Richard D. Burton

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.

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.

Metal to ligand charge transfer photochemistry of Re(I)-alkyl complexes

Abstract A preliminary study has been carried out concerning the photochemistry and photophysics of a series of (bpy)ReI(CO)3-R complexes, where bpy is 2,2′-bipyridine and R=−CH3,−CH2Ph and −Ph. Photolysis of the complexes into the lowest energy absorption band (436 nm) leads to a highly efficient ReC bond homolysis reaction for R=−CH3 and −CH2Ph. Absorption and low-temperature emission studies of the complexes suggest that the lowest excited state is based on dπ (Re)→π* (bpy) metal to ligand charge transfer (MLCT). Nanosecond transient absorption studies confirm that the MLCT stare is formed after near-UV excitation of (bpy)Re(CO)3-Ph; however, only the reactive intermediate which is formed as a result of ReC bond homolysis is observed for (bpy)Re(CO)3−CH3 and (bpy)Re(CO)3−CH2Ph. The results are interpreted by a photochemical model in which the bond homolysis reaction occurs either directly from an unrelaxed MLCT excited state or from a spectroscopically silent state which is reached during non-radiative relaxation of the Franck-Condon MLCT state.

Fourier transform ion cyclotron resonance mass spectrometry and theoretical studies of gas phase SN2 nucleophilic substitution reactions at sp3-carbon atoms

Gas phase SN2 intramolecular displacements are reported in which neutral nucleophiles displace neutral leaving groups within cationic substrates. In the intramolecular SN2 reaction of N-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (11), the cationic product 12 was detected directly. Intramolecular reactions of N-(ω-aminoalkyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salts 13a–c, N-(5-hydroxypentyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salt 14 and N-(ω-aminoalkyl)-2,4,6-triphenylpyridinium salts 20a,b afforded the corresponding protonated acridinium cation (15) or pyridinium cation (21) and, presumably, a neutral heterocycle. This interpretation is supported by isotopic labeling control experiments. No evidence has been obtained for any intermolecular gas phase SN2 reaction with a pyridine as leaving group; theoretical calculations suggest an explanation for these experimental results.

Cation tagging for monitoring gas phase reactions. Electrospray FTICR/MS study of ester pyrolysis

Abstract Seven different tetraphenylpyridinium esters were chosen as model compounds for studying gas phase pyrolysis reactions by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR/MS) Collisionally Activated Dissociation (CAD) using cation tagging. In this technique a stable positive charge (tetraphenyl pyridinium) is introduced to one end of the system so the reaction that occurs at the other end can be monitored easily using mass spectrometry. Threshold fragmentation energies and rate constants were calculated for all seven starting materials and a comparison of their observed relative fragmentation rates was made.