EP1052672B1 - Time-of-flight mass spectrometer ion source for gas sample analysis - Google Patents

Abstract

In accordance with the invention, the ion source of a time-of-flight mass spectrometer includes an electron gun having an electron source and at least one electrode for conditioning the flow of electrons, followed by at least one microchannel wafer for generating a pulsed secondary electron beam containing a greater number of electrons from a pulsed primary electron beam. The secondary electron beam enters a gas ionization area of an ion gun which produces a flow of ions which is then passed through the flight tube in order to be analyzed by an ion detector. This provides a high-performance ion source which is compact, sensitive and easy to integrate.

Description

The present invention relates to ionization means of gaseous samples for analysis in a mass spectrometer.

In a mass spectrometer, a sample is analyzed gaseous by bombarding the sample with a stream of electrons, then moving the ions thus obtained to differentiate them then according to their trajectory or their speed.

There is interest in producing an important ionization of the gaseous sample, in order to increase the sensitivity of the measurement, and increase the resolution of the mass spectrometer.

In time-of-flight mass spectrometers, the ions produced by the ion source are launched at the entrance to a tube of flight in which they maintain a constant speed, and detects at the exit of the flight tube the flight time corresponding to each type of ions in the gaseous sample to be analyzed, to deduce their nature. It is necessary for that to launch at the entrance of the flight tube a previously accelerated ion packet, identify the departure time of the packet of ions, and identify the arrival times of the different ions at the other end of the flight tube.

It is then advantageous to generate ion packets of lowest possible duration, including a maximum number of ions. This is achieved by a pulsed ion source.

Ionic sources commonly used in mass spectrometers include an electron gun having a source of electrons and one or more electrodes of conditioning the electron flow to generate a flow of suitable electrons directed to a gas ionization zone in which ions are formed which are subjected to one or more electrodes ion flow conditioning. The flow of electrons is usually directed to the gas ionization zone in a direction perpendicular to the direction of the flight tube of the mass spectrometer. This results in significant bulk, and a difficulty of integration. The quantity of ions produced is relatively low, which limits the sensitivity of the device.

The problem proposed by the present invention is design a new ion source structure for mass spectrometer, having a greater compactness and a greater sensitivity, being easily integrated with other components of a mass spectrometer.

To reach these objects as well as others, a source ionic system for a mass spectrometer according to the invention comprises a electron gun having an electron source and one or more electrons flow conditioning electrodes to generate a appropriate electron flow directed to a gas ionization zone in which ions are formed which are subject to one or more ion flow conditioning electrodes; downstream Electron flow conditioning electrodes, interposed in the electron stream one or more microchannel pancakes, so that from a pulsed primary electron beam relatively few electrons, a beam is generated secondary electronic pulsed containing a lot of electrons.

The microchannel pancakes ensure a multiplication of flow of electrons, so the subsequent ionization of the gaseous sample is also multiplied. Sensitivity and resolving power of the device are thus greatly increased.

It is advantageous to dispose, downstream of the zone occupied by the microchannel pancake (s), at least one additional electrode adapted to disperse the beam secondary electronics in order to preserve its qualities while improving its spatial qualities.

Thus, the increase of the ionization is still favored of the gaseous sample, and therefore the sensitivity of a device incorporating the ion source.

Preferably, the gas ionization zone is located between an upstream repulsion electrode traversed by the beam secondary electronics and retaining the electrons by pushing back the ions, and a downstream acceleration electrode that attracts the ions.

Thanks to this arrangement, we can place the source Ionic in alignment with the axis of the flight tube at the entrance of the tube theft of a time-of-flight mass spectrometer. We obtain better integration of the ion source, and greater compactness of the device.

The ionization zone should preferably be close immediate or microchannel slabs, so that the beam secondary electronics retains its temporal qualities and remains dense, so that all the ions in a packet of ions penetrate substantially at the same time in the flight tube.

One can use as a source of electrons a filament heated to a suitable temperature to generate a flow of electrons by thermoemission, in a traditional way. The primary electron beam is then pulse modulated by a deflection electrode.

Alternatively, the electron source can advantageously to be a microtip field emission cathode, producing a pulse-modulated primary electron beam.

The invention can find particular application in the production of time-of-flight spectrometers incorporating such ion source.

• la figure 1 illustre le schéma de principe d'un spectromètre de masse à temps de vol selon un mode de réalisation de la présente invention ;
• la figure 2 est une vue schématique en perspective en partie découpée d'une galette à microcanaux pour amplification du flux d'électrons ; et
• la figure 3 est une coupe longitudinale d'un canal de la galette à microcanaux de la figure 2, illustrant le principe de l'amplification du flux d'électrons.

Other objects, features and advantages of the present invention will become apparent from the following description of particular embodiments, with reference to the accompanying drawings, in which:

• Fig. 1 illustrates the block diagram of a time-of-flight mass spectrometer according to an embodiment of the present invention;
• Figure 2 is a schematic perspective partially cut away view of a microchannel wafer for amplification of the electron flow; and
• Figure 3 is a longitudinal section of a channel of the microchannel wafer of Figure 2, illustrating the principle of the amplification of the electron flow.

Referring to Figure 1, a mass spectrometer to flight time according to the embodiment shown comprises of generally an electron gun 1, followed by an ion gun 2, itself followed by a flight tube 3 whose output communicates with a ion detector 4.

The electron gun 1 comprises an electron source 5. In the embodiment illustrated in the figure, the electron source 5 is a filament such as a tungsten filament fed by a heating generator 6 to be heated to a temperature sufficient to provide ion thermoemission. Electrons emitted by the electron source 5 are solicited by one or more electron flow conditioning electrodes 7, for example an acceleration electrode 71 and one or more electrodes of focus 72.

In the case of an electron source 5 in the form of thermoemission filament, a deflection electrode 73 makes it possible to pulse modulating the outgoing electron flow 8.

Alternatively, we can use as a source of electrons A microtip field emission cathode, comprising a conductive substrate on which microtips are made conductors engaged in cavities of an insulating layer interposed between the substrate and a positively polarized gate. Such a microtip field emission cathode makes it possible to modulate by itself the outgoing flow of electrons, without requiring of deflection electrode 73.

Downstream of the flow conditioning electrodes of electrons 7, one interposes according to the invention in the flow of electrons 8 one or more microchannel patties. In the embodiment illustrated in Figure 1, a first microchannel slab 9 and a second microchannel slab 10, separated from each other by an intergalette electrode 11. A from an 8-pulsed primary electronic beam containing relatively few electrons, microchannel slabs 9 and 10 generate a pulsed secondary electron beam containing many electrons, giving a gain of 100 to several thousands.

In practice, the primary electron beam can be equivalent to an electric current of the order of 1 to 10 μ A, and the secondary electron beam can correspond to several milliamps, according to the gain of the microchannel slabs 9 and 10.

Primary 8 and Secondary 12 electron beams can for example be formed of pulses whose duration is the order of the nanosecond.

The constitution and the principle functioning of microchannel patties are explained in connection with the figures 2 and 3. As seen in Figure 2, a microchannel slab 9 is a generally flat element having a thickness E of the order of 0.5 mm, and consisting of side-by-side juxtaposition a very large number of very small glass capillary tubes diameter, comprising for example the tube 13, oriented according to axes perpendicular to the general plane of the slab 9. The tubes capillaries may have a diameter e of about 12 microns, and they are open at both ends on the faces The main faces of the slab 9. The main faces of the slab 9 are metallized, to constitute, as illustrated in FIG. an input electrode 14 and an output electrode 15, subjected to a potential difference VD. The potential of the electrode output 15 is greater than the potential of the input electrode 14. The inner wall of the capillary tube 13 is treated to present a suitable resistance, and forms an electron multiplier independent secondary. When an electron beam primary electronics 8 gets into tube 13 it can come hit the wall of the tube 13 and pick up one or more others electrons that are accelerated by the present electric field between the input electrodes 14 and output 15. The electrons thus detached will strike themselves the opposite wall of the tube 13, picking up other electrons that are themselves accelerated, and he results from step by step the multiplication of electrons in movement, producing a secondary electron beam 12 containing a lot of electrons.

Referring again to Figure 1, the beam secondary electronics 12 propagates to an ionization zone 16 inside the ion gun 2. In this ionization zone 16, the electrons hit the atoms of the gaseous sample at analyze, and transform them into ions. The gas ionization zone 16 is located between a repulsive electrode 17 upstream traversed by the secondary electron beam 12 and which retains the electrons by repelling ions, and an accelerating electrode downstream 18 which attracts the ions.

The ion stream 19 thus produced is sent to the inlet 20 of the flight tube 3, then travels the length of the flight tube 3 to exit through its exit 21 and enter the ion detector 4. Thus, as illustrated in FIG. 1, the ion source is arranged in line at the entrance of flight tube 3 of the spectrometer of mass in flight time.

The ion detector 4 may comprise wafers to microchannels 22 and 23, generating a multiplied electron flow from strike a target electrode 24. The measurement is made by detecting the electrical pulses collected by the target electrode 24.

In the embodiment illustrated in FIG. 1, provision is made further downstream of the area occupied by the at least one microchannels 9 and 10 of the electron gun, at least one electrode additional 25 adapted to disperse the electron beam secondary 12 in order to preserve its temporal qualities while by improving its spatial qualities. We thus increase ionization in the ionization zone 16.

Preferably, the ionization zone 16 is close immediate microchannel slab 10, of which it is separated by a reduced distance, for example from 1 to 2 mm approximately.

The present invention is not limited to the modes of which have been explicitly described, but include the various variants and generalizations covered by the claims.

Claims (8)

1. An ion source for mass spectrometers, the source including an electron gun (1) having an electron source (5) and at least one electrode (7) for conditioning the flow of electrons to generate an appropriate flow of electrons directed towards a gas ionization area (16) in which ions are formed which are acted on by at least one electrode (17, 18) for conditioning the flow of ions, the source being characterized in that at least one microchannel wafer (9, 10) is disposed in the flow of electrons (8) downstream of the electrodes (7) for conditioning the flow of electrons so that a pulsed secondary electron beam (12) containing many electrons is generated from a pulsed primary electron beam (8) containing relatively few electrons.
2. An ion source according to claim 1, characterized in that it includes an additional electrode (25) downstream of the area occupied by the microchannel wafer(s) (9, 10) for dispersing the secondary electron beam (12) to retain its temporal properties and improve its spatial properties.
3. An ion source according to either claim 1 or claim 2, characterized in that the gas ionization area (16) is between an upstream repulsion electrode (17) through which the secondary electron beam (12) is passed and which retains the electrons by repelling the ions and a downstream acceleration electrode (18) which attracts the ions.
4. An ion source according to any one of claims 1 to 3, characterized in that it is aligned with the entry of the flight tube (3) of a time-of-flight mass spectrometer.
5. An ion source according to any one of claims 1 to 4, characterized in that the gas ionization area (16) is in the immediate vicinity of the microchannel wafer(s) (9, 10).
6. An ion source according to any one of claims 1 to 5, characterized in that the electron source (5) is a filament heated to an appropriate temperature to generate a flow of electrons by thermal emission and the primary electron beam (8) is pulse modulated by a deflector electrode (73).
7. An ion source according to any one of claims 1 to 5, characterized in that the electron source (5) is a micropoint-type field-emission cathode producing a pulse modulated primary electron beam.
8. A time-of-flight mass spectrometer characterized in that it includes an ion source according to any one of claims 1 to 7.

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