Simultaneous fragmentation of multiple ions using IMS drift time dependent collision energies

Ion mobility spectrometry coupled with mass spectrometry (IMS-MS) was utilized to evaluate an ion collision energy ramping technique that simultaneously fragments a variety of species. To evaluate this technique, the fragmentation patterns of a mixture of ions ranging in mass, charge state, and drift time were analyzed to determine their optimal fragmentation conditions. The precursor ions were pulsed into the IMS-MS instrument and separated in the IMS drift cell based on mobility differences. Two differentially pumped short quadrupoles were used to focus the ions exiting the drift cell, and fragmentation was induced by collision induced dissociation (CID) between the conductance limiting orifice behind the second short quadrupole and before the first octopole in the mass spectrometer. To explore the fragmentation spectrum of each precursor ion, the bias voltages for the short quadrupoles and conductance limiting orifices were increased from 0 to 50 V above nonfragmentation voltage settings. An approximately linear correlation was observed between the optimal fragmentation voltage for each ion and its specific drift time, so a linear voltage gradient was employed to supply less collision energy to high mobility ions (e.g., small conformations or higher charge state ions) and more to low mobility ions. Fragmentation efficiencies were found to be similar for different ions when the fragmentation voltage was linearly ramped with drift time, but varied drastically when only a single voltage was used.     

Ion Mobility Sensor In Environmental Analytical Chemistry—Concept And First Results

Abstract

Ion mobility spectrometry is a technique for generating ions at atmospheric pressure via ion-molecule reactions, and for analysing them in an ion drift tube.

The time required for the ions to traverse the length of the drift tube is mainly a function of the mass and the charge of the ions. Besides, ion shape and polarizability also affect the drift time.

Ion mobility spectrometry does not allow structural identification and quantification of unknown substances in mixtures. However, under certain boundary conditions it provides selective fingerprints of the substances to be observed, and operates at the ppbv concentration level and the millisecond time scale.

Through further miniaturization of a recently developed instrument of this type an ion mobility sensor is to be constructed. This sensor includes drift channel, operating shutter, collecting electrode, electronic data acquisition and translation board. The sensor makes possible to obtain real-time ion mobility spectra.

We present and discuss the concept of a small ion mobility spectrometer, its operation principle and first results on the way towards its further miniaturization.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

A Mass-Selective Variable-Temperature Drift Tube Ion Mobility-Mass Spectrometer for Temperature Dependent Ion Mobility Studies

A hybrid ion mobility-mass spectrometer (IM-MS) incorporating a variable-temperature (80–400 K) drift tube is presented. The instrument utilizes an electron ionization (EI) source for fundamental small molecule studies. Ions are transferred to the IM-MS analyzer stages through a quadrupole, which can operate in either broad transmission or mass-selective mode. Ion beam modulation for the ion mobility experiment is accomplished by an electronic shutter gate. The variable-temperature ion mobility spectrometer consists of a 30.2 cm uniform field drift tube enclosed within a thermal envelope. Subambient temperatures down to 80 K are achievable through cryogenic cooling with liquid nitrogen, while elevated temperatures can be accessed through resistive heating of the envelope. Mobility separated ions are mass analyzed by an orthogonal time-of-flight (TOF) mass spectrometer. This report describes the technological considerations for operating the instrument at variable temperature, and preliminary results are presented for IM-MS analysis of several small mass ions. Specifically, mobility separations of benzene fragment ions generated by EI are used to illustrate significantly improved (greater than 50%) ion mobility resolution at low temperatures resulting from decreased diffusional broadening. Preliminary results on the separation of long-lived electronic states of Ti⁺ formed by EI of TiCl₄ and hydration reactions of Ti⁺ with residual water are presented.     

Ion mobility-mass spectrometry

This review article compares and contrasts various types of ion mobility-mass spectrometers available today and describes their advantages for application to a wide range of analytes. Ion mobility spectrometry (IMS), when coupled with mass spectrometry, offers value-added data not possible from mass spectra alone. Separation of isomers, isobars, and conformers; reduction of chemical noise; and measurement of ion size are possible with the addition of ion mobility cells to mass spectrometers. In addition, structurally similar ions and ions of the same charge state can be separated into families of ions which appear along a unique mass-mobility correlation line. This review describes the four methods of ion mobility separation currently used with mass spectrometry. They are (1) drift-time ion mobility spectrometry (DTIMS), (2) aspiration ion mobility spectrometry (AIMS), (3) differential-mobility spectrometry (DMS) which is also called field-asymmetric waveform ion mobility spectrometry (FAIMS) and (4) traveling-wave ion mobility spectrometry (TWIMS). DTIMS provides the highest IMS resolving power and is the only IMS method which can directly measure collision cross-sections. AIMS is a low resolution mobility separation method but can monitor ions in a continuous manner. DMS and FAIMS offer continuous-ion monitoring capability as well as orthogonal ion mobility separation in which high-separation selectivity can be achieved. TWIMS is a novel method of IMS with a low resolving power but has good sensitivity and is well intergrated into a commercial mass spectrometer. One hundred and sixty references on ion mobility-mass spectrometry (IMMS) are provided.     

Ion trap mass spectrometry in the structural analysis of haemoglobin peptides modified by epichlorohydrin and diepoxybutane

Ion trap mass spectrometry has been shown to be particularly suitable for the structural analysis of high molecular weight peptides directly fragmented in the mass analyser without needing further sub-digestion reactions. Here we report the advantages of using multi-stage ion trap mass spectrometry in the structural characterisation of haemoglobin alkylated with epichlorohydrin and diepoxybutane. Alkylated globins were digested with trypsin and the peptide mixtures were analysed by MS(3). This technique allows the sequential fragmentation of peptides under analysis, giving rise to MS(3) product ion spectra with additional information with respect to MS(2) mass spectra. The results obtained complete the previously reported structural characterisation of alkylated haemoglobin, demonstrating the potential of ion trap mass spectrometry.     

Mannose7 Glycan Isomer Characterization by IMS-MS/MS Analysis

The isomers of the Man₇GlcNAc₂ glycan obtained from bovine ribonuclease B have been characterized by ion mobility spectrometry-tandem mass spectrometry (IMS-MS/MS). In these experiments, [Man7 + 2Na]²⁺ precursors having different mobilities are selected by ion mobility spectrometry and analyzed by MS/MS techniques in an ion trap. The fragmentation spectra obtained for various precursor ions are specific, suggesting the isolation or enrichment of different glycan isomers. One fragment ion with a mass-to-charge ratio (m/z) of 903.8 is found to correspond to the loss of an internal mannose residue of a specific isomer. Extracted fragment ion drift time distributions (XFIDTDs) yield distinctive precursor ion drift time profiles indicating the existence of four separate isomers as proposed previously.     

Characterization of a DAPI-RIT-DAPI System for Gas-Phase Ion/Molecule and Ion/Ion Reactions

The discontinuous atmospheric pressure interface (DAPI) has been developed as a facile means for efficiently introducing ions generated at atmospheric pressure to an ion trap in vacuum [e.g., a rectilinear ion trap (RIT)] for mass analysis. Introduction of multiple beams of ions or neutral species through two DAPIs into a single RIT has been previously demonstrated. In this study, a home-built instrument with a DAPI-RIT-DAPI configuration has been characterized for the study of gas-phase ion/molecule and ion/ion reactions. The reaction species, including ions or neutrals, can be introduced from both ends of the RIT through the two DAPIs without complicated ion optics or differential pumping stages. The primary reactant ions were isolated prior to reaction and the product ions were mass analyzed after controlled reaction time period. Ion/molecule reactions involving peptide radical ions and proton-transfer ion/ion reactions have been carried out using this instrument. The gas dynamic effect due to the DAPI operation on internal energy deposition and the reactivity of peptide radical ions has been characterized. The DAPI-RIT-DAPI system also has a unique feature for allowing the ion reactions to be carried out at significantly elevated pressures (in 10^–1 Torr range), which has been found to be helpful to speed up the reactions. The viability and flexibility of the DAPI-RIT-DAPI system for the study of gas-phase ion reactions have been demonstrated.

Ion mobility spectrometry: a personal retrospective

With a background in mass spectrometric studies of gas-phase ion chemistry the atmospheric pressure technology of ion mobility spectrometry (IMS) presented me with challenges and opportunities. Fundamental studies of the parameters that influence the mobility of ions in a low electric field yielded insights about the effects of temperature, drift gas composition and the conformation of ions on the collision cross section. The inadequacy of current rigid-sphere, polarization limit and hard-core models to predict the mobility of ions particularly at low temperature and in heavy drift gases, led to inclusion of additional terms to the hard-core model to account for these effects. These studies eventually resulted in the two monographs entitled “Ion Mobility Spectrometry” and “Ion Mobility Spectrometry –Second Edition” co-authored with Prof. Gary Eiceman and published by Taylor & Francis, CRC Press in 1994 and 2005, respectively. Novel applications for biological and medical applications were developed on the basis of measurement of biogenic amines by IMS, in particular the rapid, accurate and inexpensive diagnosis of vaginal infections.     

Gated trapped ion mobility spectrometry coupled to fourier transform ion cyclotron resonance mass spectrometry

Analysis of molecules by ion mobility spectrometry coupled with mass spectrometry (IMS-MS) provides chemical information on the three dimensional structure and mass of the molecules. The coupling of ion mobility to trapping mass spectrometers has historically been challenging due to the large differences in analysis time between the two devices. In this paper we present a modification of the trapped ion mobility (TIMS) analysis scheme termed “Gated TIMS” that allows efficient coupling to a Fourier Transform Ion Cyclotron Resonance (FT-ICR) analyzer. Analyses of standard compounds and the influence of source conditions on the TIMS distributions produced by ion mobility spectra of labile ubiquitin protein ions are presented. Ion mobility resolving powers up to 100 are observed. Measured collisional cross sections of ubiquitin ions are in excellent qualitative and quantitative agreement to previous measurements. Gated TIMS FT-ICR produces results comparable to those acquired using TIMS/time-of-flight MS instrument platforms as well as numerous drift tube IMS-MS studies published in the literature.     

Ion mobility spectrometer—field asymmetric ion mobility spectrometer-mass spectrometry

Since the development of electrospray ionization (ESI) for ion mobility spectrometry mass spectrometry (IMMS), IMMS have been extensively applied for characterization of gas-phase bio-molecules. Conventional ion mobility spectrometry (IMS), defined as drift tube IMS (DT-IMS), is typically a stacked ring design that utilizes a low electric field gradient. Field asymmetric ion mobility spectrometry (FAIMS) is a newer version of IMS, however, the geometry of the system is significantly different than DT-IMS and data are collected using a much higher electric field. Here we report construction of a novel ambient pressure dual gate DT-IMS coupled with a FAIMS system and then coupled to a quadrupole ion trap mass spectrometer (QITMS) to form a hybrid three-dimensional separation instrument, DT-IMS-FAIMS-QITMS. The DT-IMS was operated at ~3 Townsend (electric field/number density (E/N) or (Td)) and was coupled in series with a FAIMS, operated at ~80 Td. Ions were mobility-selected by the dual gate DT-IMS into the FAIMS and from the FAIMS the ions were detected by the QITMS for as either MS or MSⁿ. The system was evaluated using cocaine as an analytical standard and tested for the application of separating three isomeric tri-peptides: tyrosine-glycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosine-tryptophan-glycine (YWG). All three tri-peptides were separated in the DT-IMS dimension and each had one mobility peak. The samples were partially separated in the FAIMS dimension but two conformation peaks were detected for the YWG sample while YGW and WGY produced only one peak. Ion validation was achieved for all three samples using QITMS.     

A novel approach to collision-induced dissociation (CID) for ion mobility-mass spectrometry experiments

Collision induced dissociation (CID) combined with matrix assisted laser desorption ionization-ion mobility-mass spectrometry (MALDI-IM-MS) is described. In this approach, peptide ions are separated on the basis of mobility in a 15 cm drift cell. Following mobility separation, the ions exit the drift cell and enter a 5 cm vacuum interface with a high field region (up to 1000 V/cm) to undergo collisional activation. Ion transmission and ion kinetic energies in the interface are theoretically evaluated accounting for the pressure gradient, interface dimensions, and electric fields. Using this CID technique, we have successfully fragmented and sequenced a number of model peptide ions as well as peptide ions obtained by a tryptic digest. This instrument configuration allows for the simultaneous determination of peptide mass, peptide-ion sequence, and collision-cross section of MALDI-generated ions, providing information critical to the identification of unknown components in complex proteomic samples.     

An ion trap-ion mobility-time of flight mass spectrometer with three ion sources for ion/ion reactions

This instrument combines the capabilities of ion/ion reactions with ion mobility (IM) and time-of-flight (TOF) measurements for conformation studies and top-down analysis of large biomolecules. Ubiquitin ions from either of two electrospray ionization (ESI) sources are stored in a three dimensional (3D) ion trap (IT) and reacted with negative ions from atmospheric sampling glow discharge ionization (ASGDI). The proton transfer reaction products are then separated by IM and analyzed via a TOF mass analyzer. In this way, ubiquitin +7 ions are converted to lower charge states down to +1; the ions in lower charge states tend to be in compact conformations with cross sections down to ～880 Ų. The duration and magnitude of the ion ejection pulse on the IT exit and the entrance voltage on the IM drift tube can affect the measured distribution of conformers for ubiquitin +7 and +6. Alternatively, protein ions are fragmented by collision-induced dissociation (CID) in the IT, followed by ion/ion reactions to reduce the charge states of the CID product ions, thus simplifying assignment of charge states and fragments using the mobility-resolved tandem mass spectrum. Instrument characteristics and the use of a new ion trap controller and software modifications to control the entire instrument are described.     

数字化矩形离子阱质谱仪的设计及性能

将数字化离子阱技术和矩形离子阱(RIT)技术相结合,建立了数字化矩形离子阱质谱仪.此技术和装置既具有数字化电源的结构简单、输出稳定和易精确控制等特点,又结合了矩形离子阱的高离子存储效率、结构简单以及加工和装配容易等优点.构建了基于电喷雾(ESI)电离源的数字化矩形离子阱质谱仪系统,并使用Fenfluramine和PPG2000分别对此系统的质量分辨率和质量范围进行了测试.研究结果表明:一个用印刷线路板(PCB)制作的简单矩形离子阱,在200 V(半峰值)的数字束缚电压的驱动下,获得了大于500的质量分辨率和超过2600 Th的质量范围.实验证明,数字化离子阱技术的应用可以显著提高矩形离子阱的性能,特别是质量范围等关键的质谱仪指标.     

Nanoflow LC/IMS-MS and LC/IMS-CID/MS of protein mixtures

A simple ion trap/ion mobility/time-of-flight (TOF) mass spectrometer has been coupled with nanoflow liquid chromatography to examine the feasibility of analyzing mixtures of intact proteins. In this approach proteins are separated using reversed-phase chromatography. As components elute from the column, they are electrosprayed into the gas phase and separated again in a drift tube prior to being dispersed and analyzed in a TOF mass spectrometer. The mobilities of ions through a buffer gas depend upon their collision cross sections and charge states; separation based on these gas-phase parameters provides a new means of simplifying mass spectra and characterizing mixtures. Additionally it is possible to induce dissociation at the exit of the drift tube and examine the fragmentation patterns of specific protein ion charge states and conformations. The approach is demonstrated by examining a simple three-component mixture containing ubiquitin, cytochrome c, and myoglobin and several larger prepared protein mixtures. The potential of this approach for use in proteomic applications is considered.     

MALDI-MS/MS with Traveling Wave Ion Mobility for the Structural Analysis of N-Linked Glycans

The preference for singly charged ion formation by MALDI makes it a better choice than electrospray ionization for profiling mixtures of N-glycans. For structural analysis, fragmentation of negative ions often yields more informative spectra than fragmentation of positive ones but such ions are more difficult to produce from neutral glycans under MALDI conditions. This work investigates conditions for the formation of both positive and negative ions by MALDI from N-linked glycans released from glycoproteins and their subsequent MS/MS and ion mobility behaviour. 2,4,6-Trihydroxyacetophenone (THAP) doped with ammonium nitrate was found to give optimal ion yields in negative ion mode. Ammonium chloride or phosphate also yielded prominent adducts but anionic carbohydrates such as sulfated N-glycans tended to ionize preferentially. Carbohydrates adducted with all three adducts (phosphate, chloride, and nitrate) produced good negative ion CID spectra but those adducted with iodide and sulfate did not yield fragment ions although they gave stronger signals. Fragmentation paralleled that seen following electrospray ionization providing superior spectra than could be obtained by PSD on MALDI-TOF instruments or with ion traps. In addition, ion mobility drift times of the adducted glycans and the ability of this technique to separate isomers also mirrored those obtained following ESI sample introduction. Ion mobility also allowed profiles to be obtained from samples whose MALDI spectra showed no evidence of such ions allowing the technique to be used in conditions where sample amounts were limiting. The method was applied to N-glycans released from the recombinant human immunodeficiency virus glycoprotein, gp120.     

Corona Discharge Ion Mobility Spectrometry of Ten Alcohols

Ion mobility spectra for ten alcohols have been studied in an ion mobility spectrometry apparatus equipped with a corona discharge ionization source. Using protonated water cluster ions as the reactant ions and clean air as the drift gas, the alcohols exhibit different product ion characteristic peaks in their ion mobility spectra. The detection limit for these alcohols is at low concentration pmol/L level according to the concentration calibration by exponential dilution method. Based on the measured ion mobilities, several chemical physics parameters of the ion-molecular interaction at atmosphere were obtained, including the ionic collision cross sections, diffusion coefficients, collision rate constants, and the ionic radii under the hard-sphere model approximation.     

Multistage electrospray ionization mass spectrometric analyses of sulfur-containing iridoid glucosides in Paederia scandens

Paederia scandens is a traditional Chinese herbal medicine that possesses important bioactive sulfur-containing iridoid glucosides. This study reports the investigation of the fragmentation behavior of four sulfur-containing iridoid glucosides isolated from P. scandens by ion trap electrospray ionization tandem mass spectrometry (ESI-MSn). Both multistage electrospray ionization (ESI-MSn) experiments in an ion trap instrument and high-resolution and accurate mass measurement in an ESI-Q-TOF mass spectrometer in positive mode were used to elucidate the main fragmentation pathways of these compounds. These experiments yielded essentially the same product ions in both ion trap and Q-TOF instruments, and their fragmentation patterns were found useful for their characterization, which were applied to elucidate a trace compound in the extracts of P. scandens by on-line LC/ESI-MSn. Major and diagnostic product ions have been identified and their origins are proposed.     

Dual buffer gases for ion manipulation in a miniature ion trap mass spectrometer with a discontinuous atmospheric pressure interface

The discontinuous atmospheric pressure interface (DAPI) has been developed to allow a direct transfer of ions from atmosphere into an ion trap mass spectrometer with minimum pumping capability. Air is introduced into the trap with ions and used as a buffer gas for the ion trap operation. In this study, a method of introducing helium as a second buffer gas was developed for a miniature mass spectrometer using a dual DAPI configuration. The buffer gas effects on the performance of a linear ion trap (LIT) with hyperbolic electrodes were characterized for ion isolation, fragmentation and a mass-selective instability scan. Significant improvement was obtained with helium for resolutions of mass analysis and ion isolation, while moderate advantage was gained with air for collision-induced dissociation. The buffer gas can be switched between air and helium for different steps within a single scan, which allows further optimization of the instrument performance for tandem mass spectrometry.     

Effects of ion/ion proton transfer reactions on conformation of gas-phase cytochrome <Emphasis Type="Italic">c</Emphasis> ions

Positive ions from cytochrome c are studied in a 3-D ion trap/ion mobility (IM)/quadrupole-time-of-flight (TOF) instrument with three independent ion sources. The IM separation allows measurement of the cross section of the ions. Ion/ion reactions in the 3-D ion trap that remove protons cause the cytochrome c ions to refold gently without other degradation of protein structure, i.e., fragmentation or loss of heme group or metal ion. The conformation(s) of the product ions generated by ion/ion reactions in a given charge state are similar regardless of whether the cytochrome c ions are originally in +8 or +9 charge states. In the lower charge states (+1 to +5) cytochrome c ions made by the ion/ion reaction yield a single IM peak with cross section of ～1110 to 1180 Ų, even if the original +8 ion started with multiple conformations. The conformation expands slightly when the charge state is reduced from +5 to +1. For product ions in the +6 to +8 charge states, ions created from higher charge states (+9 to +16) by ion/ion reaction produce more compact conformation(s) in somewhat higher abundances compared with those produced directly by the electrospray ionization (ESI) source. For ions in intermediate charge states that have a variety of resolvable conformers, the voltage used to inject the ions into the drift tube, and the voltage and duration of the pulse that extracts ions from the ion trap, can affect the observed abundances of various conformers.