INVESTIGATION OF SPACE-CHARGE INTERACTIONS WHICH ARISE DURING SIMULTANEOUS CONFINEMENT OF POSITIVE AND NEGATIVE-IONS IN AN ION-TRAP MASS-SPECTROMETER

While positive and negative ions are created simultaneously in a quadr upole ion trap during electron ionization, there is a marked preponder ance of the former species. With prolonged ionization and a relatively high concentration of substrate molecules, positive ion species of lo w mass/charge ratio can give rise to space charge potentials that can neutralize and exceed the normal trapping potentials for positive ions of high mass/charge ratio, leading to the ejection of positive ions, but not negative ions, from the ion trap. The onset of space charge is manifested by the ejection of positive ions of m/z > 600; as the spac e charge potential is increased incrementally, ejection of positive io ns is extended to those of lower mass/charge ratio, that is, to m/z ap proximate to 300. Negative ions are not affected by the space charge i nduced by the positive ions; within the limits of experimentation, neg ative ions in the mass/charge ratio range m/z 352-633 remain confined within the ion trap and can be ejected dating a normal analytical scan . Under the influence of positive ion-induced space charge, there is a n enhancement of the storage of negative ions having low values of the trapping parameter q(z). These observations can be rationalized using a theoretical approach based on a pseudo-potential web model. Althoug h not investigated here, the simultaneous storage of positive ions and negative ions permits tbe interactions between these species to be st udied. (C) 1997 by John Wiley & Sons, Ltd.