Ion mobility spectrometer with non-radioactive ion source

Details Ion mobility spectrometer with non-radioactive ion source Ion mobility spectrometer with non-radioactive ion source

2002-04-25

2003-07-01

Lee, John R. (Department: 2881)

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

C250S281000, C250S282000, C250S286000, C250S42300F, C250S42300F, C250S424000, C250S435000

Reexamination Certificate

active

06586729

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to ion mobility spectrometry and, more specifically, to the non-radioactive electron source of an ion mobility spectrometer (IMS).
BACKGROUND OF THE INVENTION
In the prior art there exists an ion mobility spectrometer having an evacuated electron source chamber which contains a non-radioactive electron source. The electron source is connected to the negative pole of an accelerating voltage source, and an x-ray anode is connected to the positive pole of the accelerating voltage source. X-ray radiation produced by electrons from the electron source impinging on the x-ray anode enters an adjoining reaction chamber of the IMS through a gas-tight x-ray window, which is impermeable to the electrons generated by the electron source. An IMS or method such as this is known from U.S. patent Ser. No. 09/617,716, which is incorporated by reference herein in its entirety.
The electron source of such an IMS must be operated in a vacuum. To ensure reliable and lasting operation, the pressure must be maintained at less than 10
−3
mbar, and in some cases at less than 10
−5
mbar. In such systems, the measurement process must be interrupted if the maximum permissible pressure is exceeded. However, because supply energy is very limited, it is undesirable to run a vacuum pump continuously. The volume of the electron source chamber is very small, and due to the presence of electrical vacuum feedthroughs, and the need for an x-ray window that is very thin, leakages into the chamber tend to occur. In addition, the risk of an unexpected additional micro-leak is always present.
SUMMARY OF THE INVENTION
In accordance with the present invention, an ion mobility spectrometer is provided that has a control electrode, in particular a Wehnelt cylinder, between the electron source and the anode of the electron source vacuum chamber. The anode current is regulated via the voltage on the control electrode, and an electrical circuit is provided that monitors the anode current between the positive and negative pole during operation. The invention also provides for a safety circuit to shut down the electron source in an emergency if a fault current between the anode and the control electrode exceeds a specified limit such as, for example, a current in the range from 1 nA to 1 &mgr;A.
The primary task of the control electrode, which has a more negative potential than the cathode during operation, is to control the intensity of the anode current. However, it can also be used to measure the undesirable ion current arising from the ionized background molecules. This serves as a measure of the residual gas pressure in the vacuum of the electron source chamber.
The electron source chamber may contain one or more getter pearls provided with electrical heat supplies. When thermally activated, one of these getter pearls will absorb the molecules of any residual gas coming into contact with it, thus improving the vacuum. In this way, each one of these pearls can, generally in a number of separate steps, improve the vacuum several times over. Altogether, each pearl can absorb several liters. In addition, a negative potential can be applied to the getter pearls so that the electrons are repelled and the positive residual gas ions are absorbed.
In one embodiment, the IMS contains an electron source in the form of a thermionic cathode, which can be electrically heated so that the anode current can be regulated via the heating power of the thermionic cathode. This can be used to shift the anode current within wide ranges. In particular, it can be significantly increased for short periods, if necessary, in order to increase the sensitivity of the measurement.
In an alternative embodiment, the electron source contains a cold emitter so that the anode current can be regulated via the potential of the control electrode. Unlike thermionic electrodes, the cold-emitter technology has the advantage that it consumes less energy and has a longer service life. In particular, the surface structure of the cold emitter still offers many possibilities for adaptation to the actual problem as well as optimization. The potential of the cold emitter, in relation to the control electrode, may lie between +5 V and +50 V. Within this range, the anode current can be adjusted very well by varying the potential of the control electrode.
Also provided by the present invention is the use of an electrical circuit that interrupts the operation of the IMS for a specified time period. Such a time period may be between 5 and 15 minutes. The circuit also switches on the getter heating system in the event of the fault current exceeding a threshold value, such as between 1 nA and 1 &mgr;A. If the system is switched off at a fault current of approximately 1 &mgr;A, the IMS may require several hours to reach its optimum operating state, after the subsequent gettering. During this time, the anode current may be increased by, for example, increasing the heating of the thermionic cathode in order to provide sufficient sensitivity. However, it is possible to perform the gettering even when the pressure is significantly lower than the maximum permissible pressure when the fault current is within the range from 1 to 10 nA. In such a case, the operation of the IMS does not have to be interrupted.
When the IMS is operating, it is advantageous to regulate the anode current to a set-point value, in particular, within the range of 1 to 500 &mgr;A, and to maintain this value via the control voltage or, if necessary, the heating system for the thermionic cathode. At the same time, the spectra will be produced at a constant sensitivity so that they can be easily compared with one other.
When starting up the IMS or restarting the IMS after the gettering or after maintenance work, the anode current should be slowly regulated to the set-point value within a period of 1 to 10 minutes while keeping to the maximum permissible fault current. This has the advantage that the instrument cannot unexpectedly get into an operating state which would damage the electron source.
With the thermionic cathode, a thermistor (in particular a TNA type) can be integrated into the heating current circuit so that the heating current can only increase slowly. This is due to the fact that the ohmic resistance of the thermistor is initially large and decreases slowly only when the thermistor heats up under constant current loading, so that the heating current continuously increases until it reaches the equilibrium value. Of course, the anode current can still be regulated via the heating power even in the presence of the thermistor. It limits the rate at which the heating current increases only via the hardware.
In an illustrative embodiment of the invention, the electron source chamber is made predominantly from metal, for example stainless steel. Although a housing made from glass might provide better performance in the areas of vacuum tightness and degassing, a metal housing is by far the simpler to manufacture, can be made with greater precision and is easier to fit to the x-ray window mounting and other components of the IMS. The above-mentioned measures can be fully exploited to improve and monitor the vacuum. Metallic materials, such as stainless steels, may be used that are optimized in regard to their degassing properties by total or surface pretreatment, using thermal, mechanical or chemical methods.
The x-ray window is preferably made from beryllium, possibly with a thickness between 10 &mgr;m and 100 &mgr;m and with an effective diameter between 3 mm and 20 mm. Beryllium is used as the window material because of its low atomic number. This metal has the required vacuum tightness and mechanical stability for the given thicknesses and diameters under a pressure difference of 1 bar.
In one embodiment, an arrangement of the components in the electron-source chamber and the x-ray window is such that no electrons emitted by the electron source reach the x-ray window. This is achieved, for example, by a configuration in wh

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