Internal glow discharge-fourier transform ion cyclotron resonance mass spectrometry

A glow discharge (GD) ion source has been developed to work within the high magnetic field of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Characterization of this source revealed that the optimum operating voltage, pressure, and current are significantly lower than those for normal glow discharges. The sputter rate was lowered to 1/30th of that found with a normal glow discharge source operated external to the high magnetic field region. Operation of the GD source closer to the FTICR analyzer cell than with previous experimental designs resulted in improved ion transport efficiency. Preliminary results from this internal GD source have established detection limits in the low parts per million range for selected elemental species.     

Improved ion extraction from a linear octopole ion trap: SIMION analysis and experimental demonstration

Externally generated ions are accumulated in a linear octopole ion trap before injection into our 9.4 T Fourier transform ion cyclotron resonance (FT-ICR) mass analyzer. Such instrumental configuration has previously been shown to provide improved sensitivity, scan rate, and duty cycle relative to accumulated trapping in the ICR cell. However, inefficient ion ejection from the octopole currently limits both detection limit and scan rate. SIMION 7.0 analysis predicts that a dc axial electric field inside the linear octopole ion trap expedites and synchronizes the efficient extraction of the octopole-accumulated ions. Further SIMION analysis optimizes the ion ejection properties of each of three electrode configurations designed to produce a near-linear axial potential gradient. More efficient extraction and transfer of accumulated ions spanning a wide m/z range promises to reduce detection limit and increase front-end sampling rate (e.g., to increase front-end resolution for separation techniques coupled with FT-ICR mass analysis). Addition of the axial field improves experimental signal-to-noise ratio by more than an order of magnitude.     

A high-resolution scanning microprobe matrix-assisted laser desorption/ionization ion source for imaging analysis on an ion trap/Fourier transform ion cyclotron resonance mass spectrometer

A new scanning microprobe matrix-assisted laser desorption/ionization (SMALDI) ion source for high spatial resolution has been developed for linear ion trap and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The source is fully compatible with commercial ion trap flanges (such as the LTQ series, Thermo Fisher Scientific). The source is designed for atmospheric pressure (AP) operation but is also suitable for mid-pressure operation. The AP mode is especially useful for investigating volatile compounds. The source can be interchanged with other ion sources within a minute when operated in the AP mode. Combining high-lateral resolution MALDI imaging with high mass resolution and high mass accuracy mass spectrometry, available in the FT-ICR mode, provides a new quality of analytical information, e.g. from biological samples. First results obtained with the new ion source demonstrate a maximum lateral resolution of 0.6 by 0.5 microm. Depending on the limit of detection of the chosen mass analyzer, however, the size of the focus had to be enlarged to a diameter of up to 8 microm in the FT-ICR mode, in order to create enough ions for detection. Mass spectra acquired for analytical imaging were obtained from single laser pulses per pixel in all the experiments. This mode allows us to investigate biological thin sections with desorption focus diameters in the micrometer range, known to cause complete evaporation of material under the laser focus with a very limited number of laser pulses. As a first example, peptide samples deposited in microstructures were investigated with the new setup. A high quality and validity of the acquired images were obtained in the ion trap mode due to the low limit of detection. High mass resolution and accuracy but poorer image quality were obtained in the ICR mode due to the lower detection sensitivity of the ICR detector.     

A novel fourier transform ion cyclotron resonance mass spectrometer with improved ion trapping and detection capabilities

A novel Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been developed for improved biomolecule analysis. A flared metal capillary and an electrodynamic ion funnel were installed in the source region of the instrument for improved ion transmission. The transfer quadrupole is divided into 19 segments, with the capacity for independent control of DC voltage biases for each segment. Restrained ion population transfer (RIPT) is used to transfer ions from the ion accumulation region to the ICR cell. The RIPT ion guide reduces mass discrimination that occurs as a result of time-of-flight effects associated with gated trapping. Increasing the number of applied DC bias voltages from 8 to 18 increases the number of ions that are effectively trapped in the ICR cell. The RIPT ion guide with a novel voltage profile applied during ion transfer provides a 3- to 4-fold increase in the number of ions that are trapped in the ICR cell compared with gated trapping for the same ion accumulation time period. A novel ICR cell was incorporated in the instrument to reduce radial electric field variation for ions with different z-axis oscillation amplitudes. With the ICR cell, called trapping ring electrode cell (TREC), we can tailor the shape of the trapping electric fields to reduce dephasing of coherent cyclotron motion of an excited ion packet. With TREC, nearly an order of magnitude increase in sensitivity is observed. The performance of the instrument with the combination of RIPT, TREC, flared inlet, and ion funnel is presented.     

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 trajectories in an electrostatic ion guide for external ion source fourier transform ion cyclotron resonance mass spectrometry

An electrostatic ion guide (EIG) that consists of concentric cylinder and central wire electrodes can transport ions efficiently from an external ion source to an ion cyclotron resonance (ICR) ion trap for mass analysis, with several advantages over current injection methods. Because the electrostatic force of the EIG captures ions in a stable orbit about the wire electrode, ions with initially divergent trajectories may be redirected toward the ICR ion trap for improved ion transmission efficiency. SIMION trajectory calculations (ion kinetic energy, 1–200 eV; elevation angle, 0.30 °; azimuthal angle, 0.360°) predict that ions of m/z 1000 may be transmitted through a strong (0.01 → 3.0-T) magnetic field gradient. Judicious choice of ion source position and EIG potential minimizes the spread in ion axial kinetic energy at the ICR ion trap. Advantages of the EIG include large acceptance angle, even for ions that have large initial kinetic energy and large radial displacement with respect to the central z-axis, low ion extraction voltage (5–20 V), and efficient trapping because ions need not be accelerated to high velocity to pass through the magnetic field gradient.     

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.     

Improved ion optics for introduction of ions into a 9.4‐T Fourier transform ion cyclotron resonance mass spectrometer

Enhancements to the ion source and transfer optics of our 9.4 T Fourier transform ion cyclotron resonance (ICR) mass spectrometer have resulted in improved ion transmission efficiency for more sensitive mass measurement of complex mixtures at the MS and MS/MS levels. The tube lens/skimmer has been replaced by a dual ion funnel and the following octopole by a quadrupole for reduced ion cloud radial expansion before transmission into a mass‐selective quadrupole. The number of ions that reach the ICR cell is increased by an order of magnitude for the funnel/quadrupole relative to the tube lens/skimmer/octopole. Copyright © 2015 John Wiley & Sons, Ltd.     

A glow discharge ion source with fourier transform ion cyclotron resonance mass spectrometric detection

A glow discharge (CD) ion source has been coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using a four-element electrostatic lens to accelerate and focus ions generated external to the instrument’s high magnetic field into its analyzer cell. Like other CD mass spectrometers, GD-FT-ICR can provide a quantitative measure of bulk analyte concentration with good precision and accuracy. Although detection limits currently attainable are several orders of magnitude higher than the commercially available magnetic sector-based instrument, CD-FT-ICR holds promise for ultrahigh resolving power elemental mass analysis. Several schemes are proposed to lower the detection limits of the technique while still providing high enough resolution to resolve isobaric interferences.     

Fourier transform ion cyclotron resonance mass spectrometry with NanoLC/microelectrospray ionization and matrix-assisted laser desorption/ionization: analytical performance in peptide mass fingerprint analysis

Protein identifications by peptide mass fingerprint analyses with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) were performed using microelectrospray ionization coupled to nano liquid chromatography (NanoLC), as well as using matrix-assisted laser desorption/ionization (MALDI). Tryptic digests of bovine serum albumin (BSA), diluted down to femtomole quantities, have been desalted by fast NanoLC under isocratic elution conditions as the high resolving power of FT-ICR MS enables peptides to be separated during the mass analysis stage of the experiment. The high mass accuracy achieved with FT-ICR MS (a few ppm with external calibration) facilitated unambiguous protein identification from protein database searches, even when only a few tryptic peptides of a protein were detected. Statistical confidence in the database search results was further improved by internal calibration due to increased mass accuracy. Matrix-assisted laser desorption/ionization and micro electrospray ionization (ESI) FT-ICR showed good mass accuracies in the low femtomole range, yet a better sensitivity was observed with MALDI. However, in higher femtomole ranges slightly lower mass accuracies were observed with MALDI FT-ICR than with microESI FT-ICR due to scan-to-scan variations of the ion population in the ICR cell. Database search results and protein sequence coverage results from NanoLC FT-ICR MS and MALDI FT-ICR MS, as well as the effect of mass accuracy on protein identification for the peptide mass fingerprint analysis are evaluated.     

A minimum thickness gate valve with integrated ion optics for mass spectrometry

A minimum thickness gate valve design for mass spectrometry is described in detail. The ion optics required to transmit ions from the source to the ICR cell are integrated into the design to minimize fringe field effects on the ions as they travel through the gate valve. The total thickness of the complete gate valve assembly is 1.03 in. (26.2 mm) with a maximum fringe field distance of 0.065 in. (1.7 mm). The gate valve is able to maintain a vacuum of <10(-10) mbar at the ICR cell when the source is vented to atmosphere and the estimated ion transfer efficiency is >95%.     

Protein identification via surface-induced dissociation in an FT-ICR mass spectrometer and a patchwork sequencing approach

Surface-induced dissociation (SID) and collision-induced dissociation (CID) are ion activation techniques based on energetic collisions with a surface or gas molecule, respectively. One noticeable difference between CID and SID is that SID does not require a collision gas for ion activation and is, therefore, directly compatible with the high vacuum requirement of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers. Eliminating the introduction of collision gas into the ICR cell for collisional activation dramatically shortens the acquisition time for MS/MS experiments, suggesting that SID could be utilized for high-throughput MS/MS studies in FT-ICR MS. We demonstrate for the first time the utility of SID combined with FT-ICR MS for protein identification. Tryptic digests of standard proteins were analyzed using a hybrid 6-tesla FT-ICR mass spectrometer with SID and CID capabilities. SID spectra of mass-selected singly and doubly charged peptides were obtained using a diamond-coated target mounted at the rear trapping plate of the ICR cell. The broad internal energy distribution deposited into the precursor ion following collision with the diamond surface allowed a variety of fragmentation channels to be accessed by SID. Composition and sequence qualifiers produced by SID of tryptic peptides were used to improve the statistical significance of database searches. Protein identification MASCOT scores obtained using SID were comparable or better than scores obtained using sustained off-resonance irradiation collision-induced dissociation (SORI-CID), the conventional ion activation technique in FT-ICR MS.     

Fourier transform ion cyclotron resonance detection of multiphoton ionization spectroscopy

Fourier transform ion cyclotron resonance (FT-ICR) detection was tested for resonanceenhanced multiphoton ionization (REMPI) spectroscopy. The (2+1) REMPI spectra of acetaldehyde were obtained in the wavelength range 364–354 nm via a two-photon resonant 3s ← n Rydberg transition. The space-charge effects on the REMPI spectra were examined in the vicinity of the 0 ₀ ⁰ transition. The trapping efficiency measurement shows that all the ions produced from REMPI dissociation processes are arrested in the ion cyclotron resonance cell even in the presence of space-charge interactions. Axial kinetic energy release distributions of ions were extracted from the trapping efficiency data obtained under a new space-charge-free condition. FT-ICR peak heights were measured as a function of pressure at different laser powers, magnetic field strengths, and ion excitation methods to test for the detection linearity. The FT-ICR detection responds linearly to the number of ions in a low pressure limit. The product branching ratio was measured by using various ion excitation methods and was compared with the previous quadrupole mass spectrometric study. FT-ICR detection yields the mass-selected REMPI spectra and the product branching ratio in the absence of kinetic shifts.     

Fourier transform ion cyclotron resonance spectroscopy

An ion cyclotron resonance (ICR) absorption spectrum has been obtained by exciting an ICR spectral segment with a fixed-frequency electric field pulse, followed by broad-band detection, digitization of the (time-domain) transient response, and digital Fourier transformation to produce the (frequency-domain) absorption spectrum. For a given signal-to-noise ratio and resolution, the FT-ICR method generates a spectrum in a time which is two orders of magnitude shorter than that required in conventional slow-sweep ICR detection. In the present example, a signal-to-noise ratio of 8:1 and a mass resolution of about 0.005 amu for CH₄⁺ (from CH₄ at a pressure of 8 X 10^?7 torr) have been achieved, using a single data acquisition period of 25.6 msec.     

Fourier transform ion cyclotron resonance mass spectrometry in a high homogeneity 25 tesla resistive magnet

Many performance parameters of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry improve dramatically with increasing magnetic field. Our prior results from a 20 tesla resistive magnet showed that performance was limited by the large spatial inhomogeneity in spite of the high field. In this paper, we compare matrix-assisted laser desorption/ionization (MALDI) mass spectra at the same magnetic field for two resistive magnets with different field spatial homogeneity. In addition, we report MALDI spectra at 25 tesla—the highest magnetic field for FT-ICR to date. The first broadband FT-ICR mass spectrum [poly(ethylene glycol) 2000] from a resistive magnet is accurately fitted by the standard ICR mass calibration function.     

Electron capture and transfer dissociation: Peptide structure analysis at different ion internal energy levels

We decoupled electron-transfer dissociation (ETD) and collision-induced dissociation of charge-reduced species (CRCID) events to probe the lifetimes of intermediate radical species in ETD-based ion trap tandem mass spectrometry of peptides. Short-lived intermediates formed upon electron transfer require less energy for product ion formation and appear in regular ETD mass spectra, whereas long-lived intermediates require additional vibrational energy and yield product ions as a function of CRCID amplitude. The observed dependencies complement the results obtained by double-resonance electron-capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and ECD in a cryogenic ICR trap. Compared with ECD FT-ICR MS, ion trap MS offers lower precursor ion internal energy conditions, leading to more abundant charge-reduced radical intermediates and larger variation of product ion abundance as a function of vibrational post-activation amplitude. In many cases decoupled CRCID after ETD exhibits abundant radical c-type and even-electron z-type ions, in striking contrast to predominantly even-electron c-type and radical z-type ions in ECD FT-ICR MS and especially activated ion-ECD, thus providing a new insight into the fundamentals of ECD/ETD.     

Automatic gain control in mass spectrometry using a jet disrupter electrode in an electrodynamic ion funnel

We report on the use of a jet disrupter electrode in an electrodynamic ion funnel as an electronic valve to regulate the intensity of the ion beam transmitted through the interface of a mass spectrometer in order to perform automatic gain control (AGC). The ion flux is determined by either directly detecting the ion current on the conductance limiting orifice of the ion funnel or using a short mass spectrometry acquisition. Based upon the ion flux intensity, the voltage of the jet disrupter is adjusted to alter the transmission efficiency of the ion funnel to provide a desired ion population to the mass analyzer. Ion beam regulation by an ion funnel is shown to provide control to within a few percent of a targeted ion intensity or abundance. The utility of ion funnel AGC was evaluated using a protein tryptic digest analyzed with liquid chromatography Fourier transform ion cyclotron resonance (LC-FTICR) mass spectrometry. The ion population in the ICR cell was accurately controlled to selected levels, which improved data quality and provided better mass measurement accuracy.     

Fourier transform ion cyclotron resonance mass spectrometer with coaxial multi‐electrode cell (‘O‐trap’): first experimental demonstration

The conceptual design of the O‐trap Fourier transform ion cyclotron resonance (FT‐ICR) cell addresses the speed of analysis issue in FT‐ICR mass spectrometry. The concept of the O‐trap includes separating the functions of ion excitation and detection between two different FT‐ICR cell compartments. The detection compartment of the O‐trap implements additional internal coaxial electrodes around which ions with excited cyclotron motion revolve. The expected benefits are higher resolving power and the lesser effect of the space charge. In this work we present the first experimental demonstration of the O‐trap cell and its features, including the high ion transfer efficiency between two distinct compartments of an ICR cell after excitation of the coherent cyclotron motion. We demonstrate that utilization of the multiple‐electrode detection in the O‐trap provides mass resolving power enhancement (achieved over a certain time) equal to the order of the frequency multiplication. In an O‐trap installed in a 5 T desk‐top cryogen‐free superconducting magnet, the resolving power of R = 80 000 was achieved for bradykinin [M + 2H]²⁺ (m/z 531; equivalent to 100 000 when recalculated for m/z 400) in 0.2 s analysis time (transient length), and R = 300 000 at m/z 531 for a 1 s transient. In both cases, detection on the third multiple of the cyclotron frequency was implemented. In terms of the acquisition speed at fixed resolving power, such performance is equivalent to conventional FT‐ICR detection using a 15 T magnet. Copyright © 2010 John Wiley & Sons, Ltd.     

Simplified electron ejection in Fourier transform ion cyclotron resonance mass spectrometry by suspended trapping

Suspended trapping is used to eject electrons in negative-ion Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric experiments. In contrast to electron ejection by resonant excitation of the trapping motion, suspended trapping involves allowing the electrons to escape along the z-axis (perpendicular to the trap plates) while the trapping potential is briefly removed. The duration of this event is sufficiently short (～10 μs) so that ion losses are negligible; the overall effect is that of a ‘high-pass mass filter’. Suspended trapping is simpler to implement and more generally applicable to various cell geometries than resonant electron ejection. The effectiveness of the suspended trapping technique is not compromised by the anharmonicity of the potential well in ‘elongated’ ICR traps, but depends simply on the time it takes the electrons to escape the cell. Finally, a small, positive offset potential (～+0.25 V) applied to the trap plates during the suspended trapping event increases the efficiency of the ejection.     

A wide-angle secondary ion probe for organic ion imaging

A secondary ion source has been developed for an organic ion microprobe capable of imaging samples up to 2 em in diameter. The source uses a focused 5 keY Cs⁺ ion beam which is rastered across the sample surface, and secondary ions from each point on the sample are collected and formed into a low energy beam to be analyzed by a quadrupole mass filter. Dynamic emittance matching is employed to deflect ions from off-axis points on the sample back onto the mass analyzer axis. Rastering and dynamic emittance matching are rapidly controlled by assembly language programs using an IBM/AT (80286) type computer. A low energy ion monitor was used to tune and evaluate the secondary ion source by providing a magnified cross-sectional image of the ion beam at the source exit aperture. A well-focused and centered secondary ion beam was obtained from each point on the sample, indicating that large-scale dynamic emittance matching with high collection efficiency is possible. Mass resolved images of grids and glycerol samples are shown to demonstrate the performance of the integrated secondary ion source mass analyzer and control system.