EP0633602B1 - High sensitivity, wide dynamic range time-of-flight mass spectrometer provided with a gas phase ion source - Google Patents

Abstract

A high particle density in the exhaust volume of a gas-phase ion source and simultaneously a very low particle density in the flight path of the time-of-flight mass spectrometer results in a high sensitivity while simultaneously maintaining a large dynamic range of the intensity display. In order to achieve this, it is necessary to divide the time-of-flight mass spectrometer into two or more regions of different pressure, the different regions being separated by a gas-flow impedance. A maximum particle density in the exhaust volume while simultaneously maintaining a minimum particle density in the flight path can be obtained by integrating the gas-flow impedances (3, 6) directly into the electrodes (1, 2) of the ion source. <IMAGE>

Description

The invention relates to a time-of-flight mass spectrometer with a gas phase ion source according to the preamble of claim 1.

In the time-of-flight mass analysis, there is a start time from which started a group of ions in the time-of-flight mass spectrometer becomes. At the end of a flight route, the time is measured needed each incoming ion and from this the mass of the concerned Ions determined.

In a gas phase ion source of a time-of-flight mass spectrometer is understood as the withdrawal volume the spatial area of the ion source, from which, starting from the start, ion orbits to the Guide the surface of the detector of the time-of-flight mass spectrometer.

• den Zeitpunkt, in dem neutrale Teilchen eines im Abzugsvolumen befindlichen zu untersuchenden Gases durch den Puls einer das Abzugsvolumen durchstrahlenden Laserstrahl- oder Elektronenstrahlquelle ionisiert werden.
• den Zeitpunkt des Anschaltens der Elektrodenspannungen der Ionenquelle. In diesem Fall handelt es sich meist darum, Ionen zu untersuchen, da Ionen nur dann in das Abzugsvolumen gelangen können, wenn an den Elektroden der Ionenquelle keine Spannungen anliegen.

The start time of the flight time analysis can be given, for example, by

• the point in time at which neutral particles of a gas to be examined in the discharge volume are ionized by the pulse of a laser beam or electron beam source radiating through the discharge volume.
• the time at which the electrode voltages of the ion source are switched on. In this case, it is usually a matter of examining ions, since ions can only get into the withdrawal volume if there are no voltages at the electrodes of the ion source.

As a side function you can also use it in a time of flight mass spectrometer the generated electrons are detected. For the electrons can you can also define a deduction volume by analogy. The deduction volume for the ions does not have to match the withdrawal volume for the electrons be congruent. However, these two volumes will at least be different partially overlap. Usually the electrons are in the opposite Subtracted towards the ions from the source.

Since the much more common case is the detection of ions, the Following mainly dealt with it. If, however, in the following Ions and their orbits are discussed, so meets in corresponding Analogy the same for electrons and their orbits too.

In any case, in the ion source, following the start time, the first acceleration phase of those arriving at the detector Ions instead. Often the ions in the ion source are up to the top speed accelerates. It may be that the ion source electrodes for focusing the ions arriving at the detector contains. But it may also be that the electrodes for focusing are arranged separately, i.e. the ions arriving at the detector Leave source in a direction and location distribution, which for the further transport through the mass spectrometer is unsuitable, and for this reason a separate focus is still necessary.

There is a high particle density in the withdrawal volume at the start time advantageous because the number of ions arriving at the detector is proportional to this density. So the size of the deduction volume and the density of the particles contained therein is a direct measure of the Time-of-flight mass spectrometer sensitivity.

Another important quality feature of a time-of-flight mass spectrometer is its dynamic range. Here is the dynamic range Factor meant by which the signal of a certain mass is smaller than the signal of other masses may be without going through at wrong times incoming ions of these other masses to be covered.

1. Stöße, welche die Geschwindigkeit oder Richtung der Ionen derart ändern, daß sie nicht mehr am Detektor ankommen. Sofern diese Art von Stoß nur bei einem geringen Anteil der Ionen auftritt, wird hierdurch der dynamische Bereich und die Empfindlichkeit des Massenspektrometers nicht wesentlich verringert.

2. Stöße, welche die Geschwindigkeit oder Richtung der Ionen nur geringfügig verändern, so daß sie immer noch am Detektor ankommen, jedoch zu falschen Zeiten. Diese Stöße verringern zwar die Empfindlichkeit nur in ebenso geringem Maße wie Stöße der ersten Sorte. Da der dynamische Bereich proportional zum Quotient (richtig ankommende)/(falsch ankommende) Ionen ist, und die Anzahl der falsch ankommenden Ionen hier im Nenner steht, ist der Einfluß dieser Art Stöße auf den dynamischen Bereich des Flugzeit-Massenspektrometers sehr groß.

Die Anzahl von Stößen der Ionen mit Molekülen oder Atomen auf ihrer Bahn zum Detektor ist proportional zum vakuumtechnischen Restgasdruck in den entsprechenden Bereichen der Flugbahn.These two quality features are affected by collisions of the ions with molecules or atoms on their path to the detector. Two types of bumps must be kept apart:

1. collisions which change the speed or direction of the ions in such a way that they no longer reach the detector. If this type of collision occurs only with a small proportion of the ions, the dynamic range and the sensitivity of the mass spectrometer are not significantly reduced.

2. Shocks that change the speed or direction of the ions only slightly so that they still arrive at the detector, but at the wrong times. These impacts only reduce the sensitivity to the same extent as impacts of the first kind. Since the dynamic range is proportional to the quotient (correctly arriving) / (incorrectly arriving) ions, and the number of incorrectly arriving ions is in the denominator here, the impact of this type of impact on the dynamic range of the time-of-flight mass spectrometer is very great.

The number of collisions of ions with molecules or atoms on their path to the detector is proportional to the vacuum-technical residual gas pressure in the corresponding areas of the trajectory.

To make the time-of-flight mass spectrometer highly sensitive so it is necessary to have a high particle density in the discharge volume to reach. Around a high dynamic range of the time-of-flight mass spectrometer to cause, the lowest possible residual gas pressure be achieved. If both quality features are to be optimized, This is how time-of-flight mass spectrometry arises in many applications the problem with gas phase particles that a high particle density in the withdrawal volume also a high load with undesired gas ballast, which increases the residual gas pressure means.

Usually the time-of-flight mass spectrometer is divided into several Areas of different pressure divided by the sample introduction, i.e. the generation of the gas or ion beam to be examined, to the ion source and along the flight path in the time-of-flight mass spectrometer are sorted by decreasing pressure. In order to neither the gas or ion beam to be examined nor the ions on it Path from the withdrawal volume to the detector, are hindered adjacent areas connected by gas flow impedances. This The procedure allows a high particle density in the discharge volume, and nevertheless a low residual gas pressure or low impact probability on the flight path of the time-of-flight mass spectrometer.

Gas flow impedances are to be understood here as small openings Cross-section, which are large enough to keep the ions on their orbits to pass to the detector, but their conductance for gases is essential is lower than the pumping capacity of the pump in the area with the lower pressure.

In the simplest case, it is a gas flow impedance around an opening of certain cross section in the partition between Areas of different pressure. Have pipes or pipe-like structures but have a much lower gas conductance than openings Cross-section and are therefore preferable in many cases.

Skimmers are conical structures with an opening in the tip, which the Gas flow opposes. Skimmers have a similar gas conductance as openings same cross section and are preferable if the gas flow has a high density.

The publication by Michael et al. (Review of Scientific Instruments, Volume 63 (10), pages 4277-4284, 1992) it can be seen that the time-of-flight mass spectrometer in several areas with different Pressure is divided, the area in which the trigger volume has a higher residual gas pressure than parts of the ion trajectory. However, how to take chapter "C. TOF operation" can, the ion source, a gas flow impedance ("A restriction of 1 in. Tubing is placed between the flight tube and the main chamber "), and the focusing electrodes individually and separately arranged units.

The disadvantage of this separate arrangement of the ion source and the gas flow impedance is that the ions still have to travel a relatively long distance through the dense gas in the ion source and there is therefore a high probability of collision for ions with residual gas particles. Incidentally, with the gas flow impedance mentioned above, the diameter seems to be too large or the length is too small, since the pressure difference between the two areas is less than a factor of 4 (2 × 10 ^-6 or 6 × 10 ^-7 ).

The published patent application DE 41 08 462 A1 and the publication of Rohwer et al. (Zeitschrift für Naturforschung, Volume 43a, pages 1151-1153, 1988) show how a skimmer is separated from the ion source in front of the Ion source is arranged. Here is the distance between the opening of the skimmer and deduction volume relatively large.

This is disadvantageous for the following reasons: You want the gas or ion beam to be examined crosses the discharge volume, because from here the ions on their trajectory into the mass spectrometer be started. If parts of the gas or ion beam to be examined do not cross the discharge volume, so do not wear these parts to increase sensitivity, they only increase the residual gas pressure and thus reduce the dynamic range of the time-of-flight mass spectrometer. Since the gas or ion beam to be examined is always more or less divergent, the proportions that are Do not cross the discharge volume, the greater the distance Skimmer / trigger volume is. A large distance is disadvantageous since there is a large gas load in the ion source area, and thus high residual gas pressure, can only achieve a lower particle density in the discharge volume. This has a reduced sensitivity and a lower dynamic range of the Time-of-flight mass spectrometers result.

In WO-A-92 04728 an apparatus for the analysis of chemical species is used a supersonic ion beam and a time-of-flight mass spectrometer, in which are two areas with different pressures from each other by a skimmer are separated and the ion source is arranged in one of these areas and in the other Area the time-of-flight mass spectrometer is arranged. The same arrangement is published in the publication by C.H. Sin et al. in Analytical Chemistry, Vol. 63, No. 24, pages 2897-2900.

The invention is accordingly based on the object of a time-of-flight mass spectrometer specify with gas phase ion source, which is equally high sensitivity and has a high dynamic range.

In particular, it is an object of this invention to provide a time-of-flight mass spectrometer with a gas phase ion source specify which allows a high particle density in the discharge volume, but at the same time a low residual gas pressure on the flight path of the ions from the withdrawal volume to the detector.

This object is achieved by the characterizing features of claim 1.

The device according to the invention is in two or more areas of different pressure split, with gas flow impedances connecting two areas. The gas flow impedance (s) is / are to be as close as possible to the withdrawal volume approach, integrated directly into the electrodes of the ion source. This has the intent part that a maximum particle density in the withdrawal volume with a minimum impact probability can be achieved in the flight path of the mass spectrometer.

Advantageous embodiments of the invention are specified in the subclaims.

The following is now based on the embodiments shown in the drawings the invention described and explained in more detail.

Fig. 1 shows the simplest possibility of the gas flow impedance in one of the electrodes to integrate. The accelerating field will defined here by a repeller electrode (1) and an acceleration electrode (2). These two electrodes define this in this example accelerating field of the ion source.

In this embodiment, a flow impedance (3) is only integrated into the acceleration electrode (2). The acceleration electrode separates the area of the acceleration field with the higher pressure p 1 from the area of the flight path in the time-of-flight mass spectrometer with lower pressure p 2. The gas flow impedance can, for example, as shown in FIG. 1 and in claim 2, to be a pinhole.

As shown in FIG. 1 , the gas or ion beam (10) to be examined can be shot into the ion source perpendicular to the direction of acceleration. Ionized particles, which are in the withdrawal volume (11) at the start time, are accelerated along the drawn paths (12) into the time-of-flight mass spectrometer.

The direction of acceleration is understood here to be the direction in which the ions are then accelerated to at the start time.

In the embodiment of FIG. 1 , the orbits (12) of the ions are divergent according to the gas flow impedance (3) and have to be focused afterwards. This can be achieved by already known lens designs and is therefore not described in more detail here.

Fig. 2 corresponds essentially to Fig. 1 , instead of a pinhole, the flow impedance (3) is formed by a tube. A pipe has a much lower gas conductivity than a pinhole with the same cross-section.

Fig. 3 shows an example of an embodiment according to claim 14 or 16. Here, the additional electrode (4) between the repeller electrode (1) and the acceleration electrode (2) serves to the ions on parallel paths (12) by the flow impedance (3) to steer. Under certain circumstances, it may be advantageous to add further electrodes behind the gas flow impedance.

Should be a laser or electron beam for ionization through the withdrawal volume openings must be shot for this be provided in the electrode (4). It is also possible to use the electrode (4) to be broken down into two parts, one closer to the repeller electrode (1), and one is closer to the accelerating electrode (2). The beams can be aimed between these two parts.

This arrangement is shown in Fig. 4 , which thus also gives an example according to claims 14 and 16, respectively. The two electrodes (4, 5) between the repeller electrode (1) and the acceleration electrode (2) serve to direct the ions on intersecting paths (12) through the flow impedance (3). Under certain circumstances, it may be advantageous to add further electrodes behind the gas flow impedance. It is also possible to choose different radii to the axis of the ion source for the two additional electrodes (4, 5).

If the electrodes ( 4 , 5) are divided into two symmetrical halves along a normal plane of the gas or ion beam (10) to be examined, which is marked with a dash ( B - B ' ) in FIG. 4 , a transverse electric field can be created , also called the deflection field. This deflection field can change the transverse velocity components of the charged particles.

• Zieht man die zylindersymmetrischen Anteile des Feldes von dem gesamten elektrischen Feld ab, d.h. setzt man die linken und rechten Hälften der geteilten Elektroden(4,5) auf gegengleiche Potentiale, und die übrigen, ungeteilten Elektroden(1,2) auf Massepotential, so entsteht in einem großen Bereich entlang der Achse ein elektrisches Feld, dessen Feldstärke in transversaler Richtung nur schwach von den transversalen Koordinaten abhängt.
• Zieht man die transversalen Anteile des Feldes von dem gesamten elektrischen Feld ab, d.h. setzt man die linken und rechten Hälften der geteilten Elektroden(4,5) auf gleiche Potentiale, so verbleibt als Rest ein nahezu zylindersymmetrisches elektrisches Feld. In einem zylindersymmetrischen Feld werden die Ionen isotrop fokussiert bzw. defokussiert, und somit sind dann nach der Ionenquelle keine anisotropen Linsenelemente nötig. Anisotrope Linsenelemente sind generell aufwendiger, teurer und schwerer zu justieren als zylindersymmetrische Linsenelemente.

In addition to a necessary, small gap between the two halves, the electrodes (4, 5) then retain their cylindrical symmetrical shape. This has the following advantages:

• Subtracting the cylindrically symmetrical portions of the field from the total electric field, ie if the left and right halves of the divided electrodes (4,5) are set to opposite potentials, and the remaining undivided electrodes (1,2) to ground potential, the result is in a large area along the axis there is an electric field whose field strength in the transverse direction depends only weakly on the transverse coordinates.
• If the transverse portions of the field are subtracted from the total electric field, ie if the left and right halves of the divided electrodes (4, 5) are set to the same potential, the remainder remains an almost cylindrical symmetrical electric field. The ions are isotropically focused or defocused in a cylindrically symmetrical field, and thus no anisotropic lens elements are then necessary after the ion source. Anisotropic lens elements are generally more complex, expensive and difficult to adjust than cylindrical symmetrical lens elements.

In addition to the optimal field properties, the has cylindrical symmetry Training the deflection electrodes the further advantage that the Deflection electrodes can initially be manufactured as a turned part. In in a subsequent operation, they can then be broken down into two parts become.

Fig. 5 shows an embodiment according to claim 20. Here, the generated electrons are drawn off along the shown electron paths (13) by a gas flow impedance (6) in the repeller electrode (1). Due to the gas flow impedance (6) along the electron tracks (13), as seen in FIG. 5 , to the left of the repeller electrode (1), the pressure p 3 is lower than the pressure p 1 in the acceleration path.

In the embodiment of FIG. 5 , the electron beam (13) is divergent according to the gas flow impedance (6) and must then be focused. This can be achieved by already known lens designs and is therefore not described in more detail here.

6 shows an embodiment according to claim 10. Here, the gas or ion beam (10) to be examined is injected into the ion source parallel to the direction of acceleration by the skimmer (6). For this embodiment of the invention, the pressure p 3 in front of the skimmer is greater than the pressure p 1 in the acceleration section.

Electrodes that separate partitions between areas simultaneously Pressure must be connected to the housing, to be able to fulfill their function. If the electrode in question Ground or housing potential, this is easy. If an electrode, which are simultaneously a partition between different areas To represent pressure, is not at ground potential, must be between an insulator can be provided for this electrode and the housing. If this insulator is glued flat between the electrode and the housing problems, e.g. by degassing the adhesive, gas inclusions between insulator and electrode, etc. arise.

FIG. 7 shows a possible solution if an electrode, which is also intended to represent a partition between areas of different pressure, is not at ground potential. As shown, the electrode (2) and the housing wall (31) overlap, but do not touch. The distance between the two, as shown here by way of example, is determined by a sapphire ball (32). The gap between the electrode (2) and the housing wall (31) should be chosen so small that the conductance for gases is significantly smaller than the pumping capacity of the pump in the area with the lower pressure. It is understood that the electrode (2) must be pressed against the housing wall. This can be brought about by already known methods, which is why it is not dealt with in more detail here.

Claims (20)

1. A time-of-flight mass-spectrometer

   with two or more regions of different gas pressure,

   neighboring regions being connected via gas flow restrictions(3,6),
   with a gasphase ion source,

   being bounded by a number of electrodes(1,2,4,5) for producing electrical fields that accelerate the ions,

   containing a repeller-electrode(1)
   characterized by
   at least one of the accelerating electrodes(1,2,4,5) forming a boundary between regions of different gas pressure, the gas pressure being reduced in the direction of flight of the ions, a gas flow restriction(3,6) integrated into said electrode, this electrode being positioned in the path of the ions behind the repeller electrode(1).
2. A time-of-flight mass-spectrometer with gasphase ion source according to claim 1, characterized by a gas flow restriction(3,6) in an electrode(1,2), said flow restriction being a hole in said electrode.
3. A time-of-flight mass-spectrometer with gasphase ion source according to claim 1, characterized by a gas flow restriction(3,6) in an electrode(1,2), said flow restriction being a tube integrated into said electrode.
4. A time-of-flight mass-spectrometer with gasphase ion source according to claim 1, characterized by a gas flow restriction(3,6) in an electrode(1,2), said flow restriction being a scimmer integrated into said electrode.
5. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by the gas flow restriction being covered by a metal mesh.
6. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by the gas flow restriction not being covered by a metal mesh.
7. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by several electrodes(1,2) with openings, said openings representing gas flow restrictions, some of said openings being covered with metal meshes, and some of said openings not being covered with metal meshes.
8. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by an electrical field between the electrodes(1,2,4,5), said electrical field being independent of time.
9. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 1 through 7, characterized by an electrical field between the electrodes(1,2,4,5), said electrical field being time-dependent.
10. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by the direction of flight of the analyte gas or ion beam(10), said direction of flight being parallel to the direction into which the ions are accelerated within the ion source.
11. A time-of-flight mass-spectrometer with gasphase ion source according to claim 10, characterized by a further gas flow restriction(6), said further gas flow restriction being integrated into the repeller electrode(1).
12. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 1 through 9, characterized by the direction of flight of the analyte gas or ion beam(10), said direction of flight being orthogonal to the direction into which the ions are accelerated within the ion source.
13. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 1 through 9, characterized by the direction of flight of the analyte gas or ion beam(10), said direction of flight having some arbitrary angle to the direction into which the ions are accelerated within the ion source.
14. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by one or several gas flow restrictions (3,6), one or several additional electrodes(4,5), and said additional electrodes being arranged before - as seen in the direction of flight for ions or electrons - said flow restriction.
15. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by one or several gas flow restrictions(3,6), one or several additional electrodes, and said additional electrodes being arranged behind - as seen in the direction of flight for ions or electrons - said flow restriction.
16. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 1 through 13, characterized by one or several gas flow restrictions(3,6), one or several additional electrodes, and said additional electrodes being arranged before or behind said flow restriction.
17. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by electrodes(1,2,4,5), said electrodes defining the acceleration field, and further electrodes, said further electrodes creating a transverse field, said transverse field changing the transverse velocity component of charged particles.
18. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 14 through 16, characterized by additional electrodes(e.g. 4,5), said additional electrodes being arranged before or after the gas flow restriction(3,6), and

    said additional electrodes being split along a plane normal to the direction of the analyte gas or ion beam into symmetrical half-parts, said half-parts producing a transverse electrical field, said transverse field changing the transverse velocity component of charged particles,

    said additional electrodes, except for being split into two half-parts, having a form of rotational symmetry around an axis, said axis pointing in the direction of acceleration of said gasphase ion source.
19. A time-of-flight mass-spectrometer with gasphase ion source according to one of the claims 17 or 18, characterized by electrodes defining a transverse electrical field, said electrodes being additionally split symmetrically along a plane, said plane being defined by two vectors, one of said vectors being the direction of the analyte gas or ion beam, the other of said vectors being the direction of acceleration in the ion source.
20. A time-of-flight mass-spectrometer with gasphase ion source according to one of the previous claims, characterized by ions and electrons that are both drawn out of the ion source, and a gas flow restriction(6) on the electron paths(13) within the ion source.

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