EP0124440A1 - High luminosity mass spectrometer - Google Patents

Abstract

Mass spectrometer in which both the aperture aberrations in the radial plane and the inclination of the image plane (P) are corrected. The correction comes from the simultaneous adjusting of at least three parameters: the radii of curvature R1 and R2 of the entrance and exit faces of the magnetic sector (5) of the spectrometer, and the coupling coefficient, k, of a sextupole (6) disposed between the magnetic sector and the image plane (P) of the spectrometer. When focusing, this results in solving a system of three equations (the aperture aberrations being defined by two coefficients A and B and the angle of inclination by a coefficient I) in which the variables are the three parameters R1, R2 and k. <IMAGE>

Description

The present invention relates to mass spectrometers, whatever their type (thermionization, gas, ...), and relates more particularly to the brightness of the magnetic sector and the possibility of multicollection, that is to say the possibility of simultaneously placing several analysis collectors in the image plane of the spectrometer.

For a given size of the magnetic sector of a mass spectrometer, the brightness of the magnetic sector is limited, for a given resolution, by the opening aberrations of the magnetic sector. It is known to reduce and even cancel these opening aberrations by a suitable choice of the radii of curvature of the input and output faces of the magnetic sector, but this generally results in an increase in the angle of inclination of the image plane with respect to the normal to the optical axis of exit of the magnetic sector; this angle can then reach a large value (80 ° and more) a multicollection analysis is no longer possible because the different collectors relating to the different masses must be placed in the image plane without the collectors closest to the sector magnetic screen in front of those who are furthest away from it; and, even when possible, problems with electron rebounds and secondary electrons distort the analysis. It is also known to use spectrometers comprising an input sextupole and an output sextupole and to cancel the opening aberrations by adjusting the excitation of the input sextupole or the curvature of the input face of the sector magnetic and the curvature of the exit face of the magnetic sector or the excitation of the exit sextupole and keeping fixed the two other parameters; in such sextupoles it is also known to cancel the angle of inclination of the image plane by modifying the distances sample-magnetic sector and magnetic sector-image plane, but this adjustment completely disturbs an adjustment which would have been made to cancel the aberrations and these can be significant when the tilt of the image plane is reduced to zero.

The object of the present invention is to avoid these drawbacks and to allow a multicollection analysis in a mass spectrometer whose aperture aberrations are corrected in the radial plane and whose image plane is rectified normally at the optical output axis. .

This is obtained by means of a spectrometer comprising at least one sextupole between its magnetic sector and its image plane.

According to the invention, a mass spectrometer comprising, in series on its optical axis, a magnetic sector, an output sextupole, an image plane, is characterized in that the coupling coefficient k of the sextupole and the radii of curvature, RI , R2, input and output faces of the magnetic sector have values such that, simultaneously, the aberration coefficients A and B, relating to the shifts in the radial plane, due to the aberrations in the radial plane and in the plane axial, are less than 1 in absolute value, and the tilt angle of the image plane is less than 10 ° in absolute value.

• - la figure 1, un spectromètre selon l'invention,
• - la figure 2, un schéma relatif aux aberrations dans un spectromètre,
• - la figure 3 une vue plus détaillée de l'un des éléments de la figure 1.

The invention will be better understood and other characteristics will appear from the following description and the figures relating thereto which represent:

• FIG. 1, a spectrometer according to the invention,
• FIG. 2, a diagram relating to aberrations in a spectrometer,
• - Figure 3 a more detailed view of one of the elements of Figure 1.

In the different figures the same elements are designated by the same references.

• - un filament, 1, sur lequel est déposé l'échantillon à analyser,
• - une optique, dite de transfert, 2, schématisée par deux lentilles successives,
• - un écran à fente, 3, déterminant ce qui est communément appelé une fente-source, et dont le point central de la fente est repéré H,
• - un premier sextupole 4,
• - un secteur magnétique, 5, pour dévier les ions issus de la fente et provenant de l'échantillon,
• - un second sextupole, 6,
• - et un ensemble, 7, de trois collecteurs d'ions disposés dans le plan image P du spectromètre.

Figure 1 is a schematic view of a thermionization mass spectrometer according to the invention. This spectrometer includes in series:

• - a filament, 1, on which the sample to be analyzed is deposited,
• an optic, called a transfer lens, 2, shown diagrammatically by two successive lenses,
• a slit screen, 3, determining what is commonly called a source slit, and the central point of the slit of which is marked H,
• - a first sextupole 4,
• - a magnetic sector, 5, for deflecting the ions coming from the slit and coming from the sample,
• - a second sextupole, 6,
• - And a set, 7, of three ion collectors arranged in the image plane P of the spectrometer.

• - l'axe optique XX du spectromètre, avec sa courbure, de rayon R et son angle de déviation a, dus au secteur magnétique 5,
• - les longueurs LI et L2 des sextupoles 4 et 6 ainsi que leurs diamètres Dl, D2,
• - la distance L3 entre la fente-source 3 et le sextupole 4,
• - la distance L4 entre le sextupole 4 et la face d'entrée du secteur magnétique
• - la distance L5 entre la face de sortie du secteur magnétique et le sextupole 6,
• - la distance L6 entre le sextupole 6 et le plan image P,
• - les rayons de courbure RI et R2 et les angles d'inclinaison bl, b2 des faces d'entrée et de sortie du secteur magnétique 5,
• - l'angle d'inclinaison, i, du plan image P ainsi que le point J où l'axe optique XX couple le plan image.

In FIG. 1, it has been indicated:

• the optical axis XX of the spectrometer, with its curvature, of radius R and its deflection angle a, due to the magnetic sector 5,
• - the lengths LI and L2 of sextupoles 4 and 6 as well as their diameters Dl, D2,
• - the distance L3 between the source slot 3 and the sextupole 4,
• - the distance L4 between the sextupole 4 and the entry face of the magnetic sector
• - the distance L5 between the exit face of the magnetic sector and the sextupole 6,
• - the distance L6 between the sextupole 6 and the image plane P,
• - the radii of curvature RI and R2 and the angles of inclination bl, b2 of the entry and exit faces of the magnetic sector 5,
• - the angle of inclination, i, of the image plane P as well as the point J where the optical axis XX couples the image plane.

The spectrometer according to FIG. 1 differs from conventional spectrometers by the presence of both the curvatures of the input and output faces of the magnetic sector and of the two sextupoles 4 and 6 whose roles will be indicated in the following.

In a mass spectrometer the sensitivity results, in particular, from the luminosity of the magnetic sector. However, the brightness is limited mainly by the aberrations of opening in the radial plane, that is to say in the plane of FIG. 1.

Figure 2 is a geometric diagram relating to the opening aberrations in a spectrometer. The magnetic sector of radius R in FIG. 1 gives, with certain ions, from point H (point of the optical axis situated at the level of the screen) an image situated, in the image plane, at J. This image is marred by an opening aberration whose width in the radial plane is
RA a ² _{+ R. B.} β ² the term RA a corresponds to the fact that a ray coming from the point H, contained in the radial plane and making an angle a with the optical axis, does not pass through the point J after having crossed the magnetic sector but remains in the radial plane and intersects the axis Jy (Jy perpendicular to the optical axis in the radial plane) at a distance RA α ² from J; as for the term RB β ² it corresponds to the fact that a ray from point H, contained, at the exit of the source slit, in the axial plane (plane, or more exactly curved surface, perpendicular to the radial plane, which it cuts along the optical axis ) and making an angle Il with the optical axis, is not contained, after the magnetic sector, in the axial plane: the projection, parallel to Jz, of the intersection of this ray with the plane Jyz intersects the axis Jy at a distance RB β ² from J.

As indicated above, by choosing radii of curvature R1, R2 and suitable angles of inclination for the input and output faces of the magnetic sector 5, it is known to cancel opening aberrations at a ² and 132, but this can lead to a significant increase in the angle i (FIG. 1) of inclination of the image plane P. This makes a multicollection analysis difficult, if not impossible.

The spectrometer according to FIG. 1 makes it possible to simultaneously correct the opening aberrations defined by the coefficients A and B which were discussed during the description of FIG. 2, and the angle of inclination i of the image plane P usually defined, in the specialized literature, by a coefficient I.

These two aberrations and this inclination can, as experience proves, be corrected by fields of the sextupolar type. Because three coefficients, A, B, I, are to be corrected, at least three degrees of freedom, and therefore three sextupolar devices are necessary. These three devices can be constituted by the curvatures of the entry and exit faces of the magnetic sector and by an electrostatic or magnetic sextupole (6, figure 1) which must be placed in an area where there is dispersion of the ion beam. to analyze, that is to say after the magnetic sector; an additional sextupole (4, figure 1) can also be placed before the magnetic sector, so as to give an additional degree of freedom and therefore more flexibility during the development of the spectrometer. It is also possible to take as sextupolar devices the sextupoles 4 and 6 of FIG. 1 as well as the curvature of the exit face of the magnetic sector and to take a plane entry face (infinite RI) for the magnetic sector; but for industrial production it will generally be preferable to take as devices sextupoles, the curvatures of the entry and exit faces of the magnetic sector and a sextupole placed after the magnetic sector.

R R où ^C ₁ =

et C₂ =

, où k₁ et k₂ sont les coefficients d'excitation des sextupoles (représentatifs des tensions d'excitation des sextupoles 4 et 6, avec k =

, où r =

, =

étant le rayon du cercle tangent r o intérieurement aux barreaux du sextupole, où V_e = V = tension d'excitation du sextupole, où V_o = charge des ions) et où A_o, A₁, A₂ , A₃, A₄, B_o, B₂, B₄, I_o, I₂, 14 sont des coefficients dépendant des paramètres géométriques du spectromètre. Ces coefficients sont obtenus par décomposition de la matrice de transfert du deuxième ordre du système optique ainsi constitué ; cette matrice de transfert est celle qui est obtenue par multiplication des matrices de transfert des différents constituants (secteur magnétique, sextupoles, espaces L3, L4, L5, L6 entre les éléments) ; ces matrices se trouvent dans la littérature spécialisée (par exemple : K.L. BROWN "A First- and Second-Order Matrix Theory for the Design of Beam Transport Systems and Charged Particle Spectrometers". Advances in Particle Physics, vol.l, pages 71-134, 1967).The study of the coefficients A, B, I, relative to FIG. 1, by the theory of second order transfer matrices, shows that these coefficients can be written:

RR where ^C ₁ =

and C ₂ =

, where k ₁ and k ₂ are the excitation coefficients of the sextupoles (representative of the excitation voltages of sextupoles 4 and 6, with k =

, where r =

, =

being the radius of the circle tangent ro internally to the bars of the sextupole, where V _e = V = excitation voltage of the sextupole, where V _o = ion charge) and where A _o , A ₁ , A ₂ , A ₃ , A ₄ , B _o , B ₂ , B ₄ , I _o , I ₂ , 14 are coefficients depending on the geometric parameters of the spectrometer. These coefficients are obtained by decomposition of the second order transfer matrix of the optical system thus formed; this transfer matrix is the one obtained by multiplying the transfer matrices of the different constituents (magnetic sector, sextupoles, spaces L3, L4, L5, L6 between the elements); these matrices can be found in the specialized literature (for example: KL BROWN "A First- and Second-Order Matrix Theory for the Design of Beam Transport Systems and Charged Particle Spectrometers". Advances in Particle Physics, vol.l, pages 71-134 , 1967).

a été recherchée sans le sextupole d'entrée, 4, en faisant varier C₁, C2 et K₂.In the example describes the solution of the system of three equations

was searched without the input sextupole, 4, by varying C ₁ , C2 and K ₂ .

dans lequel l'entrefer du secteur magnétique 5 a une hauteur de 15 mm tandis que les six barreaux du sextupole 6 (voir figure 3) ont un diamètre de 15 mm et dans lequel l'énergie cinétique des ions V est de 4500 eV, le choix ci-après

a donné

les valeurs de A et B sont relatives à un système de mesure où les angles sont exprimés en radians.With a spectrometer with the following geometric characteristics (compare with Figure 1):

in which the air gap of the magnetic sector 5 has a height of 15 mm while the six bars of the sextupole 6 (see FIG. 3) have a diameter of 15 mm and in which the kinetic energy of the V ions is 4500 eV, the choice below

gave

the values of A and B relate to a measurement system where the angles are expressed in radians.

It should be noted that, in practice, perfectly satisfactory operating conditions of the spectrometer are reached as soon as A and B are less than 1 and that i is less than 10 °.

The optimum excitation voltage of the sextupole corresponding to the cancellation of A is between 11.4 and 11.5 volts; the optimal adjustment was made by studying the shape of the analysis peaks. Under these conditions the coefficient B was not entirely zero: B = - 0.3; to make B zero it would have been necessary to resume the adjustment of the radii of curvature of the input and output faces of the magnetic sector but this was not necessary since the low value of B leads to negligible, lower line enlargements at 10 ^-2 mm for R = 200 mm and β = 10 ^-2 radians.

Regarding the adjustment, during the experiment, of the radii of curvature RI and R2, it should be noted that it was done using parts removable, located (see Figure 1) between the concave entry face of the magnetic sector and a plane M and between a plane N and the convex exit face of the magnetic sector. As for the other geometrical characteristics of the spectrometer and in particular the angles bl and b2, they were chosen according to the experience acquired previously on the spectrometers.

Figure 3 shows how is made the sextupole 6 which was discussed in the description of Figure 1. This sextupole has six metal bars, cylindrical, 60 to 65, regularly arranged around the optical axis XX, with their main axis , not shown, parallel to the optical axis; the bars 60, 62, 64 are joined, by metal spacers located at one third of their length, to a metal ring 66 brought to the potential + V; similarly the bars 61, 63, 65 are joined, by metal spacers located two-thirds of their length, to a metal ring 67 brought to the potential -V. The insulating parts which join the crowns 66 and 67 to make the sextupole a rigid unit, have not been shown in order to better show the way in which the bars 60-66 are joined by two groups of three.

The present invention is not limited to the example described; it applies in particular as well to the use of electrostatic sextupoles as to that of magnetic sextupoles; it also applies to the use of several sextupoles arranged behind the magnetic sector (between the magnetic sector and the image plane) and even to the use of several sextupoles arranged in front of the magnetic sector. It should be noted that the sextupoles also bring, by their shape and their position on the optical axis XX, other parameters for adjusting the spectrometer, but these are parameters which are much more difficult to modify than the voltages d excitement of the sextupole.

The present invention also applies to mass spectrometers using, in addition to the magnetic sector considered in the description above, one or more electrostatic and / or magnetic sectors; it is used to correct the opening aberrations and the tilt angle of the image plane.

Claims (2)

1. Mass spectrometer comprising, in series on its optical axis, a magnetic sector (5), an output sextupole (6), an image plane (P), characterized in that the coupling coefficient k of the sextupole and the rays of curvature, RI, R2, of the input and output faces of the magnetic sector have values such that, simultaneously, the aberration coefficients A and B, relating to the shifts in the radial plane, due to the aberrations in the radial plane and in the axial plane, are less than 1 in absolute value, and the angle of inclination (i) of the image plane is less than 10 ° in absolute value. 2. Mass spectrometer comprising, in series on its optical axis, an input sextupole (4), a magnetic sector (5), an output sextupole (6), an image plane (P), characterized in that the coupling coefficients kl, k2 of the input and output sextupoles and the radii of curvature, RI, R2, of the input and output faces of the magnetic sector have values such that, simultaneously, the aberration coefficients A and B, relating to the shifts in the radial plane of the spectrometer, due to aberrations in the radial plane and in the axial plane of the spectrometer, are less than 1 in absolute value, and the angle of inclination (i) of the image plane is less at 10 ° in absolute value.

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