AU2005284150B2

AU2005284150B2 - Flight time mass spectrometer

Info

Publication number: AU2005284150B2
Application number: AU2005284150A
Authority: AU
Inventors: Uwe Dahlke; Erwin Hoffmann; Christian Ludke; Carsten Roland; Jochen Skole; Rainer Sorge; Jorg Wollbrandt
Original assignee: Analytik Jena GmbH and Co KG
Current assignee: Analytik Jena GmbH and Co KG
Priority date: 2004-09-17
Filing date: 2005-09-12
Publication date: 2011-05-12
Anticipated expiration: 2025-09-12
Also published as: AU2005284150A8; EP1817788B1; WO2006029999A3; AU2005284150A1; EP1817788A2; DE102004045315A1; WO2006029999A2; ATE517428T1

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 5 Technical field The invention relates to a device for diverting ions with selected ion masses from an ion flight path in a time-of-flight mass spectrometer. 10 Ions are accelerated in an electrical field in a time-of-flight mass spectrometer. Depending on its mass the ions have a different time-of-flight. Accordingly, the ions arrive at the detector at different times. The time-resolved signal, therefore, provides information about the amount and kind of ions entered into the mass spectrometer. The longer the time-of-flight of the ions the better is the resolution. An improved 15 resolution means, that ions having a different mass can be more securely distinguished from each other. If more ions meet the detector, the signal-to-noise ratio is improved. A good signal-to-noise ratio means, that even smallest amounts of an analyt in the sample can still be detected and that small differences can be determined in different samples. It is, therefore, a principle object of each measurement to measure as many 20 ions of a sample as possible and to achieve a high resolution. Known time-of-flight mass spectrometers operate with a ion source in the form of a plasma torch (Inductively Coupled Plasma = ICP). Such ion sources are known. The sample which shall be analysed is ionised together with a gas, normally Argon, in the 25 torch. It is a challenge to enter the positive ions generated there under atmospheric pressure into the highly evacuated vacuum of the mass spectrometer. For this purpose a transfer assembly, an interface is provided. The interface comprises the so called ,,ion lenses" used to transfer the ion beam through a chamber with a pre 30 vacuum into the mass spectrometer by suitably designing electrical fields. A negative potential difference between the ion extraction device and the ion source is generated in an ion extraction device with a voltage source. The positive ions are accelerated in the direction of the ion extraction device. However, it is not possible to apply an unlimited high voltage, because an undesired electric discharge is effected otherwise. The amount of extracted sample ions is, therefore, limited. In addition to the ion source for the generation of a ion beam of sample ions the time 5 of-flight mass spectrometers comprises a repeller for accelerating the focused ion beam. For a short period of time the repeller generates a strong electric field to accelerate the ions. These will fly through a drift tube for the separation of the ions with different mass. The time resolved measurement of the intensity of the ion beam is carried out at the detector. 10 An improved resolution can be achieved in that the time peaks are as narrow as possible. This means that ions having the same mass simultaneously hit the detector. This can be achieved, however, also by increasing the drift time. Known mass spectrometers having a high resolution are provided with very long drift tubes. It is 15 disadvantageous that the diameters of the entire device are large. With 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, where the longitudinal axis of the reflector forms an angle with the longitudinal axis of the first 20 drift tube for the incident ion beam. The ion beam exiting the reflector enters a further drift tube having a detector arranged at its end. As the ion beam forms a parabola or a parabola-like flight path in the reflector the drift tubes are usually arranged with an angle to the longitudinal axis of the reflector. They quasi form a V-shaped assembly. With long drift tubes to achieve a high resolution this leads to considerably large 25 device diameters. In addition to the ion source for the generation of an ion beam of sample ions time-of flight mass spectrometers comprise focusing means for focusing the ion beam. Thereby all ions obtain the same ,,starting conditions". In order to accelerate the 30 focused ion beam a repeller is provided with an angle to the original ion beam direction. The repeller generates a strong electric field for a short period of time to accelerate the ions. These fly through the drift tube to separate ions having a different mass. The time-resolved measurement of intensities of the ion beams is carried out with the detector.

In order to achieve a high resolution and a high accuracy of the measurement results the time peaks at the detector must be as narrow as possible. This means that ions having the same mass arrive at the detector at the same time if possible. For this 5 purpose it is a condition that all ions simultaneously start from the same position. Therefore, good focusing is necessary. The ions always have a temperature distribution so that for ions having the same mass there will be always a distribution of the initial velocity. Furthermore the ion source is finite. This means that the ions are possibly not accelerated from the same position. It is always a constant goal to 10 achieve a good focusing of the ions in the repeller in time and space. Known spectrometers rotation-symmetrically focus the beam along a line. This, however, leads to an undesired ,,smearing" in the flight direction. In order to obtain a particularly good detection limit it is necessary not to measure 15 interfering ions from the matrix or the carrier gas, i.e. Argon. For this purpose ion diverters are provided. If the interfering ion package with a known mass passes the ion diverter an electric potential is applied. The ions are subjected to a force deviating them from their original path. Therefore, they do not arrive at the detector. Such a device for diverting ions with selected ion masses from a ion flight path must be 20 switched very quickly, it may not be expensive to produce and it may not influence the other ions when it is switched off. Grids are required for the generation of the respective potentials in the course of the ion flight path. The grids must be extremely even and they may not, if possible, 25 change during the life time of the device. Usually the grid consists of a wire mesh clamped into a frame. The voltage is applied then to the frame. The production of such grids is expensive. Disclosure of the invention 30 It is an object of the invention to provide a time-of-flight mass spectrometer with an improved time resolution with lower detection limits and high sensitivity.

According to an aspect of the invention there is provided a device for diverting ions with selected ion masses from an ion flight path in a time-of-flight mass spectrometer, comprising 5 (a) a carrier (156) with holding devices (b) metal combs (146) with a basis (158), teeth (150) provided at the basis and recesses (148) therebetween wherein the teeth are each provided with a longitudinal slit (152) and the metal combs are held with their basis (158) in the holding devices of 10 the carrier (156), (c) metal strips (144) arranged essentially parallel, which are elastically suspended in the longitudinal slits (152), and 15 (d) means for generating an electric voltage at the metal combs (146). In preferred embodiments, in order to remove interfering ions, for example argon ions, from the ion beam an ion diverter is provided the ion diverter having a carrier with holding devices and metal combs with a basis, teeth provided at the basis and 20 recesses therebetween wherein the teeth are each provided with a longitudinal slit and the metal combs held with their basis in the holding devices of the carrier. The ion diverter comprises metal strips arranged essentially parallel, elastically suspended in the longitudinal slits. Furthermore means for generating an electric voltage are provided at the metal combs. Two metal combs, respectively, are arranged such that 25 each tooth of the first metal comb is in the position of a gap of the second metal comb. In this design an ion diverter may be particularly simply produced. The potential is extremely even and durable in the production. 30 Preferably two metal combs are provided in pairs at the ends of the carrier and the metal strips are fixed to the respective corresponding metal combs. The metal strips are soldered to the metal combs when they are mechanically biased. Thereby the strips maintain their eveness.

Preferably the metal comb consists of a 1.5 to 2.5 cm wide and/or 0.30 to 0.4 mm thick steel sheet. It can be very precisely processed and produced by laser cutting. In a further modification of preferred embodiments of the invention the slits at the 5 tooth ends are enlarged to form recesses at the ends. Thereby the entering of the metal strips is facilitated. Preferably, the device for diverting the ions can be advantageously arranged between the two deflectors. The carrier can be arranged within the spectrometer relative to the 10 ion beam in such a way that the longitudinal sides of the metal strips run parallel to one of the deflectors. In a further preferred embodiment of the invention the metal strips are provided at the position of the first focusing of the ion beam where a spatial focusing is effected. 15 Undesired ions can be removed with a short pulse in the focus without essentially influence the other ions. Preferably, there is also provided a method for producing the device for diverting ions for time-of-flight mass spectrometers preferably comprises the following steps: 20 (a) Laser-cutting of metal strips in a metal sheet, (b) Separating the strips from the metal sheet, 25 (c) fixing metal combs with teeth provided with slits in a carrier, (d) inserting the metal strips into the slits of the metal combs, (e) soldering the metal strips when they are mechanically biased. 30 For the generation of even grids a grid carrier for even grids can be used comprising a tensioning hoop with two coaxially arranged hoop portions adapted to be connected with abutting surfaces. An annular groove is provided in the first hoop portion provided with an annular projection projecting at its inner edge. An annular groove corresponding to said annular groove is provided in the second hoop portion. The grid can be tensioned over the annular projection between the hoop portions with a rubber ring in the annular grooves. 5 Due to the use of the annular projection the grid can be tensioned without tearing. The use of two hoop portions enable a particularly cheap production. The annular groove in the second hoop portion can have a rectangular cross section. The annular groove in the first hoop portion, however, advantageously has 10 symmetrical, upwardly opening, inclined surfaces. The inner edge of the annular projection at the intermediate portion between the inclined surface and the grid plane is preferably round. The hoop portions are preferably screwed together. However, any other way of connecting is possible also. In a particularly preferred embodiment the angle at the inclined surface of the annular groove next to the annular projection 15 forms an angle in the range of 50 to 600, especially 550 with the grid plane. Further modifications of the invention are subject matter of the subelaims. An embodiment of the invention is described below in greater detail with reference to the accompanying drawings. 20 Brief description of the drawings Fig. 1 schematically shows a time-of-flight mass spectrometer generally denoted with numeral 10. 25 Fig. 2 shows a cross section through the ion source of Fig. 1 in detail. Fig. 3 shows a schematic cross section through the interface and the ion optics shown in Fig. 1 in detail. 30 Fig. 4 shows a cross section in a radial direction through an ion flight tube and a tube lens in detail.

Fig. 5 is a perspective representation of the entrance slit arrangement at the repeller. Fig. 6 shows the distribution of the electric flux lines and ion paths in the range 5 of the repeller lens and in the repeller. Fig. 7 is a perspective representation of the components used in the repeller. Fig. 8 is a schematic view of the repeller and the drift tube of Fig. I with the 10 ion flight path in a high resolution operation. Fig. 9 is a perspective view of the arrangement with the repeller and the drift tube of Fig. 8. 15 Fig. 10 is a perspective view of the selector-arrangement for diverting undesired ion packages. Fig. 11 shows a metal comb for the elastic fixing of metal strips in a selector according to Fig. 10. 20 Fig. 12 is a perspective view of the reflector and a portion of the drift tube of Fig. 1. Fig. 13 shows the time dependence of the potential at the repeller, the reflection 25 grids and the amount of useful ions in the reflector for an arrangement in high resolution operation. Fig. 14 is a perspective view of the detector. 30 Fig. 15 is a cross section through the edge of the grid carrier with a grid. Fig. 16 is a cross section through the ion source of Fig. 2. Fig. 17 illustrates the angles between the drift tubes and the reflector.

Description of the embodiment Fig. 1 schematically shows a time-of-flight mass spectrometer generally denoted with 5 numeral 10. The mass spectrometer 10 comprises an ion source 12. The ions from the ion source 12 are transferred from atmospheric pressure to the evacuated mass spectrometer 16 with an interface 14. The mass spectrometer 16 comprises an ion optical arrangement 18 for focusing the ion beam in the repeller chamber 20. After accelerating the ions in an ion acceleration path 22 the ions enter the ion drift tube 24. 10 The ions change their flight direction in a reflector 26 and are detected afterwards with an ion detector 28. Ions having a different mass have different flight times. Accordingly the time resolved detector signal provides information about the kinds of ions present in the ion source 15 and their amounts. Ions with a smaller ion mass provide a signal at an earlier point in time than ions having a larger mass. The longer the drift time the higher is the resolution, i.e. masses with small mass difference can be distinguished better. The more ions from the ion source hit the detector the higher the intensity will be. 20 The ion source 12 comprises an inductively coupled Plasma (ICP). For the generation of the plasma an injection tube 30 from ceramics or quartz and a tube shaped torch 33 made of quartz is provided. The ion source is shown in Fig. 2 and Fig. 16 again in greater detail. The torch is mounted on a carrier 32. An induction coil 34 is arranged around the torch 33. The carrier gas Argon is entered through the injection tube 30 25 into the plasma range within the torch 33. There, it is ignited with a spark. An oscillating magnetic field is induced inside the torch 33 with the coil 34, the magnetic field causing the further ionisation of the gas present inside the torch. The gas temperature within the plasma is in the range of about 6000*C. Argon is lead to the outer ranges of the plasma through a line 37 to cool the components. The plasma gas 30 Argon is lead to the plasma through a line 36. At these plasma temperatures the sample ions inserted with the carrier gas are mainly singly ionised and they have a velocity distribution corresponding to the temperature.

A grounding sheet 38 is provided between the torch 33 and the induction coil 34. The grounding sheet 38 is also tube shaped. The grounding sheet 38 is provided with a slit 39 in an axial direction. This is shown in Fig. 16. During the ignition of the plasma the grounding sheet 38 has no potential. After the ignition the sheet 38 is grounded. 5 For this purpose a contact 40 is provided to contact with ground. The contact 40 is pneumatically operated. The grounding sheet 38 is arranged between two quartz tubes 42 and 44. The quartz tubes are interconnected at their front ends 46. Furthermore, the quartz tubes 42 and 44 are connected along a narrow web 41 in an axial direction. Thereby the quartz tubes 42 and 44 form a double tube avoiding high frequency 10 discharges towards the grounding sheet 38. The double tube is shifted over the torch 33. A distance of several millimeters is formed between the upper end 48 (Fig. 1) of the coil 34 and the upper end 50 of the grounding sheet 38. The sampler 54 of the interface 14 is positioned in a distance of 2 mm of the upper 15 end 50 of the grounding sheet in the direction of the ion flight path 52 (Fig. 1). The sampler 54 forms the entrance side closure of the interface 14. In Fig. 3 the interface 14 and the ion optical arrangement 18 is shown again in greater detail. The individual components are schematically drawn apart from each other. The sampler 54 is a rotation symmetric, conic nickel plate having a relatively large opening angle of 150* 20 and an aperture with a diameter of 1 mm. A further rotation symmetric plate, the interface-skimmer 56 is arranged in a distance of 7 mm, The space 58 between the sampler 54 and the skimmer 56 is evacuated to a pressure of about I mbar with a pre vakuum pump (not shown) through a connection 57. The interface-skimmer 56 is also conic having an opening angle of 50*. The opening has a diameter of 1.2 mm. A 25 shutter 60 (Fig. 1) is placed directly behind the interface skimmer 56. The high vacuum portion of the mass spectrometer 16 can be vacuum-tight closed outside measuring times with this shutter. The shutter 60 essentially comprises two pneumatically operated gates. 30 The space 62 between the interface-skimmer 56 and the following ion extraction assembly 64 is evacuated to a pressure of about 10 3 mbar during measurements. Accordingly, there is a corresponding pressure drop.

-U The ion extraction assembly 64 comprises a further plate, the ion extraction skimmer 66. The ion extraction skimmer is also conical and is provided with an aperture of 1.2 mm. The opening angle of the cone is 500. The ion extraction skimmer 66 is directly connected to the ion flight rube 68. At its other end the ion flight tube 68 the entrance 5 slit 70 of the ion optical arrangement is provided. For this purpose an insulation 69 is provided. The sampler 54 and the interface-skimmer 56 have a potential of 0 V relative to ground potential. The ion extraction skimmer 66 and the ion flight tube 68 have a very 10 high, negative potential of -2 kV. In order to avoid undesired discharging between the ion extraction skimmer 66 or the ion flight tube 68 and the interface-skimmer 56 the ion flight tube 68 is connected to the voltage source 74 (Fig. 3) through a high resistance resistor 72 of 1 MO. Thereby the current is limited to a range where the discharging is very much handicapped. 15 Using the resistor 72 a high, negative voltage of -2kV can be applied. The oppositely charged, positive ions are attracted by the negative potential in the tube. Thereby a high extraction rate is achieved. 20 A tube lens 76 is arranged in the ion flight tube 68 in the entrance range behind the ion extraction skimmer 66. For this purpose three elongated recesses 78 with a semi circular cross section are provided in the ion flight tube 68, extending among the length of the tube lens 76 in an axial direction. This can be seen in Fig. 4. Electrically insulating ceramic sticks 80 are provided in the recesses 78. The tube lens 76 is held 25 between these ceramic sticks 80. A contact with the tube lens 76 is established through an opening 82. The contact 84 is insolated against the ion flight tube 68 by an insulation 86 provided in the opening 82. The contact 84 is connected to a potential of -300V through a resistor of I MO. Thereby the positive ions of the ion beam are ,,pushed together", i.e. focused. Neutral and negatively charged particles are not 30 focused. A focusing is achieved right at the exit-side tube end with a tube length of the ion flight tube 68 of 8 cm, a length of the tube lens 76 of 1.5 cm and the above mentioned potential conditions.

11 There the entrance slit 70 is positioned in the ion optical arrangement 20. The entrance slit 70 is shown in detail in the enlargement of Fig. 5. Two slit jaws 88 and 90 are screwed onto a flange 92. The slit jaws 88 and 90 form a fixed slit 94 having a width of 0.5 mm. The slit is vertical and perpendicular to the flight direction 96 of the 5 ions. Accordingly, there is no rotational symmetry anymore. In Fig. 5 the backside of the entrance slit assembly is shown. There, a recess 98 is in the flange 92. The recess 98 is about 6 mm wide and extends along a horizontal diameter on the backside of the flange 92. The recess 98 forms flow-off channels. 10 Non-focused particles, such as, for example, undesired neutral particles hitting the entrance slit assembly 70 on the front can leave the ion flight tube 68 sideways through the flow-off channels. Thereby sediments are avoided caused by neutral particles, gas and residue droplets, the sediments possibly deposing and drying in the following repeller. Such sediment deposits can form islands of an insulating layer 15 leading to changes of the potential distribution in the repeller. The problems occur particularly in the repeller because the particles have no or only a very small velocity there rather than in the ion flight tube. In the ion flight tube, however, the ions are fast, which is why there are not so many problems with undesired particles. 20 The flange 92 is connected to the ion extraction tube 68. The inside of the ion extraction tube 68 is evacuated with a pressure in the range of 10-mbar. Fig. 3 illustrates the individual pressure ranges 58, 62 and 100. The slit jaws have a potential of -2kV and form an opening angle of 80*. The angle is 25 larger than the opening angle of the ion extraction skimmer 66. Thereby a particularly good potential distribution for focusing the ions is achieved in the following repeller space 20. Due to the vertical geometry of the slit 94 the ions are not focused in one point but in 30 a vertically extending plane 102. The plane 102 is positioned in the repeller space 104 behind the repeller lens 106 between the repeller plate 108 and the repeller grid 110. Fig. 6 shows a cross section through the repeller space 104 again in greater detail. The slit jaws 112 and 114 of the entrance slit have a high extraction potential of -2kV. Accordingly the ions are accelerated through the slit 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 the distance piece 118 (Fig. 7). The distance between the entrance slit 70 and the repeller lens 106 is 5 mm. The opening of the repeller lens 106 is 8 mm wide and extends like the 5 entrance slit in a vertical direction over a length of 12 mm. In Fig. 6 some electric flux lines 116 and ion paths 117 in the range of the repeller lens are shown. According to the potential distribution the ions are slowed down very much in the plane 102. A spatial focusing of the ions of all masses is achieved, 10 because apart from the temperature distribution all masses have the same energy corresponding to the acceleration voltage of 2kV. As the ion distribution in the plane 102 is possible in a direction perpendicular to the flight direction 122, a particularly good spatial focusing is possible in the flight direction 122. 15 In Fig. 7 a perspective representation of the components used for the repeller space is shown. The ions enter through the repeller lens 106. This is represented by an arrow 120. They leave the repeller space in the direction of the arrow 122. The connections for two high vacuum pumps are denoted with numeral 124 and 126 (Fig. 1). 20 The ions slowed down and focused in the plane 102 are diverted in a perpendicular direction 122 by a short positive voltage pulse of 800V. At the same time, they are accelerated in this direction. Apart from the temperature distribution which is present at all times the ions have the same starting conditions at this point where they (almost) have no velocity and start in the same plane. They fly through the repeller grid 110. A 25 further grid 130, the so-called suppressor grid, is behind the repeller grid 110. A small positive voltage of +15 V is applied to the suppressor grid 130. Thereby an opposite field is generated. Ions having low energy are retained by this opposite field. Such ions can, for example, enter the range of the suppressor grid out of the focusing plane before the voltage pulse at the repeller plate due to the kinetic energy caused by 30 diffusion, scattering and temperature distribution. Such ions would generate an undesired background in the spectrum without the suppressor grid 130 because they are accelerated starting from a different plane. Ions having a high energy which are already accelerated by the voltage pulse easily overcome the potential wall of the suppressor grid.

1.) The ions are then accelerated on the acceleration path 132 in the direction of an acceleration grid 134. A high negative voltage of -1870 V is applied to the acceleration grid. The positive ions are accelerated by this negative voltage. 5 A baffle 135 is arranged behind the acceleration grid 134. A small negative voltage of -300V is applied to the baffle. Peripheral beams are guided into the measuring range by this baffle. Thereby an increased intensity is achieved. 10 A grid 137 is arranged behind the baffle 135. A potential of -1870 V is applied to the grid 137. By this grid 137 it is achieved that the electric effect of the baffle 135 does not cause deformations of the electric field in the range of the X-deflector 138. A perspective representation of the assembly is illustrated again in greater detail in 15 Fig. 9. The entrance-side portion of the drift tube 166 can be seen. The entrance of the tube is formed by the grids 134 and 137 and the baffle 135 arranged therebetween. Insulated holding rings 167 and 169 are arranged around the drift tube 166. The holding rings 167 and 169 consist of Polyacryl-meth-acrylate (PMMA). They are used to bear the drift tube 166 having a high potential in the housing of the device. A 20 selector 142 which is described below in greater detail is arranged in the drift tube with a lock against rotation 171. An x-deflector 138 and an y-deflector 140 are arranged inside the drift tube 166 (Fig. 1) along the flight path 136 of the ions. Depending on the voltage applied to the 25 deflectors the direction of the ion beam can be adjusted. In such a way mechanical manufacturing tolerances can be compensated. The components may be produced with relatively high manufacturing tolerances. The fine adjustment of the ion beam on the detector is carried out by applying a suitable voltage to the deflectors. Furthermore a sawtooth voltage, i.e. a voltage increasing with time, is applied to the x-deflector 30 138 and the y-deflector 140. Thereby the ion beam direction is optimized for light and heavy ions. The different ion masses obtain the same deflection angle which is optimal for the measurement.

A selector generally denoted with numeral 142 is arranged between the two deflectors. It is a device for diverting certain kinds of ions from the ion beam, for example ions with high intensity. the selector essentially comprises parallely arranged metal strips 144 having small diameters in the flight direction and being spaced apart 5 with small distances. The metal strips 144 run parallel to the y-deflector 140. The strips are arranged with distances of 0.5 mm therebetween. Each strip 144 is 1 mm wide and 50 pm thick. Normally all strips have the same potential. In this case the ion beam is not influenced. However, if an ion package, for example unwanted argon ions, shall be diverted in order to avoid interferences with the measurement a positive 10 voltage of 200 V is applied to every second strip through capacitive coupling. The voltage is applied for a period of time, where the unwanted ions are present in the range of the selector 142. Thereby the ions are diverted in the direction of the walls of the drift tube. Short switching times and thereby a high selectivity can be achieved with such an assembly. 15 In order to achieve a potential distribution which is as even as possible the metal strips must be straight and flat. They may not heat up or deform even when they are exposed to ions. Their edges may not be bent and should run parallel. To ensure the required geometric stability each individual strip is elastically suspended. The sheet strips are 20 cut from a 50 pm metal sheet with a suitable laser. The laser can be controlled very well. Therefore, the manufacturing accuracy of the laser strips is very high. The edge does not show deformations as it is the case with strips cut with a blade. Metal combs 146 are used for an elastic suspension. An enlargement of such a metal 25 comb is shown in Fig. 11. The metal comb 146 consists of a 2 cm wide steel sheet with recesses 148 cut therein by means of a laser 16. In such a way 17 teeth 150 are generated. A slit 152 is provided in each tooth 150. The slitwidth of the slit 152 increases at the tooth end to form an opening 154. This opening 154 facilitates the insertion of the metal strips 144. 30 Four metal combs 146 are fixed in a carrier 156 with their side 158 opposite to the slits. This is shown in Fig.10. Two combs, for example 146 and 160, are arranged in such a way that each tooth 150 of the comb 146 faces a recess 148 of the comb 160. A metal strip 144 held in a slit 152 in the comb 146 can be guided through the recess 148 in the comb 160 without touching it. The metal strip is then fixed in a corresponding comb 162 on the opposite side of the carrier 156. The combs 162 and 164 on the opposite side are also arranged in a shifted way. The metal strips 144 are soldered to the combs in a biased state. Thereby they maintain their geometric 5 stability even when they are exposed to ions. The carrier is mounted relative to the ion beam in such a way that the metal strips run vertically. The metal strips 144 are placed at the position of the first focus of the ion beam where a spatial focusing is effected. Only a contact with the comb must be 10 established to apply a voltage. This can be switched for short times also. The carrier 156 is electrically insulated. Fig. 1 shows the further path of the ion flight path. A separation of the ions having a different mass but the same energy is effected in the drift tube 166 due to the different 15 run times caused by their different velocities. Light ions fly faster, heavy ions fly slower. At the end of the drift tube 166 a reflector is positioned generally denoted with numeral 168. The reflector 168 consists of a sequence of concentric metal rings 170 which are isolated from each other. A perspective view of the reflector 168 is shown in Fig. 12. The metal rings 170 are connected through resistors 171. A voltage 20 is applied to the metal rings. The reflector is closed with a plate 174 having a potential of +800V applied thereto. A voltage of -1000 V is applied to a grid 176. A potential with constantly increasing intermediate values is applied to the metal rings therebetween. The metal ring 172 at the very front has a potential of -900 V, the last metal plate a voltage of +800V. 25 According to the potential distribution within the reflector the ion path with the positive ions runs along the way as shown in Fig. 1. The ions change their direction. Thereby a focusing in time is achieved. Due to the focusing in time run time differences of ions having the same mass caused by differences of the velocity at the 30 beginning of the drift path are compensated. A suppressor grid 176 is arranged before the reflector. A negative voltage of -lkV is applied to this grid. The ions are slowed down in the potential of this grid so that the length of the ion flight path to the turning point 178 is shortened. Thereby the required length of the reflector 168 is shortened. The reflector is, furthermore, provided with a grid 177 for closing. After leaving the reflector 168 the ions fly through a shortened portion of a further drift tube 180. Afterwards the ions hit the detector 28. The detector is perspectively shown again in Fig. 14. 5 A further grid 182 is arranged before the detector. A potential of -2.8kV is applied to the grid 182. Peripheral beams are focused on the detector by this grid 182. Thereby a higher intensity is achieved with a loss of resolution. 10 The reflector 168 is arranged in such a way that a small angle a or B, respectively, of 2* is formed between the longitudinal axes 205 and 203 and the parallel drift tubes 166 and 180 and the longitudinal axis 201 of the reflector. This is illustrated in Fig. 17. The reflector and all the metal rings running parallel with the grid 176 are slightly inclined relative to the drift tubes. Thereby it is achieved, that the flight path of the 15 exiting ion beam runs essentially parallel with the flight path of the incident ion beam. The drift tubes 166 and 180 can be parallely arranged. No angle must exist between the drift tubes to adapt them to the flight path. Furthermore, the drift tube 180 is essentially shorter than the drift tube 166. Both drift tubes only require a relatively small diameter. Particularly small diameters are achieved for this device and the 20 volume to be evacuated is relatively small. At the ends of the drift tube 166 two grids are arranged, respectively. This is shown in Fig. 8. The grids 190 and 192 are arranged at the repeller-side end and the grids 194 and 196 at the reflector-side end. Normally they have the same potential of -1830 V as 25 the drift tube 166. The grids 194 and 192 provide an even potential in the drift tube. With some analytic tasks a particularly high resolution is required. A high resolution means that masses with small mass differences can still be distinguished. This is particularly difficult if the signals are spread, for example due to the temperature 30 distribution (peak spreading). A better separation of the masses, i. e. a better resolution is achieved with longer flight times. If, therefore, a higher resolution is required, the grid 196 is set to a high potential of +800V. Thereby the ions are repelled and fly in the opposite direction back through the drift tube. This is illustrated by the flight path 198. When the returning ion package reaches the grid 190, the grid '/ is also set to a high potential of +800 V. The ions change their direction again. This is illustrated by the flight path 200. The grid 196 is set back to the potential of -1830 V to let the ions of the ion package pass into the reflector. In this high-resolution operating mode of the spectrometer the ions pass the drift tube three times, i.e. with 5 flight paths 198, 200 and 202. The drift path is almost tripled. Thereby an improved resolution is achieved. In normal operation without the respective switching of the grids 196 and 190 the ions fly along the path 204. In this mode of operation only one ion package may be measured, which enters the 10 drift tube during the short time until the grid 190 is switched. However, it is possible to measure this ion package with an increased resolution without having to apply construction measures with the device. A simple switching of the grid is sufficient. The time conditions are exemplary illustrated in greater detail in Fig. 13. At the 15 beginning a voltage pulse of +800 V is applied to the repeller plate 108. This is denoted with numeral 206. After a further 160 ns the reflexion grid 196 is switched from a potential of -1830 V to a positive potential of +800 V for the duration of 1000 ns. This is denoted with numeral 208. The ions hitting the grid 196 during this time will return. Finally a positive voltage of +800 V is applied to the repeller-side 20 reflexion grid 190, also. This is denoted with numeral 210. The voltage is set to the voltage of the drift tube until the ions return to the grid 196. This is denoted with numeral 212. The amount of usable ions is represented in the lowest representation, reaching the reflector and accordingly detected by the detector. Before switching the voltage at the grid 196 the ions reach the reflector. This is denoted with numeral 214. 25 Each time, when the voltage is switched back, further ions enter the reflector. This is denoted with numerals 216 and 218. The masses, for example 20 and 31 are spaced apart further, because this mass package runs through the drift tube several times. Depending on the required resolution a multiple reflection at the grids 190 and 196 is possible. 30 The grids 110, 130, 134, 190, 192, 194 and 196 must be particularly even to avoid that the ions are diverted in a sideways direction by the respective potential. For this purpose the grid is clamped in a ring having a cross section as it is illustrated in Fig. 15 (very much enlarged). The clamping ring generally denoted with numeral 220 consists of two ring portions 222 and 224. The two ring portions are screwed together. For this purpose a threaded bore hole 226 is provided in the ring portion 224. A conic bore 228 is provided in the ring portion 222 to countersink the screw for tightening. Otherwise the ring portions abut with plane surfaces 230 and 232 when they are 5 screwed together. On the inside next to the bore hole 226 an annular groove 234 is provided in the ring portion 224. The annular groove 234 has symmetric, upwardly opening, inclined surfaces 236. The inner edge at the transition portion between the inclined surface 236 10 and the grid plane 240 is rounded. Thereby a slightly projecting, annular projection is formed. The grid 240 is biased and clamped over this annular projection. The grid is clamped with a rubber ring 242. The rubber ring 242 is held in the annular groove with the second ring portion 222. For this purpose a corresponding annular groove 244 is provided in the second ring portion 222, the groove having a rectangular cross 15 section. With this grid assembly it is possible to durably bias the grid extremely even. The foregoing describes only one embodiment of the present invention and 20 modifications, obvious to those skilled in the arts, can be made thereto without departing from the scope of the present invention. The term "comprising" (and its grammatical variations) as used herein is used in the inclusive sense of "including" or "having" and not in the exclusive sense of 25 "consisting only of'.

Claims

3. Device according to claim 2, characterized in that two metal combs (146, 160) 25 are provided in pairs at the ends of the carrier and the metal strips are fixed to the respective corresponding metal combs.
4. Device according to any of the preceding claims, characterized in that the metal strips (144) are soldered to the metal combs when they are mechanically 30 biased.
5. Device according to any of the preceding claims, characterized in that a contact is provided for switching the electric voltage.
6. Device according to any of the preceding claims, characterized in that the carrier (156) is made of electrically insulating material.
7. Device according to any of the preceding claims, characterized in that the 5 metal comb (146) consists of a 1.5 to 2.5 cm wide and/or 0.30 to 0.4 mm thick steel sheet.
8. Device according to any of the preceding claims, characterized in that the slits (152) at the tooth ends are enlarged to form recesses (154) at the ends. 10
9. Device according to any of the preceding claims, characterized in that the strips are arranged in distances of 0.4 to 0.6 mm.
10. Device according to any of the preceding claims, characterized in that the 15 strips (144) are 0.8 to 1,2 mm wide and 40 to 60pm thick.
11. Time-of-flight mass spectrometer comprising a device according to any of the preceding claims. 20 12. Time-of-flight mass spectrometer according to claim 11, comprising means for the generation of a focused ion beam, a repeller and a drift tube, wherein deflectors are provided in the drift tube to change the components of the direction of the ion beam, characterized in that the device (142) for diverting ions is arranged between two deflectors. 25
13. Time-of-flight mass spectrometer according to claim i1 or 12, characterized in that the carrier is arranged within the spectrometer relative to the ion beam in such a way that the longitudinal sides of the metal strips run parallel to one of the deflectors. 30
14. Time-of-flight mass spectrometer according to any of claims 11 to 13, characterized in that the metal strips (144) are provided at the position of the first focusing of the ion beam where a spatial focusing is effected. 41
15. A device for diverting ions with selected ion masses from an ion flight path, the device being substantially as herein described with reference to any one or more of the accompanying drawings. 5 Dated this 6t day of April 2011 10 Analytik Jena AG By FRASER OLD & SOHN Patent Attorneys for the Applicant 15