Study on negative ions prepared in situ in a quadrupole ion trap

The quadrupole ion trap mass spectrometer is of great interest for chemical analysis. Nevertheless, negative ion studies using in situ ionization (negative ions created inside the ion trap) are difficult and limited. This paper describes the difficulties that occurred during negative ion analysis when using a commercial gas chromatography quadrupole ion trap mass spectrometer (GC/MS) system. Detection problems, due to simultaneous trapping of both positive and negative ions, are explained by the large kinetic energies of the ejected ions. Negative ion formation, produced by electron capture, is limited by the fundamental RF field, which imparts high kinetic energy to the electrons, precluding the possibility that the electrons will reach the thermal energies needed for electron capture processes. In addition, simultaneous confinement of negative and positive ions affects the recorded mass spectra. Space charge potentials that exist inside the ion trap (due to the ionization conditions necessary to detect negative ions) induce the destabilization of positive ions at higher mit ratios, while negative ions in the same mit range are stabilized. Moreover, loss of resolution and decalibration could occur for negative ions when they are ejected in the presence of positive ions with higher mit ratios. At the very least, ion-ion reactions could limit the observation of negative ions. The understanding of these phenomena, viz. detection, formation, and simultaneous confinement, will permit the proposal of solutions for negative ion analysis with quadrupole ion trap mass spectrometers using in situ ionization.