ICR spectrometry — a review of new developments in theory, instrumentation, and applications. I. 1983–1986

FT-ICR mass spectrometry: Superconducting magnet, external ion source, ion-molecule reactions, and ion-ion traps

The world of Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry has witnessed, especially in the last 30 years significant advances in many fields of science, such as electronics, magnets, new ICR cell designs, developed ICR event sequences, modern external ionization sources, and linear ion beam guides, as well as modern vacuum technology. In this review, a brief account is given focusing especially on the studies performed in Wanczek's group and ICR research laboratory at the University of Bremen. An FT-ICR mass spectrometer has been developed with a high magnetic field superconducting magnet, operating at 4.7 T. At this magnetic field, a trapping time of 13.5 h was obtained with 30% efficiency. For the tetrachloromethane molecular ion, m/z 166, a mass-resolving power m/Δm = 1.5 × 10⁶ was measured at a pressure of 2 × 10^-8  Torr. The transition from magnet sweep to frequency sweep and the application of Fourier-transform has greatly enhanced the ICR technology. External ion sources were invented and differential pumping schemes were developed for enabling ultrahigh vacuum condition for ICR detection, while guiding ions at relatively higher pressures, during their flight to the ICR cell. With the external ion source, a time-of-flight ICR tandem instrument is built. A method to measure the ion flight time and to trap the ions in the ICR cell is described. Many ICR cell characteristics such as z-axis ion ejection and coupling of radial and axial ion motions in a superposed homogeneous magnetic and inhomogeneous trapping electric field were extensively studied. Gas-phase ion-molecule reactions of several reactive inorganic compounds with a focus on phosphorous and sulfur as well as silicon chemistry were also studied in great detail. The gas-phase ion chemistry of several trifluoromethyl-reagents such as trifluoromethyltrimethylsilane and tris(trifluoromethyl)phosphine were also investigated in ICR. Dual polarities multisegmented ICR cells were invented and deeply characterized. Sophisticated ICR pulse event programs were developed to enable long-range ion-ion interactions between simultaneously trapped positive and negative ions.

Fourier transform ion cyclotron resonance (FT ICR) mass spectrometry: Theory and simulations

Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer offers highest resolving power and mass accuracy among all types of mass spectrometers. Its unique analytical characteristics made FT ICR important tool for proteomics, metabolomics, petroleomics, and investigation of complex mixtures. Signal acquisition in FT ICR MS takes long time (up to minutes). During this time ion-ion interaction considerably affects ion motion and result in decreasing of the resolving power. Understanding of those effects required complicated theory and supercomputer simulations but culminated in the invention of the ion trap with dynamic harmonization which demonstrated the highest resolving power ever achieved. In this review we summarize latest achievements in theory and simulation of FT ICR mass spectrometers.

Definitions of terms relating to mass spectrometry (IUPAC Recommendations 2013)

This document contains recommendations for terminology in mass spectrometry. Development of standard terms dates back to 1974 when the IUPAC Commission on Analytical Nomenclature issued recommendations on mass spectrometry terms and definitions. In 1978, the IUPAC Commission on Molecular Structure and Spectroscopy updated and extended the recommendations and made further recommendations regarding symbols, acronyms, and abbreviations. The IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy’s Subcommittee on Mass Spectroscopy revised the recommended terms in 1991 and appended terms relating to vacuum technology. Some additional terms related to tandem mass spectrometry were added in 1993 and accelerator mass spectrometry in 1994. Owing to the rapid expansion of the field in the intervening years, particularly in mass spectrometry of biomolecules, a further revision of the recommendations has become necessary. This document contains a comprehensive revision of mass spectrometry terminology that represents the current consensus of the mass spectrometry community.

FT-ICR mass spectrometry in the drug discovery process

The high mass accuracy and resolution of Fourier transform (FT)-ion cyclotron resonance (ICR) mass spectrometry are making it an increasingly useful tool in drug discovery and development. The basics of FT-ICR are described here, including modern ion sources and fragmentation methods. Although FT-ICR is not a high-throughput method in the traditional sense, previously difficult and complex problems are being efficiently approached using steadily improving instruments and magnets. Applications are surveyed in fields such as proteomics, metabonomics, natural product analysis and non-covalent complexes.

Quantum chemical study of the electronic structure of NiCH2+ in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.

Benzocyclopropenyl Anion: A Stable 8pi-Electron Species

The conjugate base of benzocyclopropene has been generated in the gas phase. Its reactivity and thermodynamic stability were explored. The measured acidity is DeltaH degrees (acid)(benzocyclopropene) = 386 +/- 3 kcal/mol, and the electron affinity of benzocyclopropenyl radical is 0.51 eV < EA < 1.11 eV. Ab initio calculations satisfactorily reproduce the experimental results and provide additional insights. Benzocyclopropene is found to be 34.5 kcal/mol more acidic than the allylic position of cyclopropene and only 4 +/- 3 kcal/mol less acidic than toluene. These findings are explained in terms of the structure and electronic properties of benzocyclopropenyl anion.

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.

Simulated ion trajectory and induced signal in ion cyclotron resonance ion traps

We present a numerical method for computation of electrostatic (trapping) and time-varying (excitation) electric fields and the resulting ion trajectory and detected time-domain-induced voltage signal in a rectangular (or cubic) ion cyclotron resonance (ICR) ion trap. The electric potential is calculated by use of the superposition principle and relaxation method with a large number of grid points (e.g., 100 × 100 × 100 for a cubic trap). Complex ICR experiments and spectra may now be simulated with high accuracy. Ion trajectories may be obtained for any combination of trapping and excitation modes, including quadrupolar or cubic trapping in static or dynamic mode; and dipolar, quadrupolar, or parametric excitation with single-frequency, frequency-sweep (chirp), or stored waveform inverse Fourier transform waveforms. The resulting ion trajectory may be represented either as its three dimensional spatial path or as two-dimensional plots of x-, y-, or z-position, velocity, or kinetic energy versus time in the absence or presence of excitation. Induced current is calculated by use of the reciprocity principle, and simulated ICR mass spectra are generated by Fourier transform of the corresponding time-domain voltage signal.

Concentric tube vacuum chamber for high magnetic field, high-pressure ionization in a fourier transform ion cyclotron resonance mass spectrometer

A new differential pumping design for external source Fourier transform ion cyclotron resonance mass spectrometry is described. A network of concentric tubes of increasing diameter terminates at a series of conductance limits across which a pressure from atmosphere to low-10(-8) torr is achieved. This design permits high-pressure sources to be positioned within the solenoidal superconducting magnet less than 20 cm from the analyzer trapped ion cell. Ionization at high magnetic field offers the advantage of radial ion confinement and consequently delivers enhanced ion current to the trapped ion cell. Ion injection utilizing this vacuum chamber design is simpler than previously reported serial pumping stage designs because elaborate focusing optics to overcome the magnetic mirror effect are unnecessary. Two probe-mounted atmospheric pressure sources are described as evidence of the general applicability of the concentric tube vacuum chamber. An electrospray source that delivers several hundred picoamperes of ion current to the cell yields high-sensitivity spectra of proteins beyond 100 kDa. Improved pumping compared with a prototype concentric tube network configuration now permits mass resolution in excess of 20,000 for the [M + 4H](4+) ion of melittin. The resolution is sufficient to distinguish isotope peaks within a single charge state. A probe-mounted, pulsed-laser ablation source that permits cluster formation in the strong magnetic field is also demonstrated.

Trapping and detection of ions generated in a high magnetic field electrospray ionization fourier transform ion cyclotron resonance mass spectrometer

The trapping and detection parameters employed with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer that is interfaced to a high magnetic field electrospray ionization (ES11 source are presented. ES1 occurs at atmospheric pressure in a 1.5-T field, and FTICR detection occurs 25 cm away at 3.0 T in either one of two cells separated by a conductance limit and maintained at pressure differentials of 5 × 10(5) and 2 × 10(7) torr, respectively. The continuous electrospray ion current traversing the high- and low-pressure cells is 350 and 100 pA, respectively. Retarding grid studies at the high-pressure cell indicate electrospray ion kinetic energies are controllable from less than an electronvolt to more than 10 eV. These kinetic energies are a function of desolvating capillary-skimmer assembly distance and the skimmer potential. Efficient accumulation of injected ions is accomplished only when the trap-plate potential matches the ion kinetic energy. If this condition is satisfied, the trapped ion cell fills to the ion space charge limit within a few hundred milliseconds. It is concluded that even at the high pressures used, the primary trapping mechanism cannot be solely collision dependent because the rate of ion accumulation is independent of background pressure. However, optimized FTICR excitation conditions for peptides and proteins in the mass range from 10(3) to more than 10(6) kDa are found to vary strongly with pressure; this is attributed to large mass- and charge-dependent differences in ion-molecule collision frequency.

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.