EP0002430B1 - Mass spectrometer - Google Patents

Description

The invention relates to a mass spectrometer with an ion source, a mass filter and at least one ion detector, the mass filter and ion detector being arranged laterally offset from one another.

A mass spectrometer of the type considered here generally consists of an ion source of any type, for example an ionization chamber for chemical ionization, as described in DT-OS 27 37 852, a mass filter, in particular a quadrupole mass filter as described in DT-PS 444 900 described type, and an ion detector.

It is known to arrange ion sources, mass filters and detectors in alignment in one axis in the order mentioned, this arrangement being generally referred to as an "in-axis" arrangement. A disadvantage of the arrangement mentioned is that due to the direct "line of sight" of the source and ion detector thus specified, all neutral particles that come through the mass filter unaffected from the source get into the detector and, depending on the type of detector, can generate an undesirable background current there; such neutral particles can in particular be excited or metastable neutral molecules and photons in the UV or soft X-ray range. A secondary electron multiplier is usually used as the detector.

In order to reduce this disturbing background current, which occurs due to the direct "line of sight" between the source and the detector, it has been adopted to laterally offset the detector against the axis of the mass filter. This arrangement is referred to as an "off-axis" arrangement. In particular, ion current measurements that go to the detection limit of a mass spectrometer force such an "off-axis" arrangement of the detector in order to improve the signal-background ratio of the detector.

For this purpose, mass filters, in particular quadrupole mass filters, and "off-axis" ion detector have to be coupled ion-optically. In a known manner, this is mainly done by electrostatic deflection fields, which are generated by suitably shaped sheet metal electrodes. Arrangements are also known which take advantage of the positive ion attraction of the high negative potential of the first dynode of a secondary electron multiplier, this potential typically being in the range from 2000 to 3000 V.

With such deflection arrangements, however, it has the disadvantage that all the multipole mass filters known to date and used in mass spectrometers have the property that the ions to be investigated have an energy dependent on the ion mass and the phase of the multipole radio frequency field when they exit the mass filter - and angular distribution. For this reason, all "off-axis" deflection arrangements corresponding to the current state of the art and relying only on electrostatic repulsion or attraction fields, via the mass-dependent energy and angle distributions of the ions, result in a more or less large mass dependence of the ion deflection. A particular disadvantage is the loss of detection sensitivity caused by "off-axis" arrangements of the type customary today.

In contrast, the invention has for its object to improve a mass spectrometer of the type mentioned while avoiding the disadvantages described above. In particular, a mass spectrometer is to be created which ensures reliable guidance of the ions to be examined between the mass filter and the ion detector. According to the invention, this object is achieved in a mass spectrometer of the type mentioned at the outset in that a substantially uniformly distributed, substantially radially extending guide field is arranged between the mass filter and the ion detector and converts ions into screw tracks with a speed component in the direction of the axis mentioned this axis leads to the input of the detector.

A preferred embodiment is characterized in that an electrode of the guide field is formed by an essentially rectilinear conductor which extends essentially between the mass filter and the ion detector and is charged opposite to the charge of the ions to be examined.

According to an advantageous embodiment, the conductor is a metal wire, a further embodiment providing that the conductor is insulated in an elongated housing. An electrical voltage is applied between the wire and the housing, specifically in such a way that the wire is at a position opposite the sign of the ion charge, the wire being at a negative voltage, in particular for guiding positive ions.

A coaxial arrangement of electrical conductors has a potential distribution corresponding to the following equation:

unter deren Wirkung sie sich auf ellipsenartigen Bahnen um den Zentralleiter bewegen. Besitzen die geladenen Teilchen eine axiale Drift, so gehen die ebenen Bahnen in ellipsenart,ige Schraubenbahnen um den Leiter über. Die Bewegung der Ionen erfolgt damit vom Radialfeld geführt in axialer Richtung des Führungsfeldes.Here are r _; or r _a the radii of the inner or outer conductor, the outer conductor in front lying case is a housing that has a circular cross section. U _o is the potential of the inner conductor relative to the outer conductor. If charged particles (charge q) reach the area of the guide field, they experience a force of the magnitude directed radially to the conductor arrangement

under the effect of which they move on elliptical orbits around the central conductor. If the charged particles have an axial drift, the flat trajectories merge into elliptical, helical trajectories around the conductor. The movement of the ions is thus guided by the radial field in the axial direction of the guide field.

According to further embodiments of the invention, it is provided that the housing has lateral openings for the entry or exit of the ions to be examined and any interfering neutral particles that may be present.

It is advantageous to clamp the conductor only on one side, in particular the free end of the conductor being bent in the direction of the axis of the mass filter towards the mass filter.

In a preferred embodiment it is provided that a pusher plate which repels the ions to be examined is arranged in the region of the entry point of the ions to be examined into the guide field substantially perpendicular to the axis of the guide field.

One embodiment of the invention provides that two ion detectors are staggered relative to the mass filter and that the mass filter is connected to each detector by an elongated guide field. It can preferably be provided that an electrically biased conductor is arranged between the mass filter and each of the two ion detectors, the conductors being insulated from one another. If the conductors are then provided with voltages of opposite signs, positive and negative ions can be measured simultaneously in this way.

In one embodiment, which is characterized in that one of the ion detectors is highly amplifying and the other is relatively little amplifying, the double design of the particle guide arrangement for two separate ion detectors, in particular secondary electron multipliers, opens up the possibility of a quasi-simultaneous measurement of single ion currents and total ion currents through rapid switching of the voltage of the wires is achievable. Sensitive single-current measurements are carried out with the strongly amplifying detector, while the low-amplifying detector is provided for measuring the total ion current. By simply reversing the polarity of the voltages of the two wires insulated from one another and simultaneously switching the single-mass filter to a high-pass filter which allows all higher masses to pass from a limit mass, the respective ion currents are supplied to the assigned detectors.

• Fig. 1 eine schematische Schnittdarstellung eines ersten Ausführungsbeispiels der Teilchen- führungsanordnung des erfindungsgemäßen Massenspektrometers mit einem durchgezogenen Führungsdraht;
• Fig. 2 eine schematische Darstellung einer anderen Teilchenführungsanordnung des erfindungsgemäßen Massenspektrometers, wobei der Führungsdraht im Bereich des Massenfilters frei endet und ihm gegenüber eine Pusher-Platte angeordnet ist;
• Fig. 3 die schematische Darstellung einer weiteren Teilchenführungsanordnung des erfindungsgemäßen Massenspektrometers mit zur Achse des Massenfilters hin gebogenem freien Ende des Führungsdrahtes; und
• Fig. 4 die schematische Darstellung einer Teilchenführungsanordnung des Massenspektrometers mit zwei lonendetektoren.

Further advantages of the invention result from the following exemplary embodiments, which are explained in detail with reference to the drawing. It shows:

• 1 shows a schematic sectional illustration of a first exemplary embodiment of the particle guide arrangement of the mass spectrometer according to the invention with a solid guide wire;
• 2 shows a schematic illustration of another particle guide arrangement of the mass spectrometer according to the invention, the guide wire terminating freely in the area of the mass filter and a pusher plate being arranged opposite it;
• 3 shows the schematic representation of a further particle guide arrangement of the mass spectrometer according to the invention with the free end of the guide wire bent towards the axis of the mass filter; and
• Fig. 4 is a schematic representation of a particle guide arrangement of the mass spectrometer with two ion detectors.

The mass spectrometer shown in parts in FIG. 1 consists of a quadrupole mass filter 10 and a secondary electron multiplier (SEV) 12 offset as an ion detector to its axis 11, which is indicated by dash-dotted lines. Quadrupole mass filter 10 and secondary electron multiplier 12 are ion-optically connected by a particle guide arrangement 13. The particle guide arrangement 13 consists of a housing 14 which extends between the quadrupole mass filter 10 and the secondary electron multiplier 12 perpendicularly to the longitudinal axes thereof and in the longitudinal axis of which a metal wire 16 is stretched, which is fastened to the end faces 17 of the housing 14 by means of insulation 18. The wire 16 is biased to an electrical potential which attracts the ions emerging from the quadrupole mass filter 10 which are detected by means of the secondary electron multiplier 12. The housing 14 each has an opening 19 and 21, which is connected to the quadrupole mass filter 10 and the secondary electron multiplier 12, the openings 19, 21 being arranged such that the major part of the ions to be examined from the quadrupole mass filter 10 through the opening 19 enter the housing 14 and can exit the housing 14 through the opening 21 to the secondary electron multiplier 12. In the direction of the axis 11 of the quadrupole mass filter, the housing 14 has an opening 22 for the exit of neutral particles which may be present or which are formed by discharging ions on the metal wire 16. The opening 22 lies opposite the opening 19 for the entry of the particles from the quadrupole mass filter 10. With the first dynode of the Sekun The electron multiplier 12 has a tubular extension 23 connected to the opening 21, which forms an ion-optical lens with the opening 21 for the ions to emerge from the particle guide arrangement 13. The tubular extension 23 is electrically attractively biased for the ions to be examined and has the effect that the ions are withdrawn from their helical path about the axis 16 and fed to the ion detector. The housing 14 is kept at a voltage which is opposite in sign to the voltage of the metal wire 16.

In order to explain the mode of operation of the mass filter according to the invention, it makes sense to say in advance that charged particles or ions are generated in a mass spectrometer in an ion source (not shown), which is located at the beginning of the quadrupole mass filter 10, which are caused by the quadrupole Mass filters 10 are filtered, so that - according to the respective transmission masses - only a certain part of the ions from the quadrupole mass filter 10 is admitted through the opening 19 into the housing 14 of the particle guide arrangement 13.

The metal wire 16 has an electrical voltage. The housing 14 is also electrically biased. In this way, a cylindrical guide field is formed in the interior of the housing 14, which exerts a force on the ions in the housing 14 in the direction of the metal wire. This force forces the ions around the metal wire 16 on elliptical paths.

Neutral particles entering the housing 14 from the quadrupole mass filter 10 and some of the neutral particles formed by discharge on the metal wire 16 exit the housing 14 through the opening 22.

2 to 4, the same reference numerals are used for the same parts of the mass spectrometer.

In the embodiment of FIG. 2, the metal wire 16 is clamped at only one end, namely in the vicinity of the secondary electron multiplier 12, insulated against the housing 14 on the end face 17 located there, while the metal wire 16 in the housing 14 at the opening 19 to the quadrupole -Mass filter 10 ends freely. Opposite the free end 26 of the metal wire 16 is a pusher plate 27 which is biased in the same direction as the charge of the ions to be examined, so that it repels them.

The mode of operation of the embodiment of the mass spectrometer or the particle guide arrangement 13 shown in FIG. 2 essentially corresponds to that of the one shown in FIG. 1, with only the particles being repelled by the pusher plate 27 and thus giving them an enlarged speed component along the metal wire 16 Secondary electron multiplier 12 is forced out.

In the exemplary embodiment of the mass spectrometer shown in FIG. 3, the metal wire 16 clamped near the secondary electron multiplier 12 on the end face 17 located there is bent with its free end at the mass filter 10 into the opening 19 for the entry of the ions to be examined into the housing 14 that its free end 28 is substantially aligned with the axis of the quadrupole mass filter 10. The ions emerging from the quadrupole mass filter 10 through the opening 19 can thus be taken up directly by the leading metal wire 16 and guided along it in elliptical screw paths up to the level of the outlet opening 21. Since the ions in the mass filter are not moved exactly on the mass filter axis 11, but e.g. in the case of a quadrupole mass filter, if they oscillate around the quadrupole axis, they have velocity components in the direction of the axis of the guide field when they exit the mass filter.

FIG. 4 shows a mass spectrometer with a double particle guidance arrangement, which is constructed symmetrically to the multipole axis. The housing 14 has, in alignment with the axis 11 of the quadrupole mass filter 10, an opening 19 for the entry of the particles and a passage opening 22 for neutral particles which may be present. At both ends of the housing 14 of the particle guide arrangement 13, a secondary electron multiplier 12, 12 'is arranged on the side of the housing 14 opposite the quadrupole mass filter 10. The ions to be examined enter the secondary electron multiplier from the housing 14 through openings 21. The ion-guiding metal wire 16, 16 'is clamped to the two end faces 17 of the housing 14 by means of insulation 18 and divided in the middle between the openings 19 and 22 in two halves 16, 16', so that both wire halves 16, 16 'are stand in alignment in the center of the entrance opening without touching. In the exemplary embodiment shown, the wire ends facing each other are mechanically stable and at the same time electrically insulated by means of a small glass bead 29.

When at least one of the wires 16 is pretensioned to a voltage which attracts the particles to be examined, the mass spectrometer of the exemplary embodiment of FIG. 4 essentially functions like that of the exemplary embodiment of FIG. 1.

In particular, however, both wires can be brought to different electrical voltages, preferably one half of the wire having a positive potential, while the second half of the wire is at a negative potential. By simply changing the polarity The ion current to be investigated is then directed to separate detectors.

In the exemplary embodiment shown, one secondary electron multiplier 12 is equipped with a high gain in order to be able to carry out sensitive single ion current measurements, while the second secondary electron multiplier 12 ′ has a correspondingly low gain for measuring the total ion current. By simply reversing the polarity of the wire voltages and simultaneously switching the quadrupole from the single-mass filter to a high-pass filter that allows all higher masses to pass from a limit mass, the possibility is opened to measure both the single ion current and the total ion current very precisely and without destroying the detectors .

When equipping the secondary electron multipliers 12, 12 'with essentially the same amplification and opposite sign selection of the bias voltages of the two wires 16, 16', there is the possibility, with corresponding polarity of the first dynodes, of positive ions of one of the secondary electron multipliers and negative ions of the same in the other secondary electron multiplier Detect mass, since the quadrupole mass filter 10 has a mass separating effect on both positive and negative ions.

Claims (10)

1. A mass spectrometer with a source of ions, a mass filter and at least one ion detector, the mass filter and the ion detector being laterally offset from one another, characterised in that a guide compartment extending substantially radially and distributed substantially uniformly along an axis is disposed between the mass filter (10) and the ion detector (12) and guides ions towards the detector input (12) around the aforementioned axis (16) and with a velocity component in the direction of the axis.

2. A mass spectrometer according to claim 1, characterised in that the axis extends substantially at right angles to the axis (11) of the mass filter (10) and to the longitudinal dimension of the ion detector (12).

3. A mass spectrometer according to claim 1 or 2, characterised in that one electrode of the guide compartment is a substantially straight conductor (16), more particularly a metal wire, extending substantially between the mass filter (10) and the ion detector (12) and charged with opposite polarity to the ions under investigation, the wire being preferably clamped at one end only and its free end being preferably bent towards the mass filter (10) and into the direction of the axis (11) thereof.

4. A mass spectrometer according to claim 3, characterised in that the conductor (16) is insulated and clamped in a long housing (14), preferably of metal, which has a circular, rectangular or square cross-section and is preferably formed with lateral openings (19,21, 22) for the entry and/or exit of the ions under investigation and for any interfering neutral particles present.

5. A mass spectrometer according to any of the preceding claims, characterised in that a pusher plate (27) repelling the ions under investigation is disposed substantially at right angles to the axis (16) of the guide compartment, near the place where the ions under investigation enter the guide compartment.

6. A mass spectrometer according to any of the preceding claims, characterised in that at least two ion detectors (12, 12') are offset relative to the mass filter (10) and to one another, the mass filter is connected to each detector (12, 12') by a respective guide compartment, which is formed by a conductor (16, 16') extending between the mass filter (10) and the ion detector (12, 12'), and the conductors (16, 16') are insulated from one another and at different voltages, preferably of opposite sign.

7. A mass spectrometer according to claim 6, characterised in that the electrically insulated conductors (16, 16') are mechanically interconnected, more particularly by a glass connection.

8. A mass spectrometer according to any of the preceding claims, characterised in that each ion detector (12, 12') is connected to a tubular attachment (23) for decoupling the ions from the guide compartment, the attachment cooperating with the outlet opening (21) of the guide housing (14) to form an ion-optical lens for the ions under investigation.

9. A mass spectrometer according to any of the preceding claims, characterised in that the mass filter (10) is a multi-pole mass filter, more particularly a quadripole mass filter.

10. A mass spectrometer according to any of the preceding claims characterised in that the ion detectors (12, 12') are secondary electron multipliers.

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