Kevin J. Hart

Relative dissociation energy measurements using ion trap collisional activation

The measured minimum resonance excitation amplitudes for decomposition of polyatomic ions in the quadrupole ion trap collisional activation experiment are shown to correlate with literature critical energies. The present article describes how experiments can be performed to derive threshold resonance excitation amplitudes via the kinetics associated with collision-induced dissociation (i.e., dissociation rate constants) in the quadrupole ion trap. The relationship between these threshold values and critical energies is established empirically by using kinetic data acquired for molecular ions with critical energies measured with other techniques. The experiments are complicated by the change in optimum resonance excitation frequency with amplitude, due presumably to contributions from higher order fields. It is proposed that the threshold resonance excitation amplitude is a measure of the change in temperature of the parent ion population required to achieve a measurable rate of decomposition. The present results indicate that the quadrupole ion trap may see new applications as a quantitative tool for the study of gaseous ion chemistry.

Reaction of analyte ions with neutral chemical ionization gas

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Ion-molecule reactions of neutral methane with analyte ions under normal methane chemical ionization conditions are discussed. Reactant ions can be generated by direct electron ionization (EI) fragmentation, chemical ionization (CI) fragmentation, or collision-induced dissociation (CID). Examples in which products of such reactions appear in mass spectra in both conventional CI sources in “beam” instruments and low pressure CI in a quadrupole ion trap are presented. Also shown is an example in which MS/MS product ions react with neutral methane used for CI in an ion trap. It is shown that it is relatively straightforward to recognize such reactions in a quadrupole ion trap and in certain cases to minimize or preclude them. Effects of various operating parameters have been investigated and are discussed.

MONITORING VOLATILE ORGANIC COMPOUNDS IN FLUE GAS USING DIRECT SAMPLING ION TRAP MASS SPECTROMETRY

Quantitative measurement of volatile organic compound concentrations in flue gas that has a high percentage of water vapor at low part-per-billion (ppb) to part-per-trillion levels represents a demanding analytical challenge. Direct sampling ion trap mass spectrometers developed at Oak Ridge National Laboratory are promising candidates to be developed into monitors of continuous emissions for these and other compounds at very low levels. Two direct sampling interfaces, a sorbent trap/thermal desorption interface and a continuous air monitor, have been tested in field studies of effluent from hazardous-waste incinerators. These interfaces demonstrate both an integrated, time-averaged and a raw gas, real-time approach to flue gas monitoring with detection limits in the ppb to sub-ppb range. The use of these interfaces for flue-gas monitoring with examples drawn from field tests are discussed.

Evidence of isomerization during ion isolation in the quadrupole ion trap

Evidence of ion isomerization during isolation in an ion trap mass spectrometer is presented. An ion-molecule reaction that is specific for the tolyl cation was used to monitor the relative abundance of this species. In particular, it has been observed that ion isolation in the ion trap can impart sufficient energy to the tolyl cation to cause it to isomerize to a form (presumably either the benzyl or the tropylium ion) that is not reactive with the neutral reagent. These results are important to consider in ion trap applications involving ion species having activation barriers for isomerization lower than the activation barriers for dissociation.