| The quadrupole ion trap, originally developed in the early 1950s by physicists Paul and Steinwedel as an ion storage device, has become one of the most powerful mass spectrometers available today. The modern quadrupole ion trap is most noted for its versatility, sensitivity and capability to provide multiple stages of mass spectrometry (MSn). There are several issues, however, that have plagued the ion trap since the introduction of the first commercial instrument. The most severe of these are compound-dependent mass shifts and a limited dynamic range due to space charge effects.;Compound-dependent mass shifts may result from the resonant ejection of fragile ions. As these ions come into resonance with the ejection frequency, they gain kinetic energy, collide with buffer gas molecules, and dissociate. The resultant fragment ions, which are lower in m/z, will no longer be stable within the ion trap, and will be immediately ejected and detected before the intact ions. The fragment ions and the non-dissociated precursor ion form one peak which has a significant amount of peak fronting and an overall broad peak shape.;The resonant ejection of fragile ions is not the only source of mass shifts. Overloading the ion trap can result in significant coulombic interactions, called space charge, leading to peak broadening and a shift to higher m/z. An ion of a particular m/z will experience a varying degree of space charge due to the number of ions of the same m/z value, the abundances and mass differences of neighboring ions, and the density of the charges in the trap controlled by the buffer gas pressure.;Space charge effects can be reduced by increasing the resonant ejection amplitude, increasing the drive frequency, or decreasing the trap pressure. At low buffer gas pressures, however, ions are allowed to venture near the endcap holes and experience a non-linear field. In a non-linear field an ion's stability is no longer purely dependent on its m/z value as the field imperfections perturb the trajectories of ions inside the ion trap. As a result, ions of a specific m/z experience multiple opportunities to be ejected, resulting in a splitting of the resultant peak. |
