ICR Ion Trap Research Articles

Simple and accurate determination of ion translational energy in ion cyclotron resonance mass spectroscopy

Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometry is the most versatile and widely used technique for analysis of gas-phase ion/molecule reaction pathways, kinetics, equilibria, and energetics and is also suitable for a wide range of analytical applications, as summarized in numerous recent reviews. A powerful tool in such studies is the collision-induced dissociation (CID, also known as CAD, collision-activated dissociation) technique, in which ions are first accelerated to higher translational energy (in this case, by increase in ICR orbital radius in an ICR ion trap), subsequent ion-neutral collisions (e.g., with Ar atoms) then result in fragmentation of the parent ion to give daughter ions. The structure of the parent ion may then be inferred from the masses of the daughter ions and/or the neutral fragments lost. In work to be reported fully elsewhere, the authors have computed analytically the detected ICR signal for ion traps of finite dimensions and various shapes (i.e., circular or square cross section, with arbitrary length-to-width (aspect) ratio). As the ICR orbital radius increases, there is a nearly linear increase in the ICR signal detected at the fundamental orbital cyclotron frequency, {omega}{sub 0} qB{sub 0}m.

Time-resolved unimolecular dissociation of styrene ion. Rates and activation parameters

The rate of unimolecular dissociation of styrene ion into benzene ion plus acetylene was measured by time-resolved photodissociation at 308 nm in the ICR ion trap. A rate constant of 1.10 {times} 10{sup 5} s{sup {minus}1} was obtained at a total ion internal energy of 4.20 eV. Using accurate heat of formation data for the reactant and products, a 0 K reaction enthalpy of 2.42 eV was assigned. RRKM rate-energy curves were calculated for comparison with the present measurement and with previous photoionization coincidence (PEPICO) data. The shape of the RRKM curve matches experiment, and quantitative rate agreement is obtained assuming an activation energy of 2.32 eV and a tight transition state. The activation parameters derived from the activated complex, {Delta}S{double dagger}(1,000 K) = {minus}6.4 eu and A{sub {infinity}} (1,000 K) = 2.3 {times} 10{sup 12} s{sup {minus}1}, are compared with values for other ion dissociations and with neutral-molecule rearrangement dissociations.

Photodissociation spectroscopy of several C 5H +6 isomeric ions

Photodissociation spectra were obtained in the ICR ion trap for the C 5H + 6 ions from five isomeric neutral C 5H 6 precursors. It is clear from the distinct spectra obtained that interconversion of the non-decomposing ions does not occur; the spectra are sufficiently characteristic to distinguish the various structures. The spectral profile is similar for the cyclopentadiene and 2-methyl-1-butene-3-yne isomers, but they are clearly differentiated by the order-of-magnitude difference in cross section. The location of the PDS peaks is in excellent accord with the PES spectra of the corresponding neutral compounds, suggesting retention of the neutral structure upon ionization in all cases. Fragment C 5H + 6 ions derived from phenol, aniline, and thiophenol by CID were studied. Photodissociation spectra of ions from the three precursors were similar; based on the spectral shape and low cross section, the four acyclic structures were ruled out, and the fragment ions were assigned the cyclopentadiene ion structure. Partial PDS spectra were obtained for the aniline, phenol, and thiophenol parent ions and were in accord with expectations from the PES spectra. MINDO/3 and MNDO calculations were carried out for a Koopmans' theorem comparison with the π — π transition energies of the C 5H + 6 ions. Both of these calculations gave substantially better values than previous SPINDO calculations: the MNDO results were within about 0.1 eV of the experimental values for the transition energies of the ene-ynes.

Photodissociation of 1,4-dioxane ions. Competitive formation of two fragment ions

Photodissociation of the dioxane parent ion, m/ z 88, in the ICR ion trap gives predominantly m/ z 57 fragments at wavelengths between 515 and 457 nm, while at wavelengths increasing from 595 to 615 nm, an increasing amount of m/ z 58 fragment is observed, reaching a 58/57 ratio of 5/1 at 615 nm. This is in contrast to the observed fragmentation of m/ z 88 ions energies produced by photoionization, where m/ z 58 is the predominant fragment for photon energies in this energy range. However, comparing monoenergetic fragmenting ions, as approximated by the first derivative of the photoionization efficiency plots, the photodissociation pattern is in qualitative agreement with photoionization; the higher 57/58 ratio in photodissociation is attributed to the electron-impact-produced ions in the ICR ion trap containing 0.3 eV of excitation in excess of thermal. The observations and interpretation are consistent with preliminary ion-beam photodissociation results from Bowers' laboratory and a consistent picture of the competitive production of these two fragment ions as a function of parent ion internal energy is obtained.

Fragmentation of n-heptane ions by photodissociation

Photodissociation of C 7H 16 + from n-heptane in the ICR ion trap yields chiefly C 4H 9 + ( m/z 57) and C 3H 7 + ( m/z 43) product ions, with the 43/57 ratio increasing with increasing photon energy in the range 2.1–3.5 eV. This fragmentation pattern is very different, and much simpler, than is observed in dissociative electron impact ionization or photoionization. Parent ion rearrangement, while possible, is considered unlikely. The remarkably different fragmentation behavior in photodissociation may reflect state-specific behavior following photoexcitation to a particular excited electronic state; but it is more attractive to postulate energy-specific behavior, in which the precise energy deposition of the incident photon places ions in an energy region dominated by the 100 → 57 and 100 → 71 → 43 fragmentation channels. This interpretation is supported by first-derivative analysis of previous photoionization results.

Collisionless relaxation of photoexcited benzene ions

The collisionless energy relaxation of benzene ions following excitation by a visible-wavelength photon has been measured by kinetic analysis of two-photon photodissociation, using a chopped laser at varying chopping rate irradiating ions in the ICR ion trap. At 488 nm the relaxation rate of the ion of 11 s −1 is faster by a factor of 4 or 5 than that calculated for neutral benzene. The relaxation rate varies with wavelength over the range 458-515 nm from 8.5 to 12.3 s −1 in the manner expected for infrared-fluorescent energy dissipation from an excited ion whose excess energy is stored as vibrational excitation.

Threshold ion photodissociation. Bromobenzene and iodobenzene ions

Photodissociation of bromobenzene and iodobenzene ions at wavelengths near the one-photon threshold was studied as a function of pressure in an ICR ion trap. An analysis was made of the effects on the observed wavelength dependence of three factors: the thermal distribution of ion internal energies, the kinetic shift attributable to IR-radiative lifetime limiting of photoexcited ions, and the contributions of two-photon dissociation at wave-lengths near or below threshold. Quantitative procedures were developed for taking all of these into account and values were obtained for the zero-Kelvin dissociation thresholds: these are calculated to be 2.81±0.07 eV (440±10 nm) for the bromobenzene ion and 2.48±0.06 (499±12 nm) for the iodobenzene ion. A value of 1142±5 kJ mol −1 is derived for Δ H fo 0(C 6H 5 +).