FR2560714A1 - Mass spectrometer and method of ionising and analysing gases. - Google Patents

Abstract

The ion source consists of a heated circular filament f and a pair of small ring magnets a. Most of the ionising impacts take place at the periphery of the ring to which the electronic trajectories are tangential. The ions are returned by the magnetic field and injected, after acceleration by E1, into the analysing system, which itself also consists essentially of a pair of ring magnets b. The ions incur the alternate action of magnetic B and electric E2 fields before collection on a multiplier situated on the axis of symmetry M. The great length of the ion injection slot q combined with the effectiveness of the ionic extraction affords great sensitivity: the confinement of the trajectories allows very compact construction whilst high resolution is afforded by the alternate action of the fields. The apparatus is intended for analysing gas mixtures under reduced pressure, a fundamental problem in the vacuum-production of numerous electronic components.

Description

MASS SPECTROGRAPHER AND METHOD
IONIZATION AND GAS ANALYSIS.

 The present invention relates to magnetic deflection mass spectrographs, which are more compact and more sensitive than existing devices.

où V est la tension d'accélération. Le faisceau dévié est reçu sur une fente étroite placée à distance: l'angle de déviation est ainsi défini avec précision et la fente sélectionne, pour chaque valeur de V, les ions de masse m correspondante

Magnetic deflection spectrographs are based on the deflection of a thin ion beam by a magnetic field., In a given field, the deflection angle varies as

where V is the acceleration voltage. The deflected beam is received on a narrow slit placed at a distance: the deflection angle is thus precisely defined and the slit selects, for each value of V, the ions of corresponding mass m

The resolution obviously increases with the distance from the selector slit to the deflecting field. The drawbacks of this type of device are obvious: a high resolution results in a large footprint. On the other hand, the use of a fine ion beam greatly limits the sensitivity of the system.

 In the new invention, the separating device has the symmetry of revolution; the ions are injected radially in the median plane of the separating device, all along a circular slit: the number of injected ions can thus between very large and these ions, due to the symmetry of the system, are all present in identical initial conditions.

A first magnetic field, created by a pair of permanent magnets in the shape of a crown, produces a first separation; the ions of different masses describe circular trajectories of different radius and are taken up, under different initial conditions, by a second field (magnetic or electrostatic) which selectively collects the ions or accentuates their separation before collection by a third field.

 Such a device makes it possible, while confining the trajectories in a reduced space, to obtain good resolution with high sensitivity.

The ionization and ion injection device is made up as follows: a circular heated filament located in the median plane of the system emits electrons which are accelerated by a radial electrostatic field; these electrons are thus injected, still in the median plane, into the magnetic field produced by a pair of magnets in the shape of a crown, just like in the separating device. The circular electronic trajectories are surrounded by a large circle of radius r. Most ionizing shocks take place in the immediate vicinity of this circle. A second radial electrostatic field, applied beyond the circle r, extracts and accelerates the ions formed under the variable voltage
V. In this ionization device, the magnetic field plays a triple role: on the one hand, it serves as a barrier to the electrons and prevents them from entering the region of variable potential where the ions are accelerated; on the other hand, a significant part of the electronic trajectories is practically confused with the envelope circle: the ions are therefore mainly formed in the immediate vicinity of the accelerating field but in a region which is still equipotential and they have the same initial kinetic energy; finally, most of the ions emitted in all directions with the speed corresponding to thermal agitation in the gas, are folded towards the region of acceleration: the magnetic field therefore acts as a field of attraction, without modifying the kinetic energy of ions.

 The invention will be better understood by referring to particular embodiments, given by way of example and illustrated by the appended drawings.

 FIG. 1 schematically represents an embodiment of the ion source. The electrons emitted by the heated filament f are accelerated by the electric field E. They penetrate into the magnetic induction B produced by the ring magnets a. They describe in the air gap circular paths s having the outer circumference b of the magnets as an envelope. The ions formed in the vicinity of this circumference are returned by induction B (trajectories in dashed lines3 towards the accelerating electric field Ep which injects them into the analysis system (not shown).

FIG. 2 represents a simplified embodiment of the analysis system. The ion source previously described is at the center of the system. The ions are injected radially into the air gap of the ring magnets b with a kinetic energy corresponding to the accelerating potential V. They describe circular trajectories of radius

For a given value V, all the trajectories corresponding to the same mass have as an envelope a large circle of radius

The ions are selected by a circular slot p of radius Ro, framed by two deflection electrodes i and j. For RrorRo, the ions can cross the slit and continue their circular trajectory towards the axis of the system and be captured by the multiplier M. (Each mass corresponds to a value of V such that R = Ro. By varying V, we can successively select all the masses).

à la sortie de b, ils recoivent une légère comp6sante axiale de vitesse par l'action de l'électrode annulaire p. Cette action est sélective et plus marquée pour les ions qui croisent cette électrode sous un angle faible. Ils sont alors repris par le champ électrique radial Ee entre les cylindres m et n.Les ions qui ont, entre les cylindres, une trajectoire quasi-circulaire ( condition

) peuvent sortir de l'espace d'analyse sans rencontrer le cylindre intérieur n et sont collectés par le multiplicateur M
La figure 4 représente un mode de réalisation inversé comme le précédent mais où le champ électrique radial est remplacé par une deuxième induction B1 , de sens opposé à B. Les trajectoires ont la forme indiquée sur la figure. Une électrode annulaire e portée à un potentiel légèrement inférieur à celui de l'espace où règne B collecte seulement les ions dont la trajectoire est au moins en partie parallèle à e.On remarquera que dans ce dernier cas ( deux inductions opposées ), on peut réaliser un circuit magnétique fermé par des armatures externes.FIG. 3 represents an inverted embodiment: the ion source, of the same type as in the previous example, is this time at the periphery of the analysis system. The ions describe in the air gap of the crown b arcs of a circle

at the exit of b, they receive a slight axial component of speed by the action of the annular electrode p. This action is selective and more marked for the ions which cross this electrode at a weak angle. They are then taken up by the radial electric field Ee between the cylinders m and n. The ions which have, between the cylinders, a quasi-circular trajectory (condition

) can leave the analysis space without meeting the internal cylinder n and are collected by the multiplier M
FIG. 4 represents an inverted embodiment like the previous one but in which the radial electric field is replaced by a second induction B1, of direction opposite to B. The trajectories have the form indicated in the figure. An annular electrode e brought to a potential slightly lower than that of the space where B reigns collects only the ions whose trajectory is at least partly parallel to e. It will be noted that in the latter case (two opposite inductions), one can realize a magnetic circuit closed by external armatures.

 In all the systems described, the resolution can be further improved by giving the induction increasing or decreasing values with the distance to the axis (use of conical irons).

valeur facile à obtenir, on trouve que

pour V variant de 1600 à 16V et le nombre de masse de 100 à 1

To fix ideas on the dimensions of the device, note that the diameter is roughly equal to three or four times the radius r of the ion trajectories. For

easy to get value, we find that

for V varying from 1600 to 16V and the mass number from 100 to 1

The outside diameter of the device is thus of the order of ten centimeters.

 These devices are mainly intended for the qualitative and quantitative analysis of gaseous mixtures at reduced pressure, a fundamental problem for the development under vacuum of many electronic components.

Claims (1)

CLAIMS Mass XspectrograDhe combining one or more magnetic deflections with the action of electrostatic fields, characterized by the fact that the system has symmetry of revolution; by the fact that the ions are injected radially from a source constituted by a heated circular filament and a pair of small corona magnets which con fi ne the ionizing electrons at the interior or exterior periphery of the analyzer system proper; says that the first pair of deflection magnets in the analyzer system is in the shape of a crown; by the fact that the selective action of the other fields, in particular electrostatic, is due to the preliminary dispersion obtained by the first magnetic induction. Mass spectrograph according to revevdication 1 characterized in that the ion source is at the center of the system in the free space inside the deflection ring; by the fact that a disc pierced with a narrow circular slot is located in the median plane between the magnets of the crown; by the fact that two annular electrodes of the same radius as the slot are placed on either side of it and are brought to a potential slightly higher than that of the disc: for each value of the accelerating potential, the ions having a mass such that their trajectories are tangent to a circle of the same radius as the slit, pass through it and turn towards the central part of the analyzer where they are captured by a multiplier. 3. Mass spectrograph according to claim1 characterized in that the ion source is at the outer periphery of the device; by the fact that a ra dial electrostatic field directed towards the axis of the system is applied between two coaxial cylinders housed in the free space left by the crown; by the fact that an annular polarized electrode and if killed between the crown and the first cylinder communicates to the ions an axial component of speed; by the fact that, for each value of the accelerating potential, the ions of suitable mass describe between the cylinders almost helical trajectories and are picked up by a multiplier located on the axis of the system, the ions of different mass being lost on the cylinder interior. 4 Mass spectrograph according to claim 1, characterized  by the fact that the ion source is on the outer periphery  re of the device; by the fact that a second pair of magnets  deflection, of cylindrical section, is housed in space  free left by the first crown and produces an induction  opposite direction to the first; by the fact that an electrode an  collection ring, polarized, is located on one of the fa  these interns of the central magnet; by the fact that the magnets  supplemented by an external armature constitutes a magnified circuit  tick closed. , Mass spectrograph according to claim 1 and 2, 3 or 4 characterized in that the magnetic inductions are  made increasing or decreasing with the distance to the axis,  by the use of conical air gap.