EP0633601B1 - Large aperture, low flight-time distortion detector for a time-of-flight mass spectrometer - Google Patents

Abstract

If ions transverse inhomogenous fields in the detector of a time-of-flight mass spectrometer, the case can arise that different flight paths (11) require different times from the entry window up to the ion-electron conversion surface (3). These flight-time distortions can be reduced by suitable deformation of the conversion surface (3). <IMAGE>

Description

The invention relates to detectors for time-of-flight mass spectrometers the preamble of claim 1.

Detectors for time-of-flight mass spectrometers are said to be incident Oppose the largest possible area of the ion beam, and yet have small flight time errors.

Each detector has an ion-electron conversion surface, at which time an ion hits a certain Probability one or more electrons are generated that are in one or several downstream electron multipliers will. This gives an electrical impulse, which with the impact of the ion on the conversion surface in a temporal context stands.

As an alternative to electron multipliers, it is also possible to use the electrons detected by scintillators with photomultipliers.

The ion-optical axis of a detector means one selected path in or near the center of the incident ion beam. If the detector has a cylindrical symmetry, you usually choose the axis of symmetry.

One can move the ion-optical axis backwards from the ion-electron conversion surface from the detector to one suitable track the selected point, and at this point normal to the ion optical Define a reference plane for the axis. The flight time along the ion-optical axis from the reference plane to the ion-electron conversion surface can be selected as the reference flight time. Become ions from points other than the axis point of the reference plane, however started in the detector at the same direction and speed, so they may need different flight times than an ion on the axis. The difference between these flight times and the reference flight time on the The ion-optical axis is called a flight time error.

The flight time errors can be specified as a function of the starting location on the reference level. In the most general case, the flight time errors are depending on the two variables that parameterize the reference level. If the detector has a cylindrical symmetry, then these are Flight time error is a function of the distance of the orbit in question the ion-optical axis in the reference plane.

Ions can within a detector with inhomogeneous electrical Field focused or scattered on a smaller or larger area will. For this reason, the usable area is suitable the ion-electron conversion area is not a measure of the sensitivity of the detector. As a measure of the sensitivity of the detector the content of the surface on the reference level is appropriate, from which consists of ions with acceptably small time-of-flight errors in the detector can be started into it.

By defining a reference level and viewing it exclusively the paths from the reference plane to the conversion surface can you mentally identify the detector and its flight time errors from the rest of the Disconnect the time of flight mass spectrometer. But it is also possible that Flight time errors on the entire flight route from the ion source to to determine the conversion area. In addition to the immediate Time-of-flight errors related to the detector structure often occur here on the track due to ion source and reflector flight time errors on which partly by tilting the ion-electron conversion surface are compensable. For this reason, the conversion area is often movably mounted.

• eine Metalloberfläche auf welcher die Ionen auftreffen, und mit einer gewissen Wahrscheinlichkeit Elektronen auslösen. Die Metalloberfläche kann auch speziell beschichtet sein, damit die auftreffenden Ionen mit größerer Wahrscheinlichkeit Elektronen auslösen.
• die Vorderseite einer Mikrokanalplatte. Die Ionen dringen zwar einige 10 µm tief in die Kanälchen der Mikrokanalplatte ein, bevor sie Elektronen auslösen, und so hat die Konversionsfläche eigentlich eine sehr komplexe Form. Für die folgende Diskussion soll dennoch als Konversionsfläche die glatte Vorderseite der Mikrokanalplatte gelten. Das Eindringen der Ionen in die Kanälchen muß im folgenden nicht mehr berücksichtigt werden, da es sich nur um einige 10 µm handelt, was im Vergleich zu den anderen hier zu behandelnden Fehlern als vernachlässigbare Ungenauigkeit erkennbar ist.

Two types of conversion surfaces are currently used:

• a metal surface on which the ions hit and with a certain probability trigger electrons. The metal surface can also be specially coated so that the impinging ions are more likely to trigger electrons.
• the front of a microchannel plate. Although the ions penetrate a few 10 µm deep into the microchannel plate before they release electrons, the conversion surface actually has a very complex shape. For the following discussion, however, the smooth front of the microchannel plate should be the conversion surface. The penetration of the ions into the tubules no longer has to be taken into account in the following, since it is only a matter of a few 10 μm, which is recognizable as negligible inaccuracy in comparison with the other errors to be treated here.

The probability with which an ion hits the Ion-electron conversion surface electrons are triggered, i.e. the electron yield depends very much on the speed at Impact. Because the speed of the ions is inversely proportional to the square root of their mass, the probability of detection increases with ions of large mass.

If a detector is to detect ions of larger mass, the ions must therefore be accelerated before being on the ion-electron conversion surface to hit with sufficient probability to release electrons from the conversion surface upon impact. Of the The detector must therefore be designed so that a strong, accelerating electric field in front of the conversion surface. Through this post-acceleration field flight time errors can occur.

Usually the flight time errors are kept small by keeps the post-acceleration field homogeneous. Has a homogeneous field a location-independent direction and strength of the electric field, whereby the time of flight from the reference plane in a detector with homogeneous fields independent of the ion-electron conversion surface from the starting point at reference level or regardless of the point of entry into the Post-acceleration field is.

Such an electric field can only be created if you do that Post-acceleration field versus the drift distance of the time-of-flight mass spectrometer demarcated with a grid. An example of one constructed detector is shown in Fig. 5 of the publication by de Heer et al. (Review of Scientific Instruments, Volume 62 (3), page 670-677, 1991) to see.

• Ionen können an Gitterstäben inelastisch gestreut werden, mit falschen Geschwindigkeiten weiter in Richtung Konversionsfläche fliegen, und so zu falschen Zeiten ankommen,
• Ionen können an Gitterstäben in großem Winkel gestreut werden, was ebenfalls die Geschwindigkeitskomponente in Richtung der Konversionsfläche ändert,
• Ionen können auf Gitterstäben auftreffen, zerplatzen, und die Bruchstücke können zu falschen Zeiten auf die Ionen-Elektronen-Konversionsfläche gelangen.

The ions to be detected can also strike the bars. If the ions in question are only removed from the ion beam as a result, the detector signal is reduced, usually only slightly. However, there are also several ways in which ions that strike lattice bars can cause a detector signal at the wrong times:

• Ions can be inelastically scattered on lattice bars, fly further towards the conversion surface at incorrect speeds, and thus arrive at the wrong times,
• Ions can be scattered at a large angle on bars, which also changes the velocity component in the direction of the conversion surface,
• Ions can hit lattice bars, burst, and the fragments can reach the ion-electron conversion surface at wrong times.

If you want to avoid grids because of the problems mentioned above, the post-acceleration field is necessarily inhomogeneous, whereby ions on different orbits with different from each other Flight times get to the ion-electron conversion surface.

As mentioned earlier, the size of the flight time error is a function the distance of the trajectory from the ion-optical axis. It is the distance to the ion-optical axis as a variable in this function on the reference level, not on the conversion surface. in the best case, i.e. when the conversion surface is movably suspended the size of this flight time error is proportional to the square of the Distance from the ion-optical axis.

In this case, in order to keep the flight time errors small, to load the detector only near the ion-optical axis. Here this means that ions from the reference plane only close to the ion-optical axis can be started into the detector. It doesn't matter if the ion trajectories are on within the detector a smaller or larger area can be focused or defocused. The measure of the sensitivity of the detector is the content of that Area on the reference plane from which ion trajectories are acceptable small time-of-flight errors can be started into the detector can.

This solution can be e.g. in Fig. 1 of the publication by Steffens et al. (Journal of Vacuum Science and Technology, Volume A3 (3), page 1322-1325, 1985) recognize. Fig. 4 of the PCT application WO 92/19367 also shows this possible solution. The disadvantage of these solutions is that they are only comparative small volume of the detector is usable, or only a comparatively small area on the reference plane of the incident ion beam can be opposed. This has a reduced sensitivity of the detector.

The invention is accordingly based on the object Specify detector for time-of-flight mass spectrometer, which alike ensures high sensitivity and high mass resolution.

In particular, the object of the present invention is a detector for time-of-flight mass spectrometers, in which despite large usable area opposed to the incident beam on the reference level, the flight time errors are kept small.

This task is characterized by the characterizing features of the claim 1 solved.

According to the invention, the inhomogeneous ones prevailing in the detector electric field generated or occurring in front of the detector Flight time errors between ions with different trajectories compensated by the detector itself. This happens because in Detector uses a curved ion-electron conversion surface becomes. The curvature causes that occurring in each trajectory Flight time depending on the lateral position on the ion beam varies so, i.e. is either lengthened or shortened by that caused the inhomogeneous field or those occurring in front of the detector Flight time errors can be compensated or at least minimized. For one Flight path, for example, their flight time when using a flat conversion surface through the electric field opposite that of the other trajectories would have been lengthened, causes the curvature of the conversion surface now a corresponding shortening, so that the result Flight times are adjusted to each other.

1. Man lege sich auf einen Entwurf der Nachbeschleunigungsoptik fest: Ein Beispiel dafür zeigt Fig. 1. Dabei nehme man zuerst, wie in Fig. 1 gezeigt, eine ebene Ionen-Elektronen-Konversionsfläche(3) an.

2. Man lege die Spannungen der Elektroden fest: Entlang der Nachbeschleunigungsstrecke befinde sich hier nur eine einzige Ringelektrode(1), welche sich auf dem Potential der Driftstrecke des Flugzeit-Massenspektrometers befinde. Die Halterung(2) für die Ionen-Elektronen-Konversionsfläche(3) befinde sich auf dem Nachbeschleunigungspotential U, wodurch ein inhomogenes Nachbeschleunigungsfeld vor der Konversionsfläche erzeugt wird.

3. Man bestimme eine Reihe von Ionenbahnen(11) mit folgenden Bedingungen:

• Alle Bahnen starten von einer Startfläche(12) senkrecht zur Detektorachse.
• Alle Bahnen starten parallel zur Detektorachse mit der gleichen Geschwindigkeit in den Detektor hinein.
• Alle Bahnen werden für die gleiche Flugzeit bestimmt. Als Flugzeit soll die Zeit gewählt werden, welche ein Ion auf der Achse benötigt um von der Startfläche(12) bis auf die Konversionsfläche(3) zu gelangen.

4. Der Endpunkt der Achsenbahn liegt dann auf der Mitte der Konversionsfläche. Die Endpunkte der außeraxialen Bahnen beschreiben dann die notwendige Form(20) der Konversionsfläche. Dies ist vergrößert in Fig. 2 gezeigt.

5. Man ändere nun in dem Entwurf entsprechend dem vorherigen Schritt die Form der Konversionsfläche und fahre fort mit Schritt 3.

Da eine Änderung der Form der Konversionsfläche eine Änderung des elektrischen Feldes und damit eine Änderung der Flugzeitfehler mit sich zieht, sollte die obige Prozedur so oft wiederholt werden, bis der verbleibende Flugzeitfehler eine vorher gewählte Grenze unterschreitet.In order to determine the specific shape of the ion-electron conversion surface, one can proceed according to method claim 9 as follows:

1. Commit yourself to a design of the post-acceleration optics: An example of this is shown in FIG . First, as shown in Fig. 1 , assume a flat ion-electron conversion surface (3).

2. Determine the voltages of the electrodes: There is only a single ring electrode (1) along the post-acceleration path, which is at the potential of the drift path of the time-of-flight mass spectrometer. The holder (2) for the ion-electron conversion surface (3) is at the post-acceleration potential U , which creates an inhomogeneous post-acceleration field in front of the conversion surface.

3. Determine a series of ion orbits (11) with the following conditions:

• All tracks start from a starting surface (12) perpendicular to the detector axis.
• All tracks start parallel to the detector axis at the same speed in the detector.
• All lanes are determined for the same flight time. The time that an ion on the axis needs to travel from the start area (12) to the conversion area (3) should be selected as the flight time.

4. The end point of the axis path then lies on the middle of the conversion surface. The end points of the off-axis paths then describe the necessary shape (20) of the conversion surface. This is shown enlarged in FIG. 2 .

5. Now change the shape of the conversion surface in accordance with the previous step and continue with step 3.

Since a change in the shape of the conversion area entails a change in the electrical field and thus a change in the flight time errors, the above procedure should be repeated until the remaining flight time error falls below a previously selected limit.

It is also possible to change the shape of the conversion surface as a power series to specify finite order. This would mean that you cannot takes the exact shape of the area determined in step 5, but this area optimally approximates with a power series, and with this Surface then continues at step 3.

Instead of the lanes (11) determined in step 3, lanes can also be used use that with initial conditions corresponding to the actual Operation of the time-of-flight mass spectrometer started from the ion source will. That means that in principle such flight time errors, as in the ion source and in the remaining parts of the time-of-flight mass spectrometer arise in determining the curvature of the Ion-electron conversion area can be included. At the determination of the end face (20) one must take into account that the space of the initial conditions in this case 6 coordinates, so 3 for initial speeds and 3 for initial coordinates. Because the end face is a 2-parameter area in 3-dimensional space is, the end face (20) must be adapted to the end points of the tracks (11) in this way be the average distance of the web endpoints to the end face (20) is minimal.

Alternatively, the method can also be designed in such a way that first on a design of the detector electrodes including one determines the curvature of the ion-electron conversion surface, and then the voltages varied until the flight time error fall below a predetermined limit. This procedure corresponds Process claim 10.

Preferred embodiments are now explained in more detail with reference to the drawings: FIG. 3 shows the simplest embodiment of a detector according to the invention. In this embodiment, the time-of-flight errors of off-axis paths are compensated for by a curved conversion surface (3). As in Fig. 1 , the only ring electrode (1) is at the potential of the drift path.

If the holder (2) of the conversion surface (3) is movably mounted, then This embodiment also corresponds to claim 7. By tilting a movably mounted bracket, it is possible to certain in the detector Flight time errors of the ion source, the reflector and / or the drift path of the time-of-flight mass spectrometer.

Fig. 4 shows a detector design in which the field of the post-acceleration path can be adjusted by additional ring electrodes (4). In this way, the necessary curvature of the conversion surface (3) can be kept smaller at a certain voltage than in the design of FIG. 3 . Alternatively, a higher post-acceleration voltage can be set with the same curvature of the ion-electron conversion surface (3).

The additional ring electrodes (4) reduce the flight time errors off-axis Trajectories by moving the areas of greater field curvature through them Areas are placed where the speed of the ions is already is bigger. The ring electrodes are placed on potentials, their values between the drift path potential and the potential of the ion-electron conversion surface (3) lie. Instead of two or more Ring electrodes (4) would also be a single additional ring electrode conceivable.

With greater post-acceleration potential the flight time errors become of off-axis orbits larger. In addition, the ion orbits also bent more towards the ion-optical axis. Both require that the curvature of the ion-electron conversion surface increases Post-acceleration potential must increase. If the ion orbits are so strongly bent towards the ion-optical axis that they all hitting a point on the conversion surface, it is no longer so possible to correct the flight time errors by curving the conversion surface compensate. This is only with even greater post-acceleration potential possible if the ion trajectories are in front of the conversion surface cross.

If a detector is to be operated with a large post-acceleration potential, it is advantageous, as shown in FIG. 5 , to operate it according to method claim 8. In this mode of operation, arbitrarily high post-acceleration voltages can be achieved with a comparatively small curvature of the ion-electron conversion surface (3) by ensuring that the ion paths (11) cross in front of the conversion surface by suitable arrangement of the electrodes and suitable adjustment of the voltages. Since a number of possibilities are known for arranging electrodes or adjusting voltages in such a way that an electric field with the required properties results, the electrodes are not shown here.

FIG. 6 shows a detector design according to claim 6, in which the electrons generated on the curved ion-electron conversion surface (3) are withdrawn transversely to the detector axis by a field superimposed on the post-acceleration field. The electron tracks (15) are shown in dashed lines.

The ion trajectories (11) are shown twice in the middle part of the post-acceleration section, since here, similar to Fig. 5 , it is possible to cause crossing (11a) ion trajectories, or the ion trajectories essentially parallel (11b) to the ion Lead electron conversion surface (3).

Because of the field of rotation for the electrons the rotational symmetry the arrangement is broken, the optimal curvature of the conversion surface possibly no longer rotationally symmetrical. The proof The generated electrons can be scanned using a multi-channel plate or scintillator or similar be effected.

Claims (11)

1. A detector for time-of-flight mass-spectrometers,

   with one or several electrodes(1,2,4) for postaccelerating ions, and

   with an ion-electron conversion surface(3),

   characterized by

   an ion-electron conversion surface(3), which is not flat,

   the curvature of the ion-electron conversion surface(3) reducing flight-time errors of the ions.
2. A detector according to claim 1, characterized by an ion-electron conversion surface(3), that is manufactured from metal.
3. A detector according to claim 1, with a microchannel plate as ion-electron conversion surface(3).
4. A detector according to one of the previous claims, characterized by electrodes(1,2,4) for the postacceleration of ions that have rotational symmetry and an ion-electron conversion surface(3) that has rotational symmetry.
5. A detector according to one of the claims 1 through 3, characterized by electrodes(1,2,4) for postacceleration of ions that are not rotationally symmetric and/or an ion-electron conversion surface(3) that is not rotationally symmetric.
6. A detector according to one of the previous claims, characterized by an electric field for drawing out the electrons which are created at the ion-electron conversion surface(3) and that said field is superposed over the postacceleration field.
7. A detector according to one of the previous claims, characterized by an ion-electron conversion surface(3) that can be tilted.
8. Time-of-Flight Mass-Spectrometer with a detector according to one or several of the previous claims.
9. A method of operating a detector of a time-of-flight mass-spectrometer, that

   has a number of electrodes(1,2,4) for postaccelerating ions, and

   an ion-electron conversion surface(3) that is not flat,

   characterized by

   an ion-electron conversion surface(3), which is not flat,

   the curvature of the ion-electron conversion surface(3) reducing flight-time errors of the ions, and

   the fact that the off-axis ion paths(11) are so strongly bent toward the ion optical axis by the electrodes(1,2,4) and their potentials, such that said ions hit the ion-electron conversion surface on the opposite side of the axis.
10. A method of determining the curvature of the ion-electron conversion surface(3) of a detector having a number of electrodes(1,2,4), comprising the steps of:

    a) fixing all forms of the electrodes(1,2,4), except the curvature of the ion-electrode conversion surface(3),

    b) to start with, fixing the ion-electron conversion surface(3) to some convenient form,

    c) fixing the voltages of all electrodes(1,2,4)

    d) determining the potential from the given electrode-forms and -voltages,

    e) determining a number of paths(11) into the detector, either

    starting from a reference plane(12) normal to the ion optical axis, parallel to the ion optical axis with the same initial velocity, or

    starting from the ion source of the time-of-flight mass spectrometer, choosing paths with initial conditions corresponding to normal operation of the mass spectrometer,

    taking for all paths the same flight time, which is the time that an ion needs from the reference plane(12) i.e. the ion source of the time-of-flight mass spectrometer to the conversion surface(3), said ion flying on the ion optical axis,

    f) the surface(20),

    in case of two dimensional initial conditions for the determined paths, being defined the endpoints of the previously determined paths(11),

    in case of more than two dimensional initial conditions for the determined paths, being defined by a plane most closely approximating all the the endpoints of the previously determined paths(11),

    being the necessary form of the ion-electron conversion surface,

    g) taking the surface(20) determined in step f) either

    exactly as the new form for the conversion surface(3), or

    approximate this surface by an expansion defined by a finite number of parameters,

    and continue at step d),

    repeating step d) through f) until the difference between the surface(20) determined in step f) and the actual conversion surface(3) falls below some predetermined limit, whereas if a finite expansion is used to define the new converion surface in step (g, then at least that number of parameters for the determination of the conversion surface(3) should be used that allows the difference between the surface(20) determined in step f) and the actual conversion surface(3) to fall below some predetermined limit.
11. Method for determining the electrode voltages of a detector, comprising the steps of:

    a) fixing all forms of the electrodes(1,2,4),

    b) selecting a set of voltages for the electrodes,

    c) determining the potential from the electrode-forms and -voltages given,

    d) determining a number of paths(11) into the detector, either

    starting from a reference plane(12 normal to the ion optical axis, parallel to the ion optical axis with the same initial velocity, or

    starting from the ion source of the time-of-flight mass spectrometer, choosing paths with initial conditions corresponding to normal operation of the mass spectrometer,

    taking for all paths the same flight time, which is the time that an ion needs from the reference plane(12) i.e. the ion source of the time-of-flight mass spectrometer to the conversion surface(3), said ion flying on the ion optical axis,

    e) and modifying the electrode voltages such as to minimize the difference between surface defined by the endpoints of the ion paths(11) determined in step d) and the ion-electron conversion surface(3).

Images