A mass analyser for use in a mass spectrometer, the mass analyser having: a set of sector electrodes spatially arranged to provide an electrostatic field in a 2D reference plane suitable for guiding ions along an orbit in the 2D reference plane, wherein the set of sector electrodes extend along a drift path that is locally orthogonal to the reference plane so that, in use, the set of sector electrodes provide a 3D electrostatic field region; and an injection interface configured to inject ions into the mass analyser via an injection opening such that the ions injected into the mass analyser are guided by the 3D electrostatic field region along a 3D reference trajectory according to which ions perform multiple turns within the mass analyser whilst drifting along the drift path, wherein each turn corresponds to a completed orbit in the 2D reference plane. The injection interface includes at least one injection deflector located within the mass analyser, the at least one injection deflector being configured to deflect ions injected into the mass analyser in the direction of the drift path, wherein the injection interface is preferably configured so that ions guided along the 3D reference trajectory are, after injection into the mass analyser, kept adequately distant from the injection opening such that they are substantially unaffected by electric field distortions around the injection opening.