Fundamental studies of ion injection and trapping of electrosprayed ions on a quadrupole ion trap

The quadrupole ion trap is a highly versatile and sensitive analytical mass spectrometer. Because of the advantages offered by the ion trap, there has been intense interest in coupling it to ionization techniques such as electrospray which form ions externally to the ion trap. In this work, simulation and experiment were employed to study the injection of electrosprayed ions into the quadrupole ion trap in a Finnigan MAT LCQ LC/MSn mass spectrometer. SIMION v6.0 was chosen for the simulation studies because it allowed the actual ion trap electrode geometry including endcap holes to be simulated. The endcap holes weaken the rf trapping field in the region near the holes; this distortion of the field is important when ions have large axial excursions as they do in ion injection. In addition, the field penetrates out the endcap holes and affects ions as they approach the ion trap. The results of simulating ions started outside the ion trap agreed well with experimentally obtained ion injection data of the effect of q(z) during injection. From these simulations, a model for the process by which injected ions are trapped was developed. Injected ions can be trapped even with the modest helium buffer gas pressures commonly used (approximate to 1 mTorr) because ions naturally oscillate for long times (and therefore distances) at certain q(z) values and initial rf phases. This allows enough collisions to occur to damp the ions' excess kinetic energy. (Int J Mass Spectrom 190/191 (1999) 81-102) (C) 1999 Elsevier Science B.V.