Applications of a travelling wave-based radio-frequency-only stacked ring ion guide

The use of radio-frequency (RF)-only ion guides for efficient transport of ions through regions of a mass spectrometer where the background gas pressure is relatively high is widespread in present instrumentation. Whilst multiple collisions between ions and the background gas can be beneficial, for example in inducing fragmentation and/or decreasing the spread in ion energies, the resultant reduction of ion axial velocity can be detrimental in modes of operation where a rapidly changing influx of ions to the gas-filled ion guide needs to be reproduced at the exit. In general, the RF-only ion guides presently in use are based on multipole rod sets. Here we report investigations into a new mode of ion propulsion within an RF ion guide based on a stack of ring electrodes. Ion propulsion is produced by superimposing a voltage pulse on the confining RF of an electrode and then moving the pulse to an adjacent electrode and so on along the guide to provide a travelling voltage wave on which the ions can surf. Through appropriate choice of the travelling wave pulse height, velocity and gas pressure it will be shown that the stacked ring ion guide with the travelling wave is effective as a collision cell in a tandem mass spectrometer where fast mass scanning or switching is required, as an ion mobility separator at pressures around 0.2 mbar, as an ion delivery device for enhancement of duty cycle on an orthogonal acceleration time-of-flight (oa-TOF) mass analyser, and as an ion fragmentation device at higher wave velocities.     

ETD in a Traveling Wave Ion Guide at Tuned Z-Spray Ion Source Conditions Allows for Site-Specific Hydrogen/Deuterium Exchange Measurements

The recent application of electron transfer dissociation (ETD) to measure the hydrogen exchange of proteins in solution at single-residue resolution (HX-ETD) paves the way for mass spectrometry-based analyses of biomolecular structure at an unprecedented level of detail. The approach requires that activation of polypeptide ions prior to ETD is minimal so as to prevent undesirable gas-phase randomization of the deuterium label from solution (i.e., hydrogen scrambling). Here we explore the use of ETD in a traveling wave ion guide of a quadrupole-time-of-flight (Q-TOF) mass spectrometer with a “Z-spray” type ion source, to measure the deuterium content of individual residues in peptides. We systematically identify key parameters of the Z-spray ion source that contribute to collisional activation and define conditions that allow ETD experiments to be performed in the traveling wave ion guide without gas-phase hydrogen scrambling. We show that ETD and supplemental collisional activation in a subsequent traveling wave ion guide allows for improved extraction of residue-specific deuterium contents in peptides with low charge. Our results demonstrate the feasibility, and illustrate the advantages of performing HX-ETD experiments on a high-resolution Q-TOF instrument equipped with traveling wave ion guides. Determination of parameters of the Z-spray ion source that contribute to ion heating are similarly pertinent to a growing number of MS applications that also rely on an energetically gentle transfer of ions into the gas-phase, such as the analysis of biomolecular structure by native mass spectrometry in combination with gas-phase ion-ion/ion-neutral reactions or ion mobility spectrometry.     

Characterization of microchannel plate detector response for the detection of native multiply charged high mass single ions in orthogonal-time-of-flight mass spectrometry using a Timepix detector

Time-of-flight (TOF) systems are one of the most widely used mass analyzers in native mass spectrometry (nMS) for the analysis of non-covalent multiply charged bio-macromolecular assemblies (MMAs). Typically, microchannel plates (MCPs) are employed for high mass native ion detection in TOF MS. MCPs are well known for their reduced detection efficiency when impinged by large slow moving ions. Here, a position- and time-sensitive Timepix (TPX) detector has been added to the back of a dual MCP stack to study the key factors that affect MCP performance for MMA ions generated by nMS. The footprint size of the secondary electron cloud generated by the MCP on the TPX for each individual ion event is analyzed as a measure of MCP performance at each mass-to-charge (m/z) value and resulted in a Poisson distribution. This allowed us to investigate the dependency of ion mass, ion charge, ion velocity, acceleration voltage, and MCP bias voltage on MCP response in the high mass low velocity regime. The study of measurement ranges; ion mass = 195 to 802,000 Da, ion velocity = 8.4 to 67.4 km/s, and ion charge = 1+ to 72+, extended the previously examined mass range and characterized MCP performance for multiply charged species. We derived a MCP performance equation based on two independent ion properties, ion mass and charge, from these results, which enables rapid MCP tuning for single MMA ion detection.     

The influence of drift gas composition on the separation mechanism in traveling wave ion mobility spectrometry: insight from electrodynamic simulations

The influence of three different drift gases (helium, nitrogen, and argon) on the separation mechanism in traveling wave ion mobility spectrometry is explored through ion trajectory simulations which include considerations for ion diffusion based on kinetic theory and the electrodynamic traveling wave potential. The model developed for this work is an accurate depiction of a second-generation commercial traveling wave instrument. Three ion systems (cocaine, MDMA, and amphetamine) whose reduced mobility values have previously been measured in different drift gases are represented in the simulation model. The simulation results presented here provide a fundamental understanding of the separation mechanism in traveling wave, which is characterized by three regions of ion motion: (1) ions surfing on a single wave, (2) ions exhibiting intermittent roll-over onto subsequent waves, and (3) ions experiencing a steady state roll-over which repeats every few wave cycles. These regions of ion motion are accessed through changes in the gas pressure, wave amplitude, and wave velocity. Resolving power values extracted from simulated arrival times suggest that momentum transfer in helium gas is generally insufficient to access regions (2) and (3) where ion mobility separations occur. Ion mobility separations by traveling wave are predicted to be effectual for both nitrogen and argon, with slightly lower resolving power values observed for argon as a result of band-broadening due to collisional scattering. For the simulation conditions studied here, the resolving power in traveling wave plateaus between regions (2) and (3), with further increases in wave velocity contributing only minor improvements in separations.     

Mapping the ion kinetic energy in a helium microwave plasma orthogonal acceleration time-of-flight mass spectrometer.

The ion kinetic energy of a helium microwave plasma is studied using an orthogonal acceleration time-of-flight mass spectrometer. The ions produced in the plasma are extracted into the mass spectrometer in an 'off-cone' mode (i.e. the helium plasma plume is off the sampler cone), and enter the repelling zone in the x-direction, which is perpendicular to the flight tube. The information of ion initial kinetic energy was obtained from both theoretical calculations and experimental results. The potential influence of two x-direction steering plates (X-steering plates) on the ion energy and signal intensity was examined. The influence of gas composition on the ion kinetic energy was also investigated. The calculated results show that ions with different m/z have different velocity and kinetic energy when they enter the ion modulation zone, and lighter ions have higher velocity and lower kinetic energy. The experimental results obtained demonstrate that the ion signals of different m/z produced with an 'off-cone' sampling helium microwave plasma show similar trends as calculated with the potential difference of the X-steering plates, revealing their narrow kinetic energy distribution in the x-direction. Under typical operating conditions, the x-direction kinetic energy of ions detected mostly range from about 14.9 eV for (7)Li(+) to 16.8 eV for (208)Pb(+).     

Elimination of Anti-spiral Waves by Local Inhomogeneity in Oscillatory Systems

Anti-spiral waves are controlled in an oscillatory system by using a local inhomogeneity. The inhomogeneity acts as a wave source, and gives rise to the propagating plane waves. It is found that there is a critical pacemaking domain size below which no wave will be created at all. Two types of ordered waves (target waves and traveling waves) are created depending on the geometry of the local inhomogeneity. The competition between the anti-spiral waves and the ordered waves is discussed. Two different competition mechanisms were observed, which are related to the ordered waves obtained from different local inhomogeneities. It is found that traveling waves with either lower frequency or higher frequency can both eliminate the anti-spiral waves, while only the target waves with lower absolute value of frequency can eliminate the anti-spiral waves.This method also applies to outwardly rotating spiral waves. The control mechanism is intuitively explained and the control method is easily operative.     

Evaluating the multiple benefits offered by ion mobility-mass spectrometry in oil and petroleum analysis

Ion mobility-mass spectrometry is starting to be considered as a useful tool in the deconvolution of complex oil and petroleum samples. While ultrahigh resolution mass spectrometry is the incumbent technology in this field, ion mobility offers complementary information related to species size and shape, and also the ability to resolve structural isomers. In this work, a sample of the resins portion of the Saturates, Aromatics, Resins, and Asphaltenes (SARA) fractions of crude oil was analysed using an orthogonal acceleration quadrupole time-of-flight mass spectrometer (oa-QToF MS) that incorporates a travelling wave ion mobility spectrometry (TWIMS) region. The ion mobility data were compared with previously acquired ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) data and various nitrogen containing families were identified. Ion mobility data were processed in the typical way for the oil and petroleum industry; and the use of high resolution exact mass coupled with mobility data to provide enhanced species resolution was examined. Double bond equivalence (DBE) and carbon number groups were identified using patterns in the ion mobility data, which demonstrated the utility of ion mobility for discovering species relationships within the crude oil sample. The ability to calibrate the ion mobility cell and generate sizes for the detected ions was also recognised as potentially having particular value for the implementation of conversion or hydrotreatment processes in the oil industry.     

Dynamic Gradient Directed Molecular Transport and Concentration in Hydrogel Films

Materials which selectively transport molecules along defined paths offer new opportunities for concentrating, processing and sensing chemical and biological agents. Here, we present the use of traveling ionic waves to drive molecular transport and concentration of hydrophilic molecules entrained within a hydrogel. The traveling ionic wave is triggered by the spatially localized introduction of ions, which through a dissipative ion exchange process, converts quaternary ammonium groups in the hydrogel from hydrophilic to hydrophobic. Through a reaction–diffusion process, the hydrophobic region expands with a sharp transition at the leading edge; it is this sharp gradient in hydrophilicity that drives the transport of hydrophilic molecules dispersed within the film. The traveling wave moved up to 450 μm within 30 min, while the gradient length remained 20 μm over this time. As an example of the potential of molecular concentration using this approach, a 70‐fold concentration of a hydrophilic dye was demonstrated.     

An accurate model for sample droplet acceleration in an air—acetylene flame

The experimentally observed velocity of an aerosol droplet in an air—acetylene flame is compared to the velocity calculated through a new mathematical model. The experimental droplet velocity is measured by means of stroboscopic photography of a stream of individual droplets sent into a flame at a known frequency and traveling very reproducible trajectories. Because the temporal spacing between these droplets is known, a measure of the instantaneous droplet location enables their velocity to be calculated. The droplet acceleration model was modified by incorporating into it a radially changing flame rise velocity, which was assumed constant in earlier treatments. Excellent agreement is found between calculated and measured droplet acceleration in the flame.     

A miniature mass spectrometer for liquid chromatography applications

A miniature mass spectrometer capable of detecting analytes eluting from a high-performance liquid chromatography (HPLC) system is described and demonstrated for the first time. The entire instrument, including all pumps and the computer, is contained within a single enclosure that may be conveniently accommodated at the base of the HPLC stack. The microspray ion source, vacuum interface, ion guide, and quadrupole ion filter are all microengineered. These components are fabricated in batches using microelectromechanical systems (MEMS) techniques and considered to be consumables. When coupled to a standard HPLC system using an integrated passive split, the limit of detection for reserpine while scanning the full mass range is 5 ng on-column (1 pg of which is passed to the microspray). The mass range is m/z 100-800, and each spectrum is typically acquired at a rate of 1 scan per second.     

Variable low-mass filtering using an electrodynamic ion funnel

An adjustable low-mass filter has been developed for an electrospray ionization (ESI) source to block ions associated with unwanted background species from entering the mass spectrometer. The low-mass filter is made by using an adjustable potential energy barrier from the conductance-limiting plate of an electrodynamic ion funnel, which prohibits species with higher ion mobilities from exiting the ESI source. We show that this arrangement provides a linear voltage adjustment for low-mass filtering from m/z 0 to 500. Mass filtering above m/z 500 is also performed; however, higher-mass species are attenuated. The mass filter was tested with a liquid chromatography/mass spectrometry (LC/MS) analysis of a bovine serum albumin (BSA) tryptic digest and resulted in the ability to block low-mass, background species, which accounted for 40-70% of the total ion current immediately behind the ESI source during peak elution and detection.     

Ideal velocity focusing in a reflectron time-of-flight mass spectrometer

A single-stage ion mirror in a time-of-flight (TOF) mass spectrometer (MS) can perform first order velocity focusing of ions initially located at a start focal plane while second order velocity focusing can be achieved using a double-stage reflectron. The situation is quite different when an ion source extraction field is taken into account. In this case which is common in any practical matrix-assisted laser desorption/ionization (MALDI) TOF-MS a single-stage reflectron, for example, cannot perform velocity focusing at all. In this paper an exact, analytic solution for an electric field inside a one-dimensional reflectron has been found to achieve universal temporal focusing of ions having an initial velocity distribution. The general solution is valid for arbitrary electric field distributions in the upstream (from the ion source to the reflectron) and downstream (from the reflectron to an ion detector) regions and in a decelerating part of the reflectron of a reflectron TOF mass spectrometer. The results obtained are especially useful for designing MALDI reflectron TOF mass spectrometers in which the initial velocity distribution of MALDI ions is the major limiting factor for achieving high mass resolution. Using analytical expressions obtained for an arbitrary case, convenient working formulas are derived for the case of a reflectron TOF-MS with a dual-stage extraction ion source. The special case of a MALDI reflectron TOF-MS with an ion source having a low acceleration voltage (or large extraction region) is considered. The formulas derived correct the effect of the acceleration regions in a MALDI ion source and after the reflectron before detecting ions.     

Improving the quality of the ion beam exiting a quadrupole ion guide

The effect of fringing fields on the divergence of the ion beam exiting an RF quadrupole ion guide was studied using a computer simulation. It was shown that reducing the strength of the RF field towards the ion guide exit reduces ion beam divergence. Further improvement was demonstrated when creating a DC gradient towards the exit. The results of the numerical simulation were verified experimentally using a time-of-flight (TOF) mass analyzer with orthogonal acceleration. Decreasing the ion beam divergence resulted in considerably improved mass resolution of the instrument.     

How Constant Momentum Acceleration Decouples Energy and Space Focusing in Distance–of–Flight and Time–of–Flight Mass Spectrometries

Resolution in time–of–flight mass spectrometry (TOFMS) is ordinarily limited by the initial energy and space distributions within an instrument’s acceleration region and by the length of the field–free flight zone. With gaseous ion sources, these distributions lead to systematic flight–time errors that cannot be simultaneously corrected with conventional static–field ion–focusing devices (i.e., an ion mirror). It is known that initial energy and space distributions produce non–linearly correlated errors in both ion velocity and exit time from the acceleration region. Here we reinvestigate an old acceleration technique, constant–momentum acceleration (CMA), to decouple the effects of initial energy and space distributions. In CMA, only initial ion energies (and not their positions) affect the velocity ions gain. Therefore, with CMA, the spatial distribution within the acceleration region can be manipulated without creating ion–velocity error. The velocity differences caused by a spread in initial ion energy can be corrected with an ion mirror. We discuss here the use of CMA and independent focusing of energy and space distributions for both distance–of–flight mass spectrometry (DOFMS) and TOFMS. Performance characteristics of our CMA–DOFMS and CMA–TOFMS instrument, fitted with a glow–discharge ionization source, are described. In CMA–DOFMS, resolving powers (FWHM) of greater than 1000 are achieved for atomic ions with a flight length of 285 mm. In CMA–TOFMS, only ions over a narrow range of m/z values can be energy–focused; however, the technique offers improved resolution for these focused ions, with resolving powers of greater than 2000 for a separation distance of 350 mm.      

Spontaneous and deflected drift-trajectories in orthogonal acceleration time-of-flight mass spectrometry

Orthogonal acceleration is a method for gating ions from an ion beam into a time-of-flight (TOF) mass spectrometer. The technique involves a pulsed electric field to apply acceleration directed orthogonally to an ion beam. This approach is useful for coupling continuous ion sources to TOF mass analyzers. Most instruments of this type, which have been described in the literature, use steering electrodes after the orthogonal acceleration step. Those velocity components of ions originating from the ion beam velocity are minimized so that the deflected drift-trajectory is parallel to a transverse flight tube. In an alternative geometry the ion beam velocity is conserved and the drift-trajectory after the orthogonal acceleration step is spontaneous. The differences between the space-time focusing ability with spontaneous and deflected drift-trajectories are discussed and investigated. Trajectory calculations indicate that deflection fields placed after the orthogonal acceleration step distort the ion packet because, in this geometry, the flight-time to the detector is dependent on the position that the ions enter the steering optics. Increasing the duty-cycle efficiency by sampling longer sections of the continuous ion beam leads to a degradation of resolving power. Employing a spontaneous drift-trajectory after orthogonal acceleration provides the advantage that the arrival time spread for isobaric ions is, in principle, independent of the length of the ion beam sampled. The major implication of these findings is that simultaneously optimized sensitivity and resolving power may not be achievable with the deflected drift-trajectory instruments. The calculations are in agreement with results from the published data of a number of groups who have built instruments based on the orthogonal acceleration principle.     

Optimizing signal and mass resolution for matrix-assisted laser desorption utilizing a linear time-of-flight mass spectrometer.

Results are presented for various instrumental configurations employed for matrix-assisted laser desorption mass spectrometry. Mass resolution is determined for a linear time-of-flight mass spectrometer for various lengths of the field-free region. A wire ion guide is utilized and is shown to improve ion transport efficiencies for longer field-free regions. It is also determined experimentally that a modest mass resolution increase is often obtained in configurations employing the wire ion guide when compared to the mass resolution obtained with the same geometry without the wire ion guide. Optimal applied potentials are determined for the wire ion guide. No mass dependence on the optimal applied potential (-100 V) for the wire ion guide is observed for samples of equine myoglobin (MW 16,951.5 Da) and a bacterial protease (MW 27,228.4 Da). The optimal applied voltage was also found to be identical (-100 V) for the singly through quadruply charged molecular ion species of rabbit gamma globulin (MW approximately 150,000 Da). It is shown that a 2 m flight tube with a wire ion guide provides better signal-to-noise mass spectra than a 1 m flight tube without the wire ion guide and can more than double the mass resolution obtainable. Utilization of a 4 m flight tube gives minimal mass resolution enhancement at the expense of signal-to-noise.     

Detection of Large Ions in Time-of-Flight Mass Spectrometry: Effects of Ion Mass and Acceleration Voltage on Microchannel Plate Detector Response

In time-of-flight mass spectrometry (TOF-MS), ion detection is typically accomplished by the generation and amplification of secondary electrons produced by ions colliding with a microchannel plate (MCP) detector. Here, the response of an MCP detector as a function of ion mass and acceleration voltage is characterized, for singly charged peptide/protein ions ranging from 1 to 290 kDa in mass, and for acceleration voltages from 5 to 25 kV. A nondestructive inductive charge detector (ICD) employed in parallel with MCP detection provides a reliable reference signal to allow accurate calibration of the MCP response. MCP detection efficiencies were very close to unity for smaller ions at high acceleration voltages (e.g., angiotensin, 1046.5 Da, at 25 kV acceleration voltage), but decreased to ~11% for the largest ions examined (immunoglobulin G (IgG) dimer, 290 kDa) even at the highest acceleration voltage employed (25 kV). The secondary electron yield γ (average number of electrons produced per ion collision) is found to be proportional to mv^3.1 (m: ion mass, v: ion velocity) over the entire mass range examined, and inversely proportional to the square root of m in TOF-MS analysis. The results indicate that although MCP detectors indeed offer superlative performance in the detection of smaller peptide/protein species, their performance does fall off substantially for larger proteins, particularly under conditions of low acceleration voltage.

A Hybrid Constant and Oscillatory Field Ion Mobility Analyzer Using Structures for Lossless Ion Manipulations

Here we explore the combination of constant and oscillatory fields applied in a single device to affect the continuous separation and filtering of ions based on their mobilities. The device explored allows confining and manipulating ions utilizing a combination of radio frequency (rf), direct current (DC) fields, and traveling waves (TW) in a structures for lossless ion manipulations (SLIM) module. We have investigated theoretically and experimentally a concept for continuous filtering of ions based on their mobilities where ions are mobility separated and selected by passage through two regions, both of which incorporated combined TW and constant fields providing opposing forces on the ions. The SLIM module was composed of two surfaces with mirror-image arrays of electrodes and had two regions where the different TW and opposing DC fields could be applied. The filtering capabilities are determined by the applied DC gradient and the TW parameters, such as speed, amplitude, and the TW sequence (i.e., the duty cycle of the traveling wave). The effects of different parameters on the sensitivity and the ion mobility (IM) resolution of the device have been investigated. By appropriately choosing the DC gradient and TW parameters for the two sections, it is possible to transmit ions of a selected mobility while filtering out others of both higher and lower mobility. The novel device described here provides a basis for the targeted analysis of compounds based upon the continuous selection of ions according to their mobility and without the need for high electric fields or pulsed injection.

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Graphical abstract ?

     

Scanwave: A new approach to enhancing spectral data on a tandem quadrupole mass spectrometer

A new type of mass analyzer is described, which allows lowresolution axial ion ejection to be obtained from a traveling wave based, stacked ring collision cell. Linking this ejection temporally with the scanning of the second quadrupole of a tandem quadrupole mass spectrometer provides an improvement in sampling duty cycle, which results in significant signal intensity improvements for scanning acquisitions such as product ion spectra. A near 100% storage efficiency is enabled by a split cell design, which allows ion fragmentation and accumulation to be performed in one section of the collision cell whilst previously accumulated ions are simultaneously ejected from the rear of the cell. These characteristics combine to give an m/z-dependent signal gain of 7–20X over a conventional scanning quadrupole for a 1000 Th scan. The ability to swap very rapidly from a non-enhanced mode of operation to an enhanced mode whilst retaining the existing sensitivity, speed, and functionality of a conventional tandem quadrupole mass spectrometer is also described.     

Coupling electrospray corona discharge, charge reduction and ion mobility mass spectrometry: From peptides to large macromolecular protein complexes

We present the design and implementation of a home-built point-to-plane corona discharge probe, which rapidly and efficiently charge reduces biological ions generated by electrospray ionization (ESI). The molecules analysed ranged from small peptides such as Glu-fibrinopeptide B (1.5 kDa), small proteins such as myoglobin (16.9 kDa), polymers such as polyethylene glycol (PEG 10 k) which all showed intense singly charged ions; to large native multiprotein complexes such as GroEL (802 kDa) which show a broad range of charge-reduced species. The corona discharge probe operates at atmospheric pressure and was directly interfaced with a standard-ESI or nanoflow-ESI source of quadrupole ion mobility time-of-flight mass spectrometer. The corona discharge probe is completely modular and could potentially be mounted to any commercial or research grade mass spectrometer with an ESI source. The level of charge reduction is precisely controlled by the applied voltage and/or probe gas flow rate and when in operation, results in approximately a 50 % reduction in total ion current. We also present the combination of corona discharge and travelling wave ion mobility and assign helium collision cross-section values (Ω_He) to the charge reduced species of the native protein complex pyruvate kinase. It would appear that the Ω_He of the +20 charge state for pyruvate kinase is approximately 20 % smaller than the +35 charge state. Finally, we discuss the potential benefits and concerns of utilising charge reduced protein species as a means of extending the travelling wave collision cross-section calibration range over that which is already published.