Ion guide

An ion guide for guiding ions along an ion channel, comprising:

• a plurality of electrodes arranged on a surface of the ion guide and configured to apply a time-varying potential to repel the ions from the surface;
• and
• a counter electrode arrangement comprising a main counter electrode portion and having an opening extending along the counter electrode arrangement, the counter electrode arrangement configured to apply a direct current (DC) counter potential to force the ions towards the surface, the time-varying potential and the DC counter potential together confining the ions in the ion guide,
• wherein the counter electrode arrangement is configured to apply a stronger DC counter potential in a region adjacent to the main counter electrode portion than in a region adjacent to the opening, thereby confining the ions in an ion channel corresponding to the opening.