High-frequency fourier transform ion cyclotron resonance mass spectrometry

The experimental Fourier transform ion cyclotron resonance (FT/ICR) frequency range has been extended to 107 MHz. We report the observation of FT/ICR signals from electron-ionized species of mass-to-charge ratio 8, 7, 6, 5, 4, 3, 2, and 1 μ per elementary charge. We show that moderately high charge states of atomic ions (e.g., N³⁺) are easily generated and detected. Several applications for high-frequency FT/ICR mass spectrometry are proposed and discussed.     

Quantitation of ion abundances in fourier transform ion cyclotron resonance mass spectrometry

To improve the analytical usefulness of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), an extensive survey of various methods for quantitation of peak magnitudes has been undertaken using a series of simulated transient response signals with varying signal-to-noise ratio. Both peak height (five methods) and peak area (four methods) were explored for a range of conditions to determine the optimum methodology for quantitation. Variables included dataset size, apodization function, damping constant, and zero filling. Based on the results obtained, recommended procedures for optimal quantitation include: apodization using a function appropriate for the peak height ratios observed in the spectrum (i.e., Hanning for ratios of about 1:10, three-term Blackman-Harris for ratios of ～1:100, or Kaiser-Bessel for ratios of ～1:1000); zero filling until the peaks of interest are represented by 10–15 points (generally obtained with one order of zero filling); and use of the polynomial y=(ax ²+bx+c) ⁿ and the three data points of highest intensity of the peak to locate the peak maximum, Y _max=(?b ²/4a+c) ⁿ . In this peak fitting procedure, which we have termed the “Comisarow method,” n is 5.5, 9.5, and 12.5 for the Hanning, three-term Blackman-Harris, and Kaiser-Bessel apodization functions, respectively. Accuracy of quantitation using an optimal peak height determination is about equal to that for peak area measurements. These recommendations were found to be valid when tested with real FTICR-MS spectra of xenon isotopes.     

Atmospheric pressure photoionization proton transfer for complex organic mixtures investigated by fourier transform ion cyclotron resonance mass spectrometry

To further clarify the role of dopant solvent in proton transfer in atmospheric pressure photoionization (APPI), we employ ultrahigh-resolution FT-ICR mass analysis to identify M(+*), [M + H](+), [M - H](-), and [M + D](+) species in toluene or perdeuterotoluene for an equimolar mixture of five pyrrolic and pyridinic nitrogen heterocyclic model compounds, as well as for a complex organic mixture (Canadian Athabasca bitumen middle distillate). In the petroleum sample, the protons in the [M + H](+) species originate primarily from other components of the mixture itself, rather than from the toluene dopant. In contrast to electrospray ionization, in which basic (e.g., pyridinic) species protonate to form [M + H](+) positive ions and acidic (e.g., pyrrolic) species deprotonate to form [M - H](-) negative ions, APPI generates ions from both basic and acidic species in a single positive-ion mass spectrum. Ultrahigh-resolution mass analysis (in this work, m/Deltam(50%) = 500,000, in which Deltam(50%) is the mass spectral peak full width at half-maximum peak height) is needed to distinguish various close mass doublets: (13)C versus (12)CH (4.5 mDa), (13)CH versus (12)CD (2.9 mDa), and H(2) versus D (1.5 mDa).     

External accumulation of ions for enhanced electrospray ionization fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization (ESI) in combination with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry provides for mass analysis of biological molecules with unrivaled mass accuracy, resolving power and sensitivity. However, ESI FTICR MS performance with on-line separation techniques such as liquid chromatography (LC) and capillary electrophoresis has to date been limited primarily by pulsed gas assisted accumulation and the incompatibility of the associated pump-down time with the frequent ion beam sampling requirement of on-line chromatographic separation. Here we describe numerous analytical advantages that accrue by trapping ions at high pressure in the first rf-only octupole of a dual octupole ion injection system before ion transfer to the ion trap in the center of the magnet for high performance mass analysis at low pressure. The new configuration improves the duty cycle for analysis of continuously generated ions, and is thus ideally suited for on-line chromatographic applications. LC/ESI FTICR MS is demonstrated on a mixture of 500 fmol of each of three peptides. Additional improvements include a fivefold increase in signal-to-noise ratio and resolving power compared to prior methods on our instrument.     

In-cell matrix-assisted laser desorption-ionization fourier transform ion cyclotron resonance mass spectrometry

A new internal matrix-assisted laser desorption-ionization (MALDI) Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS) method is introduced. The target is directly positioned at one trapping electrode of a single cylindrical ion cyclotron resonance (ICR) cell and becomes a part of it. The ionization occurs inside the ICR cell in contrast to external or near-cell MALDI-FTICR-MS techniques. Very efficient trapping and mass resolving power better than unit resolution of singly charged peptides and proteins ions up to 2000 u is possible by using only basic FTICR-MS techniques. The sole application of a pulsed retarding potential increases the mass range to 6000 u. No collisional cooling and quadrupolar excitation was done. Sensitivities below 1 fmol, and ion storage times of more than 15 s are shown. High resolving powers of 16,000 and 56,000 are obtained on bovine insulin (5.7 ku) and gramicidin D (1.9 ku), respectively.     

Characterization of polyphosphoesters by fourier transform ion cyclotron resonance mass spectrometry

FT-ICR mass spectrometry, together with collision-induced dissociation and electron capture dissociation, has been used to characterize the polyphosphoester poly[1,4-bis(hydroxyethyl)terephthalate-alt-ethyloxyphosphate] and its degradation products. Three degradation pathways were elucidated: hydrolysis of the phosphate-[1,4-bis(hydroxyethyl)terephthalate] bonds; hydrolysis of the phosphate-ethoxy bonds; and hydrolysis of the ethyl-terephthalate bonds. The dominant degradation reactions were those that involved the phosphate groups. This work constitutes the first application of mass spectrometry to the characterization of polyphosphoesters and demonstrates the suitability of high mass accuracy FT-ICR mass spectrometry, with CID and ECD, for the structural analysis of polyphosphoesters and their degradation products.     

Milestones in fourier transform ion cyclotron resonance mass spectrometry technique development

The present range and power of Fourier transform ion cyclotron resonance mass spectrometry rest on a number of prior technique developments. In this article, selected developments in neutral/ion introduction, ionization methods, excitation/detection, ion trap configuration/operating modes, ion dissociation and MS/MS, ion cooling techniques, theory and data reduction are briefly explained and chronicled. Evidence for the value of these techniques is provided by a compilation of current world records for mass resolution, mass resolving power and mass accuracy. With these capabilities, it becomes possible to resolve and identify up to thousands of components of a complex mixture, often without prior wet chemical separation, thereby potentially changing the whole approach to dealing with chemical and biological complexity.     

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.     

Speciation of nitrogen containing aromatics by atmospheric pressure photoionization or electrospray ionization fourier transform ion cyclotron resonance mass spectrometry

We determine the elemental compositions of aromatic nitrogen model compounds as well as a petroleum sample by atmospheric pressure photoionization (APPI) and electrospray Ionization (ESI) with a 9.4 Tesla Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. From the double-bond equivalents calculated for the nitrogen-containing ions from a petroleum sample, we can infer the aromatic core structure (pyridinic versus pyrrolic nitrogen heterocycle) based on the presence of M(+.) (odd-electron) versus [M+H](+) (even-electron) ions. Specifically, nitrogen speciation can be determined from either a single positive-ion APPI spectrum or two ESI (positive- and negative-ion) spectra. APPI operates at comparatively higher temperature than ESI and also produces radical cations that may fragment before detection. However, APPI fragmentation of aromatics can be eliminated by judicious choice of instrumental parameters.     

Ion kinetic energy modulation for improved ion trapping in electrospray ionization fourier transform ion cyclotron resonance mass spectrometry

A new technique for manipulating the kinetic energy distribution of electrospray ions that arrive at a Fourier transform ion cyclotron resonance trapped-ion cell is presented. Narrow kinetic energy distributions can complicate the selection of appropriate trapping conditions for electrospray ions and introduce charge discrimination in resulting mass spectra. Modulation of the applied skimmer potential controllably broadens the kinetic energy distribution, which improves the reproducibility of acquired spectra and eliminates charge discrimination. Mass spectra of horse heart cytochrome c are presented to demonstrate the utility of the technique. For example, applied static skimmer potentials of 12 and 9 V yield charge state distributions ranging from [M+19H]⁺¹⁹ to [M+12H]⁺¹² and [M+15H]⁺¹⁵ to [M+7H]⁺⁷, respectively. A 12 ± 2 V, 100-Hz modulation of the skimmer potential yields an electrospray spectrum with charge states that range from [M+19H]⁺¹⁹ to [M+7H]⁺⁷, which is more representative of the source distribution.     

MICRA: a compact permanent magnet Fourier transform ion cyclotron resonance mass spectrometer

MICRA, a compact Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer is described. The amount of miniaturisation in this device, based on a 1.24 T permanent magnet, remains compatible with genuine FT-ICR performance and analytical power in the mass range 2-1000 m/z, with a mass resolving power of 73,000 at mass 132. A first application of the transportability is the repetitive coupling of MICRA with a large-scale source of IR photons, the free electron laser CLIO.     

Electron capture dissociation implementation progress in fourier transform ion cyclotron resonance mass spectrometry

Successful electron capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) applications to peptide and protein structural analysis have been enabled by constant progress in implementation of improved electron injection techniques. The rate of ECD product ion formation has been increased to match the liquid chromatography and capillary electrophoresis timescales, and ECD has been combined with infrared multiphoton dissociation in a single experimental configuration to provide simultaneous irradiation, fast switching between the two techniques, and good spatial overlap between ion, photon, and electron beams. Here we begin by describing advantages and disadvantages of the various existing electron injection techniques for ECD in FT-ICR MS. We next compare multiple-pass and single-pass ECD to provide better understanding of ECD efficiency at low and high negative cathode potentials. We introduce compressed hollow electron beam injection to optimize the overlap of ion, photon, and electron beams in the ICR ion trap. Finally, to overcome significant outgassing during operation of a powerful thermal cathode, we introduce nonthermal electron emitter-based electron injection. We describe the first results obtained with cold cathode ECD, and demonstrate a general way to obtain low-energy electrons in FT-ICR MS by use of multiple-pass ECD.     

Remeasurement at high resolving power in fourier transform ion cyclotron resonance mass spectrometry

The Fourier transform ion cyclotron resonance mass spectrometry remeasurement experiment is demonstrated and evaluated under high resolution conditions. Signal-to-noise enhancement is observed for isotopically resolved bovine insulin peaks at a resolution of ～ 31,000 (full width at half height). The experiment is sensitive to spacecharge effects and resultant changes in scan-to-scan signal-to-noise and resolution. Coulombic repulsion in the ion cloud during the high resolution remeasurement experiment can cause the cyclotron frequency to shift through the duration of the experiment, which results in broadened peak shapes when individual remeasurement spectra are coadded. By either reducing the number of ions in the cell or allowing the ion cloud to diffuse during the lifetime of the experiment, high resolution remeasurement spectra can be coadded without peak broadening or degradation of signal-to-noise ratio.     

Fullerenes from kerogen by laser ablation fourier transform ion cyclotron resonance mass spectrometry

Raw oil shale, kerogen (demineralized shale) and carbonaceous residues from kerogen pyrolysis in the range 350–700°C (at 50°C intervals) were studied by laser ablation Fourier transform ion cyclotron resonance mass spectrometry using the fundamental frequency of Nd: YAG laser (1064 nm). Normally, pyrolysis of the raw materials produces oil and the resulting residues have decreased hydrogen to carbon ratios and exhibit relative increases in aromatic carbons. Raw shale and kerogen give positive-ion spectra with mainly protonated species of m/z 100–400. Laser ablation positive-ion mass spectra of the pyrolysis products of the kerogen show the presence of C₆₀, C₇₀ and other fullerene ions with a distribution of higher mass fullerene ions up to m/z 4000. Using high laser powers (100–3000 MW cm^?2), the residue from pyrolysis at 350°C initially did not produce any fullerene ions (apart from traces of C₆₀ and C₇₀), but after continued ablation a cavity was formed in the target and a wide distribution of fullerene ions was obtained with subsequent laser pulses. Residues obtained from the pyrolysis of kerogen at 400–500°C produced fullerene ions at both low (4–200 kW cm^?2) and high laser powers. The 550°C pyrolysis residue gave only small amounts of C₆₀ and C₇₀ positive ions at low laser power whereas residues from the pyrolysis of kerogen above 550°C did not give fullerene ions over a wide range of laser powers. It is proposed from the above results that the changes in the aromatic nature of the kerogen residues with increasing pyrolysis temperature are directly related to the ease of fullerene formation. This is possibly due to the formation of large polycyclic aromatic systems at pyrolysis temperatures above 400°C, formed in the residues. It should be noted that the shale samples (raw or pyrolysed) did not generate fullerene ions under any of the conditions employed in these experiments.     

Experimental evaluation of a hyperbolic ion trap for fourier transform ion cyclotron resonance mass spectrometry

The Penning ion trap, consisting of hyperbolically curved electrodes arranged as an unbroken ring electrode capped by two end electrodes whose interelectrode axis lies along the direction of an applied static magnetic field, has long been used for single-ion trapping. More recently, it has been used in “parametric” mode for ion cyclotron resonance (lCR) detection of off-axis ions. In this article, we describe and test a Penning trap whose ring electrode has been cut into four equal quadrants for conventional dipolar ICR excitation (on one pair of opposed ring quadrants) and dipolar ICR detection (on the other pair). In direct comparisons to a cubic trap, the present hyperbolic trap offers somewhat improved ICR mass spectral peak shape, higher mass resolving power, and comparable frequency shift as a function of trapping voltage. Mass measurement accuracy over a wide mass range is improved twofold and mass discrimination is somewhat worse than for a cubic trap. The relative advantages of parametric, dipolar, and quadrupole modes are briefly discussed in comparison to screened and unscreened cubic traps.     

Initial implementation of an electrodynamic ion funnel with fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has become a widely used method to study biopolymers. The method, in combination with an electrospray ionization (ESI) source has demonstrated the highest resolution and accuracy yet achieved for characterization of biomolecules and their noncovalent complexes. The most common design for the ESI interface includes a heated capillary inlet followed by a skimmer having a small orifice to limit gas conductance between a higher pressure (1 to 5 torr) source region and the lower pressure ion guide. The ion losses in the capillary-skimmer interface are large (estimated to be more than 90%) and thus reduce achievable sensitivity. In this work, we report on the initial implementation of a newly developed electrodynamic ion funnel in a 3.5 tesla ESI-FTICR mass spectrometer. The initial results show dramatically improved ion transmission as compared to the conventional capillary-skimmer arrangement. An estimated detection limit of 30 zeptomoles (approximately 18,000 molecules) has been achieved for the analysis of the proteins with molecular weights ranging from 8 to 20 kDa.     

Electrospray Lonization Fourier Transform Ion Cyclotron Resonance Mass Spectrometric Study on Sodium Azide Cluster Ions

Sodium azide has rarely been studied in gas phase or in the form of cluster ions and as a model of solid energetic substances and inorganic azide salt was ionized by electrospray ionization (ESI) and studied by high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) systematically. This paper highlights the effects of experimental conditions on the formation of salt cluster and the collision activation dissociation pathways of cluster ions to develop a microscopic understanding of inorganic azide salt clusters.     

High performance fourier transform ion cyclotron resonance mass spectrometry via a single trap electrode

An open-ended cylindrical cell with a single annular trap electrode located at the center of the excitation and detection region is demonstrated for Fourier transform ion cyclotron resonance mass spectrometry. A trapping well is created by applying a static potential to the trap electrode of polarity opposite the charge of the ion to be trapped, after which conventional dipolar excitation and detection are performed. The annular trap electrode is axially narrow to allow the creation of a potential well without excessively shielding excitation and detection. Trapping is limited to the region of homogeneous excitation at the cell centerline without the use of capacitive coupling. Perfluorotributylamine excitation profiles demonstrate negligible axial ejection throughout the entire excitation voltage range even at an effective centerline potential of only ?0.009 V. High mass resolving power in the single-trap electrode cell is demonstrated by achievement of mass resolving power of 1.45 × 10⁶ for benzene during an experiment in which ions created in a high pressure source cubic cell are transferred to the low pressure analyzer single-trap electrode cell for detection. Such high performance is attributed to the negligible radius dependent radial electric field for ions cooled to the center of the potential well and accelerated to less than 60% of the cell radius. An important distinction of the single-trap electrode geometry from all previous open and closed cell arrangements is exhibition of combined gated and accumulated trapping. Because there is no potential barrier, all ions penetrate into the trapping region regardless of their translational energy as in gated trapping, but additional ions may accumulate over time, as in accumulated trapping. Ions of low translational kinetic energy are demonstrated to be preferentially trapped in the single-trap electrode cell. In a further demonstration of the minimal radial electric field of the single-trap electrode cell, positive voltages can be applied to the annular trap electrode as well as the source cell trap electrode to achieve highly efficient transfer of ions between cells.