DE3025764C2 - Time of flight mass spectrometer - Google Patents

Description

45

The present invention relates to a time of flight mass spectrometer according to the preamble of claim 1. Such a time-of-flight mass spectrometer is from »Int. J. Mass Spectrom. Ion Phys. «9 (1972) 357-373.

In transit time mass spectrometers are usually Ions of the same energy but of different mass, separated from one another by the fact that they traverse the same path length at different speeds and one after the other at an ion collector or an ion SEV (secondary electron multiplier) arrive. It is important here that all ions are at the same point in time at the beginning of the route to be started.

A comparatively improved version for a time-of-flight mass spectrometer (Fig. 1) was by »Sov. Phys.-Tech. Phys. «16 (1972), 1177-1179, whereby the ion energy no longer has to be the same for all ions. The ions are deflected by almost 180 ° by an electrostatic mirror after they have traveled through the path length (L) , so that they have to fly back the path before they can be registered in an ion SEV located directly next to the ion source. It is essential that ions of somewhat higher energy penetrate deeper into the ion mirror formed from grid plates; they therefore have to cover a longer distance overall than ions of somewhat lower energy. By means of a suitable potential distribution in the ion mirror, it was possible to achieve that the transit time of the ions from the source to the SEV depends only on the ion mass and no longer on the ion energy. Since the mass resolution in a time-of-flight mass spectrometer is proportional to the length of the flight path, one would like to make it large. Even with the use of an ion mirror, that is to say with double utilization of the structural length, however, quite extensive systems result, in which the diameter of the ion collectors and ion mirrors must also be large.

Another disadvantage of both from »Sov Phys.-Tech. Phys. «As well as from US-PS 37 27 047 known time-of-flight mass spectrometer is that the ion mirror of parallel metal grids are formed, which are penetrated by the ion bundles. This leads to considerable ion current losses, so that z. B. a multiple kinking of the ion bend with such ion mirrors not more is possible.

From the above-mentioned reference "International Journal of Mass Spectrometry and Ion Physics" 9 (1972) pp. 357-373, ion mirrors are known that do not require metal nets. These ion levels however, include complex and therefore difficult to manufacture cylindrical, spherical or torus surfaces, which also have a large space requirement.

The subject of the application, on the other hand, is based on the task of focusing on the running time and location to be realized with simple means. The inventive solution to this problem is in the characterizing Features of claim 1 specified.

In a time-of-flight mass spectrometer built in this way, the ion mirrors consist of simple components that have both reflective and focussing properties. Similar to a Möllenstedt filter lens operated in transmission, such an ion mirror operated in reflection and made up of pinhole diaphragms or tubes delivers focusing effects, as can be shown in numerical calculation. Even in the case of very long flight paths, that is to say with repeated kinking of the ion flight path, a disadvantageous increase in the diameter of the ion beam does not occur. In addition, ion mirrors made up of focussing meshless apertured diaphragms or tubes do not cause losses in the ion beam, which are unavoidable when the ion beam passes through conventional networks. The ion flight distance is thus lengthened and the resolution of the overall system is improved with the same ion pulse length with relatively little expenditure on equipment. By using a number N of such ion mirrors (with 100% reflection), the overall length can be used not only twice, but (N + 1) times. The number N of ion mirrors can either be an even number or an odd number.

Focusing lens mirrors can be made particularly small if the focal lengths of the mirrors are all equal to L / 2 , where L denotes the distance between two opposing mirrors. The perforated diaphragms or tubes can thereby avoid interference potentials at a point that is not caused by ions

more is achieved, to be completed by grids. The focusing properties of the mirrors can e.g. B. can be achieved in that their entrance or exit-side perforated diaphragms or tubes as a single lens are switched.

The invention will now be explained in more detail on the basis of exemplary embodiments

F i g. 1 is a schematic view of a time of flight water spectrometer according to the prior art cited above,

Fig. 2 is a schematic oblique view of a Time-of-flight mass spectrometer according to the invention in a flat linear arrangement,

F i g. 3 shows an oblique view of a time-of-flight mass spectrometer according to the invention in a circular cylinder arrangement;

F i g. 4 shows the principle of a time-of-flight mass spectrometer according to the invention with switchable ion mirrors,

F i g. 5 shows the principle of a time-of-flight mass spectrometer according to the invention with electrostatically tiltable ion mirrors.

The in Fig. 1 has a total of 10 time-of-flight mass spectrometers, as shown schematically indicated, an ion source 12 and a steering device 14 for the ion bundle, which is attached to an ion mirror 16 is bent with grid plates 18 and passed to an ion collector (SEV) 20.

The F i g. 2 and 3 now show how the ion mirrors Ri, R 2 , etc., which fold the beam path and consist of perforated diaphragms or tubes, are arranged next to one another on a straight line (FIG. 2) or on a circle (FIG. 3). Generally everyone must i. The mirror can be set in such a way that, under electronic control, this can happen electrostatically or (electro-) magnetically, so that the ion beam hits the center of the following (i + 1). Mirror is steered. In the case of FIG. 2 this means that the mirrors in the two planes I and II are each arranged linearly and parallel in such a way that the ion beam is reflected back and forth in a transverse plane until it reaches the ion collector 20. In the exemplary embodiment in FIG. 3, the individual ion mirrors R 1, R 2, etc. are arranged on a circle in each of the two planes I, II, so that the corresponding envelope surface can be a cylinder or a conical surface. An essentially parallel arrangement of all ion mirrors is also possible in this case by interposing a push electrode 22 which deflects the ion beam somewhat towards the center of the overall arrangement with each passage.

It can be seen that the arrangements according to FIG. 2 and 3 use the overall length, which essentially corresponds to L , with the aid of ion mirrors (N + 1) times as the ion path.

However, according to FIG. 4 let the ion beam oscillate N times between two ion mirrors Ri, R 2. In this case, precautions must then be taken to smuggle the ion packet into this path (= * L) between the two mirrors A1, R2 and ultimately to discharge it again. In a system according to FIG. 4, the ion mirror potentials can be pulsed by suitable control instead of being applied continuously. For example, the ion packet can find the mirror R 1 grounded first, ie without any influence, while the mirror R 2 reflects the ions. The ion packet returning from mirror R 2 is then returned by mirror R 1 , which has been switched on in the meantime. After passing through the path between the mirrors R 1 and R 2 N times, the electronic control ensures that the ions at (N + 1). Approach mirror R 2 to find it grounded, ie without influence.

Therefore, they can exit and reach the ion SEV 20.

In the system according to FIG. 5 electronically briefly tiltable mirrors Ri, R 2 , etc. are used, in which the potential distribution is briefly controlled so that

i "the lonenbündel is reflected at a different angle than before the first receiver (ion SEV) 20 can be a moderately high-resolution time-of-mass spectrum register toggle now the mirror R 1 for a short time, thus enters the lonenbündel to the mirror R 2..

During the passage of the ion packet, the latter is tilted in such a way that it directs the ion beam onto the mirror R 3; then the mirror R 2 folds back and the ion packet is reflected back and forth N times between the mirrors R 2 and R 3 , until a short time

: n Pivoting the mirror A3 onto the ion beam second catcher 20 'is steered. There is therefore a small range of ionic masses from that in the first Receivers 20 registered mass spectrum had been masked out, registered with high mass resolution. Such beam tilts can of (not shown) additional, z. B. electrostatic or magnetic tilting elements.

In a transit time mass spectrometer, on the one hand, the mass resolution increases when the ion pulse length decreases. On the other hand, the ion intensity usually decreases as the ion pulse length decreases. A high mass resolution can be achieved with high intensity by providing a further flight path of length D between the ion source and the start of the path (length L) (FIGS. 4 and 5). If the source now emits ions of lower energy at the beginning of an ion pulse than at a later point in time, the faster ions can catch up with the slower ions that started earlier at point E, given the appropriate dimensioning or control. A significantly shorter ion pulse with a relatively large energy width is therefore created there without any loss of ion current. In particle accelerator technology, this process is known as ion bunching.

In the exemplary embodiments according to FIGS. 4 and 5 are means for realizing ion bunching intended. The decisive factor for this is such a dimensioning and control that on the one behind the ion source 12 lying recovery point E (Fig. 4) or £ 1 (Fig. 5)

μ ions of higher energy started later have overtaken other ions of the same mass but lower energy that were brought onto the orbit earlier. The ion mirrors R 1, R 2 , etc., made up of perforated diaphragms or tubes (not shown) have such a focusing effect that ions are bunched at a second recovery point E 2 on the ion collector 20 (or at an N th recovery point Ev on the ion collector 20 ') arrive. Thanks to the intermediate distance D (between the ion source 14

w and the first recovery point £ 1) that the total flight time on the subsequent distance L only depends on the ion mass, but not on the ion energy, so that minimally short ion impulses arise at the catcher for ions of a single mass. It leaves

^{b5 it} can also be achieved that the first recovery point after the one located behind the ion source 14 by the distance D is only at the ion collector 20 or 20 '.

For this purpose 3 sheets of drawings

Claims (6)

Patent claims: 1. Time-of-flight mass spectrometry with an ion source and focussing ion mirrors, at detn, starting from a starting point, ions of different masses and energies are brought essentially simultaneously starting onto a flight path, at the end of which there is an ion collector, the ion mirrors bending the flight path in this way several times that the ion flight time between the starting point and the ion collector is independent of the ion energy and that the ions from the starting point are spatially focused on the ion collector, characterized in that each ion mirror (RX, R 2 , etc.) has apertured diaphragms or tubes which have different potentials applied to them Generate spherical equipotential lines. 2. Time-of-flight mass spectrometer according to claim 1, characterized in that the pinhole or Pipes are closed off by grids at a point that can no longer be reached by ions. 3. Time-of-flight mass spectrometer according to claim 1 or 2, characterized in that the spatial focal length of the ion mirrors (R 1, R 2, etc.) is half as large as their distances. 4. Time of flight mass spectrometer according to one of claims 1 to 3, characterized in that the Input or output side perforated diaphragms or tubes of the ion mirrors as single lenses are switched. 5. transit time mass spectrometer according to one of claims 1 to 4, characterized in that at Ion mirrors that reflect the ion trajectory in themselves, the potential of the pinhole or Tubes of an ion mirror is briefly changed, so that the ions are introduced and discharged can pass through. 6. Time-of-flight mass spectrometer according to one of claims 1 to 4, characterized in that the * o The deflection angle of the ion mirror can be changed for a short time for the introduction or discharge.