EP1817788A2

EP1817788A2 - Flight time mass spectrometer

Info

Publication number: EP1817788A2
Application number: EP05792139A
Authority: EP
Inventors: Uwe Dahlke; Christian LÜDKE; Carsten Roland; Jochen Skole; Rainer Sorge; Jörg WOLLBRANDT
Original assignee: Angewandter Optik Optoelektronik Quantenelektronik und Spektroskopie Ev Gesell zur Forderung; Der Analytischen Wissenschaft Ev Gesell zur Forderung; GES ZUR FOERDERUNG ANGEWANDTER OPTIK OPTOELEKTRONIK QUANTENELEKTRONIK & SPEKTROSKOPIE EV; Hoffmann Erwin; Leibniz Institut fuer Analytische Wissenschaften ISAS eV
Current assignee: Analytik Jena AG
Priority date: 2004-09-17
Filing date: 2005-09-12
Publication date: 2007-08-15
Anticipated expiration: 2025-09-12
Also published as: AU2005284150A1; WO2006029999A3; DE102004045315A1; ATE517428T1; AU2005284150B2; WO2006029999A2; EP1817788B1; AU2005284150A8

Abstract

A device for producing an ion beam (120) from positively charged ions in an evacuated flight time mass spectrometer (10) containing an ion source (12), an interface (14) for transferring the ion beam of atmospheric pressure into the mass spectrometer (10), and an ion extraction device (64) with a voltage source (74) for the production of a negative potential difference between the ion extraction device (64) and the ion source (12), characterized in that the negative potential difference amounts to at least one - 1 kV and the ion extraction device (64) is connected to the voltage source (74) via a high ohm resistor (72), the value of which is selected in such a way that no spontaneous discharge occurs between the ion source (12) and ion extraction device (64) when voltage is applied.

Description

Time of Flight Mass Spectrometer

Technical area

The invention relates to a device for generating an ion beam of positively charged ions in an evacuated time-of-flight mass spectrometer

(a) an ion source,

(b) an interface for transferring the ion beam from the ambient pressure to the mass spectrometer, and

(c) an ion extraction device having a voltage source for generating a negative potential difference between the ion extraction device and the ion source. The invention further relates to a time of flight mass spectrometer with such a device.

In a time-of-flight mass spectrometer, ions are accelerated in an electric field. Depending on the mass, the ions have a different time of flight.

Accordingly, the ions arrive at different times on the detector. The time-resolved signal therefore provides information about the amount and type of ions that have been introduced into the mass spectrometer. The longer the time of flight of the ions, the better the resolution. An improved resolution means that ions of different masses can be distinguished from one another more securely. The more

Ions strike the detector, the better the signal-to-noise ratio. A good signal-to-noise ratio means that even the smallest amounts of an analyte in a sample can still be measured and small differences can be determined for different samples. It is thus the fundamental goal of each measurement to measure as many ions of a sample as possible and to achieve a high resolution.

Known time of flight mass spectrometers use an inductively coupled plasma (ICP) as an ion source. Such ion sources are known. In this case, the sample to be analyzed together with a gas - usually argon - in the

Torch ionized. It poses a challenge to introduce the positive ions generated there under atmospheric pressure into the highly evacuated vacuum of the mass spectrometer.

For this purpose, a transition arrangement, an interface is provided. The interface comprises so-called "ion lenses" with which the ion beam is transferred via a chamber with a pre-vacuum into the mass spectrometer by suitable design of electric fields.

In an ion extraction device, a voltage source becomes a negative

Potential difference between the ion extraction device and the ion source generated. The positive ions are accelerated towards the ion extraction device. However, an arbitrarily high voltage can not be applied since otherwise an unwanted electrical flashover occurs. The amount of extracted sample ions is therefore limited.

Time-of-flight mass spectrometers also contain a repeller for accelerating the focused one, in addition to the ion source for generating an ion beam of sample ions

Ion beam The repeller briefly generates a strong electric field in which the ions are accelerated. These then fly through a drift tube to separate ions of different mass. The time-resolved measurement of the intensity of the ion beam then takes place at the detector.

An improved resolution can be achieved on the one hand by the fact that the time peaks are as narrow as possible. This means that ions of the same mass impinge on the detector as simultaneously as possible. But it can also be achieved by increasing the drift time. Known high-resolution time of flight mass spectrometers therefore have very long drift tubes. The disadvantage here is that the overall dimensions of the device are large.

In some time-of-flight mass spectrometers, a first drift tube is provided, followed by a reflector. The reflector serves to time focus and fold the ion beam, wherein the longitudinal axis of the reflector forms an angle with the longitudinal axis of the first drift tube for the incident ion beam. The ion beam emerging from the reflector then passes into another drift tube, at the end of which the detector is arranged. Since the ion beam assumes a parabola or parabolic-like trajectory in the reflector, the drift tubes are usually arranged at an angle to the longitudinal axis of the reflector. They form a quasi-V-shaped arrangement. For longer drift tubes to achieve a high resolution, this leads to considerable device dimensions.

Time-of-flight mass spectrometers also contain means for focusing the ion beam in addition to the ion source for generating an ion beam of sample ions. This provides all the ions with the same "starting conditions." To repel the focused ion beam at an angle to the original ion beam direction, a repeller is provided, which generates a strong electric field for a short time which the ions are accelerated. These then fly through a drift tube to separate ions of different mass. The time-resolved measurement of the intensity of the ion beam then takes place at the detector.

In order to obtain a high resolution and a high accuracy of the measurement results, the time peaks at the detector must be as sharp as possible. This means that ions of the same mass should as far as possible arrive at the detector "simultaneously", provided that all ions "start" from the same place at the same time. This requires the best possible focus. The ions are always subject to a temperature distribution, so that even with ions of the same mass, the initial velocity is subject to a distribution. Furthermore, the ion source is finite. This means that the ions may not all be accelerated from the same location. So it is a constant effort to achieve the best possible temporal and spatial focusing of the ions in the repeller. Known spectrometers focus the beam rotationally symmetric along a line. However, this results in an undesirable "smearing" in the direction of flight.

In order to achieve a particularly good detection limit, it is necessary, as far as possible, no interfering ions from matrix or carrier gas, i. To measure argon at the detector. For this purpose, so-called "ion deflectors" are provided

Ionpaktet known mass past the ion deflector, an electrical potential is applied. The ions experience a force that detracts them from the original orbit. They therefore no longer reach the detector. Such a device for the deflection of ions with selected ion masses from an ion trajectory must be quickly switchable, must not be too expensive to manufacture and must not have any influence on the remaining ions in the off state.

To generate the respective potentials in the course of the ion trajectory grids are required. The grids must be extremely flat and should not change during the lifetime of the device. Usually, the grid consists of a wire mesh, which is stretched in a frame. The voltage is then applied to the frame. The production of these grids is expensive. Disclosure of the invention

It is an object of the invention, a time-of-flight mass spectrometer with improved

Time resolution with reduced detection limits and high sensitivity to create. According to the invention the object is achieved in that the negative potential difference between the ion extraction device and the ion source has an amount of at least - 1 kV and the ion extraction device via a high-impedance

Resistor is connected to the voltage source whose value is selected so that when applied voltage no spontaneous discharge between the ion source and the ion extraction device takes place.

By using the resistor, a particularly high, negative potential difference can be achieved without a spontaneous discharge takes place. With such a potential difference, significantly more ions can be extracted from the ion source. However, the extraction rate has a direct effect on the signal-to-noise ratio, and thus on the detection limits and sensitivity of the device. These can therefore be significantly improved.

Preferably, the ion source is an inductively coupled plasma, which is producible in a tubular torch with an induction coil, and a tubular

Grounding plate for grounding the plasma potential between the torch and the

Induction coil is arranged. This grounded the plasma. The components of

Ion extraction devices may be at a potential of -1.8 kV to -2.5 kV relative to the potential of the ion source. In contrast, only a maximum potential of -600 V is possible without the use of the resistor according to the invention.

An increased resolution can be achieved in an arrangement of the type mentioned not only in that the drift tube is extended, but also by

(E) additional means for generating a relation to the drift tube increased potential in a first and a second region of the drift tube, with a for a change of direction of incoming ions sufficient potential difference compared to the potential of the drift tube,

(f) wherein the first region is upstream of the second region with respect to the direction of flight, and

(G) means for controlling the time course of the potential thus generated.

This ensures that the ions pass through the drift tube several times. The ion packet, which is to be measured with increased resolution, reverses its direction when it comes to the second region and there is a sufficient positive potential with respect to the drift tube. This potential can be, for example, 2 kV. It then passes through the drift tube in the reverse direction a second time. If it comes in the upstream first area, there is also a sufficiently high potential created there. The ion packet reverses again and traverses the drift tube a third time. Now the second area is put back to the original drift tube potential. This allows the ion packet to pass in the direction of the detector.

Preferably, the means for generating a potential of each one arranged substantially perpendicular to the ion trajectory grid formed, which is connected to a voltage source.

This embodiment for increasing the resolution is particularly advantageous because the simple way of installing additional gratings in a conventional device, the resolution can be increased. The resolution can be increased for ion packets from any mass ranges. The device dimensions remain low.

The potential of the drift tube may be below -1700 V relative to the potential of the ion source and the potential in the first and second regions above +400 V.

In a particularly preferred embodiment of the invention, the first region at the entrance and the second region are arranged at the exit of the drift tube. This will realizes a particularly long drift distance. This leads to a particularly high resolution. The ion packet measured at this high resolution is large.

The means for controlling the time profile of the potential may be switching means for switching on and off the potential difference with switching times in the range of up to 100 ns. As a result, a particularly precise circuit of the potentials is possible. The shorter the switching times, the greater the measured ion packet may be.

The operation of a time-of-flight mass spectrometer with increased resolution thus takes place with the following steps:

(a) generating an ion beam;

(b) spatially focusing the ion beam;

(c) accelerating the ion beam with a voltage pulse;

(d) selecting an ion packet with selected masses from the ion beam;

(e) generating a potential difference in the second region when the selected one

Ion packet enters this area, such that the ion packet changes its direction of flight,

(f) generating a potential difference in the first region when the selected ion packet enters that region, such that the ion packet again

Flight direction changes.

Preferably, the ion packet runs back in itself when changing its direction of flight substantially in itself. However, it is also an arrangement suitable in which the ion beam is folded only at an acute angle. Then the beam zigzags back and forth. The potential differences can also be generated alternately in the first and second regions for a limited period of time. Then the ion packet does not run three times, but five, seven, nine ... times back and forth. The resolution increases accordingly.

Time-of-flight mass spectrometers employing a reflector can achieve high resolution with small dimensions even if the longitudinal axis of the drift tube for the outgoing ion beam and the longitudinal axis of the reflector form an angle less than or equal to the angle between the longitudinal axis of the reflector and is the longitudinal axis of the drift tube for the incident ion beam. in the

In extreme cases, the two drift tubes run parallel. The surface which coincides with the reflector aperture then forms a small angle with the surface formed by the two drift tube ends.

The reflector is preferably arranged so that the angle between the longitudinal axis of the reflector and the longitudinal axis of the drift tube for the incident ion beam is 1.5 to 2.5 °. It has surprisingly been found that exactly in this angular range, the signal at the detector is particularly high.

Preferably, a brake grid is arranged in front of the reflector, on which a

Voltage from -800 to -1200 V is applied. By using a brake grid, the reflector can be made relatively small, thereby further reducing the device dimensions.

In a particularly preferred embodiment of the invention, one of the drift tubes is for a maximum of half, in particular a quarter times as long as the drift tube between the repeller and reflector. Then the ion beam can also assume large aperture angles at the reflector.

The resolution of time-of-flight mass spectrometers can also be increased if the time peaks are as narrow as possible. This is achieved in that the means for focusing the ion beam are formed such that the ion beam is focused in a plane perpendicular to the direction of acceleration in the repeller. The ions are allowed So spread out virtually perpendicular to the propagation plane. The focusing takes place only in the direction of flight. Due to the deviation from the rotational symmetry at this point, a particularly good focus is achieved.

Preferably, the means for focusing comprise an ion tube within which is provided a tube lens having a potential which is positive with respect to the potential of the ion guide tube and an ion optic having a gap which is substantially vertical and perpendicular to the direction of flight of the ions and the potential of the ionic tube lies. With such a gap, which closes off the ion tube, can be achieved in a particularly suitable manner focusing in a plane.

Preferably, the gap is formed by two split jaws which are provided on a flange, which is connected to the ion tube. In the flange discharge channels may be provided in the form of a recess along a horizontal diameter on the back of the flange. Neutral particles, gas, residual droplets and the like settle over a long period of time, dry out. The salt components then form an insulating layer or islands of an insulating layer. This leads to potential changes in the repeller. In the repeller, the particles have in contrast to the tube no or only a low speed. It is therefore primarily in the repeller problems with deposits here. In the ionic tube, however, the ions are fast, so there are not so many deposits there.

The split jaws preferably have an opening angle between 78 ° and 82 °, in particular of 79.8 °. It has been shown that the focusing is particularly good at this angle.

In order to remove interfering ions, for example argon ions, from the ion beam, there is provided an ion deflector having a holder with retainers and metal combs having a base, teeth adjacent to the base, and recesses therebetween, the teeth each having a longitudinal slot. and the metal crests are held with the base in the holders of the holder. The ion deflector comprises substantially parallel arranged metal strips, which are suspended resiliently in the longitudinal slots. Furthermore, means for generating an electrical voltage to the metal combs are provided. Two metal combs are arranged offset from each other so that each tooth of the first metal comb is at the height of a bulge of the second metal comb.

An ion deflector is particularly easy to manufacture in this embodiment. The potential is extremely even and durable to produce.

Preferably, two metal combs are provided in pairs at the ends of the holder and the metal strips are attached to respective metal combs. The metal strips are soldered under mechanical tension to the metal combs. As a result, the strips keep their flatness.

Preferably, the metal comb consists of a 1.5 to 2.5 cm wide and / or 0.30 to 0.4 mm thick stainless steel sheet. It can be processed or produced very accurately by means of laser cutting.

In a further embodiment of the invention, the slots are widened at the tooth end to indentations. This facilitates the threading of the metal strips.

The device for deflecting ions can advantageously be arranged between the two deflectors. In this case, the holder may be arranged within the spectrometer to the ion beam, that the longitudinal sides of the metal strip is arranged parallel to one of the deflectors.

In a further embodiment of the invention, the metal strips are provided at the location of the first focusing of the ion beam, at which a spatial focusing takes place. In one focus, the unwanted ions can be removed by a short pulse, without the other ions being significantly affected.

The method for producing an ion deflection device for time-of-flight mass spectrometers according to the invention preferably comprises the steps: (a) laser cutting metal strips into a metal sheet,

(b) separating the strips from the metal sheet,

(c) holding metal combs with teeth provided with slots in a holder,

(d) inserting the metal strips into the slots of the metal combs,

(e) soldering the metal strips under mechanical tension.

To produce the most level grid possible a grid holder for flat grid can be used, which contains a clamping ring with two coaxially arranged ring parts which are connectable to each other with surfaces. In a first ring part, an annular groove is provided, which has at its inner edge a projecting, circumferential annular nose. In the second ring part an annular groove corresponding to the annular groove is provided. The grid can be tensioned via this ring nose between the ring parts with a rubber ring in the ring grooves.

By using the annular nose, the grid can be tensioned without it tearing. The Verwenung of two ring parts allow a particularly cost-effective production.

The annular groove in the second ring part can have a rectangular cross-section. The annular groove in the first ring part, however, advantageously has symmetrical, upwardly opening inclined surfaces. The inner edge with the annular nose at the transition between the inclined surface to the lattice plane is preferably rounded. The ring parts are preferably screwed together. However, any other type of connection is possible. In a particularly preferred embodiment, the angle of the annular nose adjacent to the inclined surface of the annular groove with respect to the lattice plane has a

Angle in the range of 50 to 60 °, in particular of 55 °. Embodiments of the invention are the subject of the dependent claims. An exemplary embodiment is explained in more detail below with reference to the accompanying drawings.

Brief description of the drawings

Fig. 1 generally shown at 10 designated time-of-flight mass spectrometer shown schematically.

Fig. 2 shows a cross section through the ion source of Figure 1 in detail.

3 shows a schematic section through the interface shown in Fig.l and the ion optics in detail.

4 shows a cross-section in the radial direction through an ion tube and a tube lens in detail.

Fig. 5 is a perspective view of the entrance slit arrangement on the repeller.

FIG. 6 shows the course of the field lines and ion paths in the area of

Repeller lens and in the repeller.

Fig. 7 is a perspective view of the repeller space used

Components.

Fig. 8 shows a schematic representation of the repeller and the drift tube

Fig.1 with ion trajectory in high-resolution operation.

9 is a perspective view of the arrangement with repeller and drift tube of FIG.

10 is a perspective view of the selector arrangement for deflecting unwanted ion packets 11 shows a metal comb for the resilient mounting of metal strips in a selector from FIG.

Fig. 12 is a perspective view of the reflector and part of the drift tube of Fig. 1

FIG. 13 shows the temporal potential profile of repellers, reflection gratings and the

Number of useful ions in the reflector for an arrangement in high-resolution operation.

Fig. 14 is a perspective view of the detector.

Fig. 15 is a cross-sectional view of the edge of a lattice support with grid

Fig. 16 is a cross-sectional view of the ion source of Fig. 2

Fig.17 illustrates the angular relationships between the drift tubes and the

reflector

Description of the embodiment

A time-of-flight mass spectrometer, generally designated 10, is shown schematically in FIG. The mass spectrometer 10 comprises an ion source 12. Via an interface 14, the ions from the ion source 12 are evacuated from atmospheric pressure

Mass spectrometer 16 transferred. The mass spectrometer 16 comprises an ion optics 18 for focusing the ion beam in a repeller space 20. After the acceleration of the ions in an ion acceleration path 22, the ions pass into an ion drift tube 24. In a reflector 26, the ions change their direction of flight and are subsequently detected with an ion detector 28 ,

Ions of different mass have a different time of flight. Accordingly, the time-resolved detector signal provides information about those present in the ion source Ion types and their quantity. Smaller ion mass ions provide a signal earlier than larger mass ions. The longer the drift time, the higher the resolution, ie the better the masses can be distinguished with little difference in mass. The more ions from the ion source reach the detector, the higher the intensity.

The ion source 12 comprises an inductively coupled plasma (ICP). To generate the plasma, an injection tube 30 made of ceramic or quartz and a tubular torch 33 (Torch) made of quartz is provided. The ion source is shown again in detail in FIGS. 2 and 16. The torch sits on a support 32.

An induction coil 34 extends around the torch 33. The argon carrier gas is supplied through the injection tube 30 into the plasma region within the torch 33. There it is ignited by means of a spark. With the coil 34, an oscillating magnetic field is induced in the torch 33, which leads to further ionization of the gas in the torch. The gas temperature within the plasma is in the range of approx. 6000 ° C. Argon is passed through a feed 37 for cooling the components in the edge region of the plasma. The plasma gas argon is passed via a feed 36 into the plasma. At the plasma temperatures, the sample ions introduced with the carrier gas are predominantly simply ionized and have a velocity distribution corresponding to the temperature.

Between the torch 33 and the induction coil 34, a grounding plate 38 is provided. The grounding plate 38 is also tubular. The grounding plate 38 has a slot 39 in the axial direction. This is shown in FIG. 16. During the ignition of the plasma, the grounding plate 38 is potential-free. After ignition, the sheet 38 is grounded. For this purpose, a contactor 40 is provided, via which the connection to the earth is made. The contactor 40 is pneumatically operated. The grounding plate 38 is disposed between two quartz tubes 42 and 44. The quartz tubes 42 and 44 are connected to each other at the front side 46. Furthermore, the quartz tubes 42 and 44 are connected together along a narrow web 41 in the axial direction. As a result, the quartz tubes 42 and 44 form a double tube, which avoids high-frequency flashovers to the grounding plate 38. The double tube is pushed over the torch 33. There is a distance of a few millimeters between upper end 48 (Fig.l) of the coil 34 to the upper end 50 of the grounding plate 38th

At a distance of 2 mm from the top end 50 of the grounding plate in the direction of the ion trajectory 52, the sampler 54 of the interface 14 is arranged (Fig.l). The sampler

54 forms the input-side termination of the interface 14. In FIG. 3, the interface 14 and the ion optical system 18 are again shown in detail. The individual components are schematically pulled apart. The sampler 54 is a rotationally symmetrical, rear cone-shaped nickel aperture with a comparatively large aperture angle of 150 ° and an aperture with a diameter of 1 mm.

At a distance of 7 mm, another rotationally symmetrical diaphragm, the interface skimmer 56 is arranged. The space 58 between sampler 54 and skimmer 56 is evacuated via a port 57 to a pressure of about 1 mbar with a fore-vacuum pump (not shown). The interface skimmer 56 is also cone-shaped with an opening angle of 50 °. The opening has a diameter of 1.2 mm.

Immediately behind the interface skimmer 56, a closure device 60 (Fig.l) is arranged. With this closure device located behind the interface skimmer 56 high-vacuum part of the mass spectrometer 16 can be vauumdicht closed outside the measurement times. The closure device 60 essentially comprises two slides, which are pneumatically actuated.

The space 62 between the interface skimmer 56 and the adjoining ion extraction arrangement 64 is evacuated during the measuring operation to a pressure of about 10 ^-3 mbar There is a corresponding pressure gradient.

The ion extraction assembly 64 includes another orifice, the ion extraction skimmer 66. The ion extraction skimmer 66 is also cone shaped and has an opening of 1.2 mm. The opening angle of the cone is 50 °. The ion extraction skimmer 66 is directly connected to an ion tube 68. At the other end of the ionic flight tube 68, the entrance slit 70 is provided for ion optics. For this is one

Insulation 69 provided. The sampler 54 and interface skimmer 56 are at a potential of 0 V with respect to ground. The ion extraction skimmer 66 and the ionic tube 68 are at a very high negative potential of -2 kV. In order to avoid unwanted discharges between the ion extraction skimmer 66 or ion trajectory tube 68 and the interface skimmer 56, the ionic flight tube 68 is connected to the voltage source 74 via a high-impedance resistor 72 of 1 MΩ (FIG. 3). As a result, the current is limited to a range in which the discharge is severely hampered.

By using the resistor 72, a high, negative voltage of -2 kV can be used. The oppositely charged positive ions are attracted to the tube by the negative potential. This achieves a high extraction rate.

Within the ion trajectory tube 68, a tube lens 76 is disposed in the entrance area behind the ion extraction skimmer 66. In the ion trajectory 68 three elongated bulges 78 are provided with semicircular cross section for this purpose, which extend over the length of the tube lens 76 in the axial direction.

This can be seen in FIG. In the bulges 78 are electrically insulating

Ceramic rods 80 provided. The tube lens 76 is held between these ceramic rods 80. Through an opening 82, a contact 84 is made to the tube lens 76.

The contact 84 is isolated from the ionic tube 68 by an insulation 86 provided in the opening 82. The contact 84 is above a resistance of 1MΩ at a potential of -300V. The positive ions of the ion beam are thereby

Neutral and negatively charged particles are not focussed, with a tube length of the ionic flight tube 68 of 8 cm, a length of the

Tube lens 76 of 1.5 cm and said potential ratios is a

Focusing just reached the output side pipe end.

There, the entrance slit 70 is provided in the ion optical system 20. The entrance slit 70 is shown enlarged in Fig. 5 in detail. Two split jaws 88 and 90 are on one

Flange 92 screwed. The split jaws 88 and 90 form a fixed gap 94 of 0.5 mm wide. The gap is arranged vertically and perpendicular to the direction of flight 96 of the ions. Accordingly, there is no longer rotational symmetry.

In Fig. 5, the back of the entrance slit assembly is shown. There is a recess 98 in the flange 92. The recess 98 is about 6mm wide and extends along a horizontal diameter on the back of the flange 92. The recess 98 forms outflow channels. Unfocused particles, such as unwanted neutral particles striking the entrance slit assembly 70 from the front, may laterally exit the ionic flight tube 68 through the outflow channels. This prevents deposits that can be caused by neutral particles, gas and residual droplets that settle and dry up in the following repeller. Such deposits can form islands of an insulating layer and lead to potential changes in the repeller. The problems exist in particular in the repeller, since the particles have there, in contrast to the ionic tube no or only a low speed. On the other hand, ions are fast in the ion tube, so there are not so much problems with the unwanted particles.

The flange 92 is connected to the ion extraction tube 68. The interior of the ion extraction tube 68 is evacuated mbar to a pressure in the range of 10 ^'6. In Figure 3, the individual pressure zones 58, 62 and 100 are shown.

The gap jaws are at a potential of -2kV and form an opening angle of 80 °. The angle is greater than the opening angle of the ion extraction skimmer 66. As a result, a particularly favorable potential profile for focusing the ions in the subsequent repeller space 20 is achieved.

Due to the vertical extent of the gap 94, the ions are not focused in one point but in a vertical plane 102. The plane 102 is located in the repeller space 104 behind the repeller lens 106, between the repeller plate 108 and the repeller grid 110. In Fig. 6 is a section through the repeller space 104 again in

Detail shown. The gap jaws 112 and 114 of the entrance slit 70 are at a high extraction potential of -2 kV. Accordingly, the ions are accelerated through the gap 94. The repeller lens 106, the repeller plate 108, and the repeller grid 110 are grounded. The distance between the repeller plate 108 and the repeller grid 110 is 16 mm. It is defined by spacers 118 (FIG. 7).

The distance between entrance slit 70 and repeller lens 106 is 5 mm. The opening of the repeller lens 106 is 8 mm wide and extends like the entrance slit in the vertical direction over a length of 12 mm.

In Fig. 6, some field lines 116 and ion trajectories 117 are in the region of the repeller lens

106 drawn. In accordance with the potential curve, the ions in the plane 102 are strongly decelerated. There is a spatial focusing of the ions for all masses, since all masses except for a temperature distribution have the same energy corresponding to the acceleration voltage of 2 kV. Since an ion distribution in the plane 102 perpendicular to the direction of flight 122 is permissible, a particularly spatial focusing in

Flight direction 122 possible.

In Fig. 7 is a spatial representation of the used for the repeller space

Components shown. The ions enter through the repeller lens 106. This is shown by an arrow 120. You leave the repeller room in the direction of the arrow

122. The connections for two high vacuum pumps are labeled 124 and 126

(Rg-I) -

The ions decelerated and focused in plane 102 become orthogonal through a short positive voltage pulse of 800 V at the repeller plate 108

122 distracted. You will be accelerated in this direction simultaneously. Apart from the always existing temperature distribution, the ions at this point all have the same initial conditions, where they have (almost) no velocity and start in the same plane. They then pass through the repeller grid 110. Behind the repeller grid 110 is another grid 130, the so-called brake grid. The

Brake grille 130 is at a small positive voltage of +15 V. This creates an opposing field. The opposing field retains ions of low energy. Such ions may, for example, before the voltage pulse on the repeller plate due to the kinetic energy that exists due to diffusion, scattering and temperature distribution, it can reach the area of the brake grid from the focusing plane. Without the brake grid 130, these ions would create an undesirable background in the spectrum as they are accelerated out of another plane. High energy ions, which have already been accelerated by the voltage pulse, easily overcome the potential barrier of the brake grid.

The ions are then accelerated on the acceleration path 132 in the direction of an acceleration grating 134. The accelerator grid is at a high negative voltage of -1870 V. The positive ions are from this negative

Tension accelerates.

Behind the acceleration grid 134, a diaphragm 135 is arranged. There is a weak negative voltage of -300 V at the diaphragm. Through this aperture, the marginal rays are conducted into the measuring range. As a result, an increase in intensity is achieved.

Behind the diaphragm 135, a grating 137 is arranged. The grating 137 is at a potential of - 1870 V. By the grating 137 is achieved that the electrical effect of the diaphragm 135 does not lead in the area of the X-deflector 138 to electric field distortions.

A spatial representation of the arrangement is shown again in detail in FIG. One recognizes the input-side part of the drift tube 166. The drift tube entrance form the grids 134 and 137 and the diaphragm 135 arranged therebetween. Around the drift tube

166 insulated retaining rings 167 and 169 are arranged. The retaining rings 167 and 169 are made of polyacrylic methacrylate (PMMA). With them, the drift tube 166 lying at a high potential is mounted in the device housing. A selector 142 to be described later is arranged with an anti-twist device 171 in the drift tube.

Along the trajectory 136 of the ions, an x-deflector 138 and a y-deflector 140 are disposed within the drift tube 166 (Fig.l). Depending on the voltage at the deflectors, the direction of the ion beam is adjustable. In this way, mechanical Production inaccuracies are compensated. The components may be manufactured with comparatively high manufacturing tolerances. The fine adjustment of the ion beam at the detector then takes place by applying a suitable voltage to the deflectors. Furthermore, a sawtooth voltage, ie a time-increasing voltage, is applied to the x-deflector 138 and the y-deflector 140. This optimizes the ion beam direction for light and heavy ions. The different ion masses receive the same, for the measurement optimal deflection angle.

Between the two Deflektoren a generally designated 142 selector is arranged. This is a device for the deflection of certain types of ions, e.g. High intensity ion species from the ion beam. The selector comprises substantially parallel arranged metal strips 144 with small extension in the direction of flight and a small distance from each other. The metal strips 144 are parallel to the y-deflector 140. The strips are arranged at intervals of 0.5 mm.

Each strip 144 is 1 mm wide and 50 μm thick. Normally, all stripes are at the same potential. Then the ion beam is not affected. However, if an ion packet, for example interfering argon ions, is to be deflected so that it does not disturb the measurement, a positive voltage of 200 V is applied alternately to every other strip via a capacitive coupling. The voltage is applied for the period in which the interfering ions are in the region of the selector 142. As a result, the ions are deflected in the direction of the Driftrohrwandung. With this arrangement, short switching times and thus high selectivity can be realized.

To permanently achieve the most even possible potential course must

Be straight and smooth. If possible, they should not heat up and deform under ion bombardment. The edges should not be bent and should be parallel. To guarantee the required geometric stability, each individual strip is resiliently suspended. The metal strips are cut with a suitable laser from a 50 micron thick sheet. The laser is very easy to control. Therefore, the manufacturing accuracy of the laser strips is very high. in the Unlike strips cut with a blade, the edge shows no deformation.

For resilient suspension metal combs 146 are used. An enlargement of such a metal comb is shown in FIG. 11. The metal comb 146 consists of a 2 cm wide stainless steel sheet, in which 16 recesses 148 are produced by a laser. In this way, 17 teeth 150. In each tooth 150, a slot 152 is provided. The slot 152 is widened at the tooth end to a recess 154. This indentation 154 facilitates the threading of the metal strips 144.

Four metal combs 146 are clamped to the non-slotted side 158 in a holder 156. This is shown in FIG. In each case, two combs, for example 146 and 160 are arranged to each other so that each tooth 150 of the comb 146 is at the height of a bulge 148 of the comb 160. A metal strip 144, which is thus guided through a slot 152 in the comb 146, can through the bulge 148 in

Kamm 160 are passed without touching it. The metal strip is then secured to the opposite side of the holder 156 in the corresponding comb 162. Also, the combs 162 and 164 on the opposite side are offset from each other. The metal strips 144 are soldered under mechanical tension to the combs. As a result, they retain their geometric stability even under ion bombardment.

The holder is then inserted into the ion beam so that the metal strips are perpendicular. The metal strips 144 are then at the location of the first focusing of the ion beam, at which a spatial focusing takes place. To apply a voltage now only a contact with the comb needs to be made. This can easily switch even for short periods. The holder 156 is electrically insulating.

In Fig.l the further course of the ion trajectory is shown. In the drift tube 166 a separation of ions of different mass, but the same energy due to their different transit time as a result of their different speeds. Light ions fly faster, heavy ions fly more slowly. At the end of the drift tube 166 is a generally designated 168 reflector is arranged. The reflector 168 consists of a series of concentrically arranged and mutually insulated metal rings 170. A spatial representation of the reflector 168 is shown in FIG. The metal rings 170 are connected to one another via resistors 171. At the metal rings is a voltage. The reflector is terminated with a plate 174 which abuts a potential of +800V. At the grid 176 is a

Voltage of -1000 V on. The intervening metal rings are at potentials with steadily increasing intermediate values. At the foremost metal ring 172 is a voltage of -900 V, at the final metal plate a voltage of +800 V.

In accordance with the potential curve within the reflector, the ion trajectory with the positive ions takes the course shown in FIG. The ions reverse their direction. This "temporal" focusing compensates for propagation time differences of ions of equal mass as a result of small differences in speed at the start into the drift path.

In front of the reflector, a brake grid 176 is arranged. This grid is at a negative voltage of -IkV. The ions are decelerated in the potential of this grating, so that the length of the ion trajectory is shortened to the reversal point 178. This shortens the required length of the reflector 168. The reflector also has a grid 177 for termination. After leaving the reflector 168, the ions still fly through a shortened piece of another drift tube 180. Subsequently, the ions strike the detector 28. The detector is again shown spatially in FIG. 14.

In front of the detector another grid 182 is arranged. The grid 182 lies on one

Potential of -2,8 kV. By the grating 182, the marginal rays are focused on the detector. This achieves a higher intensity, but with loss of resolution.

The reflector 168 is arranged so that between the longitudinal axes 205 and 203 of the parallel drift tubes 166 and 180 and the longitudinal axis 201 of the reflector is a small angle α, or β of 2 °. This is illustrated in FIG. 17. Of the Reflector with all parallel to the grid 176 extending metal rings thus sits slightly tilted to the drift tubes. This ensures that the trajectory of the exiting ion beam is substantially parallel to the trajectory of the incoming ion beam. The drift tubes 166 and 180 can be arranged parallel to each other. There is no need for an angle between the drift tubes to divergent around them

Adapt trajectories. Furthermore, the drift tube 180 is substantially shorter than the drift tube 166. Both drift tubes require only a relatively small diameter. With this arrangement, particularly small device dimensions are achieved and the evacuation volume is comparatively low.

At the ends of the drift tube 166 two grids are arranged in each case. This is shown in Fig.8. At the repeller end, the grids 190 and 192 and at the reflector end, the grids 194 and 196 are arranged. Normally, these grids are at the same potential of -1830V as the drift tube 166. Grids 194 and 192 provide uniform potential in the drift tube.

Some analytical tasks require a very high resolution. A high resolution means that masses with small mass differences can still be separated. This is particularly difficult when the signals are "blurred", for example due to the temperature distribution (peak broadening) .With longer flight time, one obtains better separation of the masses, ie better resolution, so if a higher resolution is required, the grating 196 This causes the ions to be repelled and fly back through the drift tube in the opposite direction, as shown by trajectory 198. When the returning ion packet reaches grid 190, it also becomes high potential +800 V. The ions reverse again, as shown by the trajectory 200. The grating 196 is then again set to a potential of -1830 V, so that the ions of the ion packet can now enter the reflector unhindered high-resolution operation of the spectrometer fly through the ions three times, namely with the trajectories 198, 200 and 202 the Driftroh r The drift distance is approximately tripled. This achieves improved resolution. In normal operation without corresponding switching of the gratings 196 and 190, the ions fly along the track 204. In this mode of operation, although only one ion packet can be measured that passes in the short time until the switchover of the grid 190 into the drift tube 166. For this it is possible to measure this ion packet with increased resolution without having to carry out constructive measures on the device. A simple switching of the grid is sufficient.

In FIG. 13, the temporal relationships are shown by way of example in detail. Initially, a voltage pulse of +800 V is applied to the repeller plate 108. This is designated 206. Subsequently, after a further 160 ns, the reflection grating 196 is set high for a duration of 1000 ns from a potential at -1830 V to a positive potential of + 800V. This is designated 208. The ions that hit grid 196 during this time reverse. On the repeller-side reflection grating 190, a positive voltage of + 800V is then also applied. This is labeled 210. This causes the ions to pass through the drift tube a third time. Until the ions reach the grating 196, the voltage is restored to the voltage of the drift tube. This is designated 212. The bottom diagram shows the number of useful ions that reach the reflector and can be detected accordingly at the detector. Before switching the voltage on the grid 196, the ions reach the reflector. This is designated 214. Each time the voltage is switched back, more ions enter the reflector. This is labeled 216 and 218. The masses, e.g. 20 to 31 but further pulled apart, as this mass package rotates several times in the drift tube. Depending on the required resolution, a multiple reflection on the gratings 190 and 196 can also take place.

The grids 110, 130, 134, 190, 192, 194 and 196 must be particularly flat, so that the ions are not deflected laterally by the respective potential. For this purpose, the grids are clamped in a ring with the cross-section shown in Fig. 15 (greatly enlarged). The clamping ring generally designated 220 consists of two

Ring parts 222 and 224. The two ring parts are screwed together. For this purpose, a threaded bore 226 is provided in the ring member 224. In the ring member 222, a conical bore 228 is provided, in which the screw for Screwing is retractable. Moreover, the ring parts lie with flat surfaces 230 and 232 in the bolted together.

Inside next to the bore 226, an annular groove 234 is provided in the ring part 224. The annular groove 234 has symmetrical, upwardly opening inclined surfaces 236. The inner one

Edge 238 at the transition between inclined surface 236 to the lattice plane 240 is rounded.

It forms as a slightly prominent, circumferential ring nose. About this ring nose 238, the grid 240 is stretched. The grid is clamped with a rubber ring 242. Of the

Rubber ring 242 is held with the second ring member 222 in the annular groove 234. For this purpose, a corresponding annular groove 244 is provided with a rectangular cross-section in the second ring member 222.

With this grid arrangement, it is possible to permanently stretch the grid extremely flat.

Claims

claims

An apparatus for generating an ion beam (120) of positively charged ions in an evacuated time-of-flight mass spectrometer (10)

(a) an ion source (12),

(B) an interface (14) for transferring the ion beam from the ambient pressure in the mass spectrometer (10), and

(C) an ion extraction device (64) with a voltage source (74) for

Generation of a negative potential difference between the ion extraction device (64) and the ion source (12),

characterized in that

(d) the negative potential difference has an amount of at least -1 kV, and

(E) the ion extraction device (64) via a high-impedance resistor (72) to the voltage source (74) is connected, whose value is selected so that when applied voltage no spontaneous discharge between the

Ion source (12) and the ion extraction device (64) takes place.

2. Apparatus according to claim 1, characterized in that the ion source (12) is an inductively coupled plasma.

3. Apparatus according to claim 2, characterized in that the inductively coupled plasma (12) in a tubular torch (33) having a Induction coil (34) is producible, and a tubular grounding plate (38) for grounding the plasma potential between the torch (33) and the induction coil (34) is arranged.

4. Apparatus according to claim 3, characterized by two connected on its front side quartz tubes (42, 44), between which the grounding plate (38) is arranged.

5. Device according to one of claims 3 or 4, characterized in that the connection between the grounding plate (38) and earth via a pneumatically actuated contactor (40) can be produced.

6. Apparatus according to claim 5, characterized by means for actuating the contactor (40) after ignition of the plasma.

7. Device according to one of the preceding claims, characterized in that the interface (14) comprises a sampler (54) and an interface Skimmer (56) which are at the same potential as the ion source (12).

8. Device according to one of the preceding claims, characterized in that the ion extraction device (64) comprises an ion trajectory tube (68) and an upstream ion extraction skimmer (66), which are at a negative potential.

9. Apparatus according to claim 8, characterized in that the negative

Potential in the range of -1.8 kV to -2.5 kV based on the potential of the ion source (12).

10. Time of flight mass spectrometer (10) characterized by a device for generating an ion beam (120) of positively charged ions according to one of the preceding claims.

11. Flight time mass spectrometer (10) containing

(a) an ion source (12) for generating an ion beam (120) of sample ions;

(b) means (76, 70, 106) for focusing the ion beam (120);

(b) a repeller (108) for accelerating the focused ion beam (120),

(c) a drift tube (166) for separating ions of different mass, and

(d) a detector (28) for time-resolved measurement of the intensity of the ion beam,

marked by

(e) additional means (190, 196) for generating a potential increased in relation to the drift tube (166) in a first and a second region of the drift tube (166), with a potential difference for the potential of the drift tube sufficient for a direction change of incoming ions,

(f) wherein the first region (190) precedes the second region (196) with respect to the direction of flight (122), and

(G) means for controlling the time course of the potential thus generated.

12. time-of-flight mass spectrometer (10) according to claim 11, characterized in that the means for generating a potential of each one arranged substantially perpendicular to the ion trajectory grating (190, 196) are formed, which is connected to a voltage source.

13. Time of flight mass spectrometer (10) according to claim 11 or 12, characterized in that the potential difference is at least 2 kV.

14. Time of flight mass spectrometer (10) according to claim 13, characterized in that the potential of the drift tube (166) below -1700 V based on the

Potential of the ion source (12) and the potential in the first and second regions (190, 196) is above +400 volts.

15. Time-of-flight mass spectrometer (10) according to one of claims 11 to 14, characterized in that the first area at the entrance and the second area at

Output of the drift tube (166) are arranged.

16. time-of-flight mass spectrometer (10) according to any one of claims 11 to 15, characterized in that the means for controlling the time profile of the potential switching means for turning on and off the potential difference with switching times in

Range of a maximum of 100 ns.

17. A method for operating a time-of-flight mass spectrometer (10) according to any one of claims 11 to 16, characterized by the steps:

(a) generating an ion beam;

(b) spatially focusing the ion beam;

(c) accelerating the ion beam with a voltage pulse (206);

(d) selecting an ion packet with selected masses from the ion beam;

(e) generating a potential difference (208) in the second region when the selected ion packet enters that region such that the ion packet changes direction of flight, (f) generating a potential difference (210) in the first region when the selected ion packet enters that region such that the ion packet again changes its direction of flight.

18. The method according to claim 17, characterized in that the ion packet at

Change in its flight direction essentially runs back in itself.

19. The method according to claim 18, characterized in that the potential differences in the first and second regions are generated alternately one or more times for a limited period of time.

20. Containing time-of-flight mass spectrometer (10)

(a) an ion source (12) for generating an ion beam (120) of sample ions;

(b) means (76, 70, 106) for focusing the ion beam (120);

(c) a repeller (108) for accelerating the focused ion beam;

(d) a drift tube (166) for separating ions of different mass,

(e) a reflector (168) for timing and folding the ion beam, the longitudinal axis of the reflector (168) forming an angle (α) with the longitudinal axis of the incident ion beam drift tube (166), and

(f) a detector (28) for time-resolved measurement of the intensity of the ion beam,

characterized in that (g) the longitudinal axis of the falling ion beam drift tube (180) and the longitudinal axis of the reflector form an angle (β) which is less than or equal to the angle (α) between the longitudinal axis of the reflector (168) and the longitudinal axis of the drift tube (166 ) for the incident ion beam.

21. Time of flight mass spectrometer (10) according to claim 20, characterized in that

(a) the reflector (168) comprises a series of concentrically arranged and mutually insulated metal rings (170) and a terminating one

Includes metal plate,

(b) the metal rings (170) are connected to one another via resistors,

(C) a voltage source is provided which is a voltage on the metal rings

(170) generates,

(D) at the foremost metal ring (172) has a voltage of at least -900 V, at the final metal plate, a voltage of at least +800 V can be applied and

(e) the intervening metal rings can be applied to potentials with steadily increasing intermediate values.

22. Time of flight mass spectrometer (10) according to claim 21, characterized in that the reflector (168) is arranged so that the angle (α) between the longitudinal axis of the reflector (168) and the longitudinal axis of the drift tube (166) for the incident ion beam 1.5 to 2.5 °.

23. Time of flight mass spectrometer (10) according to any one of claims 20 to 22, characterized in that in front of the reflector (168), a brake grid (176) is arranged, against which a voltage of -800 to -1200 V is applied.

24. Time of flight mass spectrometer (10) according to any one of claims 20 to 23, characterized in that a drift tube (180) for the out of the reflector (168) exiting ion beam is provided, is arranged at the output end of the detector (28) the drift tube (180) is at most half as long as the

Drift tube (166) between repeller (108) and reflector (168).

25. Time of flight mass spectrometer (10) according to claim 24, characterized in that the drift tubes (180; 166) are arranged parallel to one another (α = β).

26. Containing time-of-flight mass spectrometer (10)

(a) an ion source (12) for generating an ion beam of sample ions;

(b) means (76, 70, 106) for focusing the ion beam, and

(c) a repeller (108) for accelerating the focused ion beam at an angle to the original ion beam direction,

characterized in that

(D) the means (76,70,106) for focusing the ion beam are formed such that the ion beam is focused in a plane perpendicular to the direction of acceleration in the repeller (108).

27. A time-of-flight mass spectrometer (10) according to claim 26, characterized in that the means for focusing comprise an ion tube (68) within which a tube lens (76) is provided with a potential which is positive with respect to the potential of the ion tube (68). and an ion optic (20) having a slit (70) disposed substantially vertically and perpendicular to the direction of flight (96) of the ions and which is at the potential of the ion guide tube.

28. Time of flight mass spectrometer (10) according to claim 27, characterized in that the gap (70) of two split jaws (88, 90) is formed, which are provided on a flange (92) which is connected to the ion guide tube (68) is.

29 time-of-flight mass spectrometer (10) according to claim 28, characterized in that in the flange (92) outflow channels in the form of a recess (98) along a horizontal diameter on the back of the flange (92) are provided.

30 time of flight mass spectrometer (10) according to claim 28 or 29, characterized in that the gap jaws have an opening angle between 78 ° and 82 °, in particular of 79.8 °.

31 time-of-flight mass spectrometer (10) according to any one of claims 26 to 30, characterized in that behind the gap (70) a grounded repeller lens (106) is provided.

32. An apparatus for focusing an ion beam in a plane for use in a time of flight mass spectrometer according to any one of claims 26 to 31.

33. Apparatus for deflecting ions with selected ion masses from an ion trajectory in a time-of-flight mass spectrometer

(a) a holder (156) with holding devices,

(b) metal combs (146) having a base (158), teeth (150) adjacent to the base, and recesses (148) therebetween, the teeth each having a longitudinal slot (152), and the metal combs with the base (158). are held in the holders of the holder (156),

(c) substantially parallel arranged metal strips (144) which are resiliently suspended in the longitudinal slots (152), and (d) means for generating an electrical voltage across the metal combs (146).

34. Apparatus according to claim 33, characterized in that in each case two

Metal combs (146, 160) are arranged offset to one another such that each tooth (150) of the first metal comb (146) at the height of a bulge (148) of the second metal comb (160).

35. Apparatus according to claim 34, characterized in that in each case two

Metal combs (146, 160) are provided in pairs at the ends of the holder and the metal strips are attached to respective metal combs.

36. Device according to one of claims 33 to 35, characterized in that the metal strips (144) are soldered under mechanical stress to the metal combs.

37. Device according to one of claims 33 to 36, characterized in that a contact is provided, via which the application of the electrical voltage is switchable.

38. Device according to one of claims 33 to 37, characterized in that the holder (156) is made of electrically insulating material.

39. Device according to one of claims 33 to 38, characterized in that the metal comb (146) consists of a 1.5 to 2.5 cm wide and / or 0.3 to 0.4 mm thick stainless steel sheet.

40. Device according to one of claims 33 to 39, characterized in that the slots (152) at the tooth end to indentations (154) are widened.

41. Device according to one of claims 33 to 40, characterized in that the strips are arranged at intervals of 0.4 to 0.6 mm.

42. Device according to one of claims 33 to 41, characterized in that the strips (144) are 0.8 to 1.2 mm wide and 40 to 60 microns thick.

43. A time-of-flight mass spectrometer comprising a device according to one of claims 33 to 42.

44. The time-of-flight mass spectrometer according to claim 43, comprising means for producing a focused ion beam, a repeller and a drift tube, wherein in the drift tube deflectors for changing the components of the direction of the ion beam are provided, characterized in that the device (142) for deflection of ions is arranged between the two deflectors.

45. Time-of-flight mass spectrometer according to claim 43 or 44, characterized in that the holder is arranged within the spectrometer to the ion beam, that the longitudinal sides of the metal strip is arranged parallel to one of the deflectors.

46. Time-of-flight mass spectrometer according to one of claims 43 to 45, characterized in that the metal strips (144) are provided at the location of the first focusing of the ion beam, at which a spatial focusing takes place.

47. Method for producing an apparatus for the deflection of ions for

Time of flight mass spectrometer comprising the steps:

(a) laser cutting metal strips into a metal sheet,

(b) separating the strips from the metal sheet, (c) holding metal combs with teeth provided with slots in a holder,

(d) inserting the metal strips into the slots of the metal combs,

(e) soldering the metal strips under mechanical tension.

48. Gratingshell for planar gratings for use in a time-of-flight mass spectrometer comprising

(A) a clamping ring (220) having two coaxially arranged ring members (222, 224) which are connectable to each other with surfaces (230, 232), wherein

(b) a ring groove (234) is provided in a first ring part (224) which has a protruding annular ring nose (238) at its inner edge,

(c) a grid (240) can be tensioned via this annular nose (238) between the ring parts (222, 224),

(D) in the second ring member (222) to the annular groove (234) corresponding annular groove (244) is provided, and

(E) a grid (240) with a rubber ring (242) in the annular grooves (234, 244) is tensioned.

49. Grid holder according to claim 48, characterized in that the annular groove (244) in the second ring part (222) has a rectangular cross-section.

50. Grid holder according to claim 48 or 49, characterized in that the

Ring groove (234) in the first ring part (224) symmetrical, upwardly opening inclined surfaces (236).

51. Grid holder according to one of claims 48 to 50, characterized in that the inner edge with the annular nose (238) at the transition between inclined surface (236) to the lattice plane (240) is rounded.

52. Grid holder according to one of claims 48 to 51, characterized in that the ring parts (222, 224) are screwed together.

53. Grid holder according to one of claims 48 to 52, characterized in that the angle of the ring nose adjacent to the inclined surface of the annular groove relative to the lattice plane an angle, measured in a clockwise direction, in the range of 50 to 60 °, in particular of 55 °.