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Abstract No.: MoP-058
Session: Fundamentals: Theory and Experiment
Presentation date: Mon, Aug 28, 2006
Presentation time: 14:30 – 16:00

Internal Energy Distributions of Ions Produced by MAB Ion Sources

Christelle Kotanian1, Jeremie Ponthus1, Denis Lesage1, Gabriel Sanchez1, Sandra Alves1, Jean-Claude Tabet1

1 University Paris VI - LCSOB, Paris, France

Correspondence address: Christelle Kotanian, University of Pierre and Marie Curie, Mass Spectrometry Group, UMR 7613, 4, Place Jussieu, Paris, 75252 Paris Cedex 05 France.

Keywords: Energy, Internal; Instrumentation; Internal Energy; Ionization, Penning.

Novel aspect: This study is based on a new ionization source MAB (Metastable Atom Bombardment). In order to understand ionization and fragmentation process, internal energy of precursor ions was measured.

 

The internal energy of precursor ions and the time scale play fundamental roles in mass spectrometry. The aim of this study is to measure the internal energy distributions of different model odd-electron ions produced by "Metastable Atom Bombardment" ionization (MAB)1 (based on Penning ionization). This source uses the specific advantages of collisional metastable atom desexcitation allowing molecular ionization. It involves an electrophilic reaction of a metastable species (A*) with an analyte (BC) resulting in an ionic state of the analyte (BC+°), the ground state of A and an ejected electron in a continuum state (e°), described by the equation (1)
A* + BC → A + BC+° + e° (1)
The reaction occurs if the ionization energy of BC is lower than the excitation energy of A*. According to the metastable atoms used, this ionization process can enable a sharp control of the ion internal energy Eint as reported equation (2)
Eint(BC+°)= Eth(BC) + E*(A) [IE(BC) + Ekinv(e°)] (2)
where Eth represents the thermal energy of the molecule, E* the excitation energy of the metastable state depending on the gas used (varying from 8.32 eV for Xe* to 19.82 for He*), IE the ionization energy of the molecule and Ekin is the translational energy taken by the electron which is ejected. Eth and Ekin are internal energy distributions. Note that Eth may be easily calculated but it is not the case for Ekin, which may be varied between 0eV and the maximum energy available [Eth + E* - IE]. Furthermore, different contributions of metastable states for E* have to be considered for the different gas used (He*, Ne*, Ar*, Kr*, Xe* and N2*). For instance, N2* was most commonly used as the discharge gas thanks to its double energetic components (8.52 eV and 11.88 eV).

To measure the internal energy distributions of ions produced by a MAB source, different molecules (n-butylbenzene, n-ethylbenzene, dichlorobenzene, phenol etc.) were used as thermometer probe compounds. These compounds were chosen because their thermodynamical data were extensively discussed in the literature. The MAB mass spectra were obtained using two mass spectrometers, a Daxel 2C MAB-TOF/MS and a homemade ion source MAB-Quad/MS. For these small molecules studied, very few differences on the mass spectra were observed due to the low kinetic shift. These experimental mass spectra were compared to calculated spectra based on transition state theory (RRKM formalism). All the theoretical calculations have been performed using the MassKinetics Scientific Demo software (version 1.6: //www.chemres.hu/ms/masskinetics/).2 The internal energy distributions are the variable parameters used to fit experimental results. As a conclusion, the internal energy distribution deposed onto the ions is larger than expected but its mean value is perfectly defined.

1. D. Faubert, et al., Int. J. Mass Spectrom. Ion Processes 69-77, 124 (1993).
2. L. Drahos, K. Vekey, J. Mass Spectrom. 36, 237 (2001).