EP1052672A1 - 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 gas samples for analysis in a mass spectrometer.

In a mass spectrometer, we analyze a sample gaseous by bombarding the sample with a flow of electrons, then by setting in motion the ions thus obtained to differentiate them then according to their trajectory or their speed.

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

In time-of-flight mass spectrometers, ions produced by the ion source are launched at the entrance of a tube of flight in which they maintain a constant speed, and we detect at the outlet of the flight tube, the flight time corresponding to each type of gas sample ions to be analyzed, to deduce their nature. This requires launching at the entrance of the flight tube a ion accelerated packet, locate the start time of the packet of ions, and identify the times of arrival of the different ions at the other end of the flight tube.

It is therefore advantageous to generate ion packets of the shortest possible duration, including a maximum number of ions. This is obtained by an impulse ion source.

The ion sources usually used in mass spectrometers include an electron gun having a electron source and one or more electrodes of conditioning the flow of electrons to generate a flow of suitable electrons directed to a gas ionization zone in which form ions subjected to one or more electrodes ion flow conditioning. The electron flow is generally directed towards 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 integration difficulty. The quantity of ions produced is relatively low, which limits the sensitivity of the device.

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

To reach these and other objects, a source ion for mass spectrometer according to the invention comprises a electron gun having an electron source and one or more electron flow conditioning electrodes to generate a appropriate electron flow directed to a gas ionization zone in which are formed ions subjected to one or more ion flow conditioning electrodes; downstream of electron flow conditioning electrodes, we interpose in the electron flow one or more microchannel pancakes, so that from a pulsed primary electron beam containing relatively few electrons, we generate a beam secondary pulsed electronics containing a lot of electrons.

The microchannel pancakes ensure a multiplication of the flow of electrons, so the subsequent ionization of the gas sample is also multiplied. The sensitivity and resolving power of the device are found as well considerably increased.

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

Thus, we further promote the increase in ionization of the gas sample, and therefore the sensitivity of a device incorporating the ion source.

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

Thanks to this arrangement, the source can be placed ionic in alignment with the axis of the flight tube at the entrance of the tube time of flight mass spectrometer. We thus obtain better integration of the ion source, and greater compactness of the device.

The ionization zone should preferably be near immediate of the microchannel pancake (s), so that the beam secondary electronics keeps its temporal qualities and stays dense, so that all the ions in a packet of ions penetrate substantially at the same time in the flight tube.

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

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

The invention may find particular application in the production of time-of-flight spectrometers incorporating such a 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, characteristics and advantages of the present invention will emerge from the following description of particular embodiments, given in relation to the attached figures, among which:

• Figure 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 view partly cut out of a microchannel pancake for amplification of the electron flow; and
• Figure 3 is a longitudinal section of a channel of the microchannel pancake of Figure 2, illustrating the principle of amplification of the electron flow.

Referring to Figure 1, a mass spectrometer at 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 includes 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 brought to a temperature sufficient ensuring thermal emission of ions. The emitted electrons by the electron source 5 are requested 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 allows impulse modulating the outgoing electron flow 8.

Alternatively, you can use as an electron source 5 a microtip field emission cathode, comprising a conductive substrate on which microtips are produced conductors engaged in cavities of an insulating layer interposed between the substrate and a positively polarized grid. Such a microtip field emission cathode makes it possible to modulate by itself the outgoing flow of electrons, without requiring deflection electrode 73.

Downstream of the flow conditioning electrodes of electrons 7, according to the invention interposed in the flow of electrons 8 one or more microchannel wafers. In the embodiment illustrated in Figure 1, we use a first microchannel pancake 9 and a second microchannel pancake 10, separated from each other by an intergalette electrode 11. A from a pulsed primary electron beam 8 containing relatively few electrons, microchannel wafers 9 and 10 generate a pulsed secondary electron beam 12 containing lots of electrons, gaining 100 to many 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, depending on the gain of the microchannel pancakes 9 and 10.

Primary 8 and secondary 12 electronic harnesses can for example be formed by pulses whose duration is the nanosecond order.

The constitution and functioning in principle of microchannel wafers are explained in relation to the figures 2 and 3. As seen in FIG. 2, a microchannel pancake 9 is a generally flat element, having a thickness E of of the order of 0.5 mm, and consisting of the juxtaposition side by side of a very large number of very small glass capillary tubes diameter, including for example the tube 13, oriented according to axes perpendicular to the general plane of the wafer 9. The tubes capillaries can have a diameter e of about 12 microns, and they are open at their two ends on the faces principal of the pancake 9. The principal faces of the pancake 9 are metallized, to constitute, as illustrated in FIG. 3, 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 capillary tube 13 is treated to present an appropriate resistance, and forms an electron multiplier independent secondary. When an electron in the beam primary electronics 8 enters tube 13 it can come strike the wall of tube 13 and unhook one or more others electrons which are accelerated by the electric field present between the input 14 and output 15 electrodes. The electrons thus detached will strike themselves on the opposite wall of the tube 13, picking up other electrons which are accelerated themselves, and it gradually the multiplication of the electrons in movement, producing a secondary electron beam 12 containing a lot of electrons.

Referring again to Figure 1, the beam secondary electronics 12 spreads to an ionization zone 16 inside the ion gun 2. In this ionization zone 16, the electrons strike the atoms of the gas sample at analyze, and turn them into ions. The gas ionization zone 16 is located between an upstream repulsion electrode 17 crossed 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 flow 19 thus produced is sent to input 20 of flight tube 3, then travels the length of flight tube 3 to exit through its outlet 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 time-of-flight mass.

The ion detector 4 may include pancakes with microchannels 22 and 23, generating a multiplied electron flow coming strike a target electrode 24. The measurement is carried out by detecting the electrical pulses collected by the target electrode 24.

In the embodiment illustrated in FIG. 1, provision has been made in addition, downstream of the area occupied by the pancake (s) 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. So we increase ionization in the ionization zone 16.

Preferably, the ionization zone 16 is close immediately of the microchannel pancake 10, from 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 it includes the various variants and generalizations which are within the reach of the skilled person.

Claims (8)

Ion source for mass spectrometer, including an electron gun (1) having an electron source (5) and one or more multiple electron flow conditioning electrodes (7) to generate an appropriate electron flow directed to an area ionization gas (16) in which ions subjected to one or more ion flow conditioning electrodes (17, 18), characterized in that, downstream of the electrodes electron flow conditioning (7), we interpose in the flow of electrons (8) one or more microchannel wafers (9, 10), of so that, from a pulsed primary electron beam (8) containing relatively few electrons, we generate a beam secondary (12) pulsed electronics containing many electrons. Ion source according to claim 1, characterized in that we have, downstream of the area occupied by the microchannel wafers (9, 10), 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. Ionic source according to one of claims 1 or 2, characterized in that the gas ionization zone (16) is located between an upstream repulsion electrode (17) through which the secondary electron beam (12) which retains the electrons by repelling the ions, and a downstream acceleration electrode (18) which attracts ions. Ion source according to any one of Claims 1 to 3, characterized in that it is arranged in alignment at the inlet of the flight tube (3) of a mass spectrometer in flight time. Ion source according to any one of Claims 1 to 4, characterized in that the ionization zone gas (16) is in the immediate vicinity of the pancake (s) microchannels (9, 10). Ion source according to any one of claims 1 to 5, characterized in that the electron source (5) is a filament heated to a temperature suitable for generate a flow of electrons by thermoemission, and the beam primary electronics (8) is pulse-modulated by an electrode deflection (73). Ion source according to any one of claims 1 to 5, characterized in that the electron source (5) is a microtip field emission cathode, producing a pulse modulated primary electron beam. Time-of-flight mass spectrometer, characterized in what it includes an ion source according to any one of claims 1 to 7.

Images