Abstract: A rotating valve body and a fixed valve body, said rotating body and fixed body having radial surfaces in sliding contact. The radial surface of the rotating body is provided with one or more internal sampling outlet ports arranged on the circumference of a circle having a center coincident with the axis of the rotating body. The side wall of said rotating body is provided with one or more external sampling inlet ports. Pairs of unlike ports are connected by ducts in said rotating body. The external sample inlet ports are connected to a sample tube, or tubes or sample suction tube or tubes which rotate together with the rotating valve body. The radial surface of the fixed valve body is provided with a single internal sample inlet port located the same distance from the axis as the internal sample outlet ports.