Abstract
A metal-channel conversion dynode comprises: a wafer comprising a first face and a second face parallel to the first face and having a thickness less than 1000 m; and a plurality of channels passing through the wafer from the first face to the second face at an angle to a plane of the first face and a plane of the second face. In some embodiments, each inter-channel distance may be substantially the same as the wafer thickness. In some embodiments, the wafer is fabricated from tungsten. In some other embodiments, the wafer comprises a non-electrically conductive material that is fabricated by three-dimensional (3D) printing or other means and that is coated, on its faces and within its channels, with a metal or suitably conductive coating that produces secondary electrons upon impact by either positive or negative ions.
Claims
1. A metal-channel conversion dynode comprising: a wafer comprising a first face and a second faces parallel to the first face and having a thickness less than 1000 m; and a plurality of channels passing through the wafer from the first face to the second face at an angle to a plane of the first face and a plane of the second face.
2. A metal-channel conversion dynode as recited in claim 1 wherein each inter-channel distance, measured between centers of adjacent channels, in in the range of 150-1000 m.
3. A metal-channel conversion dynode as recited in claim 1 wherein the wafer comprises a non-conductive material that is coated, on its faces and within its channels, with a metal coating.
4. A metal-channel conversion dynode as recited in claim 1 wherein the metal wafer is fabricated from tungsten.
5. A metal-channel conversion dynode as recited in claim 1, wherein each inter-channel distance is substantially the same as the wafer thickness.
6. A metal-channel conversion dynode as recited in claim 1, wherein each channel comprises a square cross section at its intersection with each face.
7. A metal-channel conversion dynode as recited in claim 1, wherein the wafer is fabricated by three-dimensional (3D) printing by a 3D printer.
8. A metal-channel conversion dynode as recited in claim 1, wherein the wafer comprises either tungsten or molybdenum having chemical purity of 90-99%.
9. A metal-channel conversion dynode as recited in claim 1, wherein the wafer, including the channels passing therethrough, is fabricated by three-dimensional (3D) printing of metal.
10. A metal-channel conversion dynode as recited in claim 1, wherein there is no direct line of sight through the wafer along a sightline that is normal to the first and second faces.
11. A method of mass spectrometry comprising: causing positively-charged ions to be emitted from a mass analyzer; causing the positively-charged ions to impinge upon a metal-channel conversion dynode (MCD) by providing a negative electrical potential bias to the MCD relative to the mass analyzer; causing a first batch of secondary electrons emitted from the MCD to impinge upon a phosphor-coated optical component by providing a positive electrical potential bias to an electrode that is associated with the phosphor-coated optical component; measuring a signal derived from a first flux of photons that is emitted from the phosphor-coated optical component in response to the impingement of the first batch of secondary electrons; causing negatively-charged ions to be emitted from the mass analyzer; causing the negatively-charged ions to impinge upon the MCD by providing a positive electrical potential bias to the MCD relative to the mass analyzer; causing a second batch of secondary electrons emitted from the MCD to impinge upon the phosphor-coated optical component by providing a positive electrical potential bias to the electrode that is associated with the phosphor-coated optical component; and measuring a signal derived from a second flux of photons that is emitted from the phosphor-coated optical component in response to the impingement of the second batch of secondary electrons.
12. A method of fabricating a metal-channel conversion dynode comprising: fabricating a wafer of a non-electrically-conducting material having a thickness that is less than 1000 m, the wafer comprising: a first face and a second faces parallel to the first face; and a plurality of channels passing through the wafer from the first face to the second face at an angle to a plane of the first face and a plane of the second face; and coating the wafer, on its faces and within its channels, with a metal coating.
13. A method of fabricating a metal-channel conversion dynode as recited in claim 12, wherein the fabrication of the wafer is performed by three-dimensional (3D) printing with a 3D printer.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The above noted and various other aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings, not necessarily drawn to scale, in which:
[0025] FIG. 1 is a schematic illustration of a conventional triple-quadrupole mass spectrometer system;
[0026] FIG. 2A a schematic illustration of a known imaging detector system for a mass spectrometer that is based on the principle of photomultiplication; and
[0027] FIG. 2B is a schematic illustration of another known imaging detector system for a mass spectrometer that is based on the principle of photomultiplication;
[0028] FIG. 2C is a schematic illustration of another known imaging detector system that may be employed in conjunction with the present teachings;
[0029] FIGS. 3A and 3B are schematic cross-sectional depictions of channels in a venetian-blind style metal-channel conversion dynode and in a shadow mask, respectively;
[0030] FIG. 4 is a scanning electron microscope image of a metal-channel conversion dynode (MCD) in accordance with the present teaching and a schematic cross section of the channels within the MCD;
[0031] FIG. 5 is a pair of conventional voltage profiles as may be applied between a mass analyzer, a metal-channel conversion dynode (MCD) and a luminescent phosphor component of a mass spectrometer for the detection of positive ions emitted from the mass analyzer (uppermost profile) and the detection of negative ions emitted from the mass analyzer (lowermost profile);
[0032] FIG. 6 is a depiction of a voltage profile within a mass spectrometer that is suitable for detecting electrons emitted from a metal-channel conversion dynode in response to impacts from negatively-charge ions;
[0033] FIG. 7A is simulation of emission trajectories, in two directions, of secondary electrons emitted from a metal-channel conversion dynode (center) within a model system that also comprises a mass analyzer (left) and a biased phosphor element (right), where the emission is in response to impacts from positively-charged ions provided from the mass analyzer;
[0034] FIG. 7B is simulation of emission trajectories, in one direction, of secondary electrons emitted from the MCD of the model system of FIG. 7A, where the model system is modified by the incorporation of a mesh located at 1 mm distance from the MCD and is maintained at the same electrical potential as the MCD;
[0035] FIG. 8A is a plot of experimental results of signal intensity of detection of positively-charged ions using a voltage profile of the type depicted in the uppermost profile of FIG. 5 as a function of both ion energy and electron energy imparted to particles by the applied voltage profile;
[0036] FIG. 8B is a plot of experimental results of signal intensity of detection of negatively-charged ions using a voltage profile of the type depicted in the lowermost profile of FIG. 5 as a function of both ion energy and electron energy imparted to particles by the applied voltage profile; and
[0037] FIG. 9 is a schematic depiction of a cross section of a second metal-channel conversion dynode in accordance with the present teachings.
DETAILED DESCRIPTION
[0038] The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiments and examples shown but is to be accorded the widest possible scope in accordance with the features and principles shown and described. To fully appreciate the features of the present invention in greater detail, please refer to FIGS. 1, 2A-2C, 3A, 3B, 4-6, 7A-7B, 8A-8B and 9 in conjunction with the following description.
[0039] In the description of the invention herein, it is understood that a word appearing in the singular encompasses its plural counterpart, and a word appearing in the plural encompasses its singular counterpart, unless implicitly or explicitly understood or stated otherwise. Furthermore, it is understood that, for any given component or embodiment described herein, any of the possible candidates or alternatives listed for that component may generally be used individually or in combination with one another, unless implicitly or explicitly understood or stated otherwise. Moreover, it is to be appreciated that the figures, as shown herein, are not necessarily drawn to scale, wherein some of the elements may be drawn merely for clarity of the invention. Also, reference numerals may be repeated among the various figures to show corresponding or analogous elements. Additionally, it will be understood that any list of such candidates or alternatives is merely illustrative, not limiting, unless implicitly or explicitly understood or stated otherwise.
[0040] Unless otherwise defined, all other technical and scientific terms used herein have the meaning commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification, including definitions, will control. It will be appreciated that there is an implied about prior to the quantitative terms mentioned in the present description, such that slight and insubstantial deviations are within the scope of the present teachings. In this application, the use of the singular includes the plural unless specifically stated otherwise. Also, the use of comprise, comprises, comprising, contain, contains, containing, include, includes, and including are not intended to be limiting. As used herein, a or an also may refer to at least one or one or more. Also, the use of or is inclusive, such that the phrase A or B is true when A is true, B is true, or both A and B are true.
[0041] FIG. 4 includes, in its upper portion, a scanning electron microscope (SEM) image of a metal-channel conversion dynode (MCD) 16.3 in accordance with the present teachings. The lower portion of FIG. 4 is a schematic cross section of the channels within the MCD 16.3, which is drawn at approximately the same scale as the SEM image. The SEM image was obtained from a small portion of a wafer of tungsten of approximately 150 m thickness and comprising 4096 pores or channels 17 arranged in a 64 by 64 square grid array. Each pore or channel 17 comprises an approximately square cross section at each wafer surface having dimensions of approximately 150 m on each side. The walls between the pores/channels are approximately 30 m thick. The device depicted in FIG. 4 is here termed a micro-venetian-blind metal-channel conversion dynode (micro-venetian-blind MCD). The micro-venetian-blind MCD wafer 16.3 depicted in FIG. 4 was fabricated by laser ablation micro-machining. Alternatively, the device may be fabricated by wire electrical discharge machining (wire-EDM), by chemical etching, or by metallization of a channeled plate of a non-conductive material fabricated by three-dimensional (3D) printing or other means such as those employed to produce micro channel plates. Direct 3D printing of a metal channeled wafer is also possible for many different metals.
[0042] Since the pores or channels 17 micro-venetian-blind MCD 16.3 are all slanted at an angle of 45 degrees relative to the wafer surfaces, the relative dimensions of the apparatus assure that each pore or channel comprises a length is not appreciably longer than the length required to just prevent any direct line of sight through the wafer. As is shown in the schematic cross-section in the lower portion of FIG. 4, the wafer 16.3 comprises a planar first face 61 and a second opposite face 62 that is parallel to the first face. The distance between the two faces 61, 62 is the wafer thickness. The 45-degree slant of the channels is defined by slanted walls (e.g., slanted walls 63a, 63b, 63c) that are oriented at 45 degrees to the planar faces 61, 62 and that separate adjacent pores/channels along the x-dimension of the wafer. Each slanted wall comprises a first surface (e.g., first surfaces 65a, 65b, 65c) that is a component of the first face 61 and a second surface (e.g., second surfaces 67a, 67b, 67c) that is a component of the second face 62. The optimal geometry of the micro-venetian-blind MCD 16.3 is achieved when the first surface of each slanted wall is in alignment with a projection, normal to the planar faces, of the second surface of the respective succeeding wall. Such projections are indicated by dotted lines shown in selected channels. For example, the first surface 65a of slanted wall 63a aligns with the projection (see dotted lines) of the next slanted wall 67b in succession along the x-dimension. This geometry assures that, because of the absence of a line of sight through the wafer, all ions will impact some portion of the micro-venetian-blind MCD wafer while, at the same time, that the wafer has a minimum thickness so that there is minimal lateral shift of the image of ions' spatial distribution. With a constant slant angle of 45 degrees, then, if the inter-channel distance, D.sub.c, measured between centers of adjacent channels, is uniform across the wafer, then the above geometric relations are fulfilled for all pores/channels if the wafer thickness is equal to the inter-channel distance. Other angles are also possible if the line of sight requirement is met and the field penetration from the subsequent scintillator is sufficient to extract the secondary electrons. Indeed, some literature indicates a benefit from 60-degree impacts.
[0043] FIG. 5 is a schematic depiction of conventional voltage profiles as may be employed for the detection of ions emerging from an exit aperture of a mass analyzer 79 such as a quadrupole mass analyzer. In accordance with the mass spectrometer detector systems shown in FIGS. 2A-2B, the voltage profiles are applied between the mass analyzer 79, a metal-channel conversion diode 16 and an electrode 34 that is in contact with a phosphor material 22. The uppermost voltage profile of FIG. 5 is appropriate for the detection of positively charged ions using an imaging detection system 21, 31 comprising any of the MCD devices described herein or variants thereof, such as devices 16.1, 16.2 and 16.3. A negative electrical potential bias of the MCD relative to the mass analyzer 79, by an amount V.sub.c, causes the positive ions to be attracted to and to impact the MCD device. Secondary electrons generated by the impact energy then migrate out of the channels of the MCD 16 and towards the electrode 34 and phosphor 22 a result of the positive bias of electrode 34 by an amount, V.sub.e, relative to the MCD.
[0044] According to conventional understanding, the lowermost voltage profile of FIG. 5 would be expected to be appropriate for the detection of negatively charged ions using an imaging detection system that employs a metal-channel conversion dynode, such as systems 21 and 31. The MCD device is biased positively relative to the mass analyzer by an amount V.sub.a, in order to urge the negatively charged ions from the mass analyzer to the MCD device. Because MCD devices commonly emit protons and other positive ions in response to impacts from negatively charged ions, the phosphor electrode 34 is negatively biased by an amount, V.sub.p, relative to the MCD in anticipation of attracting the protons to the phosphor. Surprisingly, however, no secondary protons are detected under any circumstances when a voltage profile of the form shown in the lowermost profile of FIG. 5 is applied to a detection system using the micro-venetian-blind MCD 16.3 (FIG. 5). Instead, when using the micro-venetian-blind MCD 16.3, it is necessary to apply a voltage profile of the form shown in FIG. 6, in which the MCD is positively biased relative to the mass analyzer and the phosphor electrode is further positively biased relative to the MCD by an amount, V.sub.e. Such results imply that electrons are the predominant secondary particles that are emitted from the MCD 16.3 in response to an influx of negative ions.
[0045] Without being constrained to any particular theory or hypothesis of the mechanism of why no emissions of secondary protons are observed from the micro-venetian-blind MCD while receiving a flux of negatively charged ions, the inventors put forth the following hypothesis. Specifically, the inventors hypothesize that it is generally the case that both protons and electrons are generated upon initial impact with any MCD, but with protons usually being generated in excess of electrons. The inventors further hypothesize that, within conventional MCD devices, the field penetration from voltages applied on the detector side of the device is sufficient to extract both the protons and electrons from the conductor surface into the various channels. The inventors further hypothesize that, as a result of numerous particle collisions within the conventional channels, essentially all of the electrons and some proportion of the more-abundant protons are neutralized. Accordingly, in operation of conventional MCD devices, only a weak beam of protons is observed. Moreover, the inventors further hypothesize that, as a result of the miniaturized dimensions of the micro-venetian-blind MCD 16.3, the field penetration from voltages applied on the detector side of the device is insufficient to extract protons but is sufficient to extract electrons from the conductor surface. Elementary physics calculations by the inventors indicate that the field penetration is such that essentially all protons generated within a slanted channel are neutralized by collisions with the channel wall when the dynode thickness is less than a certain critical thickness, for example, 1 mm thickness. As a result, appreciable neutralization of the secondary electrons by secondary protons does not occur within the micro-venetian-blind MCD and the secondary electrons thus survive migration through the channels to be emitted and observed.
[0046] FIGS. 7A-7B are simulations, using the SIMION commercial simulation software package, of emission trajectories 19a, 19b of secondary electrons emitted from within a channel 17 of a metal-channel conversion dynode 16 within a model system that also comprises a source of ions (e.g., mass analyzer 79) and a biased phosphor element 34. These simulations assume that ions emitted from the mass analyzer are positively charged and, accordingly, that a voltage profile of the form of the uppermost profile of FIG. 5 is employed. The two simulations differ in that, in the simulation used to generate the results shown in FIG. 7B, the simulation includes the effects of an additional mesh electrode 14 that is disposed between the mass analyzer and the MCD at a distance of 1 mm from the MCD and that is maintained at the same potential as the MCD. From these simulations, it is concluded that, under the modeled experimental conditions and in the absence of the mesh electrode 14, as few as 25% of the secondary electrons created by incoming positive ions are extracted towards the phosphor by field penetration from the electrode 34. The remainder of the electrons are either trapped in the MCD or else exit the MCD channel in the direction of the source of positive ions. The inclusion of the mesh electrode 14 increases the quantity of electrons that arrive at the phosphor electrode 34 by a factor of four. The mesh electrode 14 is expected to give rise to a similar beneficial effect when used in conjunction with the voltage profile depicted in FIG. 6 to detect negative ions. This expectation has been confirmed by experiment.
[0047] FIG. 8A is a graphical plot 80 of experimental results of signal intensity of detection of singly-charged positive ions using a detector comprising the MCD 16.3 and a phosphor and using an applied voltage profile of the type depicted in the uppermost profile of FIG. 5. Curves 81, 82, 83, 84 and 85 correspond to induced ion energy values of 8 keV, 10 keV, 12 keV, 14 keV, and 16 keV, respectively. FIG. 8B is a similar graphical plot 90 that relates to detection of negative ions using a profile of the type depicted in FIG. 6. Curves 91, 92, 93, 94, 95 and 96 in FIG. 8B correspond to induced ion energy values of 1 keV, 2 keV, 4 keV, 6 keV, 8 keV and 10 keV, respectively. In general, it may be observed that, with regard to detection of either positive or negative ions, the observed signal increases with an increase in the absolute magnitude of either the induced ion energy or the induced secondary electron energy, which correspond, respectively, to the voltage differences, V.sub.a and V.sub.e (see FIGS. 5 and 6).
[0048] In any system in which high voltages are applied, the need to prevent corona discharge will lead to a practical maximum, V.sub.max, to how much voltage may be applied between any two electrodes. This maximum voltage value will generally be dependent on geometry and vacuum pressure. Since the electrical potential at the central axis of the mass analyzer may be assumed to be essentially at ground potential (0 Volts), then, with regard to the uppermost profile of FIG. 5 and the corresponding data of graphical plot 80 in FIG. 8A, it follows that 0(|V.sub.e||V.sub.a|)2V.sub.max. Similarly, with regard to the voltage profile of FIG. 6 and the corresponding data of graphical plot 90 in FIG. 8B, it follows that 0(|V.sub.e|+|V.sub.a|)V.sub.max. It may be observed from the plots in FIGS. 8A-8B that V.sub.max16 kV in the system used to obtain the data. Dotted line 99 in FIG. 8B connects all experimental points for which |V.sub.e|+|V.sub.a|=V.sub.max. The practical limitation on applied voltage gives rise to a practical optimum combination of V.sub.e and V.sub.a, depicted as point 97 in FIG. 8B, that gives rise to an optimum observed signal.
[0049] Although the micro-venetian-blind MCD device of the present teachings performs the essential first step of converting a flux of ions, either positive or negative, to a flux of electrons, amplification is required for electronic signal processing. This may be achieved by a simple stack of micro-venetian-blind MCD electron multiplier devices, but a better scheme is to incorporate the micro-venetian-blind MCD into a detection system that include receives the secondary electrons from the MCD converts that image to photons which may then be amplified by image-intensifier components as depicted in FIGS. 2A-2B. The spatial variation of flux of secondary electrons from the MCD reproduces the distribution of ions emerging from a mass analyzer and the spatial variation of photon flux reproduces the spatial variation of the secondary electron flux.
[0050] The spatial variation of photons, the flux of which is preferably amplified as discussed above with regard to FIGS. 2A-2B, may be detected as discussed in U.S. Pat. No. 9,524,855. The detector should be configured to record the spatial variation of photon flux. For example, the detection system may include a solid-state camera such as any well-known CMOS imager, a charge injection device (CID) camera or a charge-coupled device (CCD). Alternatively, one or more line-cameras and suitable cylindrical optics or an array of discrete photon detection means (e.g., silicon photomultipliers) may be employed.
[0051] FIG. 2C is a schematic illustration of a portion of an alternative detection system for digitizing a flux of secondary electrons emerging from a metal-channel conversion dynode. The detection system portion 41 depicted in FIG. 2C, details of which are taught in in U.S. Pat. No. 8,389,929, receives a flux of secondary electrons 43 from the MCD at a phosphor-coated optical component, e.g., a phosphor coated fiber optic plate 52. The flux of electrons is drawn to the phosphor-coated optical component under the influence of a positive electrical potential bias, relative to the MCD, that is applied to an electrode (not shown) that is associated with the phosphor-coated optical component. This arrangement converts the signal electrons to a plurality of resultant photons (denoted as p) that are proportional to the amount of received electrons. Generally, a fiber optic plate, as used here, comprises a bundle of a plurality of closely packed, mutually parallel, short lengths of fiber, wherein the coplanar input ends of the plurality of fibers comprise a first face of the plate and the coplanar output ends of the plurality of fibers comprise a second opposite face of the plate. The fiber-optic plate maintains a correspondence between the spatial variation of the secondary electron flux and the spatial variation of the flux of emerging photons. A subsequent photosensitive multichannel plate (MCP) 53 assembly then converts each incoming resultant photon p back into a photoelectron. Each photoelectron generates a cloud of secondary electrons 55 at the back of the photosensitive channel plate 53, which spreads and impacts as one arrangement.
[0052] The secondary electrons 55 are received at an array of detection anodes 44, such as, but not limited to, a two-dimensional array of resistive structures, a two-dimensional delay line wedge and strip design, as well as a commercial or custom delay-line anode readout. The anodes 44 are in a sealed vacuum enclosure 51 (as denoted by the dashed vertical rectangle). Each of the anodes 44 can be coupled to a respective independent electrical amplifier 45 and additional analog to digital (ADC) circuitry 46 as is known in the art.
[0053] The signals resultant from amplifier 45 and ADC 46 and/or charge integrators (not shown) can eventually be directed to a Field Programmable Gate Array (FPGA) 48 via, for example, a serial LVDS (low-voltage differential signaling) high-speed digital interface 47. An FPGA 48 is beneficial because of the capability of being a configurable co-processor to a computer processing means 50, as shown in FIG. 2C. The data processing means 50 (e.g., a computer, a PC, etc.), can be utilized with a Compute Unified Device Architecture (CUDA) parallel processing Graphics Processing Unit (GPU) subsystem.
[0054] Although the micro-venetian-blind metal-channel conversion dynode 16.3 of the present teachings has been described with 4096 channels, it is believed that lower resolution is practical and perhaps desirable in actual practice. If, for example, a detection system includes the system portion 41 depicted in FIG. 2C, a 1212 array is more reasonable, both from a cost and signal processing perspective. Such a 1212 MCD could be fabricated from 1 mm individual square devices. In the larger size, this may be approximated, as illustrated by the apparatus 110 shown in FIG. 9, by stacking a number of plates 111 with through holes 112 where each plate has a suitable pattern offset to achieve the 45-degree design or an approximation thereof. The individual holes 112 can be made by chemical etching or electroforming.
[0055] Improved apparatus and methods have been herein disclosed converting a flux of positive and/or negative ions into a flux of electrons which comprises an image of the spatial distribution of the original flux of ions. The discussion included in this application is intended to serve as a basic description. The present invention is not intended to be limited in scope by the specific embodiments described herein, which are intended as single illustrations of individual aspects of the invention, and functionally equivalent methods and components are within the scope of the invention.
[0056] Various other modifications of the invention, in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims. Any patents, patent applications, patent application publications or other literature mentioned herein are hereby incorporated by reference herein in their respective entirety as if fully set forth herein, except that, in the event of any conflict between the incorporated reference and the present specification, the language of the present specification will control.
