GB2259403A

GB2259403A - Detector for a time of flight mass spectrometer

Info

Publication number: GB2259403A
Application number: GB9217584A
Authority: GB
Inventors: Armin Holle
Original assignee: Bruken Franzen Analytik GmbH
Current assignee: Bruker Daltonics GmbH and Co KG
Priority date: 1991-09-05
Filing date: 1992-08-19
Publication date: 1993-03-10
Also published as: DE4129791A1; GB9217584D0; DE4129791C2

Abstract

In a method and apparatus for detecting heavy molecule ions in a time of flight mass spectrometer, secondary ions having a defined mass are filtered out of a spectrum of secondary ions of different masses produced at a converter plate when heavy primary ions strike the converter plate and the filtered out secondary ions form measurement ions, which are the only ions detected by a detector. As shown the apparatus includes a converter plate 12 onto which heavy molecule ions 22 strike and are at least partially resolved into secondary ions 24 upon impact. A microchannel plate detector 18 is also provided for detecting the secondary ions. The apparatus includes a mass filter in the form of a mass selection gap 14 arranged in a deflection magnetic field 20 and through which only secondary ions having a desired mass can traverse to form measurement ions. A pair of substantially transparent accelerating grids 10, 16 may be disposed in the ion path either side of the mass filter. <IMAGE>

Description

Field of th!2 Invention

The present invention relates generally to a method and apparatus for detecting heavy molecule ions in a time of flight (TOF) mass spectrometer, wherein heavy primary ions formed in the TOF mass spectrometer are, at a minimum, partially converted into lighter secondary ions. The secondary ions are then detected with a microchannei detector or other suitable detector.

BACKGROUND OF THE INVENTION

Previously, the mass resolution of time of flight mass spectrometers, employed for measuring the time of flight of ions, was limited by the different times required to traverse an individual ion detector used for detecting the ions (time smear), as well as other effects of ion detector arrangements. To overcome this problem, extremely short microchannel plate detectors are generally used, wherein the microchannel plate detectors are entirely planar and have a detection-sensitive surface arranged perpendicular to the ion beam. Electrons are emitted when incident ions are detected on the surfaces of individual microchannels. These electrons are then amplified, or multiplied, in the individual microchannels in the same manner as in a traditional electron multiplier (secondary electron multiplier).

However, due to the extremely short construction of the microchannel plate detectors generally used (appro)dmately 0.5mm), the overall running time of the converted electrons is extremely short. Thus, the time smear is extremely short, and therefore, only extremely small differences in running time can result before the emitted electrons are detected on the surfaces of the individual microchannels. Thus, only peak widths of < 2.5ns can be achieved with such microchannel plate detectors in a time of flight mass spectrorneter, as discussed in K Waiter et al, Int. Journ. Mass Spec. Ion Procs., Vol. 71, pp. 309-313 (1986).

1 Further, the ability to convert the incident ions into emitted electrons, or conversion efficiency, of a detector arranged as set forth above is critically dependent on the speed of the incident ions. However, the conversion efficiency of such detectors is not dependent on the energy of the ions. Tlius, fewer electrons are generated as the mass of the ions striking the converter plate increases. This phenomenon is due to the fact that the energy of all ions in the time of flight mass spectrometer is constant. Thus, the speed of the ions decreases with increasing ion mass.

Therefore, efforts have been made to improve the detection efficiency of heavy ions. For example, one such method calls for accelerating the heavy ions to higher energies, namely up to several thousand electron volts, over a short section of the motion path of the ion beam directed onto the converter plate and immediately prior to reaching the detector. However, recent studies, such as B. Sprengly et al, Proceedings of the ASMS 1990, p. 162; and J. Martens et a[, Proceedings of the ASMS 1991, illustrate that heavy ions, such as ions having the mass 20,000u, are either not converted into electrons when incident on the surface of an ion detector, or are only minimally converted into electrons when incident on the converter plate, independent of the material of the converter plate. Thus, rather than being converted into electrons, such heavy ions produce even larger secondary ions in addition to H. However, as secondary ions are typically significantly lower in mass than the primary ions, the mass spectrum of the secondary ions is generally more displaced toward larger masses the larger the primary ion.

In another effort to improve the detection efficiency of heavy ions calls for accelerating the secondary ions as they proceed from the converter plate using a detector of the type described above as set forth in M. Karas et al, Int. J. Mass, Spec. Ion Procs., Vol. 92, pp. 231-242 (1989). In the Karas et & publication, the secondary ions generated at the converter plate are used for detecting heavy molecule primary ions in the time of 2 A 1 fight mass spectrometer. Continuing from the Converter plate, the secondary ions are, accelerated up to several thousand electron volts. Using the conventional procedure, accelerating the secondary ions as they proceed from the converter plate produces an extremely large time smear, such that the secondary ion masses reach the detector used at different times.

In yet another effort to improve the conventional method, matrix ions are brought onto a different flight path than heavy secondary ions by using a pulsed ion field. Such a method is disclosed, for example, in R.C. Beavis et al, Rapid Comm. Mass SpQc., Vol. 3, p. 233 (1989), wherein heavy primary ions are generated in the matrix-supported laser description in an attempt to avoid premature saturation of the detector which complicates or prevents the detection of secondary ions. By using laser desorption, far more small matrix ions, having a mass ranging up to several thousands u, than heavy secondary ions are produced. The matrix ions are brought onto a different flight path with the assistance of a pulse field so that they do not reach the detector. The resulting time smear and low detection probability for heavy molecule ions resulting from the use of this procedure, however, are not avoided.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method and apparatus for detecting heavy molecule ions in a time of flight mass spectrometer, with which large ions can be detected without a time smear and with a low detection probability for small ions.

The above object is inventively achieved in a method and apparatus for detecting heavy molecule ions, wherein secondary ions having a defined mass are filtered out of the spectrum of secondary ions of varying masses, and the resulting secondary 3 ions having the defined mass, or measurement ions, are the only ions detected by the detector.

Primary ions having an energy of at least 5 KeV are provided. The primary ions are caused to impact the converter plate and are at least partially separated into secondary ions. The converter plate is arranged substantially perpendicular relative to the flight direction of the primary ions.

A highly transparent secondary ion accelerating grid is provided. The accelerating grid is arranged substantially perpendicular to the flight direction of the primary ions. The accelerating grid is used for accelerating the secondary ions proceeding the converter plate. The primary ions pass through the accelerating grid prior to impacting the converter plate.

In accordance with the principles of the present invention, a secondary ion accelerating voltage ranging from 100 through 1000 volts can be applied between the converter plate and the secondary ion accelerating grid.

Further, a deflection magnetic field is provided for selectively filtering the measurement ions out of the secondary ion spectrum. The deflection magnetic field is arranged substantially perpendicular to the moving direction of the secondary ions. A diaphragm is arranged in the deflection magnetic field. The diaphragm is arranged perpendicular to the orbit of the secondary ions forced by the deflection magnetic field.

The diaphragm can include a mass selection gap, or any other suitable arrangement, adjusted to the desired measurement ion mass. The mass selection gap can be followed by the microchannel plate detector, or any other suitable means for detecting ions.

4 In accordance with the principles of the present invention, the deflection magnetic field can provide a magnetic field strength ranging approximately from 0.05 Tesia through 0.5 Tesla.

in one embodiment, the method of the invention can include the step of accelerating measurement ions before the measurement ions reach the microchannel plate detector, or other suitable detector. In such an embodiment, the apparatus of the present invention provides a measurement ion accelerating grid preceding the microchannel plate detector, or other suitable detector, for accelerating the measurement ions.

In another embodiment of the present invention, an accelerating voltage is provided. The accelerating voltage ranges from approximately 1 KV through 3 KV and is applied between the measurement ion accelerating grid and the microchannel plate detector, or other suitable detector.

In addition, the present invention contemplates providing an arrangement and method for which the mass 25u is selected for the measurement ions to be filtered out of the spectrum of secondary ions.

An apparatus for detecting heavy molecule ions in a time of flight mass spectrometer constructed in accordance with the principles of the present invention provides a mass filter arranged in the motion path of the secondary ions. The mass filter is arranged between the converter plate and the microchannel plate detector, or other suitable detector. The mass filter selects only secondary ions having a desired mass, such that secondary ions of a desired mass can penetrate the mass filter and proceed as measurement ions.

The present invention further contemplates providing a highly transparent secondary ion accelerating grid folloWing the converter plate along the moving direction of the secondary ions. The highly transparent measurement ion accelerating grid can also be arranged in the motion path of the measurement ions between the mass filter and the microchannel plate detector, or other suitable detector.

A deflection magnetic field is also be provided in an apparatus constructed in accordance with the principles of the present invention. The deflection magnetic field can be arranged substantially perpendicular with respect to the flight direction of the primary ions and of the secondary ions emitted from the converter plate. Further, a diaphragm having a mass selection gap, wherein the mass selection gap is adjustable to the desired measurement ion mass is provided. The diaphragm is arranged in the orbital path of the secondary ions forced by the deflection magnetic field.

Additionally, an impact surface of the converter plate for the primary ions can be disposed in the same plane as an impact surface of the microchannel plate detector, or other suitable detector, for the measurement ions.

Further, the secondary ion accelerating grid and the measurement ion accelerating grid can be arranged substantially parallel to the plane of the converter plate.

Finally, the mass selection gap can be arranged substantially perpendicular to the plane of the converter plate.

Thus, in accordance with the principles of the present invention, heavy molecule ions are detected with a high detection probability, and small molecule ions are detected with a low detection probability. Additionally, a time-delay-free detection of a defined secondary ion mass occurs after the conversion of the heavy primary ions on a 6 conversion dynode. Thus, only secondary ions having a defined mass, where the mass can vary as needed are selected as measurement ions from the secondary ions generated at the converter plate which have a spectrum of varying masses. As set forth in J. Martins et al, Proceedings of the ASMA 1991, negat,ive secondary ions having the mass 25u are preferred, as such secondary ions are produced in abundance. on the other hand, when only small primary ions strike a conversion dynode, such as a converter plate with a secondary ion accelerating grid, then mainly e and H are generated, whereas very few secondary ions having the mass 25u are generated, such that the detection probability of the small primary ions is low.

Other features and advantages of the method and apparatus of the present invention are described hereinbelow in the detailed description.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 illustrates a side sectional view of an exemplary embodiment constructed in accordance with the principles of the present invention.

DETAILED DESCRIPTION

As illustrated in Figure 1, an apparatus constructed in accordance with the principles of the present invention includes a secondary ion accelerating grid 10. A converter plate 12 and a deflection magnetic field 20 are also provided. The secondary ion accelerating grid 10 is disposed between the converter plate 12 and the deflection magnetic field 20. A mass selection gap 14 is provided, and arranged within the deflectIon magnetic field 20. A measurement ion accelerating grid 16 is arranged between a microchannel plate detector 18 and the deflection magnetic field 20. The defection magnetic field 20 can have a magnetic field strength of, for example, 0.1 Tesla. As illustrated in Figure 1, the deflection magnetic field 20 is applied substantially perpendicular to the moving direction of the primary ions 22. When the primary ions impact the

7 converter plate 12, secondary ions 24 are generated. A mass spectrum of the secondary ions 24 is produced when the secondary ions 24 are generated. From this mass spectrum, measurement ions 26 having a specific mass, such as 25u, are filtered out of the mass spectrum of the secondary ions 24 by the mass selection gap 14.

1 The conversion plate 12 can have a potential of, for example, -500 volts. An impact plate of the microchannel plate detector 18 can have a potential of, for example, 2 W. Further, both the secondary ion accelerating grid 10 and the measurement ion accelerating grid 16 are grounded.

In accordance with the method of the present invention, primary ions 22 proceed toward the apparatus. The primary ions 22 can, for example, have a mass of 20,00Ou and an energy of, for example, 20 KeV. The primary ions 22 traverse the deflection magnetic field 20 in a substantially straight line because the orbit radii, given a selected magnetic field strength of, at most, for example, 0.1 Tesla, amount to a few meters. The primary ions 22 traverse the highly transparent secondary ion accelerating grid 10 and impact the converter plate 12 with either a slight increase or decrease in energy, dependent on the polarity of the ion.

The secondary ions generated at the converter plate 12 are accelerated in a direction opposite the moving direction 22 of the primary ions, and can have an energy of, for example, 500 eV.

Based on the velocity and mass of the secondary ions 24, varying orbital paths are traversed by the secondary ions 24 in the deflection magnetic field 20, wherein the radii of the orbital paths of the secondary ions 24 have a magnitude of, for example, a few centimeters. A diaphragm including the mass selection gap 14 is arranged such that only secondary ions 24 having a mass m = 25u can traverse the gap of the mass

8 t selection gap 14. The mass selection gap 14 is opened to its maximum width for allowing the secondary ions 24 having a mass m = 25u to traverse the gap, while allowing the secondary ions 24 of varying energies to also traverse the mass selection gap 14.

By arranging an apparatus in accordance with the principles of the present invention, only measurement ions 26 having a mass m = 25u can traverse the mass selection gap 14. Thus, a time smear of the secondary ions is eliminated. The measurement ions 26 proceed along their orbital path, and a 1800 rotation is exactly executed, as illustrated in Figure 1. Thus, secondary ions that are parallel-offset at the beginning, and then proceed from the converter plate 12, cover a comparable path, and strike the parallel channel plate detector 18, impact the parallel channel plate detector 18 parallel-offset. Further, the rotational frequency of the secondary ions 24 in a deflection magnetic field 20, as known to one of ordinary skill in the art, is dependent on the energy, as illustrated in the following equation:

f = eE3/m, wherein f = rotational frequency, e = elementary charge, B = magnetic field strength, and m = ion mass.

The running time for the semi-circular path of the measurement ions 26, that is the path of the measurement ions 26 from the converter plate 12 to the impact surface of the microchannel plate detector 18, is dependent on the energy distribution of the secondary ions 24 or of the measurement ions 26 that have been selected from the secondary ions 24. The energy distribution of the secondary ions 24 or of the measurement ions 26 that have been selected from the secondary ions 24 can amount to, for example, a few eV.

After completing their path through the deflection magnetic field 20, the measurement ions 26 traverse the measurement ion accelerating grid 16. The

9 measurement ions 26 are accelerated by the measurement ion accelerating grid 16 to an energy of, for example, 2 W. The measurement ions 26 are then detected by the microchannel plate detector 18. In accordance with the principles of the present invention, it is also contemplated that any suitable detector can be used in place of the microchannel plate detector described herein.

In accordance with the principles of the method of the present invention, time smears are only produced in the accelerating and field-free flight distances outside of the deflection magnetic field 20. For example, assuming that the path from the converter plate 12 to the microchannel plate detector 18 is 1cm in length and that the initial energy distribution of the secondary ions 24 is approximate 5 eV, a spread in running time of only a few ns results wherein the primary ions 22 have a mass of 20,00ou and advance toward the apparatus with an energy of 20 KeV, the magnitude of the deflection magnetic field is, at most, 0.1 Tesla, the secondary ions 24 generated at the converter plate 12 accelerate in a direction opposite the moving direction of the primary ions 22 at an energy of 500 eV, the mass selection gap 14 is arranged such that only secondary ions 24 having a specific mass, m = 25u, can traverse the mass selection gap 14, and wherein the potential of the converter plate 12 is 500 volts and the microchannel plate detector 18 is at a potential of 2 W. Thus, by constructing an apparatus in accordance with the principles of the presenting invention and by applying the method steps of the present invention, time of flight mass spectrometry having a high detection probability for heavy molecule ions and low detection probability for small molecule ions can be achieved.

Although various minor modifications may be suggested by those versed in the art, it should be understood that 1 wish to embody within the scope of the patent granted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.

1

Claims

CLAIMS:

1. A method for detecting heavy molecule ions in a time of flight mass spectrometer, wherein the heavy primary ions formed in the time of flight mass spectrometer are at least partially converted into lighter secondary ions and a detector is provided for detecting ions, comprising the steps of:

filtering secondary ions having a defined mass from a spectrum of secondary ions of varying masses to form measurement ions; and detecting said measurement ions with said detector.

2. A method according to claim 1, further comprising the step of providing a converter plate disposed substantially perpendicular to a flight direction of said heavy primary ions, wherein said heavy primary ions strike said converter plate with an energy of at least 5 KeV and said heavy primary ions are at least partially converted to said secondary ions upon striking said converter plate.

3. A method according to claim 2, further comprising the step of providing a highly transparent secondary ion accelerating grid means disposed substantially perpendicular to the flight direction of said heavy primary ions for accelerating said secondary ions, wherein said heavy primary ions traverse said secondary ion accelerating grid means before striking said converter plate.

4. A method according to claim 3, further comprising the step of applying a secondary ion accelerating voltage of 100 volts through 1000 volts between said converter plate and said secondary ion accelerating grid.

5. A method according to claim 1, further comprising the steps of: providing a deflection magnetic field arranged perpendicular to a moving 11 1 direction of said secondary ions; and providing a diaphragm arranged in said deflection magnetic field, wherein said diaphragm is disposed substantially perpendicular to an orbital path of said secondary ions, said diaphragm including a mass selection gap adjusted for filtering said secondary ions of a defined mass from said secondary ion spectrum prior to the detection of said secondary ions by said detector.

6. A method according to claim 5, wherein said deflection magnetic field has a magnitude ranging from approximately 0.05 Testa through 0.5 Testa.

7. A method according to claim 1, further comprising the step of accelerating said measurement ions prior to the step of detecting said measurement ions.

8. A method according to claim 7, further comprising the step of providing a measurement ion accelerating grid means preceding said detector, for accelerating said measurement ions.

9. A method according to claim 8, further comprising the step of applying an accelerating voltage ranging from approximately 1 KV through 3 KV between said measurement ion accelerating grid means and said detector.

10. A method according to claim 1, wherein said defined mass is m = 25u.

11. An apparatus for detecting heavy molecule ions in a time of flight mass spectrometer comprising:

12 1 a converter plate onto which said heavy molecule ions strike and are at least partially resolved into lighter secondary ions upon impact with said converter plate; a detector; and mass filter means, arranged in a motion path of said secondary ions between said converter plate and said detector, for filtering all secondary ions from a spectrum of secondary ions, except those secondary ions having a defined mass, such that the secondary ions of said defined mass traverse said mass filter 1 means to form measurement ions.

12. An apparatus according to claim 11, wherein said converter plate is arranged substantially perpendicular to a flight direction of said primary ions.

13. An apparatus according to claim 11, further comprising a highly transparent secondary ion accelerating grid arranged between said converter plate and said mass filter means.

14. An apparatus according to claim 11, further comprising a deflection magnetic field arranged substantially perpendicular to a flight direction of said primary ions.

15. An apparatus according to claim 11, wherein said mass filter means further comprises a diaphragm including a mass selection gap, wherein said mass selection gap is adjustable to said defined mass of said secondary ions, and wherein said diaphragm is arranged in an orbital path of said secondary ions forced by said deflection magnetic field.

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16. An apparatus according to claim 15, wherein an impact surface of said converter plate and an impact surface of said detector lie substantially in the same plane.

17. An apparatus according to claim 11, further comprising a highly transparent measurement ion accelerating grid arranged between said mass filter means and said detector.

18. An apparatus according to claim 11, further comprising a secondary ion accelerating grid; and a measurement ion accelerating grid, wherein said secondary ion accelerating grid and said measurement ion accelerating grid are disposed substantially in the same plane, and wherein said plane is parallel to a plane of said converter plate, said secondary ion accelerating grid is disposed between said converter plate and said mass filter means in the proximity of said converter plate, and said measurement ion accelerating grid is disposed between said mass filter means and said detector in the proximity of said detector.

19. An apparatus according to claim 15, wherein said mass selection gap is disposed substantially perpendicular to said plane of said converter plate.

20. An apparatus for detecting heavy molecule ions in a time of flight mass spectrometer, said apparatus including a converter plate for at least partially converting heavy molecule ions striking said converter plate into secondary ions, comprising:

filtering means for filtering secondary ions having a defined mass from a spectrum of secondary ions formed at said converter plate; and detector means for detecting said measurement ions, wherein said filtering means is arranged between said converter plate and said detector means.

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21. A method f or detecting heavy molecule ions in a time of flight mass spectrometer substantially as hereinbefore described with reference to the accompanying drawings.

22. An apparatus for detecting heavy molecule ions in a time of flight mass spectrometer, substantially as hereinbefore described with reference to the accompanying drawings.

23. Any novel feature or combination of features described herein.

is