the pump by bringing solvent into contact with the rotor 1 after the first few turns of the thread the likelihood of backward contamination of the solvent into the chamber will be reduced furthermore where solvent is introduced in the inlet region of the pump the pressure is such at the inlet that there is an increased risk that the solvent will flash in processes where it is necessary for the solvent to remain in liquid phase the solvent must be introduced closer towards the exhaust region of the pump where the pressures will have risen as solvent is introduced through a number of ports 2 along the length of the outer housing stator 5 the overall effect is to gradually increase the quantity of solvent present as the likelihood of residue build up on the rotor 1 increases towards the exhaust stages an additional benefit may be seen in some configurations where addition of liquid into the final turns of thread of the rotor will act to seal the clearances between the rotor and the stator in this region of the pump thus leakage of gas will be substantially reduced and performance of the pump will be improved in some processes it is not appropriate to introduce solvent during operation as the waste products from the evacuated chamber are collected at the outlet of the pump for a particular purpose and this material.