Fourier Transform Ion Cyclotron Resonance Cell Research Articles

A systematic study on the generation of multimetallic lanthanide fullerene ions by laser ablation mass spectrometry.

Laser ablation masss spectromety has been previously proved to be a powerful tool for studying endohedro metallofullerene (EMF) ions. Our previous study showed the possiblity of forming multi-metallofullerene ions containg more than six metal atoms for La, Y and Lu. Thus, it is important to conduct a systematic study on the generation of multi-metallofullerenes and their distribuitons for all lanthanide elements. Experiments were performed on a 7.0T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Laser ablation mass spectra were obtained by laser irradiation on mixtures of graphene and MCl3 on a stainless steel plate, applying a 355nm Nd:YAG laser with a typical energy of 2.5mJ/pulse. Reaction test experiments were performed by introducing O2 into the FTICR cell with a pulse valve. Multi-metallofullerene ions Ce2-4C2m+, Pr2-4C2m+, Gd2-4C2m+, Nd3C2m+, Dy2-3C2m+, Tb2-7C2m+, Ho2-6C2m+ were observed in the mass spectra. For the metals Sm and Eu, no multi-metallofullerene ion was observed. No reaction with O2 was observed in the reaction experiments, verifying that these species had endohedral structures. For the observed series of multi-metallofullerene ions, tri-metallofullerene ions dominated their mass spectra. The results were further compared with previously generated EMF ions for La, Er, Tm, Yb and Lu. Endohedral lanthanide metallofullerene ions were generated by laser ablation of graphene and the corresponding metal salts MCl3 (M=Ce, Pr, Nd, Gd, Tb, Dy and Ho) and studied with a FTICR mass spectrometer. Typically, multi-metallofullerene ions of TbnC2m+2≤n≤780≤2m≤176 and Ho6C2m+2≤n≤674≤2m≤162 were observed. The results show that the formation of multi-EMF ions containing lanthanides that have +3 and+4 oxidation states is easier than those containing +2 oxidation states in the process of laser ablation.

Fragmentation study of major spirosolane‐type glycoalkaloids by collision‐induced dissociation linear ion trap and infrared multiphoton dissociation Fourier transform ion cyclotron resonance mass spectrometry

Glycoalkaloids play a key role in the plant protection system against phytopathogens including fungi, viruses, bacteria, insects and worms. They can be toxic to humans if consumed in high concentrations causing gastrointestinal disturbances. The structural characterization of the major spirosolane glycoalkaloids, solasonine, solamargine, α-tomatine and dehydrotomatine, were investigated by positive electrospray ionization (ESI) coupled with a hybrid linear ion trap (LIT) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Tandem mass spectrometric analysis of spirosolane glycoalkaloids was performed by both collision-induced dissociation (CID) within the LIT and infrared multiphoton dissociation (IRMPD) in conjunction with the FTICR cell. Several common product ions were observed, generated by losses of the sugar moiety or aglycone fragmentation in the B- or E-ring, that can provide information on the accurate mass of aglycone and the primary sequence and branching of the oligosaccharide chains. Thanks to the multistage CID it was possible to understand the fragmentation pathways and thanks to the high resolution of IRMPD-FTICR the elemental compositions of product ions were obtained. Because the investigated tandem mass spectra data were acquired with high mass accuracy, unambiguous interpretation and determination of the chemical compositions for the majority of detected fragment ions were feasible. From these data, generalized fragmentation pathways were proposed, providing guidance for the characterization of unknown glycoalkaloids in plants. Copyright © John Wiley & Sons, Ltd.

Collision cross section measurements for biomolecules within a high-resolution FT-ICR cell: theory.

In this study, an energetic hard-sphere ion-neutral collision model was proposed to bridge-link ion collision cross section (CCS) with the image current collected from a high-resolution Fourier transform ion cyclotron resonance (FT-ICR) cell. By investigating the nonlinear effects induced by high-order electric fields and image charge forces, the energetic hard-sphere collision model was validated through experiments. Suitable application regions for the energetic hard-sphere collision model, as well as for the conventional Langevin and hard-sphere collision models, were also discussed. The energetic hard-sphere collision model was applied in the extraction of ion CCSs from high-resolution FT-ICR mass spectra. Discussions in the present study also apply to FT-Orbitraps and FT-quadrupole ion traps.

Fourier Transform Ion Cyclotron Resonance Mass Resolution and Dynamic Range Limits Calculated by Computer Modeling of Ion Cloud Motion

Particle-in-Cell (PIC) ion trajectory calculations provide the most realistic simulation of Fourier transform ion cyclotron resonance (FT-ICR) experiments by efficient and accurate calculation of the forces acting on each ion in an ensemble (cloud), including Coulomb interactions (space charge), the electric field of the ICR trap electrodes, image charges on the trap electrodes, the magnetic field, and collisions with neutral gas molecules. It has been shown recently that ion cloud collective behavior is required to generate an FT-ICR signal and that two main phenomena influence mass resolution and dynamic range. The first is formation of an ellipsoidal ion cloud (termed "condensation") at a critical ion number (density), which facilitates signal generation in an FT-ICR cell of arbitrary geometry because the condensed cloud behaves as a quasi-ion. The second phenomenon is peak coalescence. Ion resonances that are closely spaced in m/z coalesce into one resonance if the ion number (density) exceeds a threshold that depends on magnetic field strength, ion cyclotron radius, ion masses and mass difference, and ion initial spatial distribution. These two phenomena decrease dynamic range by rapid cloud dephasing at small ion density and by cloud coalescence at high ion density. Here, we use PIC simulations to quantitate the dependence of coalescence on each critical parameter. Transitions between independent and coalesced motion were observed in a series of the experiments that systematically varied ion number, magnetic field strength, ion radius, ion m/z, ion m/z difference, and ion initial spatial distribution (the present simulations begin from elliptically-shaped ion clouds with constant ion density distribution). Our simulations show that mass resolution is constant at a given magnetic field strength with increasing ion number until a critical value (N) is reached. N dependence on magnetic field strength, cyclotron radius, ion mass, and difference between ion masses was determined for two ion ensembles of different m/z, equal abundance, and equal cyclotron radius. We find that N and dynamic range depend quadratically on magnetic field strength in the range 1-21 Tesla. Dependences on cyclotron radius and Δm/z are linear. N depends on m/z as (m/z)(-2). Empirical expressions for mass resolution as a function of each of the experimental parameters are presented. Here, we provide the first exposition of the origin and extent of trade-off between FT-ICR MS dynamic range and mass resolution (defined not as line width, but as the separation between the most closely resolved masses).

Fourier Transform Mass Spectrometry

This article provides an introduction to Fourier transform-based mass spectrometry. The key performance characteristics of Fourier transform-based mass spectrometry, mass accuracy and resolution, are presented in the view of how they impact the interpretation of measurements in proteomic applications. The theory and principles of operation of two types of mass analyzer, Fourier transform ion cyclotron resonance and Orbitrap, are described. Major benefits as well as limitations of Fourier transform-based mass spectrometry technology are discussed in the context of practical sample analysis, and illustrated with examples included as figures in this text and in the accompanying slide set. Comparisons highlighting the performance differences between the two mass analyzers are made where deemed useful in assisting the user with choosing the most appropriate technology for an application. Recent developments of these high-performing mass spectrometers are mentioned to provide a future outlook.

Initial Experimental Characterization of a New Ultra-High Resolution FTICR Cell with Dynamic Harmonization

A new Fourier transform ion cyclotron resonance (FTICR) cell based on completely new principles of formation of the effective electric potential distribution in Penning type traps, Boldin and Nikolaev (Proceedings of the 58th ASMS Conference, 2010), Boldin and Nikolaev (Rapid Commun Mass Spectrom 25:122-126, 2011) is constructed and tested experimentally. Its operation is based on the concept of electric potential space-averaging via charged particle cyclotron motion. Such an averaging process permits an effective electric force distribution in the entire volume of a cylindrical Penning trap to be equal to its distribution in the field created by hyperbolic electrodes in an ideal Penning trap. The excitation and detection electrodes of this new cell are shaped for generating a quadratic dependence on axial coordinates of an averaged (along cyclotron motion orbit) electric potential at any radius of the cyclotron motion. These electrodes together with the trapping segments form a cylindrical surface like in a conventional cylindrical cell. In excitation mode this cell being elongated behaves almost like an open cylindrical cell of the same length. It is more effective in ion motion harmonization at larger cyclotron radii than a Gabrielse et al.-type (Int J Mass Spectrom Ion Processes 88:319-332, 1989) cylindrical cell with four compensation sections. A mass resolving power of more than twenty millions of reserpine (m/z 609) and more than one million of highly charged BSA molecular ions (m/z 1357) has been obtained in a 7T magnetic field.

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.

Unravelling the Intrinsic Features of NO Binding to Iron(II)- and Iron(III)-Hemes

Electrospray ionization of appropriate precursors is used to deliver [Fe (III)-heme] (+) and [Fe (II)-hemeH] (+) ions as naked species in the gas phase where their ion chemistry has been examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. In the naked, four-coordinate [Fe (II)-hemeH] (+) and [Fe (III)-heme] (+) ions, the intrinsic reactivity of iron(II)- and iron(III)-hemes is revealed free from any influence due to axial ligand, counterion, or solvent effects. Ligand (L) addition and ligand transfer equilibria with a series of selected neutrals are attained when [Fe (II)-hemeH] (+), corresponding to protonated Fe (II)-heme, is allowed to react in the FT-ICR cell. A Heme Cation Basicity (HCB) ladder for the various ligands toward [Fe (II)-hemeH] (+), corresponding to -Delta G degrees for the process [Fe (II)-hemeH] (+) + L --> [Fe (II)-hemeH(L)] (+) and named HCB (II), can thus be established. The so-obtained HCB (II) values are compared with the corresponding HCB (III) values for [Fe (III)-heme] (+). In spite of pronounced differences displayed by various ligands, NO shows a quite similar HCB of about 67 kJ mol (-1) at 300 K toward both ions, estimated to correspond to a binding energy of 124 kJ mol (-1). Density Functional Theory (DFT) computations confirm the experimental results, yielding very similar values of NO binding energies to [Fe (II)-hemeH] (+) and [Fe (III)-heme] (+), equal to 140 and 144 kJ mol (-1), respectively. The kinetic study of the NO association reaction supports the equilibrium HCB data and reveals that the two species share very close rate constant values both for the forward and for the reverse reaction. These gas phase results diverge markedly from the kinetics and thermodynamic behavior of NO binding to iron(II)- and iron(III)-heme proteins and model complexes in solution. The requisite of either a very labile or a vacant coordination site on iron for a facile addition of NO to occur, suggested to explain the bias for typically five-coordinate iron(II) species in solution, is fully supported by the present work.

Mass Spectroscopy of Chemical Reaction of 3d Metal Clusters Involved in Chemical Vapor Deposition Synthesis of Carbon Nanotubes

The chemical reactions of transition metal clusters in the gas phase have aroused considerable scientific interest and are also of critical scientific importance. For example, these reactions are involved in the synthesis of single-walled carbon nanotubes, which are considered ideal materials because of their outstanding properties. Alcohol catalytic chemical vapor deposition (ACCVD) is one of the best synthetic processes for carbon nanotubes (CNTs); however, even the initial growth mechanism is still unclear, unlike those of other synthetic processes. In this study, we used a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer to determine the initial reactions of transition metal cluster ions (iron, cobalt, and nickel) that are typically adopted in the alcohol CVD process. Metal clusters with approximately 10–25 atoms each, generated by a pulsed laser ablation system in a supersonic-expansion cluster beam source, were directly carried into the FT-ICR cell. Subsequently, ethanol was introduced into the ICR cell. We observed two different results: one was simple chemisorption observed in the iron cluster and the other was dehydrogenated chemisorption observed in the nickel cluster; however, cobalt clusters exhibited both patterns, and a sequential reaction was observed. Furthermore, the dehydrogenation of ethanol on the cobalt cluster is fully described from isotope-labeled experiments.

Infrared Multiphoton Dissociation Spectroscopic Analysis of Peptides and Oligosaccharides by Using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry with a Midinfrared Free-Electron Laser

The fragmentation of peptides and oligosaccharides in the gas phase was investigated by means of electrospray ionization Fourier transform ion cyclotron resonance (FTICR) mass spectrometry coupled with dissociation by a laser-cleavage infrared multiphoton dissociation (IRMPD) technique. In this technique, an IR free-electron laser is used as a tunable source of IR radiation to cause cleavage of the ionized samples introduced into the FTICR cell. The gas-phase IRMPD spectra of protonated peptides (substance P and angiotensin II) and two sodiated oligosaccharides (sialyl Lewis X and lacto-N-fucopentaose III) were obtained over the IR scan range of 5.7-9.5 microm. In the IRMPD spectra for the peptide, fragment ions are observed as y/b-type fragment ions in the range 5.7-7.5 microm, corresponding to cleavage of the backbone of the parent amino acid sequence, whereas the spectra of the oligosaccharides have major peaks in the range 8.4-9.5 microm, corresponding to photoproducts of the B/Y type.

Laser-Induced Fluoresence of Trapped Gas-Phase Molecular Ions Generated by Internal-Source Matrix-Assisted Laser Desorption/Ionization in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

The combination of laser-induced fluorescence with mass spectrometry opens up new possibilities both for detection purposes and for structural studies of trapped biomolecular ions in the gas phase. However, this approach is experimentally very challenging, and only a handful of studies have been reported so far. In this contribution, a novel scheme for laser-induced fluorescence measurements of ions trapped inside a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer will be introduced. It is based on an open FT-ICR cell design, continuous wave axial excitation of the fluorescence, orthogonal photon collection by fiber optics, and single photon counting detection. Rhodamine 6G ions generated by an internal matrix-assisted laser desorption/ionization source were used to develop and test the set-up. Due to photobleaching processes, the excitation laser power and the observation time window have to be carefully optimized. An ion tomography method was used to align the excitation laser. Potential applications for studying the gas-phase structure of fluorescent biomolecular ions and for investigating fluorescence resonance energy transfer of donor-acceptor pairs will be presented.

SIMION analysis of a high performance linear accumulation octopole with enhanced ejection capabilities

Here, we present the results of extensive SIMION 7.0 modelling of a new linear octopole ion trap. The octopole was designed to increase the efficiency of an electrospray ion source coupled to a Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometer. This improvement was achieved by applying a pulsed axial field to the octopole to eject the ion packet with a time and energy distribution that better match the acceptance criteria of the FTICR cell, thus increasing the trapping efficiency and sensitivity. The axial field was produced by applying a pulsed dc potential to the custom-designed ejection electrodes located between the octopole rods. The time and energy profiles of the ejected ion packets for several electrode shapes were calculated and are discussed in terms of their compatibility with efficient trapping of the ion packet in the FTICR cell. Preliminary experimental results show increased signal using the dc ejection electrodes of approximately 100%.

Extended Range Proteomic Analysis (ERPA): A New and Sensitive LC−MS Platform for High Sequence Coverage of Complex Proteins with Extensive Post-translational ModificationsComprehensive Analysis of Beta-Casein and Epidermal Growth Factor Receptor (EGFR)

We have developed a new and sensitive LC-MS platform, Extended Range Proteomic Analysis (ERPA), which is able to achieve very high sequence coverage and comprehensive characterization of post-translational modifications in complex proteins. This new platform provides advantages of both the top-down and bottom-up proteomic approaches by combining (i) digestion of the protein with an enzyme, such as Lys-C, which cuts less frequently than trypsin, leading to on average a higher molecular weight peptide size, (ii) high-performance LC separation of the resulting fragments, (iii) a new data acquisition strategy using the LTQ-FTMS, a hybrid mass spectrometer that couples a linear ion trap with a Fourier transform ion cyclotron resonance (FTICR) cell, for analysis of peptides in the range of 0.5 to 10 kDa, and (iv) new data analysis methods for assigning large peptide structures and determining the site of attachment of post-translational modifications as well as structural features from the accurate precursor mass together with MS(2) and MS(3) fragmentations. The LC retention of the Lys-C fragments is increased, relative to a tryptic digest, due to the generally greater hydrophobicity of the larger peptides, a result that is particularly important for peptides containing hydrophilic modifications such as glycosylation and phosphorylation. Furthermore, additional positively charged arginine and lysine residues in the Lys-C fragments enhance the sensitivity of the post-translationally modified phospho- and glycopeptides by at least 10-fold relative to tryptic fragments. In typical operation, the FTICR cell provides a survey scan with the high mass resolution (> 100 000) and accurate mass (<2 ppm) to characterize the higher charge-state precursor ions of the larger peptides. In parallel, the linear ion trap provides MS(2) and MS(3) fragmentation spectra, with a scan speed sufficiently fast for on-line LC-MS. Together, these data provide multiple means to determine or enhance the confidence of assignment of large or complicated peptide. Using ERPA, we demonstrate >95% sequence coverage in the analysis of two heavily phosphorylated and glycosylated proteins, beta-casein at the 50 fmole level and the epidermal growth factor receptor (EGFR) at the 1 pmole level. In summary, the combination of digestion strategy, high-performance separation, and the hybrid LTQ-FTMS instrument enables comprehensive characterization of large proteins, including posttranslational modifications.

Infrared Spectroscopy of Gas-Phase Cr+ Coordination Complexes: Determination of Binding Sites and Electronic States

Infrared spectra were recorded for a series of gas-phase Cr+ complexes using infrared multiphoton dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The functionalized aromatic ligands (acetophenone, anisole, aniline, and dimethyl aniline) offer a choice of either aromatic ring-pi or n-donor-base binding sites. Use of the FELIX free electron laser light source allowed convenient, rapid scanning of the chemically informative wavelength range from approximately 500 to 1800 cm(-1), which in many cases characterized the preferred site of metal binding, as well as the electronic spin state of the complex. Mono-complex ions, Cr+(ligand), for anisole, aniline, and dimethyl aniline and bis-complex ions, Cr+(ligand)(2), for anisole, aniline, and acetophenone were produced by ligand attachment to laser-desorbed Cr+ ions in the FT-ICR cell. The photodissociation yields plotted as a function of wavelength were interpreted as approximations to the infrared absorption spectra and were compared with computed spectra of different possible geometries and spin states. Clear-cut diagnostic features in the spectra of the acetophenone, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisole and aniline (both mono- and bis-complexes). The bis-complexes of aniline and anisole are low-spin (probably doublet) states, while the mono-complexes of these same ligands are high-spin (sextet) states. The dimethyl aniline complex gave a cluttered spectrum in poor agreement with calculations, which may reflect a mixture of binding-site isomers in this case.

Initial implementation of external accumulation liquid chromatography/electrospray ionization Fourier transform ion cyclotron resonance with automated gain control.

Capillary liquid chromatography (LC) separation coupled with external accumulation Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has recently been demonstrated to have significant potential for proteomics research. Accumulation of an excessive space charge external to the FTICR cell ion trap has been shown to result in increased mass measurement error, undesirable ion discrimination and/or fragmentation, potentially causing misrepresentation or incorrect assignments of lower abundance peptides in the acquired mass spectra. In this work we report on the capability of data-dependent adjustment of ion accumulation times in the course of LC separations, further referred to as automated gain control (AGC). Three different AGC approaches were evaluated based on the number of putative peptides from a tryptic digest of four casein proteins detected in the course of LC/FTICR separations. When compared with the conventional technique, AGC was found to increase, up to a factor of 3, the total number of peptides identified.

Flexible open‐cell design for internal‐source matrix‐assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry

AbstractA new Fourier transform ion cyclotron resonance (FTICR) cell design is described that improves the performance of internal‐source matrix‐assisted laser desorption/ionization (MALDI) applications. The design employs a capacitively coupled open FTICR cell and a ring electrode placed between the ion source and the ICR cell. The flexibility of our open‐cell design allows the use of several different trapping schemes for ion detection. Elimination of the drift time dependence in a MALDI experiment, ion accumulation, RF ion selection, and improved trapping of MALDI ions desorbed at an angle to the surface normal are some of the advantages of this design. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Instrumentation and methodology for simultaneous excitation/detection of ions in an FTICR mass spectrometer

A method for direct and continuous detection of ion motion during different perturbation events of the fourier transform ion cyclotron resonance (FTICR) experiment is demonstrated. The modifications necessary to convert an ordinary FTICR cell into one capable of performing simultaneous excitation/detection (SED) using a capacitive network are outlined. With these modifications, a 200-fold reduction in the detection of the coupled excitation signal is achieved. This allows the unique ability not only to observe the response to the perturbation but to observe the perturbation event itself. SED is used successfully to monitor the ion cyclotron transient during single-frequency excitation, remeasurement and exciter-excite experiments.

Instrumentation of Kinetic Energy-Resolved Surface-Induced Dissociation in Fourier Transform Mass Spectrometry

A Bruker 7 T Fourier transform (FT) mass spectrometer fitted with electron impact (EI) and electrospray ionization (ESI) external ion sources has been modified for surface-induced dissociation (SID) studies utilizing a self-assembled monolayer (SAM) of CF3(CF2)9C2H4SH alkanethiolate chemisorbed on gold. Operational aspects of ion sources, intermediate hexapole ion storage, time-of-flight function of the ion optics and cell trapping mechanisms are discussed for both primary and secondary ions. Different methods of ion trapping in the FT ion cyclotron resonance cell were analyzed theoretically and dynamic voltage trapping (DVT) was chosen as the most suitable. Direct measurements of primary (short time scale) and secondary (delayed) SID fragmentation and kinetic energy distributions of the SID fragments illustrate the capabilities of the instrument as a research tool for investigating SID kinetics. SID of small molecules and model peptides was investigated over a moderate collision energy range and the ion population was sampled at various decomposition delay times. Kinetic energy distributions of SID fragment ions were measured and shown to be quite low over the range of energies investigated. Most of the primary ion kinetic energy is dissipated into surface excitation and internal energy of ions recoiled from the surface.

Study of ion/molecule reactions of ion derived from tert-butyl methyl ether and trimethylsilyl methyl ether

The ion/molecule reactions of the ions (CH 3) 2C +–OCH 3 ( 1a) and (CH 3) 2Si +–OCH 3 ( 2a), which are the main fragment ions in the electron ionization mass spectrum of tert-butyl methyl ether (MTBE, 1) and trimethylsilyl methyl ether (MTSE, 2), respectively, and of the protonated MTBE ( 1H +) and protonated MTSE ( 2H +), generated by chemical ionization, with MTBE or MTSE, with acetone and with ammonia have been studied by Fourier transform ion cyclotron resonance (FTICR) spectrometry. The carbenium ion 1a is practically unreactive toward these substrates whereas the silicenium ion 2a efficiently forms adduct ions with all three reactants. Clearly 1a is a very stable tertiary carbenium ion due to the resonance stabilization of the methoxy group so that it lacks significant electrophilic character, whereas 2a does not gain much stabilization by this substituent. This interpretation is supported by the results of ab initio calculations at the restricted Hartree-Fock/6-31G( d) level of the structures and stabilities of these ions, of the tert-butyl cation and of the trimethylsilicenium ion and of their adducts with H 2O, (CH 3) 2O, and NH 3. The reactions of the protonated silyl ether 2H + are straightforward and correspond to transfer of the trimethylsilyl group to the reactants, as expected for a silylation reagent. Silyl group transfer competes effectively with proton transfer even in the case of NH 3 as reactant. The typical reaction of the protonated tert-butyl ether 1H + with the three substrates is an efficient elimination of C 4H 8 to generate a proton bound heterodimer of the attacking reactant with methanol as the primary product ion. This reaction also takes place when NH 3 is the substrate, although in this case proton transfer is the main process. In view of the easy interconversion of the protonated ether 1H + with its isomeric ion/neutral complexes consisting of proton bound isobutene and methanol (β complex), as proven by Audier et al. [J. Orig. Chem. 60 (1995) 7198], it is suggested that this fast elimination process is in fact a ligand switching process of the β complex which is either present as a stable species in the gas phase of the FTICR cell or is generated from 1H + by electrostatic activation in the encounter complex of the reactants.

Laser-induced acoustic desorption/chemical ionization in Fourier-transform ion cyclotron resonance mass spectrometry

Laser-induced acoustic desorption (LIAD) of neutral molecules coupled with electron and chemical ionization was examined as an analysis method for nonvolatile organic and biomolecules in Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometry. LIAD involved the production of a high amplitude acoustic wave by laser ablation of a copper or titanium foil from the opposite side of where the sample was deposited. The experiment was carried out with a simple probe designed for transmission mode laser desorption. Large amounts of neutral molecules were desorbed this way, but ions were not detected. The desorbed neutral molecules were ionized by 70 eV electron ionization or by reactions with reagent ions that were generated, isolated, and trapped in the ICR cell. Strong, reproducible signals were obtained in these experiments. The applicability of the method was demonstrated for a wide variety of molecules, including an organic salt, steroids, sugars, oligopeptides, nucleic acid bases, nucleosides, and synthetic polymers. For example, the tetrapeptide val-ala-ala-phe was volatilized by LIAD and ionized by a proton transfer reaction. The ions were stored in the FT-ICR cell for up to 30 s before detection. The base peak in the spectrum obtained corresponds to the protonated peptide, which indicates that the neutral peptide was desorbed intact. In contrast, thermal desorption of this peptide leads to substantial degradation. Based on this and other results obtained, LIAD combined with postdesorption ionization in an FT-ICR shows promise as a practical method for the analysis of thermally labile, nonvolatile molecules. A special advantage of this approach is that it allows better control of the ionization step (through selection of reagent ions) and a broader choice of ionization modes (e.g. group transfer, addition/elimination) than laser desorption/ionization methods, such as matrix-assisted laser desorption/ionization (MALDI). A disadvantage is lower sensitivity.