Mass-selective Ejection Research Articles

Overtone Transition 2ν1 of HCO+ and HOC+: Origin, Radiative Lifetime, Collisional Quenching.

We present spectra of the first overtone vibration transition of C-H/ O-H stretch (2ν1) in HCO+ and HOC+, recorded using a laser induced reaction action scheme inside a cryogenic 22 pole radio frequency trap. Band origins have been located at 6078.68411(19) and 6360.17630(26) cm-1, respectively. We introduce a technique based on mass selective ejection from the ion trap for recording background free action spectra. Varying the number density of the neutral action scheme reactant (CO2 and Ar, respectively) and collisional partner reactant inside the ion trap, permitted us to estimate the radiative lifetime of the state to be 1.53(34) and 1.22(34) ms, respectively, and the collisional quenching rates of HCO+(2ν1) with He, H2, and N2.

Increasing the Upper Mass/Charge Limit of a Quadrupole Ion Trap for Ion/Ion Reaction Product Analysis via Waveform Switching.

Quadrupole ion traps (QITs) are versatile platforms for performing experiments with gas-phase ions due to their abilities to store ions of both polarities and to conduct MSn experiments. The QIT is particularly useful as a reaction cell for ion/ion reactions. In the case of an ion/ion reaction experiment in a QIT, multiply charged reactant ions may initially be of relatively low m/z (e.g., m/z < 1000) whereas the product ions can be one or more orders of magnitude higher in m/z (e.g., m/z > 100,000). Several factors can limit the m/z range over which an ion/ion reaction experiment can be conducted. These include (1) the efficiency of the detector, (2) the m/z range over which oppositely charged ions can be mutually stored, and (3) the m/z range over which ions can be mass selectively ejected into an external detector. High-frequency waveforms provide larger m/z trapping ranges for mutual storage of oppositely charged ions whereas low-frequency waveforms provide better trapping for very high m/z product ions. Presented here is a method that switches from a high-frequency sine wave prior to and during an ion/ion reaction to a low-frequency square wave to eject low m/z reagent ions and improves confinement of the product ions before mass-selective ejection by scanning the frequency of the square wave. This approach addresses the third issue, which is the primary limiting factor with QITs operated at high RF (e.g., > 900MHz). Graphical Abstract.

Computer simulations of a new toroidal-cylindrical ion trap mass analyzer.

A novel toroidal-cylindrical ion trap (TCIT) design is introduced based on a compact dual ion trap system that comprises an outer toroidal ion trap (T-trap) and an inner cylindrical ion trap (CIT). These two traps have specific but different functions that can improve the performance of tandem mass analysis operation. The TCIT is studied by using a simulated mass spectrometer platform that is mainly based on SIMION modeling and extensive data processing. This platform combines different functions, such as simulation of ion motion and field calculations, as well as acquisition of a simulated mass spectrum. The main steps of the MS operation, including the full scan process, the mass-selective ejection of the ions in the T-trap, and the capture of the injected precursor ions in the CIT, are realized using the abovementioned platform. In addition, the simulated design is optimized for improved mass analysis performance. For the outer T-trap, when the outside surface of the inner cylindrical electrode is modified to include a circular arc with appropriate radius, the ion ejection efficiency that is directed into the CIT could be significantly enhanced. In the case of the inner CIT, the ion capture efficiency could be improved to more than 90% with geometry optimization. Copyright © 2016 John Wiley & Sons, Ltd.

Calibration procedure for secular frequency scanning in ion trap mass spectrometers.

Mass spectra can be recorded using ion traps by scanning the frequency of an alternating current (ac) signal that corresponds to the secular frequency of a trapped ion. There is a considerable simplification in the instrumentation needed to perform such a scan compared with conventional scans of the radiofrequency (rf) amplitude. However, mass calibration is difficult. An algorithm that can be used to achieve mass calibration is investigated and the factors that affect ion mass assignments are discussed. Time domain data, recorded using a commercial benchtop linear ion trap mass spectrometer, are converted to the m/z domain using ion Mathieu parameter qu values which are derived from the dimensionless frequency parameter βu expressed as a continuing fraction in terms of qu . The relationship between the operating parameters of an ideal ion trap and the ion m/z ratio is derived from the Mathieu equations and expressed as an algorithm which through successive approximations yields the Mathieu qu value and hence m/z values and peak widths. The predictions of the algorithm are tested against experiment by sweeping the frequency of a small supplementary ac signal so as to cause mass-selective ejection of trapped ions. Calibration accuracy is always better than 0.1%, often much better. Peak widths correspond to a mass resolution of 250 to 500 in the m/z 100-1800 range in secular frequency scans. A simple, effective method of calibration of mass spectra recorded using secular frequency scans is achieved. The effects of rf amplitude, scan rate, and ac amplitude on calibration parameters are shown using LTQ linear ion trap data. Corrections for differences in ion mass must be made for accurate calibration, and this is easily incorporated into the calibration procedure. Copyright © 2016 John Wiley & Sons, Ltd.

Mass resolution of linear quadrupole ion traps with round rods.

Auxiliary dipole excitation is widely used to eject ions from linear radio-frequency quadrupole ion traps for mass analysis. Linear quadrupoles are often constructed with round rod electrodes. The higher multipoles introduced to the electric potential by round rods might be expected to change the ion ejection process. We have therefore investigated the optimum ratio of rod radius, r, to field radius, r0, for excitation and ejection of ions. Trajectory calculations are used to determine the excitation contour, S(q), the fraction of ions ejected when trapped at q values close to the ejection (or excitation) q. Initial conditions are randomly selected from Gaussian distributions of the x and y coordinates and a thermal distribution of velocities. The N = 6 (12 pole) and N = 10 (20 pole) multipoles are added to the quadrupole potential. Peak shapes and resolution were calculated for ratios r/r0 from 1.09 to 1.20 with an excitation time of 1000 cycles of the trapping radio-frequency. Ratios r/r0 in the range 1.140 to 1.160 give the highest resolution and peaks with little tailing. Ratios outside this range give lower resolution and peaks with tails on either the low-mass side or the high-mass side of the peaks. This contrasts with the optimum ratio of 1.126-1.130 for a quadrupole mass filter operated conventionally at the tip of the first stability region. With the optimum geometry the resolution is 2.7 times greater than with an ideal quadrupole field. Adding only a 2.0% hexapole field to a quadrupole field increases the resolution by a factor of 1.6 compared with an ideal quadrupole field. Addition of a 2.0% octopole lowers resolution and degrades peak shape. With the optimum value of r/r0 , the resolution increases with the ejection time (measured in cycles of the trapping rf, n) approximately as R0.5 = 6.64n, in contrast to a pure quadrupole field where R0.5 = 1.94n. Adding weak nonlinear fields to a quadrupole field can improve the resolution with mass-selective ejection of ions by up to a factor of 2.7. The optimum ratio r/r0 is 1.14 to 1.16, which differs from the optimum ratio for a mass filter of 1.128-1.130.

Autoresonant Trap Mass Spectrometry (ART MS) for remote sensing applications

A novel mass spectrometer is described that uses purely electrostatic fields to store ionized gases within a cylindrical ion trap. Ions are generated directly inside the trap by electron ionization of gas molecules. An anharmonic trapping potential well confines the ion trajectories of all ion masses, and of a wide range of initial energies, to stable oscillatory motions along the axis of the trap. Mass selective ejection is achieved through a novel autoresonant energy pumping process. The new mass spectrometer has unlimited mass range, good sensitivity at high and ultrahigh vacuum levels, very fast scan rates, compact construction, and extremely low power requirements as it uses only static bias potentials and very small RF signal levels (in the 100 mV range). Small size, low power consumption, rugged construction and reliable operation make ART MS technology an ideal candidate for in situ mass spectrometry in a wide variety of remote sampling applications including harsh sampling environments. Fast mass spectra acquisition rates particularly attractive for sampling applications from fast moving platforms such as airplanes, rockets and boats.

Mass‐selective axial ejection from a linear ion trap with a direct current extraction field

We have developed a new mass-selective axial ejection method from a linear ion trap (LIT). In this method, a set consisting of a trap wire lens and an extraction wire lens positioned orthogonally to each other was placed between quadrupole rods. The trap wire lens confines the ions inside the trap, and the extraction wire axially extracts ions from the trap. Ions introduced into the LIT are trapped between the inlet lens and the trap wire lens. In addition to the wire lenses, a set of excitation lenses, which are aligned orthogonally to the trap wire lens, are inserted between rods. The ions are resonantly excited in the direction perpendicular to the trap wire lens by applying a supplemental alternating current (AC) to the excitation lenses. Excited ions with a large motion pass over the trap wire lens, while unexcited ions remain trapped inside. Ions that have passed over the trap wire lens are then extracted by the extraction wire lens. The characteristics of the mass-selective ejection with a direct current (DC) extraction field were investigated by both simulation and experiment. A mass resolving power of m/Deltam = 1300 was achieved at a scan rate of 500 Th/s. The dependence of the ejection efficiency on trap wire lens bias was measured, and an ejection efficiency of 20% at a scan rate of 500 Th/s was achieved by optimizing the DC bias on the trap wire lens.

Multiple scales analysis of early and delayed boundary ejection in Paul traps

We use the method of multiple scales to elucidate dynamics associated with early and delayed ejection of ions in mass selective ejection experiments in Paul traps. We develop a slow flow equation to approximate the solution of a weakly nonlinear Mathieu equation to describe ion dynamics in the neighborhood of the stability boundary of ideal traps (where the Mathieu parameter q z = q z ∗ = 0.908046 ). The method of multiple scales enables us to incorporate higher order multipoles, extend computations to higher orders, and generate phase portraits through which we view early and delayed ejection. Our use of the method of multiple scales is atypical in two ways. First, because we look at boundary ejection, the solution to the unperturbed equation involves linearly growing terms, requiring some care in identification and elimination of secular terms. Second, due to analytical difficulties, we make additional harmonic balance approximations within the formal implementation of the method. For positive even multipoles in the ion trapping field, in the stable region of trap operation, the phase portrait obtained from the slow flow consists of three fixed points, two of which are saddles and the third is a center. As the q z value of an ion approaches q z ∗ , the saddles approach each other, and a point is reached where all nonzero solutions are unbounded, leading to an observation of early ejection. The phase portraits for negative even multipoles and odd multipoles of either sign are qualitatively similar to each other and display bounded solutions even for q z > q z ∗ , resulting in the observation of delayed ejection associated with a more gentle increase in ion motion amplitudes, a mechanism different from the case of the positive even multipoles.

Mass Selective Ejection by Axial Resonant Excitation from a Linear Ion Trap

We describe a new mass selective ejection method from a linear ion trap, which we call axial resonant excitation (AREX). A set of vane lenses are inserted between each quadrupole rod to produce electrostatic potential that is approximately harmonic along the central axis of the quadrupole field. After ions with specific m/z are resonantly oscillated in the axial direction, the ions are mass selectively ejected in the axial direction. At a high scan rate of 11 Th/ms, AREX achieved a high ejection efficiency of more than 60%, which is more than three times higher than a conventional mass selective axial ejection method from a linear trap using fringing field.

A simulation study of the digital ion trap mass spectrometer

The theoretical foundation for the digital ion trap (DIT) is presented in outline where the study of ion motion in the DIT employed the a, q, β parameter framework of conventional ion trap study. A user program for SIMION has been developed to simulate the DIT operation in real environment allowing the higher-order fields to be represented. Three-dimensional random ion buffer gas collisions were modelled where Maxwellian velocity distribution of the buffer gas was considered. A mass-selective ejection scan method is described and simulated, where both the trapping quadrupole field and dipole excitation field are driven digitally and their frequencies are scanned proportionally. The performances of the stretched ion trap and a new field-modified ion trap are simulated and compared. Using a field-adjusting electrode near the end-cap aperture, the new ion trap is able to achieve higher mass resolution for both forward and reverse mass scans. The simulation studies also the relation between the ejection time and the parameters of digital dipole excitation, the influence of space charge, duty cycle modulation as an alternative ejection scan method, and ion introduction into the ion trap.

Ion trap mass spectrometry. Using high-pressure ionization.

The quadrupole ion trap, operated with a relatively high-pressure bath gas, occupies a unique place among mass analyzers. As an ion storage device, it shares many characteristics with the ICR instrument, but the use of a light bath gas and mass-selective instability for mass analysis contrasts with normal ICR operation. The bath gas facilitates the capture of ions injected into the oscillating quadrupole field and cools the ions to the center of the ion trap before mass-selective ejection. The bath gas also provides a conduit for heating ions. Mild collisional activation conditions can dissociate clusters, such as solvated analyte ions, formed by ES. Dissociation of covalently bound ions can be used to obtain structural information as part of a tandem MS experiment or to destroy polyatomic species in elemental analyses. The moderate background pressure requirements of the ion trap using mass-selective instability make it a natural mass analyzer to be coupled with high-pressure ionization methods. The ion trap can provide a significant advantage in efficiency over beam-type mass spectrometers, which translates into lower detection limits. Lower detection limits can be realized from superior duty cycle and MS/MS efficiency, although transmission for single-stage MS is comparable. The greatest advantage in efficiency can be obtained with weak ion beams that require long ion accumulation times and therefore high duty cycles. This, in part, makes the ion trap particularly attractive for ES. Bright ion sources can provide similar advantages if major ion beam components can be ejected during ion accumulation to minimize the deleterious effects caused by ion-ion interactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Quadrupole store (quistor) mass spectrometry

A new quadrupole ion store mass spectrometer has been constructed and tested. The system uses a new scanning method which incorporates both mass selective storage and mass selective ejection. The features and advantages of the system are presented and discussed.