Detector for time-of-flight mass-spectrometers with low timing errors and simultaneously large aperture

Ions passing through inhomogeneous electric fields in the detector of a time-of-flight mass-spectrometer may need different times on different paths(11) between the entrance aperture and the ion-electron conversion surface(3). These errors in flight time can be reduced by properly deforming the conversion surface(3).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to detectors used in time-of-flight 
mass-spectro-meters having some kind of ion-electron conversion surface. 
Detectors for time-of-flight mass-spectrometers should oppose the in-coming 
beam with an aperture as large as possible and even with this large 
aperture they should cause as little timing errors as possible. 
Every detector must have some kind of ion-electron conversion surface. At 
the instant that an ion impinges on that surface there is a certain 
probability that one or more electrons are created, which are amplified in 
electron-amplifiers. This amplification has as result an electrical 
impulse that gives information about the time-of-arrival of that ion. 
As an alternative to electron-amplifiers a combination of scintillator and 
photomultiplier can also be used. 
The ion optical axis is understood as one path, said path selected at or 
close to the center of the incoming ion beam. Should the detector have a 
construction of cylindrical symmetry, then usually the axis of symmetry is 
chosen. 
Starting from the ion-electron conversion surface, one can follow the ion 
optical axis in reverse direction out of the detector up to a conveniently 
chosen point. Normal to the ion optical axis one can define a reference 
plane. As reference-time-of-flight one can define the time-of-flight from 
that reference plane onto the ion-electron conversion surface. If ions are 
started from the reference plane at other points than the axis point but 
with the same direction and velocity, these ions may need different flight 
times than an ion started on the axis point would need. The difference 
between these flight times and the reference-time-of-flight are called 
time errors. 
These time errors can be given as a function of the starting location on 
the reference plane. In the most general case the time errors are a 
function of the two variables or parameters defining the reference plane. 
If the detector is constructed with rotational symmetry around a straight 
axis, the time errors are a function of the distance a path has from the 
ion optical axis in the reference plane. 
Within a detector having inhomogeneous electrical fields ions can either he 
focused onto a smaller surface or defocused onto a larger surface. For 
that reason the usable surface on the ion-electron conversion surface is 
not a good measure for the sensitivity of the detector. As a measure of 
sensitivity one can use the size of that portion of the reference plane 
from which ions can be started with acceptably low timing errors into the 
detector. 
By defining a reference plane and just considering the paths from the 
reference plane to the ion-electron conversion surface one can logically 
separate the detector and its timing errors from the rest of the 
time-of-flight mass-spectrometer. On the other hand, it is also possible 
to determine the timing errors of complete paths from the ion source to 
the conversion surface. Aside from timing errors that result directly from 
the detector and its construction, the paths may have timing errors in the 
ion source and the reflector that can be compensated by tilting the 
ion-electron conversion surface. For that reason the conversion surface is 
often supported such that its orientation can he varied under operating 
condition. 
2. Description of the Related Art 
The main types of conversion surfaces in present use are: 
a metal surface on which ions release electrons with a certain probability 
after impinging. The metal surface may have a special coating to increase 
the probability of releasing electrons. 
the front surface of a microchannel plate. Actually the ions do 
penetrate some 10 .mu.m deep into the channels before releasing electron. 
In this spirit the conversion surface really has a very complex form. For 
the discussion that follows the front surface of the microchannel plate 
will be equated to the conversion surface. The penetration of ions into 
the channels will not be considered any more, because these few 10 .mu.m 
can be neglected compared to the other timing errors involved. 
The probability of releasing electrons on the ion-electron conversion 
surface strongly depends on the velocity with which an ion hits the 
surface. Since the velocity is inversely proportional to the square root 
of the mass, the probability of detection falls off strongly for ions of 
higher mass. 
Thus, to detect ions of high mass, it is mandatory to postaccelerate these 
ions before they hit the ion-electron conversion surface. Then they will 
release electrons with a sufficiently high probability when impinging on 
the surface. The detector must have a sufficiently high accelerating field 
in front of its conversion surface. This high postaccelerating field can 
be the source of timing errors. 
It is usual practice to keep the timing errors small by making the 
postaccelerating field homogeneous. The direction and magnitude of a 
homogeneous electrical field is independent of location. In a detector 
with homogeneous electrical fields the time-of-flight from the reference 
plane to the ion-electron conversion surface is independent from where in 
the reference plane the ion is started. The time-of-flight is also 
independent of the location where the ion enters the postaccelerating 
field. 
Such an electrical field can only he produced by separating the drift space 
of the time-of-flight mass-spectrometer from the postaccelerating field by 
an electrically conducting mesh. An example of such a detector can he seen 
in FIG. 5 of the publication by de Heer et al. (Review of Scientific 
Instruments, volume 62(3), page 670-677, 1991). 
Ions entering the detector can also hit the lines of the mesh. As long as 
these ions are just removed from the ion beam, this will only cause a 
slight reduction in signal-output from the detector. However, there are 
several possibilities, that ions hitting the mesh lines cause an 
output-signal from the detector at incorrect times: 
Ions can be scattered inelastically on the mesh lines. If their path 
continues toward the conversion surface they may arrive at incorrect 
times, 
Ions can be scattered under large angles from the mesh lines, which also 
changes the velocity component toward the conversion surface. 
Ions can hit the mesh lines and break into pieces upon impact. These pieces 
can also arrive at incorrect times on the ion-electron conversion surface. 
If, because of the above named problems, it is necessary to omit the 
meshes, the postaccelerating field will necessarily be inhomogeneous. This 
causes ions on different paths to strike the ion-electron conversion 
surface after different flight times. 
As already mentioned, the magnitude of the time errors are a function of 
the distance the ion path has from the ion optical axis. The variable in 
this function is to be taken as the distance to the ion optical axis in 
the reference plane, and not on the conversion surface. In the optimum 
case, i.e. when the conversion surface can be tilted, the magnitude of 
these time errors is proportional to the square of the distance to the ion 
optical axis. 
If this is the case, and if the flight time errors should be small, one 
should let the ions enter the detector only close to the ion optical axis. 
This means starting the ions from the reference plane only close to the 
ion optical axis. It does not make a difference whether ion paths are 
focused onto a smaller area or defocused onto a larger area: measure for 
the sensitivity of the detector is the size of the area in the reference 
plane from which ions can be started with acceptably small flight time 
errors into the detector. 
An example of this solution to the problem can be seen in the publication 
of Steffens et al. (Journal of Vacuum Science and Technology, volume 
A3(3), page 1322-1325, 1985). FIG. 4 of the PCT-Application WO 92/19367 
also demonstrates this method of solving the problem. The disadvantage of 
these solutions lies in the fact, that only a comparativity small volume 
of the detector can be used, i.e. only a small area of the reference plane 
can be allowed to oppose the incoming ion beam. This will reduce the 
sensitivity of the detector. 
SUMMARY OF THE INVENTION 
Accordingly, it is the object of the invention to provide a detector for 
time-of-flight mass-spectrometers that will allow for a high sensitivity 
and at the same time allow for a high mass resolution. 
In particular, it is the object of this invention, to provide a detector 
for time-of-flight mass-spectrometers that can oppose the ion beam with a 
large usable area of the reference plane and that also has low flight time 
errors. 
The characterizing features of the invention are given in claim 1 and claim 
3. 
In accordance with the invention, the flight time errors caused by the 
inhomogeneous electrical field in the detector or even flight time errors 
arising for some reasons before the detector are compenated within the 
detector itself. This is done by placing into the detector a curved 
ion-electron conversion surface. The curvature will as a function of 
lateral position, vary the flight time of each path, i.e. will shorten or 
prolong it, in such a manner that the errors induced by the inhomogeneous 
field or the errors having arised for some reason before the detector, are 
compensated or for the least minimized. As an example, some path might 
have a longer flight time than other paths in a detector with a fiat 
conversion surface. The curvature of the conversion surface will shorten 
the flight time of this path, thus equalizing its flight time with the 
flight times of the other paths. 
As an example of the method given in claim 9, the shape of the ion-electron 
conversion surface can be determined as follows: 
1. Take some particular design of the postaccelerating optics. An example 
is shown in FIG. 1. For the beginning, assume it has, as shown in FIG. 1, 
a flat ion-electron conversion surface. 
2. Fix the electrode voltages: In this case only one ring electrode(1) is 
shown, which is to be at the potential of the drift space of the 
time-of-flight mass-spectrometer. The support(2) of the ion-electron 
conversion surface(3) is also fixed here to the postaccelerating potential 
U. The arrangement sad voltages of the electrodes create an inhomogeneous 
postaccelerating field in front of the conversion surface. 
3. Determine a number of ion paths(11) subject to the following conditions: 
All paths should start from a starting surface(12) normal to the axis of 
the detector. 
All paths should start parallel to the axis of the detector with the same 
velocity into the detector. 
All paths should be determined for exactly the same time-of-flight. Use as 
reference time that time which is necessary for an ion flying on the axis 
from the starting surface(12) to the conversion surface(3). 
4. The endpoint of the axis path is on the middle of the conversion 
surface. The endpoints of the off-axis paths then describe the necessary 
form(20) of the conversion surface. This is shown enlarged in FIG. 2. 
5. Modify the form of the conversion surface in the design of the detector 
according to the previous step and continue with step 3. 
Changing the shape of the conversion surface changes the electrical field 
and with that also the fright time errors. For that reason the above 
procedure should be repeated until the flight time errors fall below some 
predetermined limit. 
It is also possible to specify the form of the conversion surface as a 
power series of finite order. This implies not taking over the exact form 
of the surface determined in step 5 but just approximating that surface as 
close as possible, and then continuing with that approximation in step 3. 
Instead of using the paths(11) determined in step 3 it is also possible to 
use paths with starting conditions corresponding to the actual operation 
of the time-of-flight mass-spectrometer, i.e. starting paths out of the 
ion source of the mass-spectrometer. In principle, this means that also 
time errors, as they arise in the ion source and the other parts of the 
time-of-flight mass-spectrometer, are also included into determining the 
curvature of the ion-electron conversion surface. In determining the end 
surface(20) one should take into consideration the fact that the space of 
initial variables has 6 coordinates in that case, 3 initial velocities and 
3 initial coordinates. Since the end surface(20) is described by 2 
parameters in B-dimensional space, it is necessary to approximate the end 
points of the paths(11) by the end surface(20) in such a way as to 
minimize the average distance of the endpoints of the paths(11) to the end 
surface(20). 
As an alternative method, one can first fix the shape of the detector 
electrodes, also fixing the shape of the ion-electron conversion surface. 
After fixing the shape the electrode voltages should be varied until the 
time errors fall below some predetermined limit. This method corresponds 
to claim 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows the most basic implementation of a detector in accordance with 
the invention. This implementation compensates the time errors on off-axis 
paths with a curved conversion surface(3). As in FIG. 1, the only ring 
electrode(1) has the potential of drift space. 
An implementation where the conversion surface(3) has a mount(2) such that 
it can he tilted corresponds also to claim 7. By tilting the mount is is 
possible to compensate within the detector some flight time errors of the 
ion source, the reflector and/or the drift space of the time-of-flight 
mass-spectrometer. 
FIG. 4 shows an implementation with additional ting electrodes(4) to adjust 
the electrical field of the postacceleration space. In this manner the 
curvature necessary for the conversion surface(3) at some fixed value of 
the postaccelerating voltage can be kept lower as in the implentation 
shown in FIG. 3. As an alternative it is possible to work with higher 
postaccelerating voltages at the same curvature of the ion-electron 
conversion surface(3). 
The additional ring electrodes(4) reduce flight time errors on off-axis 
paths by moving regions of high field curvature into places where the 
velocity of the ions is already higher. The potentials of the ring 
electrodes have values between the potential of the drift space and the 
potential of the ion-electron conversion surface(3). Instead of using two 
or more additional ting electrodes(4) it is also possible to use just one 
additional ring electrode. 
With increasing postaccelerating potential the flight time errors become 
larger. In addition to that, the ion paths are more strongly bent toward 
the ion optical axis. Both of these effects necessitate that the curvature 
of the ion-electron conversion surface increases with increasing 
postaccelerating potential. At some value of the postaccelerating 
potential, where the ion paths are so strongly bent toward the ion optical 
axis that they meet at the center of the conversion surface, it is no 
longer possible to compensate the flight time errors by curvature of the 
conversion surface. This becomes again possible at still higher 
postaccelerating potentials, when ion paths cross before hitting the 
conversion surface. 
If it is necessary that the detector has a large postaccelerating 
potential, it is advantageous, as shown in FIG. 5, to operate it 
corresponding to claim 8. This mode of operation allows any high 
postaccelerating potential at comparitively low curvature of the 
ion-electron conversion surface(3). This is done by convenient placement 
of the electrodes and adjustment of their voltages such that the ion 
paths(11) cross before the conversion surface. Since there is quite a 
number of electrode arrangements and voltages to produce an electrical 
field with the necessary properties, an explicit electrode construction is 
not shown here. 
FIG. 6 shows a detector construction according to claim 6. The electrons 
created at the curved ion-electron conversion surface(3) are drawn off to 
the side by some electrical field superposed over the postaccelerating 
field. The electron paths(15) are shown as dashed lines. 
The ion paths(11) are shown twofold in the middle part of the 
postaccelerating region. The reason is, that similar to FIG. 5, it is 
possible to effect crossing(11a) paths or paths that are for the most part 
parallel(11b) down to the ion-electron conversion surface(3). The 
electrodes for post-acceleration of the ions need not be rotationally 
symmetric. 
Since the field that draws out the electrons will break the rotational 
symmetry of the arrangement, the optimum curvature of the conversion 
surface may also not be rotationally symmetric. The detection of the 
electrons produced can be done by multichannel plate, scintillator or the 
like.