Turbomolecular pumps for providing high and ultrahigh vacuums for scientific instrumentation are well known. Herein a vacuum is considered to be within the high vacuum region when the pressure is between 1×10−3 and 1×10−9 mbar, and is considered to be within the ultrahigh vacuum (UHV) region when the pressure is between 1×10−9 and 1×10−12 mbar.
Turbomolecular pumps are momentum transfer pumps in which gas molecules entering the pump are given momentum by impact with the moving rotor blades of the pump. The pump contains multiple stages of angled rotor and stator pairs mounted in series. Gas molecules struck by a rotor blade gain momentum and due to the angle of the blade, are given a component of motion parallel to the axis of the pump. The stator blades are stationary and are provided with a different angle with respect to the axis of the pump. The gaps between the stator blades accept the travelling molecules and pass them on to the next rotor blade where a further gain in momentum is provided. Multiple stages increase the pressure of the gas from an inlet to the exhaust of the pump. The turbomolecular pump is only fully effective operating in pressure regions in the molecular flow regime and does not exhaust to atmospheric pressure, but is backed by a forevacuum pump. The working pressure range of the turbomolecular pump is usually extended by coupling a molecular drag pump, such as a Holweck pump, to the exhaust side of the turbomolecular pump within the same pumping housing and driven by the same rotating shaft, enabling lower performance forevacuum pumps to be utilised and to allow oil-free forevacuum pumps to be used. In this case the combination of the turbomolecular pumping stage and the molecular pumping stage exhaust to a pressure of 1 mbar or so, the forevacuum pump exhausting to atmosphere.
Multiple port or split-flow pumps have been developed to enable the pumping of several chambers at different pressures, the pumps containing two to four (typically three) pumping ports spaced along the length of the pump, the length being parallel to the pump axis. The pump is usually composed of a stack of pumping stages including a multistage turbomolecular pumping unit and one or more molecular pumping stages, with different pumping ports forming inlets to the pump at different locations along the stack. Typically the highest pumping speed is available at the ultimate pumping port (the main inlet) which provides access to the lowest pressure region of the pump, whilst other pumping ports (further down the stack of pumping stages) are at higher pressures and may provide lower pumping speeds. The pumping speeds in a typical three port pump are often two ports having a similar pumping speed and the highest pressure port having a pumping speed about 1/10 of that of the others. This leads to the disadvantage that the pumping requirements for an analytical instrument are not easily met by a single split-flow pump.
Two slightly different constructions are known: a split-flow pump as disclosed in EP 603694 in which a multi-stage turbomolecular pump having multiple pumping ports is located within a dedicated pump housing, and a so-called cartridge split-flow pump as disclosed in U.S. Pat. No. 6,457,954 B1 in which the pump comprising all the functional elements including an inner housing may be combined into an outer housing adapted to a specific application.
Whilst such split-flow pumps typically comprise a combination of multi-stage turbomolecular pumps and viscous pumping stages, in particular molecular drag pumps, they are sometimes referred to only as turbomolecular pumps. Herein they are referred to as turbomolecular pump arrangements.
US 2010/0098558 A1 discloses a multiple inlet pump arrangement in which at least a first inlet surrounds a second inlet such that the second inlet seals only against the pressure within the first inlet and not against atmospheric pressure. This enables the use of metal-to-metal seals between all inlets that are surrounded by another inlet, and those seals may be of a type which does not cause plastic deformation of the metallic sealing material, eliminating the difficulties found when attempting leak-tight sealing using plastic deformation of multiple seals in parallel.
Broader penetration of mass spectrometry into routine applications is somewhat hindered by the cost and size of vacuum systems, especially for mass spectrometers utilising mass analyzers that operate in the ultrahigh vacuum regime, such as Orbitrap™, multi-reflection and multi-deflection time-of-flight mass analyzers, electrostatic traps, etc., and which incorporate atmospheric pressure ion sources, such as electrospray (ESI), atmospheric pressure chemical ionisation (APCI), matrix-assisted laser desorption/ionisation (AP-MALDI), etc. Prior art split-flow pump arrangements suffer from the disadvantage that only a limited number of stages of differential pressure may be accommodated by the multiple inlet pumps and two or more such pumps, plus one or more forevacuum pumps, are required for the mass spectrometers described above.
It is desirable to be able to pump a scientific instrument, in particular a complete mass spectrometer, by a single split-flow pump.
Against this background the invention has been made.