Abstract: A valve, which is particularly useful in the inlet system of a mass spectrometer, has a fluid inlet (60) and a fluid outlet (68). A valve seat (64) is disposed between the fluid inlet (60) and the fluid outlet (68), and a valve closure member in the form of a piston (62) sliding in a cylinder (60), and a diaphragm (56), is engageable with the valve seat.A chamber (77) for receiving coolant (e.g. liquid nitrogen) is located on the side of the diaphragm (56) opposite the valve seat (64), and by thus cooling the diaphragm, fluid material in contact with the valve closure member is also cooled, and preferably condensed or frozen.The invention removes the requirement for a U- or T-shaped cold trap as used in conventional systems.