EP0403965B1 - MS-MS-flight time mass spectrometer - Google Patents

Description

The invention relates to a time-of-flight mass spectrometer with an ion source for generating a pulsed primary ion beam, which has at least 3 diaphragms with a common axis of symmetry, at which pulsed or time-constant electrical or U₁, U _b and U _o potentials are present during operation of the spectrometer , having,
wherein the ions between the first, ion-repellent and the second, ion-attracting aperture are generated at a distance a from the second aperture, which is smaller than the distance from the first from the second aperture, and with a device for pulsed, local sharply limited influence on the ions.

MS-MS techniques in mass spectrometry allow secondary mass selection after a preferred mass from the variety of ions that is generated in the ion source has already been selected with a primary mass selector. These primarily selected ions experience an interaction of various kinds (eg excitation by collisions, light, etc.), which leads to fragmentation, the secondary fragments can be examined by a further mass analysis. Such MS-MS techniques can be used to study molecular decay kinetics, to elucidate molecular structures and to analyze unknown molecules; They represent one of the most complex, but also the most information-rich methods in these areas.

So-called double-focusing devices, which consist of a combination of magnetic and electrostatic mass analyzers, are usually used for MS-MS mass spectrometry. These conventional MS-MS devices, as well as their enhancement, MS-MS-MS devices, have reached a certain limit of their developability both in terms of their price / performance ratio and in terms of their technical possibilities.

The so-called reflectron time-of-flight spectrometers (see e.g. Mamyrin et al., Zh. Eksp. Teor. Fiz. 64 (1973) 82) overcome one of the greatest disadvantages of conventional time-of-flight mass spectrometers: the low mass resolution. With reflectrons, a resolution (50% valley) of 5000 as standard (without readjustment) and of 10,000 without serious problems can be achieved (e.g. from: Boesl et al., Anal. Instrum. 16 (1987) 151). Even a resolution of 35,000 has already been achieved (T. Bergmann, T.P. Martin, H. Schaber Rev. Sci. Instrum. 60 (1989) 347). However, the excellent advantage of time-of-flight mass analyzers, their extraordinarily high transmission and thus detection sensitivity, is almost unaffected.

The resonant laser excitation serves for the ionization of molecules with the help of a multi-photon (mostly two-photon) absorption via a resonant intermediate state. The inclusion of a molecule-specific, resonant optical transition already enables substance-selective ionization and thus a first one Step towards MS-MS methods. However, resonant laser excitation is also characterized by great flexibility: on the one hand, exceptionally gentle ionization is possible (see, for example, Grotemeyer et al., Org. Mass Spectrom. 21 (1986) 645), often even without any fragmentation, but on the other hand it can also be used with it Fragmentation can be achieved, which can be varied from the generation of a few fragments with high involvement of metastable decays to extremely hard fragmentation (see, for example, Boesl et al., J. Chem. Phys. 72 (1980) 4327 and Chem. Phys. Lett. 87 ( 1982) 1).

The combination of resonant laser excitation and modern time-of-flight mass spectrometry (U. Boesl, H.J. Neusser, R. Weinkant, E.W. Schlag, J. Phys. Chem. 86 (1982) 4857) led to a new type of mass spectrometer, the resonant laser time-of-flight mass spectrometer. Resonant laser time-of-flight mass spectrometry is currently at the beginning of its development. Neither ion-optical possibilities in the time-of-flight analyzer nor their combination with properties of laser ionization, such as short pulses, extremely low excitation volume, variation in wavelength and intensity, have been fully exploited to date.

The present invention is therefore based on the object of providing a device composed of simple components with various options, in which, after ionization and before a final time-of-flight mass analysis, a further mass selection and / or a secondary fragmentation can take place, the mass resolution, the transmission and the sensitivity of detection is not inferior to that of known devices.

und $${\text{U}}_{\text{b}}{\text{/(U}}_{\text{a}}{\text{+ U}}_{\text{b}}\text{) = (2c + 2b)/3c}$$

erfüllt sind, und daß am Ortsfokus eine Einrichtung vorgesehen ist, mit der der physikalische Zustand der Ionen pulsweise mindestens einer der folgenden Änderungen unterworfen werden kann, nämlich Änderung des Impulses, Änderung des quantenmechanischen Zustands der Elektronenhülle, chemische Reaktion oder Fragmentation, wodurch jeweils aus dem primären Ionenstrahl ein sekundärer Ionenstrahl mit neuen physikalischen Eigenschaften erzeugt wird.This object is achieved according to the invention in that the distance a between the origin - with potential U _a + U _b - of the ions and the second aperture, the distance b between the second and the third, ion-accelerating aperture, the distance c between the third aperture and a location focus, located on the trajectory of the ions in the direction of flight after the third aperture, and the ratio of the potential difference U _b between the third and the second aperture to the potential difference U _a + U _b between the third aperture and the point of origin of the ions are chosen so that the relationships $${\text{a = {c · [(c - 2b) / 3c]}}^{\text{3/2}}\text{+ b} · (c - 2b) / 2 (c + 2b)}$$

and $${\text{U}}_{\text{b}}{\text{/ (U}}_{\text{a}}{\text{+ U}}_{\text{b}}\text{) = (2c + 2b) / 3c}$$

are satisfied, and that a device is provided at the location focus with which the physical state of the ions can be subjected to at least one of the following changes in pulses, namely change in the momentum, change in the quantum mechanical state of the electron shell, chemical reaction or fragmentation, each of which results from the primary ion beam a secondary ion beam with new physical properties is generated.

With the exact definition of the location focus, in particular of the 2nd or higher order, a spatial point of optimal energy correction is available in which a very high primary mass resolution is possible. With a secondary access in exactly this location focus, a secondary mass spectrum is therefore equipped with the most favorable starting conditions. The type of secondary interaction initially does not matter; there is free choice in this regard.

The MS-MS time-of-flight mass spectrometer according to the present invention has the advantages over conventional MS-MS devices of very high transmission and thus high detection sensitivity and very high speed. Commercial reflectron time-of-flight spectrometers can be converted to an MS-MS device with minor modifications; the essential additional costs arise only through the selected, secondary access method and are far below the purchase price of the source device. As with simple time-of-flight mass spectrometers, high transmission and detection sensitivity are also intrinsic properties of the method, as is the speed: Secondary mass spectra can be carried out in the submillisecond range without losses in transmission or mass resolution. A combination with almost any access method, such as laser excitation, electron, ion, molecular and atomic beam or gas cells for shock activation is possible.

The exact definition of the location focus, ie the point of optimal energy correction of the primary ions leaving the ion source, is an essential prerequisite for the time-of-flight mass spectrometer according to the invention. While energy corrections of the first order have at most been implemented in the location focus, adherence to the above distance relationships between the diaphragms and the corresponding potential relationships presents an ion source which enables energy correction of the second order.

In embodiments of the ion source according to the invention, the particles to be ionized can be provided either simply by introducing gas into the ion source, or by evaporating the particles in the ion source itself. The latter method is also suitable for the investigation of solid substances.

In a further embodiment, the particles to be examined are introduced into the ion source by means of an atomic or molecular beam, so that the particles to be ionized located only in a narrowly defined area, which enables an exact definition of the ion site. The atomic or molecular beam can either cross the axis of symmetry of the diaphragms essentially at right angles between the first and the second diaphragm of the ion source at a distance a from the second diaphragm. In another embodiment, the atomic or molecular beam enters the ion source collinearly to the axis of symmetry of the diaphragms through the first diaphragm, so that the structures for generating the atomic or molecular beam cannot be attached to the side of the mass spectrometer, but rather in an extension of its axis. Another advantage of this arrangement is that the particles which have not yet been ionized have a beam characteristic in the direction of the later ion beam.

The particles to be examined can be ionized in the ion source either by photoeffect, by particle collisions or by field ionization. In embodiments of the invention in which photoionization takes place, it is possible to keep the residual energy in the molecular ion to a minimum during the ionization process. It is a "gentle" method of ionization, in which even sensitive large molecules can be ionized without bursting. The cheapest is the use of a light beam from an incoherent source, in particular a UV source, for example a mercury vapor lamp for the continuous generation of high light output, or commercially available flash lamps. In other embodiments of the invention, pulsed or continuous laser beams are used for the photoionization of the particles to be examined. The extraordinarily high frequency sharpness for laser light enables a high atom- or molecule-specific selectivity of photoionization. With that, quite selectively select only certain particles from a particle mixture offered in the ion source. In embodiments of the invention, pulsed lasers are used, the temporal characteristics of which are impressed on the pulsed ion beam. Another advantage of using lasers is the high power density that can be achieved, the possibility of a very sharp spatial bundling of laser beams and thus a very precise definition of the ion source, as well as the utilization of the frequency sharpness of the laser light with regard to the optical excitation of the particles to be examined.

In embodiments of the time-of-flight mass spectrometer according to the invention, the particles to be examined are ionized by collisions using a beam of charged particles. This particle beam can either be an electron beam which is inexpensive and simple, e.g. high beam intensities and a good spatial beam definition can be achieved by means of a hot cathode and simple electron optics. In other embodiments, the impact ionization takes place with the aid of an ion beam, which enables the mass spectrometric investigation of ion impact processes in the ion source.

Through the use of rays both for the provision of the particles to be examined and for the quanta causing the ionization, the ion formation site is particularly well defined at the point of intersection of the two rays.

In embodiments of the invention, the pulse characteristic of the ion beam is generated by pulsed voltages on the diaphragms of the ion source, so that a continuous supply of the to investigating particles and continuous ionization is possible. In other embodiments, the potentials are statically applied to the diaphragms, which enables considerably simpler electronics for the voltage supply to the diaphragms, but requires a pulsed ionizing beam. In order to be able to adjust the ion beam both spatially and energetically, the potentials applied to the diaphragms of the ion source can be set separately in embodiments of the invention.

The exact location focus is adjusted by shifting the ionizing beam and / or the atomic or molecular beam and by varying the potentials at the diaphragms of the ion source. In embodiments of the invention, a control device is provided which automatically adjusts the potential U _b at the second aperture at given distances a, b, c and given potential U 1 at the first aperture.

In embodiments of the time-of-flight mass spectrometer according to the invention, an ion detector with a flat impact surface is provided at a distance c in the direction of flight of the ions behind the third aperture of the ion source on the trajectory of the ions, with which the position of the location focus can be determined precisely. In one embodiment, the ion detector is arranged such that it can be moved out of the trajectory of the ions by means of a mechanical displacement device, so that the properties of the location focus can either be exploited by recording a mass spectrum in the location focus, or, after the adjustment of the location focus has been completed, the ion detector is moved out of the ion beam , and a secondary interaction can take place at the location focus.

In preferred embodiments of the invention, the secondary access to the ion beam at the location focus is modulo of a defined time delay compared to the time of generation of the ion pulses in the ion source. In order to optimally utilize the energy-correcting properties of the location focus, the ions at the location focus are influenced in a strictly localized manner. The location focus is thus the starting point for a secondary mass spectrum. In the case of a pulsed secondary access, the pulse triggering the interaction is synchronized with the primary ion pulse from the ion source.

In one embodiment, the ions are influenced by building up a pulsed electrical field which is transverse to the ion beam direction and which causes a selective deflection of ions in a specific propagation time window from the primary ion beam direction. By selecting the ion transit time, the mass of the ions to which the secondary access is made can also be selected.

In embodiments of the invention, the transverse electrical field is generated by means of a mesh network. If the mesh network is fine enough, access to the ion beam is limited in space, and there is the possibility of time modulation of the secondary access via the changes in the electrical potential applied to the mesh network.

In a special embodiment, the mesh consists of two comb-like structures, the teeth of which consist of very fine wires, the teeth of the comb-like structures lying opposite one another in the center grip without touching and all teeth belonging to a comb-like structure are connected to each other in an electrically conductive manner. In this arrangement, the electrical fields generated by the two comb-like structures cancel out at a very short distance in front of and behind the mesh, so that there is no unwanted interference of the ion beam by fields that extend far into space, as are typical for conventional grids, uncontrollable interference cause.

In embodiments of the invention, voltage pulses are applied to the two comb-like structures which are complementary to the potential U Potential applied to the third aperture of the ion source, that is to say they have the same amplitudes, the same pulse durations but opposite polarities. This can either cause the elimination of certain undesirable ion masses or a short time window for the passage of special ions, e.g. for targeted further secondary fragmentation.

In other embodiments of the invention, the secondary interaction in the location focus can take place by optical excitation, in particular with a laser beam. This can cause photodissociation and subsequent fragmentation of the primary ions. Optical excitation offers the advantage of being able to be tuned very precisely to a specific electronic transition and thus extremely high mass selectivity, and on the other hand is particularly gentle as a "soft" excitation method, so that even metastable states of larger molecules can be excited without first destroying them . On the other hand, it is variable from very soft to very hard Fragmentation and thus a variation of secondary mass spectra possible.

In further embodiments of the invention, the secondary interaction takes place in the location focus in the form of an ion shock excitation. The colliding particles come either from an electron beam or another ion beam that crosses the spatial focus perpendicular to the beam axis of the primary ions. The generation of an electron beam is particularly simple and inexpensive and does not require any complex optics. With the help of a second ion beam, on the other hand, physical scattering experiments can be carried out in the location focus. As with photon excitation, the primary ions can either be brought into an excited state or can be broken up into smaller molecular fragments with sufficient energy.

In embodiments, the ions are influenced with the aid of pulses which, owing to their short duration, ensure a sharp temporal, energetic and thus mass-specific selection of the primary ions to be influenced. In other embodiments, however, the pulse duration can be virtually infinitely long, but then the selection of the primary ions to be excited is controlled by a sharply defined energy of the exciting particles or photons, in that very specific energy levels of the electron shell of these ions are excited.

In a further embodiment, the physical state of the ions is additionally influenced in the region of the location focus, either by optical excitation of the ions by means of a laser beam or by shock excitation an electron beam, an additional ion beam or an atomic or molecular beam. This enables the acquisition of secondary mass spectra of very special ion masses that were previously selected in the location focus by a first access. Conversely, in another embodiment, the interaction of the primary ion beam can also be provided in a collision gas cell before access in the location focus. A secondary mass spectrum of the ions leaving the collision cell can then be recorded by successively displacing the time window selected in the location focus relative to the time of origin of a primary ion pulse in the ion source.

In a further embodiment of the invention, the post-acceleration of the ions is provided after the location focus. In this way, the drastic decrease in the kinetic energy of the ion fragments after fragmentation, which would negatively influence the mass resolution of the spectrometer, can be partially compensated for. In a special embodiment, a fourth diaphragm after the location focus is provided in the ion beam, which is electrically connected to the third diaphragm in the ion source via a tubular shield which encloses a field-free space. The post-acceleration is then brought about by a fifth aperture, which, viewed in the direction of flight of the ions, sits on the ion beam axis after the fourth aperture and lies at the ground potential of the time-of-flight spectrometer.

In a preferred embodiment of the invention, an ion reflector is provided after the secondary interaction zone, which has one reflector end plate and several braking fields arranged at a distance in front on a common axis of symmetry contains defining brake electrodes, the reflector end plate being arranged displaceably along the axis of symmetry of the ion reflector. When the reflector end plate is displaced, the electrical potential applied to it is tracked in such a way that the electrical field strength between the reflector end plate and the brake electrode initially located is not changed. Such an ion reflector initially serves to compensate for the time-of-flight differences of ions of the same mass but different initial energies, and thus to improve the mass resolution. The displaceable end plate makes it possible to mask out the ions of the primary beam: these have a higher kinetic energy than all fragment ions that are secondary to the primary ions, so that these primary ions have the greatest depth of penetration into the ion reflector. If you move the reflector end plate towards the incoming ion beam to such an extent that the primary ions hit the plate and are thus removed from the ion beam, only the low-energy secondary ions formed due to the secondary access leave the ion reflector. These can now be detected undisturbed by the relatively high intensity of the primary beam with high resolution. By pushing the reflector end plate further towards the last brake electrode, ions with lower kinetic energy are also hidden. In this operating mode, the ion reflector can therefore be used as an energy selector for recording secondary mass spectra.

The easiest way to track the electrical potential of the reflector end plate when it is displaced is by means of a sliding contact. In another embodiment, electronic voltage tracking is provided. In an operating variant of the method according to the invention If an electrical potential applied to one of the brake electrodes is changed, the potentials of the other brake electrodes and of the reflector end plate are adjusted in such a way that the original relationships of the potentials to one another are retained before the change. As a result, the optimal setting of the ion reflector once found is automatically maintained even when the position of the reflector end plate is changed.

In embodiments, the aperture openings of the brake electrodes are each provided with a network or a grid, which serves as a potential shield and for generating parallel equipotential surfaces. In other embodiments, instead of the nets or grids, a front panel with a larger opening diameter than that of the brake electrodes is provided. The front panel is then at the ground potential of the time-of-flight mass spectrometer and enables a controlled extraction of the electric fields from the brake electrodes into the space in front of the ion reflector, and thus controlled steering of the ions arriving in and out of the ion reflector.

The ions are normally deflected by the ion reflector from their original flight direction by an angle of more than 90 ° but less than 180 °. In one embodiment of the invention, however, the axis of symmetry of the ion reflector is collinear with the direction of flight of the incoming ions, ie the ion beam is reflected back in itself. In this embodiment, the ion detector sits on the ion beam axis between the ion reflector and the ion source and has an opening concentric to the ion beam axis for the passage of the incoming ions. This arrangement enables a very compact construction of the time-of-flight mass spectrometer.

In a further embodiment, optimal time focusing for ions with less than the average kinetic energy of the ions is achieved by reducing the potentials applied to the brake electrodes and the reflector end plate. A secondary mass spectrum, in particular for molecular ions fragmented on the flight path, can thus be generated by tuning the fields and observing them in a fixed time fixed.

Fig. 1

die schematische Darstellung eines Flugzeit-Massenspektrometers nach der Erfindung mit einer Ionenquelle, einer sekundären Wechselwirkungszone, einem Ionenreflektor und einer feldfreien Ionendriftstrecke mit Detektor,

Fig. 2a

die schematische Darstellung einer Ausführungsform der sekundären Wechselwirkungszone mit Laser- oder Teilchenstrahlzugriff,

Fig. 2b

die schematische Darstellung einer Ausführungsform der sekundären Wechselwirkungszone mit zwei ein Maschennetz bildenden kammartigen Strukturen am Ortsfokus,

Fig. 2c

das Pulsschema der an die kammartigen Strukturen angelegten Spannungspulse zum Eliminieren einer primären Masse,

Fig. 2d

das Pulsschema der an die kammartigen Strukturen angelegten Spannungspulse zur Transmission einer primären Masse,

Fig. 2e

die schematische Darstellung einer Ausführungsform der Erfindung mit abgeschirmter sekundärer Wechselwirkungszone, doppeltem sekundärem Zugriff und Vorrichtung zur Nachbeschleunigung des sekundären Ionenstrahls,

Fig. 3a

ein primäres Massenspektrum aus Laserionisation von C₆H₆,

Fig. 3b

ein sekundäres Massenspektrum aus sekundärer Laseranregung des primären Ions C₆H₆ mit Masse 78,

Fig. 3c

ein sekundäres Massenspektrum aus sekundärer Laseranregung des primären Ions C₄H₄ mit Masse 52,

Fig. 3d

ein sekundäres Massensprektrum aus sekundärer Laseranregung des primären Ions C₄H₂ mit Masse 50,

Fig. 4a

ein primäres Massenspektrum aus Laserionisation von OCS ohne sekundärer Wechselwirkung im Ortsfokus,

Fig. 4b

ein 15-fach gegenüber Fig. 4a vergrößertes primäres Massenspektrum von OCS mit sekundärer Eliminierung von OCS mit Masse 60 durch Puls am Maschennetz im Ortsfokus.

The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the drawing. The features to be gathered from the description and the drawing can be used in other embodiments of the invention individually and individually or in combination in any combination. Show it:

Fig. 1

1 shows a schematic representation of a time-of-flight mass spectrometer according to the invention with an ion source, a secondary interaction zone, an ion reflector and a field-free ion drift path with detector,

Fig. 2a

the schematic representation of an embodiment of the secondary interaction zone with laser or particle beam access,

Fig. 2b

the schematic representation of an embodiment of the secondary interaction zone with two comb-like structures forming a mesh network at the location focus,

Fig. 2c

the pulse scheme of the voltage pulses applied to the comb-like structures to eliminate a primary mass,

Fig. 2d

the pulse scheme of the voltage pulses applied to the comb-like structures for the transmission of a primary mass,

Fig. 2e

1 shows a schematic representation of an embodiment of the invention with a shielded secondary interaction zone, double secondary access and device for post-acceleration of the secondary ion beam,

Fig. 3a

a primary mass spectrum from laser ionization of C₆H₆,

Fig. 3b

a secondary mass spectrum from secondary laser excitation of the primary ion C₆H₆ with mass 78,

Fig. 3c

a secondary mass spectrum from secondary laser excitation of the primary ion C₄H₄ with mass 52,

Fig. 3d

a secondary mass spectrum from secondary laser excitation of the primary ion C₄H₂ with mass 50,

Fig. 4a

a primary mass spectrum from laser ionization of OCS without secondary interaction in the location focus,

Fig. 4b

a 15-fold enlarged primary mass spectrum of OCS with FIG. 4a with secondary elimination of OCS with mass 60 by means of a pulse on the mesh network in the local focus.

The MS-MS time-of-flight mass spectrometer shown schematically in FIG. 1 comprises an ion source A, a secondary interaction zone B, an ion reflector C, in which the incident ion beam is reflected by an angle of more than 90 °, and a field-free ion drift distance D with an ion detector 10 for detecting the ions. All components are located within an evacuable housing, not shown.

The ion source A has at least two accelerating pulsed or unpulsed electric fields which are generated with at least three diaphragms: an ion-repellent first diaphragm 1, an ion-attracting second diaphragm 2 and a post-accelerating third diaphragm 3. The diaphragms 2 and 3 have an opening for the passage of the accelerated ions. Separately adjustable voltages can be applied to all three diaphragms, namely the potential U 1 at the first diaphragm 1, the potential U _b + U ₀ at the second diaphragm 2 and the potential U ₀ at the third diaphragm 3. In the exemplary embodiment shown in FIG. 1 U₀ is identical to the ground potential of the equipment. Between aperture 1 and 2 there is an atomic or molecular beam 4 which either, as shown in FIGS. 1 and 2e, perpendicular to the axis of symmetry 20 of the diaphragm arrangement or collinear with it. In the latter case, the first aperture 1 must also have an opening for the neutral molecules to pass through. In other embodiments, the particles to be ionized are provided by introducing gas into the ion source or by evaporating the particles within the ion source.

The ionization of the particles to be examined in the ion source takes place in the embodiments shown in FIGS. 1 and 2e by means of a beam irradiated perpendicular to the axis of symmetry 20 of the diaphragm arrangement between the first diaphragm 1 and the second diaphragm 2 at a distance a from the second diaphragm 2 5. The beam 5 can be either a laser beam, an electron beam or an ion beam. Accordingly, the particles to be examined are then ionized either by absorption of photon energy in the electron shell or by particle collision. In other embodiments, a beam of incoherent light, in particular from a UV source, is used instead of the laser beam. In the embodiment shown, the ionizing beam 5 perpendicularly crosses the beam 4 of the particles to be ionized on the axis of symmetry 20 and is focused on the crossing point. The pulse behavior of the primary ion beam 25 generated in the ion source is caused by a correspondingly pulsed ionizing beam 5. In other embodiments, however, the ionizing beam 5 can also be irradiated continuously over time, the pulse behavior of the primary ion beam 25 then being impressed by pulsed electrical fields which are generated by applying corresponding potentials to the diaphragms 1, 2, 3. In further embodiments, the Ionization of the particles to be examined caused by strong electric fields.

The point of origin of the primary ions, which in the exemplary embodiment shown is located on the axis of symmetry 20 between the first diaphragm 1 and the second diaphragm 2 at a distance a from the second diaphragm 2, can be changed by correspondingly shifting the beams 4 and 5. The distance b between aperture 2 and 3 is fixed in the exemplary embodiment shown, but can be kept variable in other embodiments, e.g. by means of a second orifice 2 which can be displaced in parallel, but then the distance a of the point of origin of the primary ions from the second orifice 2 also changes accordingly.

A decisive feature of all embodiments of the present invention is the exploitation of the fact that ion sources with pulsed ion generation have a so-called location focus, which in the exemplary embodiment shown is located on the axis of symmetry 20 of the diaphragm arrangement at a distance c in the direction of the primary ion beam 25 according to FIG third aperture 3 is located.

All ions of the same mass arrive at the location focus 30 at the same time, even if they have arisen simultaneously but at different locations in the ion source A and thus with different potential energies. The location focus 30 is therefore a spatial point of optimal energy correction. The order and thus the quality of this correction depends on the type of ion source. With a one-stage ion source (only apertures 1 and 2) you can achieve a 1st order correction, with a two-stage ion source (like the one described above) a 2nd order correction. So far, even for multi-stage ions only the first order location focus is observed. Here, the second order location focus is used for the first time, which is derived below:

mit $${\text{t}}_{\text{a}}{\text{= 2a/v}}_{\text{a}}{\text{, t}}_{\text{b}}{\text{= 2b/(v}}_{\text{a}}{\text{+v}}_{\text{b}}{\text{), t}}_{\text{c}}{\text{= c/v}}_{\text{b}}$$

und mit $${\text{v}}_{\text{a}}{\text{= (2qU}}_{\text{a}}{\text{/m)}}^{\text{1/2}}{\text{, v}}_{\text{b}}{\text{= (2q(U}}_{\text{a}}{\text{+U}}_{\text{b}}{\text{)/m)}}^{\text{1/2}}\text{,}$$

wobei mit q die elektrische Ladung der Ionen und mit m ihre Masse bezeichnet ist.The total flight time t _{ges of} the ions from the ion origin to the location focus 30 is: $${\text{t}}_{\text{total}}{\text{= t}}_{\text{a}}{\text{+ t}}_{\text{b}}{\text{+ t}}_{\text{c}}$$

With $${\text{t}}_{\text{a}}{\text{= 2a / v}}_{\text{a}}{\text{, t}}_{\text{b}}{\text{= 2b / (v}}_{\text{a}}{\text{+ v}}_{\text{b}}{\text{), t}}_{\text{c}}{\text{= c / v}}_{\text{b}}$$

and with $${\text{v}}_{\text{a}}{\text{= (2qU}}_{\text{a}}{\text{/ m)}}^{\text{1/2}}{\text{, v}}_{\text{b}}{\text{= (2q (U}}_{\text{a}}{\text{+ U}}_{\text{b}}{\text{) / m)}}^{\text{1/2}}\text{,}$$

where q is the electrical charge of the ions and m is their mass.

mit x<< 1 darstellen.An energy blur at the ion site leads to a blurring of the flight times, which can be avoided as far as possible by an energy correction in the location focus 30. In order to determine the condition for such a correction, the flight time t _{tot is developed} after this initial energy blur. This can be achieved by developing the potential $${\text{U}}_{\text{a}}{\text{= U}}_{\text{a}}\text{(exact) · (1 + x)}$$

represent with x << 1.

mit den Größen A = ( m/(2qU_a) )^1/2, B = U_a/(U_a+U_b), C = U_a/U_b. Für eine Korrektur 2. Ordnung muß in obiger Gleichung sowohl der Term 1. Ordnung als auch der Term 2. Ordnung verschwinden. Aus dieser Bedingung ergeben sich für die Größen a, b, und c (siehe Fig. 1) und die Potentiale am Ionenentstehungsort (U_a+U_b) und an der zweiten Blende 2 (U_b) die Zusammenhänge: $${\text{(I.) a = {c·[(c - 2b)/3c]}}^{\text{3/2}}\text{+ b} · (c - 2b)/(c + 2b)}$$

$${\text{(II.) U}}_{\text{b}}{\text{/(U}}_{\text{a}}{\text{+ U}}_{\text{b}}\text{) = (2c + 2b)/3c}$$

The blur x = x _o + x _k includes both the blur x _o due to the location blur in the formation of ions and the initial kinetic energy of the ions. The latter term (x _k ) contributes not only to an energy blur but also to a time blur ("turn-around-time"), which must be taken into account separately. A development after x finally results

with the sizes A = (m / (2qU _a )) ^1/2 , B = U _a / (U _a + U _b ), C = U _a / U _b . For a 2nd order correction, both the 1st order and the 2nd order terms must disappear in the above equation. From this condition, the relationships result for the quantities a, b, and c (see FIG. 1) and the potentials at the ion site (U _a + U _b ) and at the second aperture 2 (U _b ): $${\text{(I.) a = {c · [(c - 2b) / 3c]}}^{\text{3/2}}\text{+ b} · (c - 2b) / (c + 2b)}$$

$${\text{(II.) U}}_{\text{b}}{\text{/ (U}}_{\text{a}}{\text{+ U}}_{\text{b}}\text{) = (2c + 2b) / 3c}$$

While the distance between the ion source and the location focus is fixed for a single-stage ion source (this corresponds to b = O, c = 2a in FIG. 1), it can be changed within very wide limits for a two-stage ion source. It is very advantageous to push the location focus 30 as far away from the ion source A as possible, or to make the distance c as large as possible. The longest possible flight distance c (e.g. 10 to 20 cm) already leads to clear flight time differences between the individual masses. In connection with a second order location focusing, a mass resolution of 500 to 1000 can already be achieved in such a location focus 30.

For a = 1.5 cm, b = 1 cm, c = 10 cm, the kinetic energy of the ions 400 eV, ion mass 100 amu, results in a flight time t _ges microseconds of approx. 5 With a pulse width of the exciting beam 5 of Dt = 5nsec, the value for resolution is R = 1 / 2.t / Dt = 500, with Dt = 2nsec the value R = 1250. 2nd order energy correction also leads to ion pulse widths smaller than Dt in the latter case and thus does not limit the resolution.

In all embodiments of the time-of-flight mass spectrometer according to the invention, such a location focus of the 2nd order is generated by a suitable choice of the aperture distances in the ion source A and by applying suitable potentials U 1, U _b and U _o to the apertures 1, 2, 3. A suitable ion detector 11 with a flat impingement surface is used for the exact adjustment of the location focus 30 to a defined distance c, which can be achieved by varying the distance a of the origin of the ions from the second aperture 2 and the voltages at the apertures 1 and 2; this detector can be brought into the ion trajectory of the primary ion beam 25 by means of a mechanical displacement device and can be removed again for MS-MS measurements.

An electronic control device (not shown) is provided for automatically tracking the potential U _b according to equation (II.) At a given potential U 1 and given distances a, b, c.

The ion reflector C shown in FIG. 1 consists of a two-stage arrangement, namely the braking electrodes 6 and 7 defining a braking field and the reflector end plate 8, which together with the second braking electrode 7 viewed in the direction of the incident ion beam defines a reflector field. In the embodiment shown here, the brake electrodes 6, 7 are designed as pinholes. The reflector end plate 8 is attached so that it can be pushed into or out of the reflector. The voltage on this end plate is adjusted so that the field strength between Brake electrode 7 and reflector end plate 8 is not changed. The brake electrodes 6 and 7 have an opening several centimeters in size, which is either provided with a network for generating parallel aquipotential surfaces; or they are operated without networks, but then with a front panel 9, which lies on the apparatus mass of the time-of-flight mass spectrometer. The voltages required for such reflectors correspond to the values known from the literature. The axis of symmetry 40 of the ion reflector C can both, as shown in FIG. 1, have an angle to the ion flight direction, and can also run collinearly with it. In the latter case, however, a special ion detector is required (see below).

The field-free ion drift distance D is simply formed by a sufficiently long, empty vacuum tube between a secondary interaction zone B and the ion reflector C, in accordance with known arrangements. Only a suitable ion detector 10 (e.g. multichannel plate detector) is located at the end of the ion trajectory, as close as possible to the secondary interaction zone B. In the case of a self-reflecting ion reflector C, the ion detector 10 is located on the incident ion flight direction with a concentric opening in the middle for the Passage of the ions coming from the ion source A and the secondary interaction zone B.

The secondary interaction zone B contains the location focus 30 and is the heart of the MS-MS time-of-flight mass spectrometer. In the embodiment shown in FIG. 2a, the focus 12 of a second laser pulse with selected wavelength and intensity is placed exactly in the location focus 30. This laser pulse can optionally be replaced by other pulsed, locally sharp access methods (eg electron beam). Varies If the time delay between the primary generation of ions in the ion source A and the secondary access in the location focus 30, the individual ions of different _{weights are} selectively excited in accordance with their different flight times t _ges and can therefore also be selectively fragmented by photodissociation. If the pulse length is only short enough and the focus 12 is only small enough (for example 0.1 mm), the maximum mass resolution possible in the location focus 30 can be obtained. Secondary access is therefore responsible for both secondary mass selection and secondary fragmentation.

If the ion reflector C is optimally corrected for the kinetic energy of the primary ions in the ion beam 25 and the second laser is not switched on, a usual primary mass spectrum is obtained. In order to obtain a secondary mass spectrum of the ions selected with the secondary access, the voltage at the ion reflector C must now be continuously reduced, the ratio of the voltages at the brake electrodes 6, 7 and the reflector end plate 8 (and possibly the front panel 9) being retained . The entire ion reflector C is thus optimally corrected for decreasing ion energies. In the case of fragmentation in field-free space, the kinetic energy of a molecular ion in relation to the mass of the fragments is distributed among them; secondary fragments with smaller mass therefore also have smaller kinetic energies. The energy correction of the ion level is thus coordinated with the decreasing masses of the secondary fragment ions. However, this means that secondary fragment ions with decreasing mass arrive at the ion detector 10 within a fixed time window. If signals of the ion detector 10 are registered only within this time window, a secondary mass spectrum is obtained with a linear mass scale. All primary molecular and fragment ions have a maximum kinetic energy, thus penetrate the furthest into the ion reflector C and can be eliminated from the mass spectrum by bumping against the reflector end plate 8 of the ion reflector C. In other words, the ion reflector C with a displaceable reflector end plate 8 and time window works as a tunable energy analyzer and thus, according to what has been said above about the fragment energies, as an analyzer for the masses of the secondary fragment ions.

3b-d show such secondary mass spectra, the ordinate being the intensity of the incident ions measured at the ion detector 10 and the abscissa the voltage of the reflector end plate 8 calibrated in ion masses m. 3a shows a primary laser time-of-flight spectrum of benzene, in which the intensity of the first laser was chosen such that, in addition to the ionization, a partial fragmentation of the benzene ions also took place. In FIG. 3b, only the molecular ions in the location focus were detected, excited and fragmented by a suitable delay of the second laser; the secondary mass spectrum can be seen here. In Fig. 3c, only the fragment ions C₄H₄ im were selectively fragmented in location focus 30, in Fig. 3d only the fragment ions C₄H₂ ⁺. The secondary mass spectra of these two fragment ions differ considerably, which can be attributed to different decay routes in accordance with theoretical considerations. A resolution of the secondary mass spectra of R = 600 has already been achieved in these preliminary spectra, although this was limited here by non-optimal reflector correction and by excessive steps in varying the voltages at the ion reflector C.

In order to be able to achieve optimum correction of the ion reflector C by eliminating the primary ions, the movable reflector end plate 8 is provided. The voltages at the ion reflector C can thus be set to optimal energy correction; then the reflector end plate 8 is pushed so far into the reflector field that the point of reversal of the primary ions lies exactly on it. In order to leave the reflector field and thus the energy correction unaffected, the reflector end plate 8 must always be at a voltage which corresponds exactly to the equipotential surface of its respective position; In embodiments, this can be done by means of a sliding contact (not shown) or electronic voltage tracking.

This means that a resolution of 5000 and above can also be achieved for secondary mass spectra, similar to the primary mass spectra.

In the embodiment shown in FIGS. 2b-d, the secondary intervention consists in a special mesh network 23, which in turn is located precisely in the location focus 30. This mesh network 23 consists of two comb-like structures 13 and 14, the "teeth" of which engage in the center without touching one another. The "teeth" consist of the finest possible wires; all "teeth" belonging to a comb are electrically connected to one another; their distances are 0.3 mm and smaller, or 0.15 mm or smaller from the "teeth" of the other comb. Complementary voltage pulses ± U (same amplitude, same length, opposite sign) are best applied to the two combs, so that ions that fly between the teeth at the right time sense a transverse electric field and are deflected laterally, so that but on the other hand, at a very short distance from the mesh network 23, the pulsed fields already cancel each other out.

Voltage pulses of 5 nsec in length and a few 100 V are sufficient to deflect the corresponding ions to such an extent that they no longer hit the ion detector 10 and are therefore eliminated from the mass spectrum (FIG. 2c). If, on the other hand, the two "combs" at different potentials + U and -U are briefly brought to the same potential by a voltage pulse (FIG. 2d), then only the ions that fly through the mesh 23 at the exact moment of the pulse are not deflected and appear as the only ones in the mass spectrum. On the one hand, this allows undesired ions to be eliminated when analyzing mixtures (e.g. because of too high intensities), but on the other hand, targeted ions can also be selected for further, secondary fragmentation. In Fig. 4a and b are first results with a prototype of the mesh network 23 (distance between adjacent "teeth" 1 mm, or 0.5 mm, pulse length 10 nsec, pulse height 100 V) on OCS⁺ and its ¹³C, ³³S and ³⁴S isotopes to see. By far the most common mass 60 (93.5%) (FIG. 4a) was eliminated except for a remainder of 6% (FIG. 4b). For the experiments in which this measurement was carried out, a complete suppression of the mass 60 was undesirable, so that this figure is only to be regarded as a demonstration of the effect.

The embodiment shown in FIG. 2e represents to some extent a combination of the embodiments from FIGS. 2a and 2b. In the embodiment of FIG. 2a, a secondary mass spectrum can only be obtained sequentially and not with a single laser pulse. In the embodiment shown in FIG. 2e, the fact is used that a two-stage ion reflector can still correct 20% energy blur, that a mass resolution of 5000 is easily achieved. In order to keep the loss of kinetic energy that occurs during ion fragmentation within this framework, the fragment ions are accelerated after the location focus 30. The kinetic energy of the ions at the location focus 30 may only be a fraction of the final kinetic ion energy. For this purpose, the ion source A with the diaphragms 1, 2 and 3, the mesh network 23 in the local focus 30 and an additional, final fourth diaphragm 15 are placed at an increased potential U₀. The post-acceleration then takes place between the fourth orifice 15 and a fifth orifice 16 which is at ground potential. Between the diaphragms 3 and 15 there is a field-free drift space with the local focus 30, shielded by a tube 17, which is at the same potential as the diaphragms 3 and 15 and corresponds to the reference potential of the mesh network 23.

If, for example, ions arise at the potential of 2000 V and f-stops 3 and 15 are at 1600 V (kinetic energy in the local focus 400 eV), then the above condition for good mass resolution is already given. As in the embodiment shown in FIG. 2b, the mesh network 23 is attached in the location focus 30; selected primary ions are selected with high mass resolution. Shortly behind (for example 1 mm) the focus 18 of a second laser or another pulsed access, for example an electron beam or ion beam, is adjusted to the ion beam leaving the mesh on the axis of symmetry 20. If the voltage pulse on the mesh network 23 and the second laser pulse 18 or, in the case of embodiments, a different access pulse are precisely synchronized with the flight time of the primary ions of defined mass to be examined, a secondary mass spectrum of these ions is obtained.

A complete secondary mass spectrum can now be obtained with a single ionization and fragmentation pulse. With the help of the sliding reflector end plate 8, all primary ions can also be hidden.

Furthermore, in embodiments, the mesh network 23 can also be replaced by the laser focus 18 (or other pulsed access methods); then, however, metastable ion decays, which take place in front of the location focus 30, can disrupt the secondary mass spectrum of the selected ion. Finally, with the mesh network 23, the secondary access can also consist of a continuous interaction, such as a continuous electron beam, molecular or atomic beam or a collision gas cell. The latter must then be installed in front of the mesh and the post-acceleration as close as possible behind the mesh.

In order to increase the mass resolution in embodiments, the secondary mass spectrum can also be divided into two or more mass ranges, the energy correction of the reflector only having to be optimized in one of these ranges and thus only to 10%, 5% etc. energy deviation. For the post-acceleration between the diaphragms 15 and 16, significantly lower voltages then suffice in comparison to the primary ion energy U _a + U _b .

It is also possible to measure pure "metastable mass spectra". A "metastable mass spectrum" here means the mass spectrum of all productions from the metastable decay of a selected precursor ion that has arisen in the ion source. This metastable decay is usually induced by the additional excitation of the primary ions at the ionization site, for example by further absorption of a photon by laser 1 in the primary ion. A suggestion, e.g. by laser 2 is then not necessary. To record "metastable mass spectra" only laser 2 is switched off. Otherwise, either the ion reflector potentials are tuned or post-acceleration is used (entire "metastable mass spectrum" with few or even one laser shot); this is quite analogous to the acquisition of secondary mass spectra. In the case of post-acceleration, however, only productions from decays that took place before post-acceleration are detected. This area can be enlarged by extending the field-free distance between the ion source and post-acceleration. Primary ions will also be eliminated for "metastable mass spectra" by bumping against the ion reflector end plate. The mesh grating deflection in or near the location focus can be used to further suppress interfering ions, for example productions from metastable decays of other primary ions.

Claims (14)

1. A time-of-flight mass spectrometer comprising an ion source (A) for generating a pulsed primary ion beam (25) and at least 3 diaphragms (1, 2, 3) with a common axis of symmetry (20) to which diaphragms, during operation of the spectrometer, pulsed or time-constant electric potentials are applied which are denoted U₁, U_b, and U₀, wherein the ions are generated at a distance a from the second diaphragm (2) between the first diaphragm (1) which repels ions and the second diaphragm (2) which attracts ions, wherein the distance a is smaller than the distance between the first (1) and the second diaphragm (2), and also comprising a device (B) for influencing the ions in a pulsed and strictly space-delimited manner,
   characterized in that
   the distance a between the point of generation having the potential U_a + U_b of the ions and the second diaphragm (2), the distance b between the second (2) and the third diaphragm (3) post-accelerating the ions, the distance c between the third diaphragm (3) and a so-called space focus (30), namely a point in space located in the trajectory of the ions, after the third diaphragm (3) in the direction of flight of the ions, and the relation of the potential difference U_b between the third diaphragm (3) and the second diaphragm (2) to the potential difference U_a + U_b between the third diaphragm and the point of generation of the ions are selected in such a way that the conditions $${\text{a = {c·[(c - 2b)/3c]}}^{\text{3/2}}\text{+ b} · (c - 2b)/2(c + 2b)}$$

   

   and $${\text{U}}_{\text{b}}{\text{/(U}}_{\text{a}}{\text{+ U}}_{\text{b}}\text{) = (2c + 2b)/3c}$$

   

   are fulfilled, and that the space focus (30) is provided with means by which the physical state of the ions can be subjected intermittently to at least one of the following changes, namely change of the momentum, change of the quantum-mechanical state of the electron shell, chemical reaction or fragmentation, whereby in each case a secondary ion beam with new physical properties is produced from the primary ion beam (25).
2. A time-of-flight mass spectrometer according to claim 1, characterized in that the potentials applied to the diaphragms (1, 2, 3, 15, 16) can be adjusted separately and the distance a can be adjusted by displacing an ionizing photon beam (5) and/or an atomic or molecular beam (4).
3. A time-of-flight mass spectrometer according to claim 1 or 2, characterized in that control means are provided for adjusting the potential U_b prevailing at the second diaphragm (2) automatically to given distances a, b, c and a given potential U₁, existing at the first diaphragm (1).
4. A time-of-flight mass spectrometer according to any one of the preceding claims, characterized in that an ion detector (11) having a plane impact surface is provided in the trajectory of the ions at a distance c after the third diaphragm (3), viewed in the direction of flight of the ions, which can be moved out of the trajectory of the ions by mechanical displacement means.
5. A time-of-flight mass spectrometer according to any one of the preceding claims, characterized in that means are provided for influencing the ions at the space focus (30) by pulses which are synchronized with the generation of the ion pulses in the ion source (A), through a defined time delay.
6. A time-of-flight mass spectrometer according to claim 5, characterized in that a wire mesh (23) is provided by which the ions can be deflected, by an electric field built up in a direction transverse to the direction of the ion beam, and wherein the wire mesh (23) consists of two comb-like structures (13, 14) with teeth consisting of very fine wires, the teeth of the oppositely arranged comb-like structures (13, 14) engaging each other centrally, without however contacting each other, and all teeth of one comb-like structure (13, 14) being interconnected in an electrically conductive manner.
7. A time-of-flight mass spectrometer according to claim 5, characterized in that a laser beam is provided for optical excitation of the electron shell of the ions by means of photons and/or a particle beam is provided for collision excitation of the electron shell of the ions for the purpose of fragmentation, the beam effecting excitation of the ions intersecting the primary ion beam (25) at the space focus (30) at a right angle and/or being focused on the space focus (30).
8. A time-of-flight mass spectrometer according to any one of the preceding claims, characterized in that a device serving to take an additional influence on the physical state of the ions by optical excitation of the ions by means of a laser beam or by collision excitation by means of an electron beam, an additional ion beam, an atomic beam or a molecular beam is provided after the space focus (30), viewed in the direction of movement of the ions.
9. A time-of-flight mass spectrometer according to any one of claims 5 to 7, characterized in that a collision gas chamber is arranged before or after the space focus (30), viewed in the direction of movement of the ions.
10. A time-of-flight mass spectrometer according to any one of the preceding claims, characterized in that a fourth diaphragm (15) is provided coaxially to the third diaphragm (3) after the space focus (30), viewed in the direction of movement of the ions, the fourth diaphragm (15) is electrically connected to the third diaphragm (3) by a shield (17) and a fifth diaphragm (16) is arranged after the fourth diaphragm (15), viewed in the direction of movement of the ions, and the fifth diaphragm (16) is connected to the ground potential of the time-of-flight mass spectrometer.
11. A time-of-flight mass spectrometer according to any one of the preceding claims, comprising an ion reflector (C) for compensating flight time differences of ions of the same mass, whose ion reflector (C) comprising a reflector end plate (8) and a plurality of retarding electrodes (6, 7) arranged in front of such end plate on a common axis of symmetry (40) and at a certain distance therefrom and defining a retarding field, characterized in that the reflector end plate (8) is arranged for displacement along the axis of symmetry (40) of the ion reflector (C).
12. A method for operating a time-of-flight mass spectrometer according to claim 11, characterized in that upon displacement of the reflector end plate (8) the electric potential applied to it is adjusted, so that the electric field strength prevailing between the reflector end plate (8) and the retarding electrode (7) nearest thereto is maintained unchanged.
13. Method according to claim 12, characterized in that, when the electric potential applied to one of the retarding electrodes (6, 7) varies, the potentials of the remaining retarding electrodes (6, 7) and of the reflector end plate (8) are adjusted in such a manner that the original potential conditions prevailing before any such variation are maintained unchanged.
14. A method for generating a mass spectrum with a time-of-flight mass spectrometer according to claims 1 and 11, characterized in that at least partial fragmentation of the ions is effected by influencing the ions at the space focus (30), that the position of the reflector end plate (8) on the axis of symmetry (40) of the ion reflector (C) is selected in a suitable way so that all arriving ions having a kinetic energy higher than a predetermined value, in particular the primary ions, are eliminated from the ion beam by impingement upon the reflector end plate (8), and that the reflector potentials are varied in such a way that secondary fragmentary ions of decreasing or increasing masses are detected continuously in a fixed time window.

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