Abstract: An ion mobility spectrometer (1) or other detection apparatus has an external dopant reservoir (22, 41, 24, 44) connected to it. The reservoir has a stainless steel base (22) with a recess (24), a heater (28) and a temperature sensor (32). The heater (28) and sensor (32) are connected to a feedback temperature control (3) to maintain a constant temperature of liquid dopant (100) in the recess (24). A lid (41) is sealed around the upper surface (23) of the base (22) and supports opposite ends of a length of vapour-permeable tubing (51) that is bent down so that a part of its length is immersed in the dopant. One end of the tubing (51) is connected with the IMS (1) and the other end opens externally so that air can be supplied along the tubing to the IMS and collect dopant vapour passed through the wall of the tubing.