Investigation of drift gas selectivity in high resolution ion mobility spectrometry with mass spectrometry detection

Recent studies in electrospray ionization (ESI)/ion mobility spectrometry (IMS) have focussed on employing different drift gases to alter separation efficiency for some molecules. This study investigates four structurally similar classes of molecules (cocaine and metabolites, amphetamines, benzodiazepines, and small peptides) to determine the effect of structure on relative mobility changes in four drift gases (helium, nitrogen, argon, carbon dioxide). Collision cross sections were plotted against drift gas polarizability and a linear relationship was found for the nineteen compounds evaluated in the study. Based on the reduced mobility database, all nineteen compounds could be separated in one of the four drift gases, however, the drift gas that provided optimal separation was specific for the two compounds.     

Identity confirmation of drugs and explosives in ion mobility spectrometry using a secondary drift gas

This work demonstrated the potential of using a secondary drift gas of differing polarizability from the primary drift gas for confirmation of a positive response for drugs or explosives by ion mobility spectrometry (IMS). The gas phase mobilities of response ions for selected drugs and explosives were measured in four drift gases. The drift gases chosen for this study were air, nitrogen, carbon dioxide and nitrous oxide providing a range of polarizability and molecular weights. Four other drift gases (helium, neon, argon and sulfur hexafluoride) were also investigated but design limitations of the commercial instrument prevented their use for this application. When ion mobility was plotted against drift gas polarizability, the resulting slopes were often unique for individual ions, indicating that selectivity factors between any two analytes varied with the choice of drift gas. In some cases, drugs like THC and heroin, which are unresolved in air or nitrogen, were well resolved in carbon dioxide or nitrous oxide.     

Optimization of Peptide Separations by Differential Ion Mobility Spectrometry

Differential ion mobility spectrometry (DIMS) has the ability to separate gas phase ions based on their difference in ion mobility in low and high electric fields. DIMS can be used to separate mixtures of isobaric and isomeric species indistinguishable by mass spectrometry (MS). DIMS can also be used as a filter to improve the signal-to-background of analytes in complex samples. The resolving power of DIMS separations can be improved several ways, including increasing the dispersion field and increasing the amount of helium in the nitrogen carrier gas. It has been previously demonstrated that the addition of helium to the DIMS carrier gas provides improves separations when the dispersion field is the kept constant as helium content is varied. However, helium has a lower breakdown voltage than nitrogen. Therefore, as the percent helium content in the nitrogen carrier gas is increased, the highest dispersion field accessible decreases. This work presents the trade-offs between increasing dispersion fields and using helium in the carrier gas by comparing the separation of a mixture of isobaric peptides. The maximum resolution for a separation of a mixture of three peptides with the same nominal molar mass was achieved by using a high dispersion field (~72 kV/cm) with pure nitrogen as the carrier gas within the DIMS assembly. The conditions used to achieve the maximum resolution also exhibit the lowest ion transmission through the assembly, suggesting that it is necessary to consider the trade-off between sensitivity and resolution when optimizing DIMS conditions for a given application. Figure

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Ion-neutral potential models in atmospheric pressure ion mobility time-of-flight mass spectrometry IM(tof)MS

The ion mobilities and their respective masses of several classes of amines (primary, secondary, and tertiary) were measured by electrospray ionization atmospheric pressure ion mobility time-of-flight mass spectrometry IM(tof)MS. The experimental data obtained were comparatively analyzed by the one-temperature kinetic theory of Chapman-Enskog. Several theoretical models were used to estimate the collision cross-sections; they include the rigid-sphere, polarization-limit, 12-6-4, and 12-4 potential models.These models were investigated to represent the interaction potentials contained within the collision integral that occurs between the polyatomic ions and the neutral drift gas molecules. The effectiveness of these collision cross-section models on predicting the mobility of these amine ions was explored. Moreover, the effects of drift gas selectivity on the reduced-mass term and in the collision cross-section term was examined. Use of a series of drift gases, namely, helium, neon, argon, nitrogen, and carbon dioxide, made it possible to distinguish between mass effects and polarizability effects. It was found that the modified 12-4 potential that compensates for the center of charge not being at the same location as the centers of mass showed improved agreement over the other collision cross-section models with respect to experimental data.     

Monte Carlo simulation of ion transport in ion mobility spectrometry

A program for simulation of ion trajectories in ion mobility spectrometry (IMS) instruments has been developed and incorporated into SIMION 7.0 [Int. J. Mass Spectrom. 200 (2000) 3–25]. Simulations were based on elastic collisions between ions and gas particles and conducted for an IMS drift tube. The program was validated by comparing the reduced mobility of helium ions derived from the simulation with the experimental data for helium ions in neon drift gas in low electric fields. Typical IMS parameters, including pressure, temperature, and flow rate of the drift gas were taken into account in the simulations. The program demonstrates capabilities of generating IMS spectra and predicting ion transport efficiency and separating ions. For the IMS drift tube studied, a correlation between imperfection of the electric field distribution and low resolution has been observed.     

迁移管的电场强度对真空紫外电离-离子迁移谱仪性能的影响

离子迁移管是离子迁移谱仪的核心部分,它用来产生均匀的电场,以使不同迁移率的离子进行分离。本研究以丙酮为例,详细研究了本课题组所研制的真空紫外电离源-离子迁移谱仪中迁移管的电场参数对离子的灵敏度和分辨率的影响,发现电压的增大灵敏度增大,但是分辨率存在一个最佳的电压,这些结果可用于迁移谱的优化设计。     

How Hot are Your Ions in TWAVE Ion Mobility Spectrometry?

Effective temperatures of ions during traveling wave ion mobility spectrometry (TWIMS) analysis were measured using singly protonated leucine enkephalin dimer as a chemical thermometer by monitoring dissociation of the dimer into monomer, as well as the subsequent dissociation of monomer into a-, b-, and y-ions, as a function of instrumental parameters. At fixed helium cell and TWIMS cell gas flow rates, the extent of dissociation does not vary significantly with either the wave velocity or wave height, except at low (<500 m/s) wave velocities that are not commonly used. Increasing the flow rate of nitrogen gas into the TWIMS cell and decreasing the flow rate of helium gas into the helium cell resulted in greater dissociation. However, the mobility distributions of the fragment ions formed by dissociation of the dimer upon injection into the TWIMS cell are nearly indistinguishable from those of fragment ions formed in the collision cell prior to TWIMS analysis for all TWIMS experiments. These results indicate that heating and dissociation occur when ions are injected into the TWIMS cell, and that the effective temperature subsequently decreases to a point at which no further dissociation is observed during the TWIMS analysis. An upper limit to the effective ion temperature of 449 K during TWIMS analysis is obtained at a helium flow rate of 180 mL/min, TWIMS flow rate of 80 mL/min, and traveling wave height of 40 V, which is well below previously reported values. Effects of ion heating in TWIMS on gas-phase protein conformation are presented.     

Altering the mobility-time continuum: nonlinear scan functions for targeted high resolution trapped ion mobility-mass spectrometry

Trapped ion mobility spectrometry (TIMS) is a versatile high resolution technique that provides the user with the flexibility to adjust the mobility range of interest, duty cycle (up to 100 %), and resolving power (up to ~300) according to the application requirements. Furthermore, TIMS offers the flexibility of operating as either a mobility-selective or conventional ion funnel, thus permitting ion mobility separations to be turned on or off. Here, we extend the flexibility of TIMS by introducing multilinear and nonlinear scanning methods that allow enhanced resolution in user-defined mobility regions. The performance of the new method is demonstrated using a variety of nonlinear scan functions that allow the resolving power to be continuously varied across the mobility spectrum. Further, we demonstrate that mobility analysis can be targeted over disparate regions using a multilinear scan function. In this example, high resolution mobility analysis is targeted on two analytes on opposite ends of a mobility range, while other ions that fall between the regions of interest remain unanalyzed. Using this approach, the resolving power for targeted species was increased by a factor of two over the conventional linear scanning approach (R ~60 versus ~120) without reducing the duty cycle of the TIMS measurement. Importantly, in such an analysis, ions in the untargeted regions are not mobility analyzed, however, they are also not discarded. Rather, these ions are ejected for downstream mass analysis. In this sense, TIMS bridges the gap between dispersive and scanning mobility techniques. That is, TIMS disperses ions according to their elution voltage, however, TIMS can also perform target mobility analyses without eliminating untargeted ions.     

Characterizing the resolution and accuracy of a second-generation traveling-wave ion mobility separator for biomolecular ions

High-accuracy, high-resolution ion mobility measurements enable a vast array of important contemporary applications in biological chemistry. With the recent advent of both new, widely available commercial instrumentation and also new calibration datasets tailored for the aforementioned commercial instrumentation, the possibilities for extending such high performance measurements to a diverse set of applications have never been greater. Here, we assess the performance characteristics of a second-generation traveling-wave ion mobility separator, focusing on those figures of merit that lead to making measurements of collision cross-section having both high precision and high accuracy. Through performing a comprehensive survey of instrument parameters and settings, we find instrument conditions for optimized drift time resolution, cross-section resolution, and cross-section accuracy for a range of peptide, protein and multi-protein complex ions. Moreover, the conditions for high accuracy IM results are significantly different from those optimized for separation resolution, indicating that a balance between these two metrics must be attained for traveling wave IM separations of biomolecules. We also assess the effect of ion heating during IM separation on instrument performance.     

Development of an ion mobility spectrometer for use in an atmospheric pressure ionization ion mobility spectrometer/mass spectrometer instrument for fast screening analysis

An ion mobility spectrometer that can easily be installed as an intermediate component between a commercial triple-quadrupole mass spectrometer and its original atmospheric pressure ionization (API) sources was developed. The curtain gas from the mass spectrometer is also used as the ion mobility spectrometer drift gas. The design of the ion mobility spectrometer allows reasonably fast installation (about 1 h), and thus the ion mobility spectrometer can be considered as an accessory of the mass spectrometer. The ion mobility spectrometer module can also be used as an independently operated device when equipped with a Faraday cup detector. The drift tube of the ion mobility spectrometer module consists of inlet, desolvation, drift, and extraction regions. The desolvation, drift and extraction regions are separated by ion gates. The inlet region has the shape of a stainless steel cup equipped with a small orifice. Ion mobility spectrometer drift gas is introduced through a curtain gas line from an original flange of the mass spectrometer. After passing through the drift tube, the drift gas serves as a curtain gas for the ion-sampling orifice of the ion mobility spectrometer before entering the ion source. Counterflow of the drift gas improves evaporation of the solvent from the electrosprayed sample. Drift gas is pumped away from the ion source through the original exhaust orifice of the ion source. Initial characterization of the ion mobility spectrometer device includes determination of resolving power values for a selected set of test compounds, separation of a simple mixture, and comparison of the sensitivity of the electrospray ionization ion mobility spectrometry/mass spectrometry (ESI-IMS/MS) mode with that of the ESI-MS mode. A resolving power of 80 was measured for 2,6-di-tert-butylpyridine in a 333 V/cm drift field at room temperature and with a 0.2 ms ion gate opening time. The resolving power was shown to be dependent on drift gas flow rate for all studied ion gate opening times. Resolving power improved as the drift gas flow increased, e.g. at a 0.5 ms gate opening time, a resolving power of 31 was obtained with a 0.65 L/min flow rate and 47 with a 1.3 L/min flow rate for tetrabutylammonium iodide. The measured limits of detection with ESI-MS and with ESI-IMS/MS modes were similar, demonstrating that signal losses in the IMS device are minimal when it is operated in a continuous flow mode. Based on these preliminary results, the IMS/MS instrument is anticipated to have potential for fast screening analysis that can be applied, for example, in environmental and drug analysis.     

Scaling of the resolving power and sensitivity for planar FAIMS and mobility-based discrimination in flow- and field-driven analyzers

Continuing development of the technology and applications of field asymmetric waveform ion mobility spectrometry (FAIMS) calls for better understanding of its limitations and factors that govern them. While key performance metrics such as resolution and ion transmission have been calculated for specific cases employing numerical simulations, the underlying physical trends remained obscure. Here we determine that the resolving power of planar FAIMS scales as the square root of separation time and sensitivity drops exponentially at the rate controlled by absolute ion mobility and several instrument parameters. A strong dependence of ion transmission on mobility severely discriminates against species with higher mobility, presenting particular problems for analyses of complex mixtures. While the time evolution of resolution and sensitivity is virtually identical in existing FAIMS systems using gas flow and proposed devices driven by electric field, the distributions of separation times are not. The inverse correlation between mobility (and thus diffusion speed) and residence time for ions in field-driven FAIMS greatly reduces the mobility-based discrimination and provides much more uniform separations. Under typical operating conditions, the spread of elimination rates for commonly analyzed ions is reduced from >5 times in flow-driven to 1.6 times in field-driven FAIMS while the difference in resolving power decreases from approximately 60% to approximately 15%.     

Overtone mobility spectrometry: Part 2. Theoretical considerations of resolving power

The transport of ions through multiple drift regions is modeled to develop an equation that is useful for an understanding of the resolving power of an overtone mobility spectrometry (OMS) technique. It is found that resolving power is influenced by a number of experimental variables, including those that define ion mobility spectrometry (IMS) resolving power: drift field (E), drift region length (L), and buffer gas temperature (T). However, unlike IMS, the resolving power of OMS is also influenced by the number of drift regions (n), harmonic frequency value (m), and the phase number (Φ) of the applied drift field. The OMS resolving power dependence upon the new OMS variables (n, m, and Φ) scales differently than the square root dependence of the E, L, and T variables in IMS. The results provide insight about optimal instrumental design and operation.     

Ion mobility adds an additional dimension to mass spectrometric analysis of solution-phase hydrogen/deuterium exchange

The goal of this study was to determine the utility of adding ion mobility spectrometry to studies probing the solution-phase hydrogen/deuterium exchange (HX) of proteins. The HX profile of the Hck SH3 domain was measured at both the intact protein and the peptic peptide levels in the Waters Synapt HDMS system which uses a traveling wave to accomplish ion mobility separation prior to time-of-flight (Tof) m/z analysis. The results indicated a similar loss of deuterium with or without use of mobility in the Synapt and a level of deuterium loss comparable with a non-mobility Q-Tof instrument. The drift time of this small protein and its peptic peptides did not noticeably change due to solution-based deuterium incorporation. Importantly, ion mobility separations provided an orthogonal dimension of separation in addition to the reversed-phase high-performance liquid chromatography (RP-HPLC). The additional dimension of separation allowed for the deconvolution of overlapping isotopic patterns for co-eluting peptides and extraction of valuable deuterium incorporation data for those peptides. Taken together, these results indicate that including ion mobility separation in HX MS analyses further improves the mass spectrometry portion of such experiments. Copyright (c) 2008 John Wiley & Sons, Ltd.     

碰撞气体的种类和压力对离子阱质谱性能的影响

基于数字离子阱技术,研究了离子阱质谱分析实验过程使用的碰撞气体种类及压力对离子阱质谱性能,如质量分辨能力、信号强度、串级质谱分析,以及低质量截止效应等的影响.实验过程中,在离子的激发和碰撞诱导解离阶段,分别采用质量数不等的氦气(质量数=4 amu)、氮气(质量数=28 amu)、氩气(质量数=40 amu)等作为碰撞气体,以及不同的气体压力,研究了它们对质谱性能的影响.结果表明,当采用质量数较大的氩气作为碰撞气体时,可以有效改善低质量数截止效应和提高离子碰撞过程中的能量转移效率,同时提高离子捕获和解离效率,但是质量分辨率会明显降低.在获得较高质量分辨率方面,氦气作为碰撞气体时效果最好.在气压相同的情况下,质量数大的碰撞气体有利于提高串级质谱分析效率,即获得更多碎片离子峰和更多有关母体离子结构的信息.     

Structural Elucidation of β‐Lactam Diastereoisomers through Ion Mobility Mass Spectrometry Studies and Theoretical Calculations

The ion mobility combined with mass spectrometry and theoretical calculations were used to characterize and separate six diastereoisomeric β‐lactams. The influence of traveling wave height and wave velocity, size of the alkali metal ion (Li⁺, Na⁺ and K⁺) and drift gases with varying masses and polarizabilities (N₂ and CO₂) on separation efficacy was additionally examined. The best separation of diastereoisomers of β‐lactams was observed for adducts with Na⁺ and Li⁺ ions, whereas other parameters had little impact on separation process. The isomeric β‐lactams were characterized by both experimental and theoretical collision cross sections. The theoretically calculated values of collision cross sections obtained from extensive molecular dynamics and density functional theory calculations for model structures agreed well with those established experimentally. The relationship between separation efficacy and the configuration at the carbon atoms C5 and C6 of β‐lactam ring was defined. Copyright © 2016 John Wiley & Sons, Ltd.     

Effect of buffer gas on the fluorescence yield of trapped gas-phase ions

We investigated the dependence of three different gases, helium, argon, and nitrogen, on the fluorescence signal intensity of rhodamine 6G cations in the gas phase. The method is based on laser-induced fluorescence of ions trapped in a Fourier transform ion cyclotron mass spectrometer. We found that the use of helium results in the highest fluorescence signal, while no fluorescence was detected when using argon under the same conditions.     

Overtone mobility spectrometry: Part 1. Experimental observations

A new method that allows a linear drift tube to be operated as a continuous ion mobility filter is described. Unlike conventional ion mobility instruments that use an electrostatic gate to introduce a packet of ions into a drift region, the present approach uses multiple segmented drift regions with modulated drift fields to produce conditions that allow only ions with appropriate mobilities to pass through the instrument. In this way, the instrument acts as a mobility filter for continuous ion sources. By changing the frequency of the applied drift fields it is possible to tune this instrument to transmit ions having different mobilities. A scan over a wide range of drift field frequencies for a single ion species shows a peak corresponding to the expected resonance time of the ions in one drift region segment and a series of peaks at higher frequencies that are overtones of the resonant frequency. The measured resolving power increases for higher overtones, making it possible to resolve structures that were unresolved in the region of the fundamental frequency. We demonstrate the approach by examining oligosaccharide isomers, raffinose and melezitose as well as a mixture of peptides obtained from enzymatic digestion of myoglobin.     

An alternative field switching ion gate for ESI-ion mobility spectrometry

In electrospray ionization (ESI)-ion mobility spectrometry, continuously generated ions must be desolvated in a first tube before short ion pulses are introduced into a second (drift) tube. Both tubes are separated by an ion-gate. The resolving power of the resulting drift time spectrum is strongly influenced by the design of the ion gate. In the case of the Bradbury-Nielsen gates typically used, an orthogonal field between oppositely charged, parallel wires blocks ions from entering the drift tube. However, the blocking field also distorts the entering ion cloud. One alternative, which eliminates these effects and therefore enables a potentially higher resolving power, is already known for spectrometers with small ionization volumes, where ions are formed between two electrodes and subsequently transferred into the drift tube by a high voltage pulse. Based on this setup, we introduce an alternative ion gate design for liquid samples, named field switching ion gate (FSIG). The continuous flow of ions generated by ESI is desolvated in the first tube and introduced into the space between two electrodes (repeller and transfer electrodes). A third (blocking) electrode prevents the movement of ions into the drift tube in the closed state. Ions are transferred during the open state by pulsing the voltages of the repeller and blocking electrodes. First results demonstrate an increase of the resolving power by 100% without intensity losses and further changes in the spectrometer setup. The parameters of the FSIG, such as electrode voltages and pulse width, are characterized allowing the optimization of the spectrometer’s resolving power.     

Modeling the resolution and sensitivity of FAIMS analyses

Field asymmetric waveform ion mobility spectrometry (FAIMS) is rapidly gaining acceptance as a robust, versatile tool for post-ionization separations prior to mass-spectrometric analyses. The separation is based on differences between ion mobilities at high and low electric fields, and proceeds at atmospheric pressure. Two major advantages of FAIMS over condensed-phase separations are its high speed and an ion focusing effect that often improves sensitivity. While selected aspects of FAIMS performance are understood empirically, no physical model rationalizing the resolving power and sensitivity of the method and revealing their dependence on instrumental variables has existed. Here we present a first-principles computational treatment capable of simulating the FAIMS analyzer for virtually any geometry (including the known cylindrical and planar designs) and arbitrary operational parameters. The approach involves propagating an ensemble of ion trajectories through the device in real time under the influence of applied asymmetric potential, diffusional motion incorporating the high-field and anisotropic phenomena, and mutual Coulomb repulsion of ionic charges. Calculations for both resolution and sensitivity are validated by excellent agreement with measurements in different FAIMS modes for ions representing diverse types and analyte classes.     

Separation mechanism of chiral impurities, ephedrine and pseudoephedrine, found in amphetamine-type substances using achiral modifiers in the gas phase

A new mechanism is proposed that describes the gas-phase separation of chiral molecules found in amphetamine-type substances (ATS) by the use of high-resolution ion mobility spectrometry (IMS). Straight-chain achiral alcohols of increasing carbon chain length, from methanol to n-octanol, are used as drift gas modifiers in IMS to highlight the mechanism proposed for gas-phase separations of these chiral molecules. The results suggest the possibility of using these achiral modifiers to separate the chiral molecules (R,S) and (S,R)-ephedrine and (S,S) and (R,R)-pseudoephedrine which contain an internal hydroxyl group at the first chiral center and an amino group at the other chiral center. Ionization was achieved with an electrospray source, the ions were introduced into an IMS with a resolving power of 80, and the resulting ion clusters were characterized with a coupled quadrupole mass spectrometer detector. A complementary computational study conducted at the density functional B3LYP/6-31g level of theory for the electronic structure of the analyte–modifier clusters was also performed, and showed either “bridged” or “independent” binding. The combined experimental and simulation data support the proposed mechanism for gas-phase chiral separations using achiral modifiers in the gas phase, thus enhancing the potential to conduct fast chiral separations with relative ease and efficiency.