John R. Eyler

Free electron laser-Fourier transform ion cyclotron resonance mass spectrometry facility for obtaining infrared multiphoton dissociation spectra of gaseous ions

A Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has been installed at a free electron laser (FEL) facility to obtain infrared absorption spectra of gas phase ions by infrared multiple photon dissociation (IRMPD). The FEL provides continuously tunable infrared radiation over a broad range of the infrared spectrum, and the FT-ICR mass spectrometer, utilizing a 4.7Tesla superconducting magnet, permits facile formation, isolation, trapping, and high-mass resolution detection of a wide range of ion classes. A description of the instrumentation and experimental parameters for these experiments is presented along with preliminary IRMPD spectra of the singly-charged chromium-bound dimer of diethyl ether (Cr(C4H10O)2+) and the fluorene molecular ion (C13H10+). Also presented is a brief comparison of the fluorene cation spectrum obtained by the FT-ICR-FEL with an earlier spectrum recorded using a quadrupole ion trap (QIT).

‘‘Magic number’’ carbon clusters: Ionization potentials and selective reactivity

The ionization potentials (IPs) of several large carbon clusters Cn (n≥48), including the enhanced abundance (‘‘magic number’’) clusters C50, C60, and C70, have been determined by Fourier transform ion cyclotron resonance (FTICR) mass spectrometric charge transfer bracketing experiments. The IPs of C50, C60, and C70 were bracketed by the same two charge transfer compounds, leading to a common value of 7.61±0.11 eV. The IPs of even numbered clusters adjacent to these magic number clusters were found to be lower by as much as 0.5 eV and all clusters between C50 and C200 were determined to have IPs greater than 6.20 eV. The reaction rates of C+60 and C+70 with metallocenes were anomalously slow in comparison to the other large carbon cluster ions. IP and reactivity results suggest that C50, C60, and C70 may indeed have different or more stable structures than neighboring clusters, which supports the hypothesis of closed‐shell, spherical species. The implications of these results for the mechanism of C+n form...

FTMS studies of mass-selected, large cluster ions produced by direct laser vaporization

Abstract A method is described for the production of large cluster ions by direct laser vaporization in a low-pressure FTMS. Production of high-mass carbon cluster ions (C n + ; 40 n x Sb y + ) cluster ions containing up to five metal atoms are reported. The observed distributions are compared with those obtained previously by both direct laser vaporization and molecular beam sources. Details of the mechanism for formation of these larger cluster ions by direct laser vaporization are discussed. The mass selectivity and long ion residence times obtainable in the FTMS may now be utilized in the study of these cluster ions. Results are presented from a limited study of the ion/molecule reactions and collision induced dissociation of the high-mass carbon cluster ions.

Determination of carbon cluster ionization potentials via charge transfer reactions

Ionization potentials (IPs) for carbon clusters containing 6–24 atoms have been determined from charge transfer reactions of carbon cluster ions with compounds of known ionization potential in a Fourier transform ion cyclotron resonance mass spectrometer. Cluster IPs generally decrease with increasing cluster size, but the IPs for clusters containing 4n+3 atoms (n=1–5) are found to be ∼0.5 eV lower than those of neighboring clusters. The relationship between cluster IP and structure is discussed.

Proton Affinities of Eight Matrices Used for Matrix‐assisted Laser Desorption/Ionization

Protonated molecules of analytes in matrix-assisted laser desorption/ionization (MALDI) are frequently the most intense ions observed, especially when the concentration of alkali metal cations is low. Examination of the laser desorption mass spectra of MALDI matrices usually shows the presence of both molecular radical ions M+• and [M + H]+ ions. With some matrices, the intensity of the [M + H]+ ion is greater than that of the molecular radical ion, e.g. with 2,5-dihydroxybenzoic acid. A logical source for the ions of protonated analyte in MALDI is proton donation from the [M + H]+ ions of the matrix, but donation could also occur from the radical molecular ions. A knowledge of the proton affinities of the common MALDI matrices might be helpful in understanding why some matrices are ‘hotter’ than others and lead to more post-source as well as prompt decay. The ground-state proton affinity of eight common MALDI matrices were determined. For each matrix, the [M + H]+ ion was generated by methane chemical ionization, trapped and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, allowed to cool for 5 s and reacted with reference compounds of known proton affinities. In some cases, the matrix proton affinities are low enough that proton transfer can occur from the ground state [M + H]+ ion to MALDI analytes; in other cases, the matrix proton affinities are so high that some other mechanism for proton transfer is required.

The coupling of direct analysis in real time ionization to Fourier transform ion cyclotron resonance mass spectrometry for ultrahigh-resolution mass analysis

Direct Analysis in Real Time (DART) is an ambient ionization technique for mass spectrometry that provides rapid and sensitive analyses with little or no sample preparation. DART has been reported primarily for mass analyzers of low to moderate resolving power such as quadrupole ion traps and time-of-flight (TOF) mass spectrometers. In the current work, a custom-built DART source has been successfully coupled to two different Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers for the first time. Comparison of spectra of the isobaric compounds, diisopropyl methylphosphonate and theophylline, acquired by 4.7 T FT-ICR MS and TOF MS, demonstrates that the TOF resolving power can be insufficient for compositionally complex samples. 9.4 T FT-ICR MS yielded the highest mass resolving power yet reported with DART ionization for 1,2-benzanthracene and 9,10-diphenylanthracene. Polycyclic aromatic hydrocarbons exhibit a spatial dependence in ionization mechanisms between the DART source and the mass spectrometer. The feasibility of analyzing a variety of samples was established with the introduction and analysis of food products and crude oil samples. DART FT-ICR MS provides complex sample analysis that is rapid, highly selective and information-rich, but limited to relatively low-mass analytes.

Acidity, basicity, and ion-molecule reactions of phosphine in the gas phase by ion cyclotron resonance spectroscopy

Gas phase ion-molecule phosphine reactions in pure and binary mixtures by ion cyclotron resonance spectroscopy, considering acidity and basicity

Fourier transform-ion cyclotron resonance mass spectrometric resolution, identification, and screening of non-covalent complexes of Hck Src homology 2 domain receptor and ligands from a 324-member peptide combinatorial library

The preferred ligands for the Hck Src homology 2 domain among a combinatorial library containing 324 different peptides were determined in a single experiment involving Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS), electrospray ionization (ESI), stored-waveform inverse Fourier transformation (SWIFT), and infrared multiphoton laser disassociation (IRMPD). These were compared with the results obtained by conventional screening of the peptide library in solution using affinity chromatography. The results reported here show that by combining ESI, FT-ICR MS, SWIFT, and IRMPD, ligands likely to bind under physiological conditions are rapidly and efficiently identified, even from complex library mixtures. In the gas phase some discrimination against hydrophobic ligands could be observed. However, the illustrated feasibility of identifying high affinity ligand via gas-phase screening of complex library mixtures should lead to broad applications in the development of ligands for proteins with interesting biological activity, the first step that must be taken to develop a therapeutic agent.

Ionization potentials of small carbon clusters

Ionization potentials (IP’s) of small carbon clusters (Cn, n=3–6) have been bracketed with an uncertainty of a few tenths of an eV by charge‐transfer methods in a Fourier transform ion cyclotron resonance mass spectrometer. Values obtained are compared to previously determined experimental and theoretical values. Several interesting reactions other than charge‐transfer were observed between the carbon cluster ions and the charge‐transfer reference compounds. The products of these reactions are reported and discussed.

Exact mass measurements using a 7 tesla fourier transform ion cyclotron resonance mass spectrometer in a good laboratory practices-regulated environment

Fourier transform ion cyclotron resonance mass spectrometry has been found to produce reliable exact mass measurements using two different internal calibration methods. For these measurements, electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) were utilized both individually and in tandem. For internal calibration with a co-dissolved polyethylene glycol standard, measurements of 41 compounds resulted in an average absolute mass determination error of 0.7 ppm, with a standard deviation of 0.9 ppm. For comparison, internal calibration was effected through the simultaneous use of ESI and MALDI, with the former being used for the introduction of analyte ions and the latter for formation of polymethylmethacrylate calibrant ions. This technique led to mass measurements with an average absolute error of 0.8 ppm and a standard deviation of 1.0 ppm. In addition, exact mass measurements of tandem mass spectrometry fragment ions were made for 35 compounds using external calibration with a single internal mass standard. The observed average absolute error was 0.7 ppm with a standard deviation of 1.0 ppm.