EP2394290B1 - Magnetic achromatic mass spectrometer with double focusing - Google Patents

Description

The present invention relates to a dual focus achromatic magnetic mass spectrometer.

Mass spectrometers are devices that allow the characterization of the chemical structure of molecules constituting a sample or analyte. Mass spectrometry is thus a micro-analysis technique typically requiring only a few picomoles of the sample, to extract characteristic information as to its molecular weight or even its molecular structure. There are different types of mass spectrometers, among which are mainly time-of-flight mass spectrometers, quadrupole mass spectrometers and magnetic mass spectrometers. For example, one can refer to a general work such as John Roboz, Introduction to Mass Spectrometry, Instrumentation and Techniques, published by Interscience publishers, 1968 , or J. Throck Watson, Introduction to Mass Spectrometry, edited by Lippincott-Raven, 1997 , presenting in particular the different types of mass spectrometers and their principles of operation. Among the magnetic mass spectrometers, it is possible to distinguish between single-focus spectrometers and dual-focus spectrometers. With regard to the theoretical aspects relating to the optical properties of mass spectrometers, it is possible to refer to H. Wollnik, Optics of charged particles, published by Academic Press, 1987 .

The term "optical" is for the future to be considered in its broadest sense, here applied to ion optics.

One particular type of mass spectrometry is commonly referred to as SIMS, according to the acronym of the term "Secondary Ion Mass Spectrometry", that is, "Secondary Emission Mass Spectrometry". One of the problems specific to this analysis technique is that accelerated ions in the mass spectrometer have a high energy dispersion. As regards the chromatic properties of mass spectrometers, and in particular the devices treating ions with a wide dispersion of energy, reference will be made to the A. Benninghoven et al., Secondary Ion Mass Spectrometry, edited by John Wiley, 1987 . This book deals in particular with the SIMS technique.

The present invention is particularly in the field of mass spectrometers of the SIMS type. In spectrometers of this type, it is known that the principle of secondary ion extraction results in a large energy dispersion of the emitted ions. It is also known that it is advantageous to introduce into SIMS type mass spectrometers, an electrostatic sector between the sample and the magnetic sector, this electrostatic sector being intended to make the mass spectrometer achromatic for at least one mass. . It is commonly possible to vary the magnetic field produced by the magnetic sector. This is easily achieved by varying the electrical excitation, for example in the case where the magnetic field is produced by an electromagnet. In this case, the condition of achromatism is not attached to a given mass, but to a particular trajectory. If this trajectory is considered as the main axis of the spectrometer, it is said that the spectrometer is achromatic on the axis, and that it is not off the axis or "off axis".

Even more particularly, the present invention may relate as well to a so-called "mono-collection" spectrometer, that is to say capable of measuring a mass in the axis, that to a mass spectrometer said to " multi-collection ", that is to say able to simultaneously measure several masses. For example, it is possible to measure several masses simultaneously by arranging a plurality of collectors on the focusing plane of the mass spectrometer. The blur observed at the point of focus of a given mass different from the mass on the axis, when the energy distribution of the ions is relatively wide, is called off-axis chromatic aberration. Using the notation of H. Wollnik, presented in the aforementioned work, this fuzziness is characterized by an aberration coefficient x / em defined by the relation: $$\mathrm{Dx}\mathrm{1}=\left(\mathrm{x}/\mathrm{em}\right)\mathrm{x}\left(\mathrm{.DELTA.E}/\mathrm{E}\right)\mathrm{x}\left(\mathrm{M}\mathrm{1}-\mathrm{M}\mathrm{0}\right)/\mathrm{M}\mathrm{0},$$

where M0 is the mass on the main axis for which the chromatic focusing is well done, ΔE the energy dispersion of the beam, and Δx1 the blur formed at the point where the trajectories of the mass M1 are focused in opening.

It is desired to reduce the fuzziness Δx1 to improve the off-axis mass resolution. To reduce this blur, we try to cancel the coefficient x / em. In multi-collection mass spectrometers, it is therefore necessary, in order to guarantee good mass resolution for different masses, to be able to eliminate or significantly reduce off-axis chromatic aberrations.

There are different types of mass spectrometers known from the state of the art, which are made to be achromatic on the axis. Among these types of spectrometers, it is possible to mention the Nier-Johnson spectrometer.

The Mattauch-Herzog spectrometer, also known from the state of the art, is characterized in particular by the fact that the output face of the magnet is aligned with the entry point. This particular configuration allows a certain number of remarkable properties, and in particular makes it possible to offer an achromatism for different masses. However, it is sometimes very advantageous for a mass spectrometer to have a large mass dispersion, and in this case, the Mattauch-Herzog spectrometer is not appropriate.

It is further known that it is preferable, in order to increase the mass resolution, to suppress or reduce second order aberrations. It is recalled here that the use in mass spectrometers of elements which do not have a symmetry of revolution around the main axis, such as the electrostatic and magnetic sectors, leads to second order aberrations. These aberrations are by definition not correctable by a focus. Four types of second-order aberrations are produced in mass spectrometers comprising a magnetic sector and an electrostatic sector, these second order aberrations are noted in accordance with the use in the field of optics or ion optics: a first aberration noted x / aa proportional to square of the angle of aperture in the radial plane, a second aberration x / bb proportional to the square of the opening angle in the transverse plane, a third aberration x / ae proportional to the opening angle in the radial plane and away in relative energy, and a fourth aberration x / ee proportional to the square of the difference in relative energy.

It is known that the respective geometrical parameters of the electrostatic sector and the magnetic sector and the other ion optics devices can be calculated in such a way that the 4 second order aberration coefficients cancel each other out. For example, it is possible to refer to the work of H.Matsuda, Double focusing mass spectrometer of second order, Journal of Mass Spectrometry and Ion Processes, 14 (1974) ). In this book, it is particularly proposed a dual focusing spectrometer designed with a set of physical and geometric parameters determined very precisely. This type of solution has several disadvantages: there is no possibility of adjusting the aberration correction, and if the calculations are not quite accurate, the aberrations are not really canceled.

In addition, it is not possible to adjust this type of spectrometer differently according to the type of performance that is to be preferred: for example, a very good mass resolution on the axis only or a fairly good mass resolution for all the masses detected by the multi-collection. Finally, this type of spectrometer is not stigmatic, that is to say that it is impossible to have at the output of the spectrometer the ionic microscope function which makes it possible to visualize an image of the sample, filtered mass. .

It is also known that hexapoles can correct second-order aberrations. For example, reference may be made to Wollnik's work cited above. A hexapole is a set of six poles arranged around the main axis, and alternately set to an electric potential + V or -V.

Spectrometers known from the state of the art are equipped with corrective electrostatic hexapoles: a single focus mass spectrometer as described in the European patent application EP 0124440 or a dual-focus mass spectrometer as described in US Pat. US 4,638,160 . The advantage of introducing electrostatic hexapoles to reduce aberrations is that it is then possible to adjust the aberration correction as finely as possible by adjusting the excitation voltage of the hexapole while observing a signal characteristic of the fineness of the spot, as for example, the signal resulting from the scanning of the beam on the edge of the exit slot of the spectrometer, arranged upstream of a counting member or the projection on an ion-photon conversion device such as a micro-channel slab, from the image of the ion beam to the plane of the exit slit.

In a stigmatic double focusing mass spectrometer, it is known that as long as a difference in magnification is not created between the image in the radial plane and the transverse plane, it is not possible to simultaneously cancel the first one. and second aforementioned second order aberrations x / aa and x / bb; but it is also known that if this difference in magnification is created with the appropriate means as described in the European patent application EP0473488 , it is then possible to simultaneously cancel these two aberrations.

It is possible to refer to the article of E. de Chambost et al., Achieving High Transmission with the Cameca IMS1270, Secondary Mass Spectrometry, SIMSX, edited by John Wiley, 1995 , where it is particularly specified that with a hexapole located between the entrance slit and the electrostatic sector, a hexapole located upstream of the magnetic sector and a hexapole located downstream of the magnetic sector, it is possible to cancel the first three aberrations of the aforementioned second order x / yy, x / bb and x / ae. This configuration makes it possible to considerably improve the transmission for a mass resolution of the order of 10000. However, the fourth aberration of the second order x / ee is not canceled, which represents a serious drawback when higher mass resolutions to 20000 are required.

The article of Yavor et al., "Beam distortions and their compensation in the field mass spectrometers", International Journal of Mass Spectrometry, vol. 171, no. 1-3, December 1, 1997, pages 203-208 , XP004107075, discloses a mass spectrometer according to the preamble of claim 1.

An object of the present invention is to overcome at least the aforementioned drawbacks, by proposing a solution for significantly reducing the four second-order aberrations, as well as the out-of-axis chromatic aberrations, while allowing modulation of the mass dispersion, for example a reduction of the mass dispersion to concentrate the masses in the focal plane in order to reduce the travel of mobile collectors or an increase in dispersion mass to be able to measure closer masses with a multi-collection system.

• un premier dispositif électrostatique placé entre la source d'ions et la sortie du secteur électrostatique, focalisant le faisceau d'ions sur l'axe principal du spectromètre de masse ;
• un second dispositif électrostatique disposé en aval du secteur magnétique, créant dans le plan de symétrie longitudinal un champ électrique radial d'autant plus élevé que le point considéré est éloigné de l'axe et dont les signes respectifs, du côté des masses faibles et du côté des masses fortes, sont opposés ;

caractérisé en ce que:

• le secteur électrostatique est un secteur électrostatique sphérique tronqué comprenant une électrode extérieure portée à une tension +Ve, une électrode intérieure, portée à une tension -Ve, l'électrode extérieure et l'électrode intérieure comprenant en outre une paire de plaques parallèles extérieures disposées de part et d'autre de l'électrode extérieure et portées à une tension V_ext, et une paire de plaques parallèles intérieures disposées de part et d'autre de l'électrode intérieure et portées à une tension V_int, lesdites paires de plaques parallèles intérieures et extérieures formant le premier dispositif électrostatique ;

les tensions V_ext, V_int appliquées respectivement sur les plaques parallèles extérieures et intérieures étant ajustées d'une même différence de tension ΔV chaque fois que le second dispositif électrostatique est activé, pour moduler la dispersion en masse ou pour annuler le chromatisme hors d'axe, de façon à ce que le faisceau d'ions correspondant à la masse sur l'axe reste toujours focalisé sur l'axe principal.For this purpose, the subject of the invention is a dual-focus magnetic mass spectrometer comprising an ion source, an entrance slit, an electrostatic sector, a magnetic sector and means for simultaneously detecting at least one mass. ionic, comprising:

• a first electrostatic device placed between the ion source and the output of the electrostatic sector, focusing the ion beam on the main axis of the mass spectrometer;
• a second electrostatic device disposed downstream of the magnetic sector, creating in the longitudinal plane of symmetry a radial electric field all the higher as the point considered is distant from the axis and whose respective signs, on the side of the weak masses and the on the side of the strong masses, are opposed;

characterized in that

• the electrostatic sector is a truncated spherical electrostatic sector comprising an outer electrode brought to a voltage + Ve, an inner electrode, brought to a voltage-Ve, the outer electrode and the inner electrode further comprising a pair of parallel outer plates arranged on both sides of the outer electrode and brought to a voltage V _ext , and a pair of parallel inner plates disposed on either side of the inner electrode and brought to a voltage V _int , said pairs of plates internal and external parallels forming the first electrostatic device;

the voltages V _ext , V _int applied respectively to the outer and inner parallel plates being adjusted by the same voltage difference ΔV each time the second electrostatic device is activated, to modulate the mass dispersion or to cancel off-axis chromatism, so that the ion beam corresponding to the mass on the axis always remains focused on the main axis.

In one embodiment of the invention, the magnetic mass spectrometer may be characterized in that the second electrostatic device comprises an electrostatic lens and / or a quadrupole and / or an octopole centered on the main axis of the mass spectrometer and whose north and south poles located in the transverse plane and on an axis perpendicular to the radial axis are brought to an electrical potential V, and the east and west poles located on an axis located in the radial plane, perpendicular to the axis defined by the North and South poles, are brought to an electric potential -V.

In one embodiment of the invention, the magnetic mass spectrometer may be characterized in that the first electrostatic device comprises a lens and / or a quadrupole and / or a multiplex activated as a stigmator.

• un premier hexapôle annulant les aberrations du second ordre en x/bb,
• un second hexapôle annulant les aberrations du second ordre en x/ee,
• un troisième hexapôle annulant les aberrations du second ordre en x/ae,
• un quatrième hexapôle annulant les aberrations du second ordre en x/aa.

In one embodiment of the invention, the mass spectrometer may be characterized in that it further comprises means for canceling second-order aberrations, said means comprising:

• a first hexapole canceling second-order aberrations in x / bb,
• a second hexapole canceling second-order aberrations in x / ee,
• a third hexapole canceling the second order aberrations in x / ae,
• a fourth hexapole canceling the second order aberrations in x / aa.

• o la figure 1, un diagramme synoptique d'un exemple de spectromètre de masse magnétique achromatique sur l'axe connu de l'état de la technique,
• o la figure 2, un schéma d'ensemble d'un exemple de spectromètre de masse magnétique selon la présente invention,
• o les figures 3a, 3b, 3c et 3d, des schémas représentant un exemple de mode de réalisation préféré pour des dispositifs équipant un spectromètre de masse magnétique selon la présente invention.

Other features and advantages of the invention will appear on reading the description, given by way of example, with reference to the appended drawings which represent:

• o the figure 1 , a block diagram of an example of an achromatic magnetic mass spectrometer on the axis known from the state of the art,
• o the figure 2 , an overall diagram of an example of a magnetic mass spectrometer according to the present invention,
• o the Figures 3a, 3b , 3c and 3d , diagrams showing an example of a preferred embodiment for devices equipping a magnetic mass spectrometer according to the present invention.

The figure 1 presents, by a synoptic diagram, an example of an achromatic magnetic mass spectrometer on the axis known from the state of the art.

A magnetic mass spectrometer 100 is shown in a section in the radial plane. The mass spectrometer 100 includes an inlet slot 101 and an exit slot 102. A diaphragm 103 is located downstream of the inlet slot 101. An electrostatic sector 104 is located downstream of the diaphragm 103. A magnetic sector 105 is disposed downstream of the electrostatic sector 104. An optical device 106 is located between the electrostatic sector 104 and the magnetic sector 105.

It is recalled first of all that the radial plane is defined as the plane of symmetry of the mass spectrometer containing the main axis of the mass spectrometer 100, perpendicular to the large dimension of the entrance slot 101 and containing the main axis In a given point, the transverse plane is defined as the plane perpendicular to the radial plane, and which also contains the main axis of the mass spectrometer 100.

For the sake of clarity, it is considered that the mass spectrometer itself is located downstream of the inlet slit 101, and the ionization device, and the ion beam shaping devices up to at the entrance slot 101 are not shown in the figure. In the same way, the collection and measurement devices located downstream of the exit slot 102 are not shown.

In the radial plane, the opening angle θ of the ion beam is designated a. The opening angle of the ion beam in the transverse plane, not shown in the figure, is designated b.

The mass spectrometer shown in the figure is a Nier-Johnson spectrometer. This type of spectrometer is an example of an achromatic mass spectrometer on the axis. A particular configuration of the physical and geometrical characteristics of the mass spectrometer 100 allows the measurement in the axis of a given mass with a given mass resolution.

The figure 2 presents an overall diagram of an example of a magnetic mass spectrometer according to the present invention.

The magnetic mass spectrometer 200 includes an input slot 101, a plurality of output slots 102 for filtering the ion beams to a plurality of collectors not shown in the figure for the sake of clarity. The mass spectrometer 200 also comprises a first diaphragm 103, an electrostatic sector 104, a magnetic sector 105. The mass spectrometer 200 further comprises, upstream of the electrostatic sector 104, a first electrostatic device 201 located downstream of the electrostatic gap. 101, a first hexapole 202 downstream of the first electrostatic device 201 and upstream of the diaphragm 103.

Downstream of the electrostatic sector 104 and upstream of the magnetic sector 105, the mass spectrometer 200 comprises, in series, a first optical device 211, a second hexapole 212, a second diaphragm 213, a second optical device 214, a third hexapole 215 and a third optical device 216.

Downstream of the magnetic sector 105 and upstream of the output slots 102, the mass spectrometer 200 comprises in series a fourth hexapole 221 and a second electrostatic device 222.

Of course, it is recalled that the configuration presented here is given by way of example, and the skilled person can envisage a multitude of equivalent configurations, optical and electrostatic devices to be considered in a broad sense as optical ion systems electrostatic, which may include lenses with symmetry of revolution, anisotropic lenses more or less effective respectively in the radial plane and in the transverse plane or multipoles for achieving achromatism on the axis, such as commonly used devices in dual mass spectrometers focusing or multipoles also to treat trajectories differently in the transverse plane, as is described for example in the book by E. de Chambost et al. supra.

There is a combination between the first electrostatic device 201 and the second electrostatic device 222, located respectively between the input slot 101 and the output of the electrostatic sector 104, and between the magnetic sector 105 and the output slots 102, which solves the problem. both the problem of the modulation of mass dispersion and the problem of suppressing off-axis chromatic aberration. In both cases, it is the second electrostatic device 222 which is active, but as it creates a defocusing of the image of the entrance slot 101 to the detection plane, it is necessary to compensate this effect with the first electrostatic device 201 It is necessary that the first electrostatic device 201 be located neither in the region where the trajectories are dispersed in mass, nor in those where they are dispersed in energy.

To cancel the chromatic aberrations out of axis x / em, the present invention proposes to arrange in the part where the trajectories are dispersed in mass, that is to say between the magnetic sector 105 and the exit slots 102, a focusing device - the second electrostatic device 222. The second electrostatic device 222 is necessarily more efficient peripherally than in the center, and its convergence is necessarily inversely proportional to the energy of the particle. In other words, such a device produces a displacement of trajectories $$\mathrm{Dx}=\mathrm{K}*\left(\mathrm{.DELTA.E}/\mathrm{E}\right)\mathrm{x}\left(\mathrm{M}\mathrm{1}-\mathrm{M}\mathrm{0}\right)/\mathrm{M}\mathrm{0}$$

Just adjust it properly so that it opposes off-axis chromatic aberration. This adjustment can for example be done using a suitable calculation program or by using appropriate measures that can characterize the fineness of the beam or the displacement of the beam resulting from an energy shift. Among the suitable calculation programs, ISIOS described by MIYavor, ASBerdnikov in "ISIOS: a program to calculate imperfect particle optical systems" (Nucl.Instr and Meth in Phys Res, Vol 363, No. 1, 1995, pp.416-422 ) or GIOS described in "Principles of GIOS and COZY, H.Wollnik et al." (AIP Conference Proceedings, C.Eminhizer, vol 177 (1988), p.74-75 ).

This same electrostatic device 222 may be used, excited otherwise, that is to say polarized with different voltages, when the problem is not to reduce out-of-axis aberrations, but to modulate the mass dispersion.

In both cases, that is to say the cancellation of out-of-axis chromatic aberrations and the modulation of mass dispersion, the parasitic side effect is the displacement of the plane of the image of the entrance slot. This parasitic side effect is compensated by the first electrostatic device 201 which has the same effect on the trajectories, whatever the mass of the ion in question.

The second electrostatic device 222, downstream of the magnetic sector 105, produces in a plane normal to the axis, an electric field all the more important that the point is considered distant from the axis. This electric field is of opposite sign according to whether one is on the side of the weak masses or the side of the strong masses. These electrostatic means may be an electrostatic lens centered on the axis, that is to say a series of revolution symmetry electrodes polarized by applied voltages. These electrostatic means may also be a quadrupole device such that the plane containing one of the pairs of opposite poles contains the radial plane of the spectrometer. These electrostatic means can also be an octopole where the poles located on the Ox and Oy axes are excited as quadrupoles, and the diagonal poles are set to zero.

The electrostatic means located between the input slot 101 and the output of the electrostatic sector 104 are intended to reconstruct the opening focus which is broken by the activation of the devices located between the magnet and the detection.

To cancel all the chromatic aberrations, it is proposed at the same time a set of 4 judiciously positioned hexapoles and a system making it possible to modify the relative magnifications of the beam according to the radial and transverse planes between the 2 extreme hexapoles, or between the first hexapole 202 and the fourth hexapole 221.

The first hexapole 202, arranged in the example of the figure between the input slot 101 and the electrostatic sector 104, is particularly dedicated to the cancellation of the second aberration of the second order x / bb.

The fourth hexapole 221, disposed between the magnetic sector 105 and the detection plane on which the output slots 102 are located, is particularly dedicated to the cancellation of the first second order aberration x / aa.

The second hexapole 212, disposed near the second diaphragm 213 forming an energy slot, is particularly dedicated to the cancellation of the fourth second order aberration x / ee.

The third hexapole 215, disposed between the second diaphragm 213 and the magnetic sector 105, is particularly dedicated to the cancellation of the third aberration of the second order x / ae.

The system for varying the magnitudes in the radial plane and in the transverse plane, comprising in particular the optical devices 211, 214 and 216, can be for example that described in the European patent application. EP0473488 supra.

The figure 3a presents a preferred embodiment for the magnetic sector 105 of the magnetic mass spectrometer 200 according to the invention.

The structure of the mass spectrometer is for example of the Nier-Johnson type. The magnetic sector 105 produces on the main axis a deflection of the trajectory of the ions by an angle of 90 °. In the magnetic sector 105, the trajectory of the ions on the main axis has a radius of curvature equal to 585 mm. The input and output faces 301 and 302 of the magnetic sector 105 have, relative to the plane normal to the axis, an angle of 27 °, in order to give the magnetic sector 105 stigmatic properties, according to a configuration in itself known from the state of the art. Point of focusing of the mass on the main axis is located at a distance of 1220 mm from the output of the magnetic sector 105.

The figure 3b presents a preferred embodiment for the second electrostatic device 222. Three different trajectories for three different masses M0, M1 and M2 are represented. M0 is the mass on the main axis. A tangent plane P is also represented; this plane P is an approximation of the surface on which the different masses M0, M1 and M2 focus.

The figure 3c presents a perspective view of a preferred embodiment of an example octopole forming the second electrostatic device 222. The octopole 222 comprises eight poles North, Northeast, East, Southeast, South, Southwest , West and Northwest formed by eight cylindrical bars arranged around the main axis. The North-South axis is perpendicular to the radial plane, and located in the transverse plane. The East-West axis is perpendicular to the North-South axis, and located in the radial plane.

For example, the octopole 222 100 mm deep may be disposed 460 mm upstream of the focal point of the mass M0. The eight poles of the octopole can be located on a circle with a radius of 75 mm. The diameters of the cylindrical poles may be about 30 mm. If we wish to compress the positive ion trajectories and reduce the mass dispersion, then the North-West, North-East, South-East and South-West poles of the octopole can be excited at 0 Volt, the poles East and West at a positive potential + V, and the North and South poles at potential -V.

For the particular geometry described above, cited by way of example, the typical value of the voltage V which cancels out-of-axis chromatic aberrations is 300 volts, for a kinetic energy of the ions of 10 keV. At the same time as the chromatic aberration compensation, the application of a voltage on the poles has the effect of tightening the dispersed paths in mass, thus reducing the mass dispersion, typically by a factor of 2/3.

If an electrostatic lens is used rather than an octopole, that is to say for example a so-called Einzel lens, with an active electrode of the same internal diameter as the octopole, the same effect can be obtained. Typically, it is necessary to apply to the active electrode a voltage of the order of one half of the ion acceleration voltage which, in addition to the cancellation of off-axis chromatic aberration, also has the effect of reduce the mass dispersion by a factor of about 2/3.

In the two cases presented above, the additional convergence introduced by the second electrostatic device 222 disrupts the focusing in opening of the entrance slot 101 to the plane of the exit slot 102. To compensate for this phenomenon, a device must be activated. focalisant located in a region where the beam is neither dispersed in mass nor dispersed in energy.

In a preferred embodiment of the invention, a stigmator function may be integrated in the electrostatic sector 104, as illustrated by FIG. figure 3d .

$${\mathrm{V}}_{\mathrm{int}}=-{\mathrm{V}}_{\mathrm{hex}}+{\mathrm{V}}_{\mathrm{stig}}$$

The figure 3d shows an exemplary embodiment of an electrostatic sector 104, in sectional view in a plane perpendicular to the main axis of the mass spectrometer. The electrostatic sector 104 comprises, for example, a pair of additional external parallel plates 341 and a second pair of internal parallel plates 342, called Matsuda plates or, according to the English terminology, "flat Matsuda", arranged on a truncated spherical electrostatic sector. consisting of an outer electrode 343 where is applied, in the case of positively charged ions, a + Ve voltage, and an inner electrode 344 where is applied, always in the case of positively charged ions, a voltage -Ve . The two outer plates 341, symmetrical with respect to the radial plane are interconnected, and the inner two plates 342 are also interconnected. The additional plates 341 and 342, give the truncated sector a full spherical symmetry if they are applied respective voltages V _ext and V _int determined by the calculation or by experience. The voltages V _ext and V _int can be expressed as form of a differential component V _hex and a common component V _stig : $${\mathrm{V}}_{\mathrm{ext}}={\mathrm{V}}_{\mathrm{hex}}+{\mathrm{V}}_{\mathrm{stig}}$$

$${\mathrm{V}}_{\mathrm{int}}=-{\mathrm{V}}_{\mathrm{hex}}+{\mathrm{V}}_{\mathrm{stig}}$$

The traditional means of analog or digital electronics can produce voltages V _ext and V _int from V _hex and V _{stig voltages} that can of course be controlled by computer.

In other words, the application on the plates 341 and 342 of a common V _stig component creates a _stigmatory effect which can be judiciously used to compensate for the defocusing created by the second electrostatic device 222 disposed downstream of the magnetic sector. 105, but without any effect on the bulk dispersion. In this embodiment, it is therefore possible to substitute the Matsuda plates 341 and 342 for the first electrostatic device 201.

• on donne une valeur donnée à l'excitation du second dispositif électrostatique 222 ;
• on règle la valeur de l'excitation du premier dispositif électrostatique 201 pour refocaliser le faisceau sur l'axe. Dans le mode de réalisation décrit précédemment, où le premier dispositif électrostatique 201 est formé par les plaques additionnelles 341 et 342, il est par exemple possible d'ajuster les tensions V_ext et V_int qui leur sont respectivement appliquées, d'une même différence de tension ΔV ;

• on réduit le second diaphragme 213 ou fente en énergie disposée entre le secteur électrostatique 104 et le secteur magnétique 105 à une faible valeur, typiquement 1 eV ;
• on balaye cette fente, typiquement dans une plage de zéro à 20 eV, c'est à dire que l'on déplace la fente en énergie par incréments réguliers à l'intérieur de cette plage ;
• on observe le déplacement induit par le faisceau au niveau d'un collecteur hors d'axe. Typiquement, en balayant le faisceau de sortie sur la fente d'entrée de ce collecteur, c'est-à-dire au niveau de la fente de sortie 102 associée à ce collecteur. Le déplacement induit est caractéristique de l'aberration chromatique hors d'axe, soit l'aberration que l'on cherche à annuler à cette étape ;
• par itérations successives, il est alors possible de déterminer la valeur de l'excitation du second dispositif 222 qui fournit une aberration chromatique hors d'axe minimale.

When it is desired to cancel off-axis chromatic aberration, it is possible to calculate, for example with the above-mentioned calculation programs, which electrical voltages are to be applied to the different electrostatic devices, but it is also possible to determine these by purely experimental methods. The proposed method can then be the following. Let us first note that the width ΔE and the position of the energy slot expressed in eV are easily convertible in units of length by using the energy dispersion coefficient K _e of an electrostatic sector, according to a calculation in itself known to those skilled in the art: dx (mm) = K _e (ΔE / E), where E is the ion acceleration energy:

• a given value is given to the excitation of the second electrostatic device 222;
• the value of the excitation of the first electrostatic device 201 is adjusted to refocus the beam on the axis. In the embodiment described above, where the first electrostatic device 201 is formed by the plates additional 341 and 342, it is for example possible to adjust the voltages V _ext and V _int which are respectively applied to them, the same voltage difference ΔV;

• reducing the second diaphragm 213 or energy slot disposed between the electrostatic sector 104 and the magnetic sector 105 to a low value, typically 1 eV;
• this slot is scanned, typically in a range of zero to 20 eV, i.e. the slit is moved into energy in regular increments within that range;
• we observe the displacement induced by the beam at an off-axis collector. Typically, by scanning the output beam on the input slot of this collector, that is to say at the outlet slot 102 associated with this collector. The induced displacement is characteristic of out-of-axis chromatic aberration, the aberration that we seek to cancel at this stage;
• by successive iterations, it is then possible to determine the value of the excitation of the second device 222 which provides a chromatic aberration out of minimum axis.

An advantage of the invention is that this method should be performed once and only once during the life of the mass spectrometer 200.

• on commence par procéder au réglage du second hexapôle 212 ;
• le premier diaphragme 103, communément appelé diaphragme de champ ou encore diaphragme d'ouverture angulaire, est fermé afin de réduire suffisamment les ouvertures a et b de façon à ce que les première, seconde et troisième aberrations du second ordre, soit respectivement (x/aa).a², (x/bb).b² et (x/ae).a.e, soient négligeables ;
• il est alors possible de faire varier l'énergie des ions, par exemple en faisant varier la tension d'accélération des ions. Ou encore, lorsque le processus d'émission des ions produit une grande distribution en énergie, il suffit de fermer le second diaphragme 213 ou fente en énergie, et de déplacer cette fente en énergie ainsi fermée. Lorsque la quatrième aberration du second ordre x/ee n'est pas annulée, il résulte de cette variation de l'énergie des ions un déplacement de l'image de la fente d'entrée 101 qu'il est possible de mesurer : il est donc possible de régler l'excitation du second hexapôle 212 pour que ce déplacement soit minimum ;
• on procède alors au réglage du premier hexapôle 202 ; par exemple en minimisant la largeur du faisceau que l'on peut observer directement sur une image du plan de sortie réalisée sur une galette de micro-canaux associée à un écran phosphore ou indirectement en observant la largeur du front S(M) obtenu en balayant le faisceau de sortie sur l'arête de la fente de sortie et en mesurant le courant ionique S en aval de la fente de sortie. On balaye le faisceau de sortie en incrémentant par pas réguliers le champ magnétique auquel est associée une valeur de la masse M de l'ion associé à la trajectoire sur l'axe.
• on procède alors au réglage du troisième hexapôle 215 ;
• on procède enfin au réglage du quatrième hexapôle 221.

As regards the cancellation of the four aberrations of the second order, each of the four hexapoles is excited by a single parameter: the voltage which is applied on 3 poles in positive and on the other 3 poles in negative. The method that allows to set these 4 hexapoles can for example be the following, again with reference to the figure 2 :

• we start by adjusting the second hexapole 212;
• the first diaphragm 103, commonly referred to as the field diaphragm or angular aperture diaphragm, is closed in order to sufficiently reduce the apertures a and b so that the first, second and third aberrations of the second order are respectively (x / aa) .a ² , (x / bb) .b ² and (x / ae) .ae, are negligible;
• it is then possible to vary the energy of the ions, for example by varying the ion acceleration voltage. Or, when the ion emission process produces a large energy distribution, it suffices to close the second diaphragm 213 or energy slot, and to move this slot into energy thus closed. When the fourth aberration of the second order x / ee is not canceled, it results from this variation of the energy of the ions a displacement of the image of the input slot 101 that it is possible to measure: it is therefore possible to adjust the excitation of the second hexapole 212 so that this displacement is minimum;
• the first hexapole 202 is then adjusted; for example by minimizing the beam width that can be observed directly on an image of the output plane made on a micro-channel slab associated with a phosphor screen or indirectly by observing the width of the front S (M) obtained by scanning the output beam on the edge of the output slot and measuring the ion current S downstream of the output slot. The output beam is scanned by incrementing in regular steps the magnetic field which is associated with a value of the mass M of the ion associated with the trajectory on the axis.
• the third hexapole 215 is then adjusted;
• the fourth hexapole 221 is finally adjusted.

Claims (3)

1. A magnetic mass spectrometer with double focusing (200) comprising an ion source, an entry slit (101), an electrostatic section (104), a magnetic section (105) and means for simultaneous detection (102) of at least one ion mass, comprising:

   • a first electrostatic device (201) placed between the ion source and the exit of the electrostatic section (104), focusing the beam of ions onto the main axis of the mass spectrometer (200);

   • a second electrostatic device (222) disposed downstream of the magnetic section (105), creating in the longitudinal plane of symmetry a radial electric field that is higher the further the point in question is from the axis and whose respective signs, on the side of the low masses and on the side of the high masses, are opposing;

   characterized in that:

   • the electrostatic section (104) is a truncated spherical electrostatic section comprising an external electrode (343) to which a +Ve voltage is applied, an internal electrode (344) to which a -Ve voltage is applied, the external electrode (343) and the internal electrode (344) furthermore comprising a pair of external parallel plates (341) disposed on either side of the external electrode (343) and to which a voltage V_ext is applied, and a pair of internal parallel plates (342) disposed on either side of the internal electrode (344) and to which a voltage V_int is applied, said pairs of internal and external parallel plates (341, 342) forming the first electrostatic device (201);

   the voltages V_ext, V_int respectively applied to the external and internal parallel plates (341, 342) being adjusted by the same voltage difference ΔV each time that the second electrostatic device (222) is activated, in order to modulate the dispersion in mass or in order to cancel the off-axis chromatism, in such a manner that the beam of ions corresponding to the on-axis mass always remains focused on the main axis.
2. The magnetic mass spectrometer (200) as claimed in claim 1, characterized in that the second electrostatic device (222) comprises an electrostatic lens and/or a quadrupole and/or an octopole centered on the main axis of the mass spectrometer (200) and whose North and South poles, situated in the transverse plane and on an axis perpendicular to the radial axis, are biased at an electrical potential V, and whose East and West poles situated on an axis situated in the radial plane, perpendicular to the axis defined by the North and South poles, are biased at an electrical potential -V.
3. The magnetic mass spectrometer (200) as claimed in either of the preceding claims, characterized in that it furthermore comprises means for canceling the aberrations of the second order, said means comprising:

   - a first hexapole (202) canceling the aberrations of the second order proportional to the square of the opening angle in the transverse plane referred to as aberrations in x/bb,

   - a second hexapole (212) canceling the aberrations of the second order proportional to the square of the relative difference in energy referred to as aberrations in x/ee,

   - a third hexapole (215) canceling the aberrations of the second order proportional to the opening angle in the radial plane and to the relative difference in energy referred to as aberrations in x/ae,

   - a fourth hexapole (221) canceling the aberrations of the second order proportional to the square of the opening angle in the radial plane referred to as aberrations in x/aa.

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