Source Fourier Transform Ion Cyclotron Research Articles

On initial ion velocities in MALDI: A novel FT-ICR MS approach

Even after more than two decades of investigation, the question whether the initial velocity of ions generated by matrix-assisted laser desorption/ionization (MALDI) depends on their molecular weight or not remains unanswered. A novel method employing an internal MALDI source Fourier-transform ion cyclotron resonance mass spectrometry is used to measure the initial ion velocities of peptides with m/z values in the range of typical proteomic experiments. Simultaneous measurement of multiple matrix and analyte ions clearly shows initial ion velocities that depend on analyte or matrix molecular weight. The measured velocity values agree well with the published data.

Initial velocity distribution of MALDI/LDI ions measured by internal MALDI source Fourier-transform ion cyclotron resonance mass spectrometry.

A new method for measuring the ion velocity distribution using an internal matrix-assisted laser desorption/ionization (MALDI) source Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer is described. The method provides the possibility of studying ion velocities without any influence of electric fields in the direction of the instrument axis until the ions reach the ICR cell. It also allows to simultaneously account for and to estimate not only the velocity distribution but the angular distribution as well. The method was demonstrated using several types of compounds in laser desorption/ionization (LDI) mode.

Excitation of Radial Ion Motion in an rf-Only Multipole Ion Guide Immersed in a Strong Magnetic Field Gradient

Radiofrequency (rf) multipole ion guides are widely used to transfer ions through the strong magnetic field gradient between source and analyzer regions of external source Fourier transform ion cyclotron resonance mass spectrometers. Although ion transfer as determined solely by the electric field in a multipole ion guide has been thoroughly studied, transfer influenced by immersion in a strong magnetic field gradient has not been as well characterized. Recent work has indicated that the added magnetic field can have profound effects on ion transfer, ultimately resulting in loss of ions initially contained within the multipole. Those losses result from radial ejection of ions due to transient cyclotron resonance that occurs when ions traverse a region in which the magnetic field results in an effective cyclotron frequency equal to the multipole rf drive frequency divided by the multipole order (multipole order is equal to one-half the number of poles). In this work, we describe the analytical basis for ion resonance in a rf multipole ion guide with superposed static magnetic field and compare with results of numerical trajectory simulations.

Motional averaging of ions for control of magnetron motion in fourier transform ion cyclotron resonance open-geometry trapped-ion cells

Open trapped-ion cell geometries have been utilized extensively in external source Fourier transform ion cyclotron resonance (FTICR) applications. A fundamental difference between open and closed cells is that in open cells the radial electric field undergoes a shift in orientation as a function of z displacement. Ions confined near the cell radial centerline in the z center of the cell experience an outward-directed destabilizing field gradient force whereas ions at large axial amplitude encounter an inward-directed stabilizing force. Radial destabilization at the center of the cell is compensated by radial stabilization at increased z amplitude through motional averaging along the z axis. This suggests that conditions can be created in the open cell for which deleterious effects due to magnetron motion can be minimized. This contrasts with the closed cell, for which the radial electric field is outward directed at every point along the z axis. This unique aspect of the open geometry trapped-ion cell is demonstrated to reduce radial drift by control of the axial amplitude of the ion cloud. Suspended trapping and resonance excitation of the trapping motion are used to increase the ion z amplitude. For ions with increased z amplitude, the effective cyclotron frequency (ω +) approaches the unperturbed cyclotron frequency (ω c ) with virtual elimination of the magnetron frequency (ω −). This is observed in benzene for which ω + is observed only 16 Hz below ω c . Additionally, resonant excitation at the trapping frequency (2ω t ) was used to increase the axial extent of the ion cloud, raising cyclotron frequency up to 550 Hz. The cyclotron frequency shift also occurs without peak broadening. This radial trajectory stabilizing characteristic of the open cell was maximized using shallow trapping wells in nested traps that yielded cyclotron frequency shifts 300 Hz higher than the unperturbed cyclotron frequency. This is a result of longer ion residence time at the turnaround points in the trapping well (where the inward-directed radial electric field is greatest in magnitude), increased axial extent of the ion cloud due to ion Coulomb repulsion, and the shallower trapping well. The presence of large numbers of charges (electrons, negative ions) in the outer trapping wells of polarity opposite the positive ions in the central trapping well may compensate for the depression of cyclotron frequency due to space charge.

Freeze-Dried Biomolecules: FT-ICR Studies of the Specific Solvation of Functional Groups and Clathrate Formation Observed by the Slow Evaporation of Water from Hydrated Peptides and Model Compounds in the Gas Phase

Solvent evaporation from extensively hydrated peptides and selected model compounds formed by electrospray ionization has been examined using an external ion source Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Water evaporation from the clusters, formed at room temperature by appropriate operation of an electrospray ion source, is initially rapid and results in evaporative cooling of the clusters to a temperature around 130−150 K, determined by the balance between evaporative cooling and heating by background blackbody radiation. In this “freeze-drying” process, it is observed that the kinetics of solvent evaporation and the cluster size distributions are highly dependent on the nature of the core ion in the cluster. In agreement with earlier studies of the hydrated proton, pure water clusters exhibit special stability characteristic of clathrate formation where, for example, a hydronium ion is encapsulated by a pentagonal dodecahedron of twenty water molecules. Similar clustering...

Salt Bridge Chemistry Applied to Gas-Phase Peptide Sequencing: Selective Fragmentation of Sodiated Gas-Phase Peptide Ions Adjacent to Aspartic Acid Residues

Salt bridge chemistry has recently been realized as a determining factor in the structures and reaction dynamics of biological molecules in the gas phase. In this paper, we further investigate salt bridge chemistry in studies of the low-energy collision-induced dissociation (CID) of sodiated peptides. MALDI and electrospray ionization are used to generate singly and multiply charged sodiated peptides which are analyzed by using an external ion source Fourier transform ion cyclotron resonance mass spectrometer. Of particular interest is the observation that sodiated peptides exhibit highly selective cleavage at aspartic acid residues. Sodiated peptides that lack acidic residues, however, undergo sequential cleavages from the C-terminus on low-energy CID. We propose a mechanism for cleavage at aspartic acid residues that involves a salt bridge intermediate in which the sodium ion stabilizes the ion pair formed by proton transfer from aspartic acid to the adjacent amide nitrogen. This proposal is supported b...

Determination of Block Length Distributions of Poly(oxypropylene) and Poly(oxyethylene) Block Copolymers by MALDI-FTICR Mass Spectrometry

Matrix-assisted laser desorption/ionization (MALDI) was performed on an external ion source Fourier transform ion cyclotron resonance mass spectrometer (FTICR-MS) to analyze the block length distributions of triblock polymers of poly(oxypropylene) and poly(oxyethylene). The first series of results presented demonstrate that the apparent molecular weight distributions are distorted. This distortion is induced by the flight-time-induced mass discrimination inherent in the experimental technique, the variation of isotopic patterns over the measured mass range, and the overlap of peaks in the spectrum. Subsequently, a method for the treatment of molecular weight distributions measured by MALDI on an external ion source FTICR-MS is developed to yield the actual molecular weight distribution and, from that, the individual block length distributions. For the first time, detailed and accurate molecular weight data were obtained on a complex sample using this methodology, which independently validates the data provided by the manufacturer. The experimentally verified random coupling hypothesis proves the validity of the methodology.

Rapid microscale analyses with an external ion source Fourier transform ion cyclotron resonance mass spectrometer

The design of a multi-purpose external ion source Fourier transform ion cyclotron resonance mass spectrometer for rapid microscale analyses of complex mixtures is presented. Rapid microscale analyses are carried out using several desorption and ionisation techniques, and benefit from a hydrocarbon-free UHV (Ultra High Vacuum) system. Two different techniques for polymer analysis will be discussed. “In source” filament pyrolysis is used for high speed, broadband screening of additives in polymer blends, and matrix assisted laser desorption and ionisation (MALDI) is used for endgroup analysis and molecular weight determination. External ion source MALDI measurements of the molecular weight distribution of an amine terminated poly(propyleneglycol) are shown to be extremely sensitive to the ion accumulation time used, as a result of the flight time distribution of the ions from ion source to analyser cell. Deconvolution of the measured transient shows that magnetron motion exhibits a strong mass dependent influence on the measured molecular weight distribution. This deconvolution and 2D-representation of the measured and Fourier transformed data provides additional insight into the ion behaviour inside the ICR cell.

High resolution end group determination of low molecular weight polymers by matrix-assisted laser desorption ionization on an external ion source fourier transform ion cyclotron resonance mass spectrometer

Matrix-assisted laser desorption ionization was performed on an external ion source Fourier transform ion cyclotron resonance mass spectrometer equipped with a 7-T superconducting magnet to analyze end groups of synthetic polymers in the mass range from 500 to 5000 u. Native, perdeutero methylated, propylated, and acetylated polyethylene glycol and polyvinyl pyrrolidone with unknown end-group elemental composition were investigated in the mass range up to 5000 u by using a 2,5-dihydroxybenzoic acid matrix. A small electrospray setup was used for the deposition of the samples. Two methods to process data were evaluated for the determination of end groups from the measured masses of the component molecules in the molecular weight ranges: a regression method and an averaging method. The averaging method is demonstrated to allow end-group mass determinations with an accuracy within 3 mu for the molecular weight range from 500 to 1400 and within 20 mu for the molecular weight range from 3400 to 5000. This is sufficient to identify the elemental composition of end groups in unknown polymer samples.

Pulsed-Gas Glow Discharge for Ultrahigh Mass Resolution Measurements with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

A new pulsed-gas glow discharge (GD) source has been developed for use with an external ion source Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. With pulsed argon gas introduction into the GD source, the gas load and pressure in the mass analyzer region were greatly reduced; this resulted in improved mass resolution. Mass resolution of greater than 1 450 000 (fwhm) has been achieved for Cu(+) ions from a brass sample, the highest reported for any type of GD mass spectrometer. The pulsed-gas GD source promises analytical usefulness for ultrahigh resolution measurements in GD mass spectrometry.

Direct Temperature Resolved HRMS of Fire-Retarded Polymers by In-Source PyMS on an External Ion Source Fourier Transform Ion Cyclotron Resonance Mass Spectrometer

Rapid microscale analysis with high mass accuracy is demonstrated by direct temperature resolved desorption and pyrolysis from a Pt/Rh filament probe inside the external ion source of a 7-T FTICR-MS. High pressure generated during desorption and pyrolysis in the ion source does not interfere with analysis in the hydrocarbon-free UHV of the ICR cell, thus allowing short observation cycles at high resolution. The typical conditions achieved, a mass resolution (m/Δm) 50% equals 50 000 at m/z 600 with cycle times of 100 ms, were used to analyze isobaric compound mixtures generated by pyrolysis of brominated fire-retarded polymers spiked with antimony-containing synergists. Unknown fire-retarded polymer blends sampled from household appliances were found to contain brominated biphenyls, brominated diphenyl ethers, tetra-bromoBisphenol-A and its butylated isomers, polystyrene, and antimony oxides. High-resolution temperature-resolved analysis by in-source pyrolysis FTICR-MS confirms the elemental composition. The resolution is sufficient to separate the nominally isobaric ions from the antimony(III) oxide (Sb 4 O 6 ) synergist and the n-butyl ether derivative of tetrabromoBisphenol-A.

Enhanced accumulated trapping efficiency using an auxiliary trapping electrode in an external source Fourier transform ion cyclotron resonance mass spectrometer

A scheme for the enhanced accumulation of ions injected into the trapped ion cell of a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer from an external electrospray ionization source is described. This method utilizes an auxiliary electrode positioned outside the trapped ion cell which increases the effective collisional path length for ions traversing the trapped ion cell. A comparison made between pressure-assisted accumulated trapping in a symmetric trapping well, an asymmetric trapping well, and a symmetric trapping well with that for the addition of an auxiliary electrode indicates that trapping efficiency can be significantly improved when an auxiliary electrode trapping configuration is utilized.

High mass resolution glow discharge mass spectrometry using an external ion source FT-ICR mass spectrometer

A glow discharge source has been adapted for use with a commercial external ion source Fourier transform ion cyclotron resonance mass spectrometer. This combination, with improvements in detection limits and accuracy, should be analytically useful for a variety of samples. Results on National Institute of Standards and Technology stainless steel standards showed that peaks due to nominally isobaric atomic isotopes can be observed with high mass resolution (m/[Delta]m[sub [1/2]] [>=] 290 000). Detection limits in the high parts per billion to low parts per million range were achieved for analysis of trace metals in the standards, and concentrations of the trace elements were determined with good accuracy (average error of 14%). 21 refs., 6 figs., 2 tabs.

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.

Principles and applications of Fourier transform ion cyclotron resonance mass spectrometry. Plenary lecture

In the past decade, much progress has been made in the development of Fourier transform ion cyclotron resonance mass spectrometry. This method is nowadays increasingly used in both basic and applied research studies. Following a description of the basic principles of the method, its performance with respect to mass resolving power and accurate ion mass and ion abundance measurements is discussed. It is then shown that the method permits either collision-induced fragmentation or photo-dissociation experiments on mass-selected ions. In this mass selection a relatively high so-called front-end resolution can be achieved, while a high to ultra-high back-end mass resolution can also be obtained for the product ions formed from the mass-selected ions on collision-induced fragmentation or photo-dissociation. Finally, the development is discussed, of dual cell and external ion source Fourier transform ion cyclotron resonance mass spectrometry, which has allowed the use of ionization techniques known to be suited for the analysis of non-volatile or thermally labile compounds, and the coupling of this mass spectrometric method with separation techniques such as gas chromatography.