Linear excitation and detection in Fourier transform ion cyclotron resonance mass spectrometry

Gas Analysis by Electron Ionization Combined with Chemical Ionization in a Compact FTICR Mass Spectrometer

In this Article, a compact Fourier transform ion cyclotron resonance (FTICR) mass spectrometer based on a permanent magnet is presented. This instrument has been developed for real-time analysis of gas emissions. The instrument is well-suited to industrial applications or analysis of toxic and complex samples where the concentrations can vary rapidly on a wide range. The novelty of this instrument is the ability to use either electron ionization (EI) or chemical ionization (CI) individually or both of them alternatively. Also in CI mode, different precursor ions can be used alternatively. Volatile organic compounds (VOCs) from the ppb level to very high concentrations (% level) can be detected by CI or EI. The magnet is composed of three Halbach arrays, and the nominal field achieved is 1.5 T. The ICR cell is a 3 cm side length cubic cell. The mass range is 12-200 u with a broad band detection. The mass accuracy of 0.005 u and the resolving power allow the separation of isobaric ions such as C₃H₈⁺ and CO₂⁺. Gas introduction via controlled gas pulses, electron ionization, ion-molecule reactions, ion selection, and detection are all performed in the ICR cell. The potential of the instrument will be illustrated by an analysis of a gas mixture containing trace components at ppm level (VOCs) and components in the 0.5-100% range (N₂, alkanes, and CO₂).

Towards analytically useful two-dimensional Fourier transform ion cyclotron resonance mass spectrometry

Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) achieves high resolution and mass accuracy, allowing the identification of the raw chemical formulae of ions in complex samples. Using ion isolation and fragmentation (MS/MS), we can obtain more structural information, but MS/MS is time- and sample-consuming because each ion must be isolated before fragmentation. In 1987, Pfändler et al. proposed an experiment for 2D FT-ICR MS in order to fragment ions without isolating them and to visualize the fragmentations of complex samples in a single 2D mass spectrum, like 2D NMR spectroscopy. Because of limitations of electronics and computers, few studies have been conducted with this technique. The improvement of modern computers and the use of digital electronics for FT-ICR hardware now make it possible to acquire 2D mass spectra over a broad mass range. The original experiments used in-cell collision-induced dissociation, which caused a loss of resolution. Gas-free fragmentation modes such as infrared multiphoton dissociation and electron capture dissociation allow one to measure high-resolution 2D mass spectra. Consequently, there is renewed interest to develop 2D FT-ICR MS into an efficient analytical method. Improvements introduced in 2D NMR spectroscopy can also be transposed to 2D FT-ICR MS. We describe the history of 2D FT-ICR MS, introduce recent improvements, and present analytical applications to map the fragmentation of peptides. Finally, we provide a glossary which defines a few keywords for the 2D FT-ICR MS field.

High mass selectivity for top-down proteomics by application of SWIFT technology

Stored waveform inverse Fourier transform (SWIFT) technology has been implemented on a commercial Fourier transform ICR mass spectrometer. Complex ejection/isolation waveforms can be generated on an arbitrary waveform generator (AWG) and applied to the instrument by use of a high speed analog switch. High mass selectivity and subsequent electron capture dissociation (ECD) of the SWIFT isolated ions has been demonstrated with analysis of intact Bovine histone H4. A mass selectivity about 0.1 m/z unit for isolation of the 18+ charge state ions was achieved. Adaptation of SWIFT on the commercial instrument provides an enhanced capability for characterizing intact proteins by top-down analysis.

Impact of ion cloud densities on the measurement of relative ion abundances in Fourier transform ion cyclotron resonance mass spectrometry: experimental observations of coulombically induced cyclotron radius perturbations and ion cloud dephasing rates

Fundamental research into the quantitative properties of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has yielded interesting observations, especially in terms of factors affecting the accuracy of relative ion abundances. However, most of the previous discussions have focused on theoretical systems, or systems of limited scope. In this paper, we document ion motion attributes of a 30 spectra (six samples, five replicates each) system previously established as linear over two orders of magnitude. Observed behaviors include the perturbation of one charged species (cyclosporin A, CsA) of low ion density to a cyclotron orbit of greater radius than that of an almost identical, but slightly mass-separated species (CsG) with a higher ion density. This radial perturbation is attributed to the coulombic repulsion between the two ion clouds as they interact during the excitation process, as previously proposed by Uechi and Dunbar. Magnitudes of the perturbation were confirmed by making cyclotron radii determinations utilizing the ratio of the third-to-first harmonics for the charged species of interest. It was found that these radial differences can account for as much as a 55% signal bias in favor of CsA for a single sample and a >20% positive bias in the slope of the regressed data set. A second behavior noted that also contributes to the potential inaccuracy of relative ion abundance measurements is the difference in signal decay rates for CsA and CsG. Damping constants and initial time domain signal amplitudes were evaluated using segmented Fourier transforms. Discrepancies in decay rates were not expected from two species that have essentially identical collisional cross-sections. However, it has been observed that the faster decay rates are observed by the species of lower ion cloud density. We have attributed this differential signal decay phenomenon to the rates of loss of phase coherence for the two ion clouds. Previously, others have reported that less dense ion clouds are more susceptible to shearing and other disruptive forces during the course of their excited cyclotron motion. Our experimental evidence supports that it is the loss of cloud coherence that accounts for the signal loss over time, with the less dense cloud de-phasing more quickly. As the ion populations of the two investigated species near equivalence, so do their time constants.

Central ring electrode for trapping and excitation/detection in Fourier transform ion cyclotron resonance mass spectrometry

The use of a central trapping ring electrode for Fourier transform ion cyclotron resonance (FTICR) mass spectrometry is demonstrated. Ions are trapped with an oppositely biased static potential superimposed on both the excite and detect electrodes and maintained throughout the experiment, including the application of a dipolar rf excite waveform and the image current ion detection event. The use of a central trapping electrode for FTICR coupled with an open cell design retains the advantages of high ion throughput and gas conductance, while simplifying the electrode geometry and reducing the overall dimensions of the cell. This allows the central trapping electrode to be of utility in volume-limited vacuum chambers including FTICR instrument miniaturization. Presented here are the preliminary experimental results using the central trapping electrode as an FTICR cell in which the excitation and detection electrodes also create a trapping depression to constrain the z-axis motion of the ions. The cell overcomes the principle limitation of an earlier single trapping electrode design by producing a 91% effective potential well depth compared to 19% for the single trapping electrode and 33% for standard open cells. This allows the central trapping electrode configuration to achieve an order of magnitude improvement in ion capacity compared to more conventional open cell designs.

A novel high-performance fourier transform ion cyclotron resonance cell for improved biopolymer characterization

A new trapped ion cell design for use with Fourier transform ion cyclotron resonance mass spectrometry is described. The design employs 15 cylindrical ring electrodes to generate trapping potential wells and 32 separately assignable rod electrodes for excitation and detection. The rod electrodes are positioned internal to the ring electrodes and provide excitation fields that are thereby linearized along the magnetic field over the entire trapped ion volume. The new design also affords flexibility in the shaping of the trapping field using the 15 ring electrodes. Many different trapping well shapes can be generated by applying different voltages to the individual ring electrodes, ranging from quadratic to linearly ramped along the magnetic field axis, to a shape that is nearly flat over the entire trap volume, but rises very steeply near the ends of the trap. This feature should be useful for trapping larger ion populations and extension of the useful range of ion manipulation and dissociation experiments since the number of stages of ion manipulation or dissociation is limited in practice by the initial trapped ion population size. Predicted trapping well shapes for two different ring electrode configurations are presented, and these and several other possible configurations are discussed, as are the predicted excitation fields based on the use of rod electrodes internal to the trapping ring electrodes. Initial results are presented from an implementation of the design using a 3.5 T superconducting magnet. It was found that ions can be successfully trapped and detected with this cell design and that selected ion accumulation can be performed with the utilization of four rods for quadrupolar excitation. The initial results presented here illustrate the feasibility of this cell design and demonstrate differences in observed performance based upon different trapping well shapes.

Electrospray ionization fourier transform mass spectrometry quantification of enkephalin using an internal standard

Fourier transform mass spectrometry (FTMS), long-known for its capabilities in structural characterization of molecules, is an emerging tool in quantification, and quantification methods using external and internal standards with electrospray ionization (ESI) FTMS have recently been demonstrated. Here, commercial ESI-FTMS is used to quantify the opioid pentapeptide methionine enkephalin using an internal standard. Linear working curves over three orders of magnitude are obtained using the internal standard, an improvement of one order of magnitude over the previous external standard ESI-FTMS quantification method for enkephalins. Low coefficients of variation (generally <6%) are observed, and inter-day and intra-day assays are compared and found to possess similar linearity and precision. The high mass accuracy advantage of FTMS can be exploited to give molecular specificity. Efforts to improve mass accuracy using internal mass calibration generally provide mass accuracies within 2.5 ppm.

Simulation for internal energy deposition in sustained off-resonance irradiation collisional activation using a monte carlo method

This paper proposes a novel computational approach employing a Monte Carlo method, aimed at an improved understanding of the dynamics and energetics of activated ions in sustained off-resonance irradiation collisionally activated dissociation (SORI-CAD) experimental events of Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). In SORI-CAD events, internal energies of activated ions are complicatedly associated with their motion undergoing off-resonance excitation (i.e. alternate accelerations and decelerations) and inherently stochastic ion-neutral collisions. Several types of pseudo-random generators were adapted to probability density functions (PDFs) which characterize the ion-neutral collision process. Simulated ion trajectories involve the realistic feature of pressurized SORI-CAD events, such as a collisional damping and those which have not been illustrated in conventional analytical approaches. The proposed method can simulate the time-varying translational and internal energies of activated ions. The present result suggests that the internal energy of a SORI-activated ion should be inversely proportional to the cube of the SORI excitation frequency offset. Copyright 1999 John Wiley & Sons, Ltd.

Quantification of biomolecules by external electrospray ionization Fourier transform mass spectrometry

Fourier transform mass spectrometry (FTMS) is well-known for its capabilities in structural characterization of molecules. Recent developments in radio frequency excitation, linearized trapping, and accumulation of ions generated from external sources have improved the potential of FTMS for quantitative analysis. Here, a commercial external electrospray ionization FTMS, employing a linearized ion trap (the Infinity Cell) and an ion accumulation procedure in which ions are deflected off-axis and injected into the trap, is evaluated as an analytical method for quantifying amino acids, peptides, and proteins. Linear response over approximately 2-3 orders of magnitude is observed for singly-charged ions with low coefficients of variation (generally < 10%), and the calibration curves generated can be used to quantify structurally similar analytes with < 4% relative error, as shown here for quantification of leucine enkephalin from curves generated by methionine enkephalin. Similar precision is obtained for multiply-charged lysozyme, but over only 1.5 orders of magnitude. Some m/z discrimination is observed as a function of trap accumulation potential for a two-component cytochrome c/lysozyme mixture. The results are promising because they suggest that quantification using liquid chromatography coupled to electrospray FTMS is possible.

Fourier transform mass spectrometry-advancing years (1992-mid. 1996)

This article is one of a series of Fourier transform mass spectrometry (FTMS) reviews that has appeared in this journal at ca. 3-4 year intervals. A comprehensive review of the recent theoretical developments, instrumental developments, electrospray ionization (ESI), and MALDI is given. Ion dissociation techniques are also discussed because of their contributions to gaining insight into chemical structure. Special sections have been devoted to discussing the emerging fields of surface analysis, polymer analysis, Buckminsterfullerenes (buckyballs), and hydrogen/deuterium exchange studies. This review, although not all-inclusive, is intended to be a starting point for those wishing to learn more about the current status of FTMS, and also as a representative cross-section of the literature for those familiar with the technique. © 1997 John Wiley & Sons, Inc.