the observed lineshapes and their time dependencies is discussed. We also report resonant ejection results obtained using multiple frequency (narrow or broad bandwidth) excitation. Multiple frequency excitation allows ions with different mass-to-charge ratio values to be ejected from the trap using one excitation waveform. The finite ion storage capacity of the ion trap is thereby reserved for the ion(s) of interest. We show that ejection of (89)Y ions can be ∼ 10(5) times more efficient than ejection of ions at either m/z 88 or 90.

Multiparticle simulation of ion motion in the ion trap mass spectrometer: Resonant and direct current pulse excitation

A PC-based program that simulates the behavior of a collection of ions is extended to include the effects of collisions with the buffer gas and enhanced visualization methods. The simulations are based on the quadrupole field associated with the actual ion trap electrode structure. Ionization is simulated in such a way as to distribute ionization events randomly over rf phase angles and yield a realistic collection of stored ions. The effects of buffer gas collisions on ion motion during both mass-selective instability and resonance ejection scans are found to include the expected dampening of spatial excursions as well as limitation of the kinetic energy of trapped ions. In both experiments, ion ejection occurs over a number of secular cycles in the vicinity of the theoretical instability point. Activation via a resonant ac signal or a short dc pulse is shown to result in phase-locking of the ions as well as the expected increase in the size of the excursions in the z direction and in ion kinetic energy. Collisions cause dephasing and loss of kinetic energy. Radial dc activation is compared with activation in the axial direction. Experimental data for dc pulse activation of the n-butylbenzene molecular ion are analyzed in phase space and the onset of surface-induced dissociation is correlated with changes in the experimental m/z 91 to m/z 92 fragment ion ratio. Poincaré sections are shown for resonantly excited ions and their value in demonstrating improvement of the resolution of these experiments over conventional mass-selective instability scans is shown.

Competition between resonance ejection and ion dissociation during resonant excitation in a quadrupole ion trap

The competition between ion dissociation and ion ejection during resonant excitation in a quadrupole ion trap is investigated. Ions of similar mass but with a range of critical energies for the onset of dissociation have been examined. The effects of the amplitude and duration of the resonant excitation, the well depth in which the ions are trapped, and the pressure and nature of the collision gas are explored. Once the onset of ion ejection is reached, the rate of ion ejection increases with increased amplitude of the resonant excitation signal. The rate of ejection decreases or stays constant as a function of the duration of the resonant excitation, depending upon the ion species being excited. Increasing the trapping well depth increases the relative amount of dissociation versus ejection as does increasing the pressure of the bath gas. Adding heavier bath gases lowers the onset of ion dissociation and raises the onset of ion ejection.

Determination of positions, velocities, and kinetic energies of resonantly excited ions in the quadrupole ion trap mass spectrometer by laser photodissociation

The effects on ion motion caused by the application of a resonance AC dipole voltage to the end-cap electrodes of the quadrupole ion trap are described. An excimer laser is used to photodissociate benzoyl ions, and its triggering is phase locked to the AC voltage to follow the motion of the ion cloud as a function of the phase angle of the AC signal. Resonantly excited ions maintain a coherent motion in the presence of He buffer gas, which dissipates energy from the ions via collisions. Maximum ion displacements, which depend upon the potential well depth (q z value), occur twice each AC cycle. Axial components of ion velocities are determined by differentiating the displacements of the distributions with respect to time. The experimental data show that these velocities are maximized when the ion cloud passes through zero axial displacement, and they compare favorably with results calculated using a simple harmonic oscillator model. Axial components of ion kinetic energies are low (<5 eV) under the chosen experimental conditions. At low values of q2 (≈ 0.2), the width of the ion distribution increases as the ion cloud approaches the center of the trap and decreases as it approaches the end-cap electrodes. This effect is created by compaction of the ion trajectories when ion velocities are decreased.

Computer simulation of single-ion trajectories in paul-type ion traps

The computer simulation of single-ion trajectories using a number of computer programs is described together with associated theory. The programs permit calculation of ion trajectories while the ion is subjected to collisions with buffer gas of variable pressure, resonance excitation in any of three modes, and static or ramped DC and radiofrequency levels. Initially, the programs were designed for the calculation of ion trajectories in a quadrupole ion trap. The programs now permit such calculations for ions confined in traps having electrodes shaped to include percentages of hexapole and octupole components in the electric field as well as electrode surface geometries for which there is no closed-form expression. The Langevin collision theory is reviewed and a theoretical treatment of the multipole trap is presented.