Ion guide

An ion guide 10 is disclosed comprising: an inlet 11 having a first longitudinal axis 12; and an outlet 13 having a second longitudinal axis 14, the first axis being substantially co-axial with the second axis; wherein the inlet, outlet and a middle section 16 therebetween together define an ion path, said path being non-linear in at least part of the middle section. The non-linear part may be V-shaped and offset from the first/second longitudinal axes by a distance at least equal to the diameter of the ion path, e.g. 4mm. The non-linear section preferably occludes a line-of-sight through the device, thereby preventing light generated by, for example, a filament in an upstream ionisation source from reaching an optical detector, such as a photomultiplier tube, located downstream thereof. A shroud (60, Fig. 4) comprising a housing (61) sized to receive an optical detector (5) is also disclosed, the housing including an aperture (66) to allow ions to be detected therethrough.