Method and ion guide assembly for modulating a stream of ions

Abstract

The invention relates to a method for, in an ion guide (), modulating a stream of ions according to a modulation function, wherein the stream of ions includes at least N different ion species, wherein N is at least 1. This ion guide () forms an ion guide path, wherein the ions of the stream of ions are conveyed along the ion guide path in a conveying direction to form the stream of ions. The ion guide () includes an ion gate () arranged at an ion gate position on the ion guide path, wherein the ion gate () is adapted to provide an open state for allowing the ions passing the ion gate position when being conveyed along the ion guide path and a closed state for preventing the ions from passing the ion gate position. The ion guide () further includes a first arrangement () of conveying electrodes () arranged along the ion guide path, the first arrangement () of conveying electrodes () extending over a first section of the ion guide path, wherein the first section of the ion guide path reaches from at least the ion gate position downstream to at least a transition position on the ion guide path, wherein the first arrangement () of conveying electrodes () is adapted for generating first travelling waves having a first travelling wave amplitude and travelling along the first section of the ion guide path at a first travelling wave velocity for conveying the ions along the first section of the ion guide path. Furthermore, the ion guide () includes a second arrangement () of conveying electrodes () arranged along the ion guide path, the second arrangement () of conveying electrodes () extending over a second section of the ion guide path, wherein the second section of the ion guide path reaches from the transition position downstream, wherein the second arrangement () of conveying electrodes () is adapted for generating second travelling waves having a second travelling wave amplitude and travelling along the second section of the ion guide path at a second travelling wave velocity for conveying the ions along the second section of the ion guide path. According to the method, the stream of ions is modulated with the ion gate () according to the modulation function and AC voltages are applied to the first arrangement () of conveying electrodes () for generating the first travelling waves and to the second arrangement () of conveying electrodes () for generating the second travelling waves for conveying the ions downstream of the ion gate () along the first section and the second section of the ion guide path in the conveying direction away from the ion gate (), wherein for each ion species of the at least N different ion species, a ratio A is the average velocity of the ions of the respective ion species in the first section of the ion guide path divided by the first travelling wave velocity, wherein for each ion species of the at least N different ion species, a ratio B is the average velocity of the ions of the respective ion species in the second section of the ion guide path divided by the second travelling wave velocity, wherein the first travelling wave amplitude, the first travelling wave velocity, the second travelling wave amplitude and the second travelling wave velocity are chosen such that for each ion species of the at least N different ion species, the ratio A is larger than the ratio B.