Bipolar time-of-flight detector, cartridge and detection method

ABSTRACT

A replaceable, electronically-isolated, MCP-based spectrometer detector cartridge with enhanced sensitivity is disclosed. A coating on the MCP that enhances the secondary electron emissivity characteristics of the MCP is selected from aluminum oxide (Al2O3), magnesium oxide (MgO), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof A mass detector is electro-optically isolated the from a charge collector with a method of detecting a particle including accelerating the particle with a voltage, converting the particle into a multiplicity of electrons and converting the multiplicity of electrons into a multiplicity of photons. The photons then are converted back into electrons which are summed into a charge pulse. A detector also is provided.

INFORMATION

DETAILED DESCRIPTION OF THE INVENTION

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is a replaceable, electronically-isolated, MCP-based spectrometer detector cartridge with enhanced sensitivity.

FIGS. 3 and 4 show a modular detector assembly assembled with a modified vacuum flange of a TOF spectrometer (not shown). FIG. 3 also shows a shield interposed between detector assembly and flange . An ionization source (not shown) directs charged or neutral particles, for example, electrons, ions and photons, toward an input end of detector assembly .

Detector assembly is adapted to be secured to a vacuum side of vacuum flange with a plurality of rods .

A plurality of connectors pass through flange . Connectors supply electrical energy to pogo pins (not shown) which contact elements (not shown) for creating electric fields in detector assembly for accelerating particles therein, as discussed below.

Shield is connected to detector assembly with threaded fasteners . Shield

shields connectors from electromagnetic interference from particles directed toward detector assembly during detection.

Referring to FIGS. 5-7, detector assembly includes a detector cartridge , a scintillator and a charge collector . Detector cartridge receives the ions which enter input end from an ionization source (not shown) and produces electrons at intervals that correspond to the respective masses of the ions, as described above. Scintillator receives output electrons from detector cartridge and produces approximately output photons for every electron absorbed. Collector receives and converts the output photons into up to 5×106 electrons and sums the electrons into a charge pulse. As discussed above, the timing of the pulses correspond to the masses of the ions, thereby aiding identification of an unknown composition.

Detector assembly includes a base , a cap and a collector mounting plate which cooperate to receive and support detector cartridge , scintillator and collector in a spaced relationship with.

Base has a stepped and tapered central opening for receiving cartridge . Base also has a stepped and tapered central opening for receiving collector . Collector mounting plate has threads which threadingly engage corresponding threads of cap , which facilitates assembling cartridge , scintillator and collector within detector assembly .

Base has a shoulder that receives and maintains cartridge in spaced relationship with respect to collector . Base has a second shoulder that receives scintillator . Base maintains scintillator in spaced relationship with respect to collector . A ring maintains scintillator against shoulder and imparts a spaced relationship between scintillator and cartridge .

Referring also to FIGS. 8-10, cartridge has an input through which ions enter cartridge from opening in cap , as shown in FIG. . Cartridge includes an insulated cartridge body having an interior chamber . Cartridge body has an interior shoulder which supports a conductive output plate . Output plate is generally circular and has an edge portion removed for providing clearance for an opening in cartridge body . An insulating centering ring , having a central opening , rests on output plate . Centering ring receives and centers an MCP , which rests on an inner annular edge of output plate . A conductive input plate sandwiches centering ring against output plate . An inner annular edge of input plate sandwiches MCP against inner annular edge . An insulated spacer rests on input plate .

A conductive grid or mesh rests on insulated spacer . Grid includes crossed wires (not shown) which define a grounded plane for MCP . A voltage between grid and the input of MCP defines a “post acceleration” potential which urges ions toward and into MCP .

A ring rests on grid . An insulating ring retainer threadingly engages with cartridge body and compresses ring , grid , spacer , input plate , MCP and output plate against shoulder , as shown in FIG. . Ring protects grid from damage which might occur if insulating ring retainer is threadingly advanced directly against grid .

As shown in FIG. 8, cartridge body has a first contact opening in registration with a contact surface of output plate . A contact member extending from input plate passes through a second contact opening of cartridge body . As shown in FIG. 5, pogo pin assemblies and respectively contact contact surface and contact member , producing a voltage across input plate and output plate , hence across MCP .

Referring also to FIG. 9, base of detector assembly has upstanding registration pins which mate with corresponding apertures in cartridge body for ensuring that the appropriate pogo pin assemblies , contact the appropriate contact surface or contact member . This ensures proper voltage polarity upon replacement of cartridge . Cartridge is easily replaceable, which reduces the downtime of dependent mass spectrometry equipment.

To provide a high post acceleration potential and safeguard mass spectrometry equipment from voltage surges, the invention employs scintillator to electro-optically isolate collector from upstream voltages. Scintillator converts electrons received from MCP into photons, on the order of 400 photons per electron The photons cross a neutral field to collector , which converts the photons into electrons which are summed into a charge pulse.

Referring again to FIG. 5, scintillator is constructed from either of specially-formulated plastics, known as Bicron and Bicron , manufactured by Bicron, Inc. These materials provide the previously unattainable bandwidth capability necessary for converting the electron clouds produced by MCP within the typical range of frequencies encountered during mass spectrometry of very massive ions. This bandwidth extends up to about 3 GHz.

Scintillator has an input working area defined by ring . Upstream of scintillator , MCP has an active area defined by the channel array. Working areas and generally are coextensive. Additionally, the voltage between MCP and the input of scintillator accelerates the electrons from MCP toward scintillator .

Referring to FIG. 7, pogo pin applies a voltage to an input side of scintillator which provides the uniform field for drawing electrons from MCP . The output of scintillator is grounded Thus, collector is electrically isolated from scintillator , preventing arcing or voltage surges from being transferred to expensive instrumentation coupled to detector assembly .

The input side of scintillator has a layer of aluminum, in the order of 1000 Å, deposited thereon. Layer also may be chrome. Metalized layer provides a field plane for attracting electrons to scintillator . Metalized layer also fosters converting electrons just under the surface thereof into photons.

Layer also functions as a mirror to reflect photons which may have a rearward or wayward trajectory toward collector . The reflective properties of layer approximately double electron-to-photon conversion capability of scintillator , thus making practical the use of scintillator for electro-optically isolating high post-acceleration voltages across detector assembly from collector , promoting high sensitivity to massive ions.

Referring again to FIG. 5, collector includes a photomultiplier which, responsive to the output photons of scintillator , generates on the order of 5×106 electrons for every photon that strike photomultiplier . Collector also includes a socket into which photomultiplier is received. Photomultiplier and socket are electrically connected with pins (not shown) extending from photomultiplier and received in electrical contacts (not shown) in socket in a known manner.

An exemplary photomultiplier is a Hamamatsu RU7400 photomultiplier tube, which; is a “fast” photomultiplier. “Fast” refers to the reaction time from when a photon strikes a dynode to when a resultant electron strikes an anode of the photomultiplier. For example, the RU7400 has a reaction time of approximately 3.2 ns FWHM. Faster reaction times improve the. dynamic range of a detector because the detector may identify individual ions, rather than groups of ions. Faster reaction times maybe possible by connecting one or more downstream dynodes with the anode.

Referring to FIG. 10A, the invention provides improved MCP sensitivity by depositing on the surface of MCP a coating . Coating also extends into each channel of MCP . Coating enhances the first strike conversion capability, or ability to convert ions into electrons, of MCP . An exemplary coating is magnesium oxide (MgO). Magnesium oxide has been found to provide superior secondary electron emissivity properties over other coatings, such as aluminum oxide. Coating also may be tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO) or diamond.

Referring to FIG. 11, in operation, detector assembly may be used to detect, for example, large negative ions. Ionization source S has multiple plates (not shown) across which a voltage repels only negative ions −i into the field free drift tube. A net +10 kV voltage exists across the gap between ionization source S and MCP , between ionization source output So, which is at ground, and MCP input voltage Pmi. Ions −i are attracted to MCP by the net positive voltage bias with respect to MCP . The voltage between ionization source S and MCP temporally separates negative ions −i by mass. Ions −i may be post-accelerated with a high voltage to increase overall ion detection efficiency.

A net positive potential, such as +1 kV, across MCP , i.e. between MCP input (Pmi=+10 kV) and MCP output (Pmo=+11 kV), accelerates electrons −e, converted from ions −i, as discussed above, through MCP . A net positive voltage, such as +2 kV, between MCP and scintillator , i.e. between MCP output (Pmo=+11 kV) and scintillator input (Psi+13 kV), accelerates electrons −e from MCP toward scintillator .

Scintillator converts electrons −e into photons P. Photons P are insensitive to electrical fields, therefore the voltage across scintillator may drop to ground. Photons P strike collector .

The photomultiplier (not shown in FIG. 11, but see FIG. 5) of collector converts photons P into electrons (not shown). A net positive voltage across collector , such as +600 kV, from collector input (Pco=600 kV) to the grounded output, urges electrons through collector . The electrons are summed into a charge pulse at the output C.

Referring to FIG. 12, detector assembly is bi-polar in that detector assembly may be operated to detect large positive ions as well as negative ions. Similar to the above, ionization source S directs only positive ions +i toward MCP . A net −10 kV voltage between ionization source S and MOP , i.e. between ionization source output So and MOP input voltage Pmi. Ions +i are attracted to MOP by the net negative voltage bias with respect to MOP .

A net positive potential, such as +1 kV,across MCP , between MOP input voltage Pmi (e.g. −10 kV) and MOP output voltage Pmo (e.g. −9 kV), likewise accelerates electrons −e through MOP .

Electrons e from MOP travel toward scintillator , driven by a net positive voltage, such as +3 kV, between MOP and scintillator , i.e. between MCP output (Pmo=9 kV) and scintillator input (Psi=6 kV).

Scintillator converts electrons −e into photons P. The output of scintillator is grounded.

Photomultiplier (not shown in FIG. 12, but see FIG. 5) in collector converts photons P into electrons (not shown), which are urged therethrough with a net +600 kV voltage and summed into a charge pulse at output C.

While the foregoing is considered to be exemplary of the invention, various changes and modifications of feature of the invention may be made without departing from the invention The appended claims cover such changes and modifications as fall within the true spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below in conjunction with the following drawings, throughout which similar reference characters denote corresponding features, wherein:

FIG. 1 is a perspective view, partially in section, of a multichannel plate;

FIG. 2 is a schematic view of a single channel of the multichannel plate of FIG. 1;

FIG. 3 is a side elevational view of a detector assembly configured according to principles of the invention assembled-with a vacuum flange of a mass spectrometer and an interposed shield;

FIG. 4 is an environmental perspective view of the embodiment of FIG. 3, without the interposed shield of FIG. 3;

FIG. 5 is a cross-sectional view, drawn along line V—V in FIG. 6, of the detector assembly of FIG. 3;

FIGS. 6 and 7 respectively are front and rear elevational views of the detector assembly of FIG. 3;

FIG. 8 is a cross-sectional view, drawn along line VIII—VIII in FIG. 9, of the detector cartridge of FIG. 5;

FIG. 9 is a front elevational view of the cartridge of FIG. 5;

FIG. 10 is an exploded, axial cross-sectional view of the cartridge of FIG. 5;

FIG. 10A is a fragmentary schematic view of a channel input having a coating, in accordance with the invention; and

FIGS. 11 and 12 are schematic views of alternative voltages across a mass spectrometer incorporating the detector assembly of FIG. .

CROSS REFERENCE TO RELATED APPLICATION

This Application incorporates and claims the benefit of U.S. Provisional Application Ser. No. 60/189,894, filed Mar. 16, 2000, by Kevin Owens et al., entitled

CLAIMS

1. Detector for a time-of-flight mass spectrometer comprising: an electron multiplier, for converting a charged particle into a multiplicity of electrons; a scintillator, for converting the multiplicity of electrons into a multiplicity of photons; and a charge collector disposed for receiving the multiplicity of photons and adapted for recovering said photons into a second multiplicity of electrons and integrating said second multiplicity of electrons into a charge pulse corresponding to the mass of the charged particle; whereby said charged collector is electro-optically isolated from said electron multiplier.

2. Detector of claim 1, wherein said charge collector comprises a photomultiplier for converting the multiplicity of protons into the second multiplicity of electrons.

3. Detector of claim 2, wherein said electron multiplier is adapted for summing the second multiplicity of electrons into the charge pulse.

4. Detector of claim 1, wherein said electron multiplier comprises a coating formed on a surface thereof, said coating being formed of a material selected from the group consisting of aluminum oxide (Al2O2), magnesium oxide (MgO), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof.

5. Detector of claim 1, wherein said electron multiplier comprises a microchannel plate.

6. Detector of claim 5 comprising a cartridge configured to receive said microchannel plate, said cartridge being readily removable from and installable in said detector.

7. Detector of claim 1, wherein said scintillator is configured to provide a frequency bandwidth which accommodates arrival times of the multiplicity of electrons.

8. Detector of claim 1, wherein said scintillator is constructed from “BICRON” 418 or “BICRON” 422b.

9. Detector of claim 1, further comprising a conductive coating on said scintillator configured to reflect photons generated therein.

10. Detector of claim 9, wherein the conductive coating on said scintillator is selected from the group consisting of aluminum, chrome and combinations thereof.

11. Method of detecting a charged particle with a time-of-flight mass spectrometer having a high portion and a detector, said method comprising the steps of: accelerating a charged particle with a voltage; converting the charged particle into a multiplicity of electrons converting the multiplicity of electrons into a multiplicity of photons; collecting the multiplicity of protons, thereby electro-optically isolating the detector from the high voltage portion of the time-of-flight mass spectrometer; converting the multiplicity of photons into a second multiplicity of electrons; and then integrating the second multiplicity of electrons into a charge pulse.

12. Method of claim 11, wherein the step of converting the charged particle is achieved by using a microchannel plate.

13. Method of claim 12, further comprising the step of enhancing secondary electron emissivity of the microchannel plate with a coating selected from aluminum oxide (Al2O2), magnesium oxide (MgO), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof.

14. Method of claim 13, wherein said converting the particle is achieved with a microchannel plate.

15. Method of claim 11, wherein the voltage ranges from −15 kV to +15 kV.

16. Method of claim 11, wherein said converting the photons is achieved with a scintillator.

17. Method of claim 16, wherein the scintillator is configured to provide a frequency bandwidth which accommodates arrival times of the multiplicity of electrons.

18. Method of claim 16, wherein the scintillator is constructed from BICRON 418 or BICRON 422b.

19. Method of claim 16, wherein the scintillator has a conductive coating thereon for reflecting photons generated therein.

20. Method of claim 16, wherein the scintillator has a conductive coating thereon selected from aluminum, chrome and combinations thereof.

21. Detector for a time-of-flight mass spectrometer comprising: an electron multiplier, for converting particles in to a multiplicity of first electrons; a scintillator, for converting the multiplicity of first electrons into a multiplicity of photons; and a photomultiplier for converting the multiplicity of photons into a second multiplicity of electrons, whereby said detector is electro-optically isolated from high voltage portion of the time-of-flight mass spectrometer.

22. Detector of claim 21, wherein said photomultiplier is adapted for summing the second multiplicity of electrons into the charge pulse.

23. Detector of claim 21, wherein said electron multiplier comprises a coating formed on a surface thereof, said coating being formed of a material selected from the group consisting of aluminum oxide (Al2O3), magnesium oxide (MgO), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof.

24. Detector of claim 21, wherein said electron multiplier comprises a microchannel plate.

25. Detector of claim 24 comprising a cartridge to receive said microchannel plate, said cartridge being readily removable from and installable in said detector.

26. Detector of claim 21, wherein said scintillator is configured to provide a frequency bandwidth which accommodates arrival times of the multiplicity of electrons.

27. Detector of claim 21, wherein said scintillator is constructed from “BICRON” 418 or “BICRON” 422b.

28. Detector of claim 21, further comprising a conductive coating on said scintillator configured to reflect photons generated therein.

29. Detector of claim 28, wherein the conductive coating on said scintillator is selected from the group consisting of aluminum, chrome and combinations thereof.

30. Detector for a time-of-flight mass spectrometer responsive to input particles, each having a corresponding mass, for producing output pulses representative of the respective masses of the particles, comprising: a biased input for differentially accelerating each input particle in accordance with its mass; a first electron multiplier, for converting the accelerated input particle into a corresponding multiplicity of first electrons; a scintillator, responsively coupled to the first electron multiplier for converting the multiplicity of first electrons into a multiplicity of corresponding photons; and a second electron multiplier responsively coupled to the scintillator for converting the multiplicity of photons into a corresponding second multiplicity of electrons, said second electron multiplier being electrically isolated from the scintillator.

31. Detector of claim 30, wherein said charge collector comprises a photomultiplier for converting the multiplicity of photons into the second multiplicity of electrons.

32. Detector of claim 31, wherein said photomultiplier is adapted for summing the second multiplicity of electrons into the charge pulse.

33. Detector of claim 30, wherein said electron multiplier comprises a coating formed on a surface thereof, said coating being formed of a material selected form the group consisting of aluminum oxide (Al2O2), magnesium oxide (MgO2), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium ton (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof.

34. Detector of claim 30, wherein said electron multiplier comprises a microchannel plate.

35. Detector of claim 34, comprising a cartridge configured to receive said microchannel plate, said cartridge being readily removable from and installable in said detector.

36. Detector of claim 30, wherein said scintillator is configured to provide a frequency bandwidth which accommodates arrival times of the multiplicity of electrons.

37. Detector of claim 30, wherein said scintillator is constructed from “BICRON” 418 or “BICRON” 422b.

38. Detector of claim 30, further comprising a conductive coating on said scintillator configured to reflect photons generated therein.

39. Detector of claim 38, wherein the conductive coating on said scintillator is selected from the group consisting of aluminum, chrome and combinations thereof.

40. Detector for a time-of-flight mass spectrometer responsive to input particles, each having a corresponding mass, for producing output pluses representative of the respective masses of the particles, comprising: a biased input for differently accelerating each input particle in accordance with its mass; a microchannel plate electron multiplier, for converting the accelerated input particle into a corresponding multiplicity of first electrons; a scintillator, responsively coupled to the microchannel plate electron multiplier for converting the multiplicity of first electrons into a multiplicity of corresponding photons; and a photomultiplier tube electron multiplier responsively coupled to the scintillator for converting the multiplicity of photons into a corresponding second multiplicity of electrons, said photomultiplier tube electron multiplier being electrically isolated from the scintillator.

41. Detector of claim 40, wherein said photomultiplier is adapted for summing the second multiplicity of electrons into the charge pulse.

42. Detector of claim 40, wherein said electron multiplier comprises a coating formed on a surface thereof, said coating being formed of a material selected from the group consisting of aluminum oxide (Al2O2), magnesium oxide (MgO), tin oxide (SnO2), quartz (SiO2), barium fluoride (BaF2), rubidium tin (Rb3Sn), beryllium oxide (BeO), diamond and combinations thereof.

43. Detector of claim 40, comprising a cartridge configured to receive said microchannel plate, said cartridge being readily removable from and installable in said detector.

44. Detector of claim 40, wherein said scintillator is configured to provide a frequency bandwidth which accommodates arrival times of the multiplicity of electrons.

45. Detector of claim 40, wherein said scintillator is constructed from “BICRON” 418 or “BICRON” 422b.

46. Detector of claim 40, further comprising a conductive coating on said scintillator configured to reflect photons generated therein.

47. Detector of claim 46, wherein the conductive coating on said scintillator is selected from the group consisting of aluminum, chrome and combinations thereof.

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