Turbo molecular pump for mass spectrometer

A turbo molecular pump and mass spectrometers are disclosed. The turbo molecular pump comprises a rotor 300 in a low pressure input region 304A which, during operation, rotates with respect to a frame. The rotor has a shaft receiving member 302 from which a first blade portion 306A extends radially outwards and is connected to a second blade portion 306B which extends substantially paraxially to, and along, the shaft receiving portion towards a high pressure output region 304B. The disclosed mass spectrometer comprises a compartment with the described pump mounted in such that the second blade portion protrudes into the compartment. A multi-port pump is also considered and a multi-compartment spectrometer with the different ports connected to different compartments. The disclosed cage-like rotor stage allows for optimised pumping speed on systems with low gas flows and low ultimate pressures. This allows for a smaller rotor as well as a smaller overall form factor.