STUDY ON NEGATIVE-IONS PREPARED IN-SITU IN A QUADRUPOLE ION-TRAP

The quadrupole ion trap mass spectrometer is of great interest for che mical analysis. Nevertheless, negative ion studies using in situ ioniz ation (negative ions created inside the ion trap) are difficult and li mited. This paper describes the difficulties that occurred during nega tive ion analysis when using a commercial gas chromatography quadrupol e ion trap mass spectrometer (GC/MS) system. Detection problems, due t o simultaneous trapping of both positive and negative ions, are explai ned by the large kinetic energies of the ejected ions. Negative ion fo rmation, produced by electron capture, is limited by the fundamental R F field, which imparts high kinetic energy to the electrons, precludin g the possibility that the electrons will reach the thermal energies n eeded for electron capture processes. In addition, simultaneous confin ement of negative and positive ions affects the recorded mass spectra. Space charge potentials that exist inside the ion trap (due to the io nization conditions necessary to detect negative ions) induce the dest abilization of positive ions at higher mit ratios, while negative ions in the same mit range are stabilized. Moreover, loss of resolution an d decalibration could occur for negative ions when they are ejected in the presence of positive ions with higher mit ratios. At the very lea st, ion-ion reactions could limit the observation of negative ions. Th e understanding of these phenomena, viz. detection, formation, and sim ultaneous confinement, will permit the proposal of solutions for negat ive ion analysis with quadrupole ion trap mass spectrometers using in situ ionization.