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 (Al 2 O 3 ), magnesium oxide (MgO), tin oxide (SnO 2 ), quartz (SiO 2 ), barium flouride (BaF 2 ), rubidium tin (Rb 3 Sn), berrylium 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.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a division of U.S. Nonprovisional Application No. 09/809,090, filed on Mar. 16, 2001, which claims the benefit of U.S. Provisional Application No. 60/189,894, filed Mar. 16, 2000, now U.S. Pat. No. 6,828,729. 

   BACKGROUND OF THE INVENTION 
   Conventional time-of-flight mass spectrometry (TOFMS) is a technique that uses electron impact (EI) ionization. EI ionization involves irradiating a gas phase molecule of the unknown composition with an electron beam, which displaces outer orbital electrons, thereby producing a net positive charge on the newly formed ion. 
   TOFMS has seen a resurgence due to the commercial development of two new ionization methods: electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). The availability of low cost pulsed extraction electronics, high speed digital oscilloscopes and ultra-high speed microchannel plate detectors have improved the mass resolution capability of the traditional TOFMS technique. 
   Mass spectrometers include three major components: (1) an ionization source; (2) a mass filter; and (3) a detector. The ionization source ionizes an unknown composition. The mass filter temporally separates the resultant ions so that lighter ions reach the detector before the heavier ions. The detector converts the ions into a charge pulse. The detector ascertains the arrival times of the charge pulses, which correspond to the masses of the ions. Identifying the masses of the ions enables identification of the unknown composition. 
   Typically, a TOF mass spectrometer also has a digitizer connected to the detector to process the signals. 
   In the MALDI technique, the analyte of interest is usually mixed in solution with a large excess of light absorbing matrix material. The sample mixture is placed on a mass spectrometer sample plate and illuminated with a pulse of light from a pulsed laser. The matrix material absorbs the laser light, the analyte molecules are desorbed from the sample surface and ionized by one of a number of ionization mechanisms. 
   In ESI, the analyte of interest is normally dissolved in an acidified solution. This solution is pumped out the end of a metallic capillary tube held at a high potential. This potential causes the evaporation of extremely small droplets that acquire a high positive charge. Through one of a number of mechanisms, these small droplets continue to evaporate until individual molecular ions are evaporated from the droplet surface into the gas phase. These ions then are extracted through a series of ion optics into the source region of the TOFMS. 
   The mass filter temporally separates ions by accelerating the ions with a bias voltage ranging up to ±30 kV. Since like charges repel, negative ions, for example, experience repulsive forces, thus tend to accelerate from, a negative potential toward a positive or less negative potential. A higher bias voltage will generate stronger repelling forces, thus greater ion acceleration. The repelling force accelerates lighter particles faster than heavier particles. Although smaller voltages foster better temporal separation, larger voltages allow for greater detection efficiency. 
   Detectors typically convert an ion into many electrons, forming an electron cloud which is more readily discernable. Three conventional types of detectors, or electron multipliers, generally have been used. The first type of electron multiplier is a single channel electron multiplier (SCEM). SCEMs typically are not used in modem TOFMS instruments because SCEMs provide limited dynamic range and temporal resolution, in the order of 20–30 nanoseconds to full width at half maximum (ns FWHM). 
   The second type of electron multiplier is a discrete dynode electron multiplier (DDEM). DDEMs exhibit good dynamic range, and are used in moderate and low resolution applications because of relatively poor pulse widths, in the order of 6–10 ns FWHM. 
   The third type of electron multiplier is a microchannel plate (MCP) electron multiplier. MCPs typically have limited dynamic range, in the order of 20 mHz/cm 2  of active area. However, MCPs provide the highest temporal resolution, in the order of 650 ps FWHM. 
   An ideal TOF electron multiplier should exhibit both high temporal resolution and high sensitivity to high-mass ions, as well as a disinclination to saturation. 
   As the present invention obtains both high temporal resolution and high sensitivity from an MCP-type electron multiplier, the following reviews the general operating characteristics of an MCP. 
     FIG. 1  shows an MCP  10 . MCP  10  typically is constructed from a fused array of drawn glass tubes filled with a solid, acid-etchable core. Each tube is drawn according to conventional fiber-optic techniques to form single fibers called mono-fibers. A number of these mono-fibers then are stacked in a hexagonal array called a multi. The entire assembly is drawn again to form multi-fibers. The multi-fibers then are stacked to form a boule or billet which is fused together at high temperature. The fused billet is sliced on a wafer saw to the required bias angle, edged to size, then ground and polished to an optical finish, defining a glass wafer  15 . Glass wafer  15  is chemically processed to remove the solid core material, leaving a honeycomb structure of millions of pores, also known as holes or channels,  20 , which extend at an angle  25  relative to the normal flight trajectory of an ion between the surfaces  30  and  32  of MCP  10 . 
   Referring also to  FIG. 2 , subsequent processing of the interior surface  35  of each channel  20  produces conductive and secondary electron emissive properties. These secondary electron emissive properties cause channel  20  to produce one or more electrons upon absorption or conversion of a particle, such as an ion, impacting surface  35 . As a result, each channel  20  functions like an SCEM, having a continuous dynode source which operates relatively independently of surrounding channels  20 . 
   Finally, a thin metal electrode  40 , typically constructed from Inconel or Nichrome, is vacuum deposited on the surfaces  30  and  32  of wafer  15 , electrically connecting all channels  20  in parallel. Electrodes  40  permit application of a voltage  45  across MCP  10 . 
   MCP  10  receives ions  50  accelerated thereto by an ion-separating voltage  55 . Ion  50  enters an input end  60  of channel  20  and strikes interior surface  35  at a point  62 . The impact on surface  35  causes the emission of at least one secondary electron  65 . Each secondary electron  65  is accelerated by the electrostatic field created by voltage  45  across channel  20  until electron  65  strikes another point (not shown) on interior surface  35 . Assuming secondary electrons  65  have accumulated enough energy from the electrostatic field, each impact releases more secondary electrons  70 . This process typically occurs ten to twenty times in channel  20 , depending upon the design and use thereof, resulting in a significant signal gain or cascade of output electrons  80 . For example, channel  20  may generate 50–500 electrons for each ion. 
   Gain impacts the sensitivity, or ability to detect an ion, of a spectrometer. A spectrometer with a high gain produces many electrons in an electron cloud corresponding to an ion, thus providing a larger target to detect. 
   To increase the gain of channel  20 , or produce a greater amount of electrons for every ion strike, channel  20  must exhibit enhanced secondary emissivity qualities or conversion efficiency. Enhancing the secondary emissivity qualities of channel  20  is a standing goal. 
   The gain of channel  20  also is a function of the length-to-diameter ratio (l/d) thereof. This allows for considerable reduction in both length and diameter which permits the fabrication of very small arrays of channels  20  in MCP  10 . 
   In conventional TOF mass spectrometers, electron clouds produced at the channel output are driven toward an anode or charge collector, such as a Faraday cup (not shown). The charge collector sums or integrates the electron charges into a charge pulse, which is analyzed by a digitizer. Because lighter ions accelerate faster than the heavier ions, the voltage pulses correspond to the masses of the respective ions. The aggregate of arrival times of the voltage pulses corresponds to the mass spectrum of the ions. The mass spectrum of the ions aids in discerning the composition of the unknown composition. 
   Detecting the masses of very massive ions requires a high “post acceleration” potential between the ionization source and the MCP. A high post acceleration potential permits sufficient high mass ion conversion efficiency to enable detection of massive ions. However, MCPs cannot withstand excessive voltages thereacross without risk of significant degradation. Accordingly, some MCP-based spectrometers “float” or electronically isolate the anode from the charge collector. To this end, the MCP output voltage is dropped to ground through a voltage divider. Unfortunately, this creates great potential for arcing or short circuiting between the output and the anode, the energy from which could damage or destroy sensitive and expensive spectrometry equipment. Thus, attaining superior temporal range with an MCP-based spectrometer which also has superior dynamic capabilities, or high sensitivity, may come at significant, unpredictable cost. 
   Another problem with MCP-based detectors is that, over time, MCPs wear and require replacement. Some mass spectrometers are constructed in a manner that does not permit field replacement of the MCPs. Thus, when an MCP requires replacement, the entire spectrometer had to be returned to the manufacturer for refurbishment. This is undesirable in terms of cost and out-of-service time for the instrument. 
   To overcome this inconvenience, U.S. Pat. No. 5,770,858 (&#39;858 patent) provides a cartridge containing MCPs which may be installed and uninstalled in the field. However, the charge collector of the &#39;858 cartridge is not electro-optically isolated from the high post acceleration potential of the MCP element therein, like the present cartridge. 
   Ideally, a TOF electron multiplier should be bipolar, or able to detect both negative and positive ions, which are common to chemical compositions. Thus, the TOF electron multiplier should accommodate positive and negative ion acceleration voltages. 
   What is needed is a replaceable, electronically-isolated, MCP-based spectrometer detector cartridge with enhanced sensitivity. 
   SUMMARY OF THE INVENTION 
   The invention overcomes the problems discussed above with a replaceable, electronically-isolated, MCP-based spectrometer detector cartridge with enhanced sensitivity. 
   The invention eliminates the potential for destruction of expensive spectrometry equipment from high-voltage power surges due to current source, vacuum or other failures by electro-optically isolating the charge collector from the high post-acceleration potential across the detector assembly. 
   The invention improves the uptime of a TOF mass spectrometry device by providing an easily replaceable, electro-optically isolated MCP cartridge. 
   The invention improves the sensitivity of an MCP-based spectroscope by providing a coating on the MCP that enhances the secondary electron emissivity characteristics of the MCP selected from magnesium oxide (MgO), tin oxide (SnO 2 ), quartz (SiO 2 ), barium flouride (BaF 2 ), rubidium tin (Rb 3 Sn), berrylium oxide (BeO), diamond and combinations thereof. 
   The invention electro-optically isolates the detector from a spectrometer 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 and summed into a charge pulse. 
   The invention also electro-optically isolates the detector from a spectrometer with an arrangement including an electron multiplier, for converting a particle into a multiplicity of electrons, and a scintillator, for converting the multiplicity of electrons into a multiplicity of photons. 
   Other features and advantages of the invention will become apparent upon reference to the following description and drawings. 

   
     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. 3 . 
   

   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  100  assembled with a modified vacuum flange  200  of a TOF spectrometer (not shown).  FIG. 3  also shows a shield  103  interposed between detector assembly  100  and flange  200 . An ionization source (not shown) directs charged or neutral particles, for example, electrons, ions and photons, toward an input end  105  of detector assembly  100 . 
   Detector assembly  100  is adapted to be secured to a vacuum side  210  of vacuum flange  200  with a plurality of rods  215 . 
   A plurality of connectors  300  pass through flange  200 . Connectors  300  supply electrical energy to pogo pins (not shown) which contact elements (not shown) for creating electric fields in detector assembly  100  for accelerating particles therein, as discussed below. 
   Shield  103  is connected to detector assembly  100  with threaded fasteners  107 . Shield  103  shields connectors  300  from electromagnetic interference from particles directed toward detector assembly  100  during detection. 
   Referring to  FIGS. 5–7 , detector assembly  100  includes a detector cartridge  700 , a scintillator  800  and a charge collector  900 . Detector cartridge  700  receives the ions which enter input end  105  from an ionization source (not shown) and produces electrons at intervals that correspond to the respective masses of the ions, as described above. Scintillator  800  receives output electrons from detector cartridge  700  and produces approximately 400 output photons for every electron absorbed. Collector  900  receives and converts the output photons into up to 5×10 6  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  100  includes a base  110 , a cap  115  and a collector mounting plate  120  which cooperate to receive and support detector cartridge  700 , scintillator  800  and collector  900  in a spaced relationship with. 
   Base  110  has a stepped and tapered central opening  112  for receiving cartridge  700 . Base  110  also has a stepped and tapered central opening  125  for receiving collector  900 . Collector mounting plate  120  has threads  122  which threadingly engage corresponding threads  124  of cap  115 , which facilitates assembling cartridge  700 , scintillator  800  and collector  900  within detector assembly  100 . 
   Base  110  has a shoulder  135  that receives and maintains cartridge  700  in spaced relationship with respect to collector  900 . Base  110  has a second shoulder  140  that receives scintillator  800 . Base  110  maintains scintillator  800  in spaced relationship with respect to collector  900 . A ring  145  maintains scintillator  800  against shoulder  140  and imparts a spaced relationship between scintillator  800  and cartridge  700 . 
   Referring also to  FIGS. 8–10 , cartridge  700  has an input  705  through which ions enter cartridge  700  from opening  130  in cap  115 , as shown in  FIG. 5 . Cartridge  700  includes an insulated cartridge body  710  having an interior chamber  715 . Cartridge body  710  has an interior shoulder  720  which supports a conductive output plate  725 . Output plate  725  is generally circular and has an edge portion  765  removed for providing clearance for an opening  767  in cartridge body  710 . An insulating centering ring  730 , having a central opening  735 , rests on output plate  725 . Centering ring  730  receives and centers an MCP  740 , which rests on an inner annular edge  745  of output plate  725 . A conductive input plate  750  sandwiches centering ring  730  against output plate  725 . An inner annular edge  755  of input plate  750  sandwiches MCP  740  against inner annular edge  745 . An insulated spacer  775  rests on input plate  750 . 
   A conductive grid or mesh  780  rests on insulated spacer  775 . Grid  780  includes crossed wires (not shown) which define a grounded plane for MCP  740 . A voltage between grid  780  and the input of MCP  740  defines a “post acceleration” potential which urges ions toward and into MCP  740 . 
   A ring  785  rests on grid  780 . An insulating ring retainer  790  threadingly engages with cartridge body  710  and compresses ring  785 , grid  780 , spacer  775 , input plate  750 , MCP  740  and output plate  725  against shoulder  720 , as shown in  FIG. 7 . Ring  785  protects grid  780  from damage which might occur if insulating ring retainer  790  is threadingly advanced directly against grid  780 . 
   As shown in  FIG. 8 , cartridge body  710  has a first contact opening  712  in registration with a contact surface  727  of output plate  725 . A contact member  760  extending from input plate  750  passes through a second contact opening  770  of cartridge body  710 . As shown in  FIG. 5 , pogo pin assemblies  150  and  155  respectively contact contact surface  727  and contact member  760 , producing a voltage across input plate  750  and output plate  725 , hence across MCP  740 . 
   Referring also to  FIG. 9 , base  110  of detector assembly  100  has upstanding registration pins  160  which mate with corresponding apertures  716  in cartridge body  710  for ensuring that the appropriate pogo pin assemblies  150 ,  155  contact the appropriate contact surface  727  or contact member  760 . This ensures proper voltage polarity upon replacement of cartridge  700 . Cartridge  700  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  800  to electro-optically isolate collector  900  from upstream voltages. Scintillator  800  converts electrons received from MCP  740  into photons, on the order of 400 photons per electron. The photons cross a neutral field to collector  900 , which converts the photons into electrons which are summed into a charge pulse. 
   Referring again to  FIG. 5 , scintillator  800  is constructed from either of specially-formulated plastics, known as Bicron 418 and Bicron 422b, manufactured by Bicron, Inc. These materials provide the previously unattainable bandwidth capability necessary for converting the electron clouds produced by MCP  740  within the typical range of frequencies encountered during mass spectrometry of very massive ions. This bandwidth extends up to about 3 GHz. 
   Scintillator  800  has an input working area  810  defined by ring  145 . Upstream of scintillator  800 , MCP  740  has an active area  746  defined by the channel array. Working areas  746  and  810  generally are coextensive. Additionally, the voltage between MCP  740  and the input of scintillator  800  accelerates the electrons from MCP  740  toward scintillator  800 . 
   Referring to  FIG. 7 , pogo pin  165  applies a voltage to an input side of scintillator  800  which provides the uniform field for drawing electrons from MCP  740 . The output of scintillator  800  is grounded. Thus, collector  900  is electrically isolated from scintillator  800 , preventing arcing or voltage surges from being transferred to expensive instrumentation coupled to detector assembly  100 . 
   The input side of scintillator  800  has a layer  805  of aluminum, in the order of 1000 Å, deposited thereon. Layer  805  also may be chrome. Metalized layer  805  provides a field plane for attracting electrons to scintillator  800 . Metalized layer  805  also fosters converting electrons just under the surface thereof into photons. 
   Layer  805  also functions as a mirror to reflect photons which may have a rearward or wayward trajectory toward collector  900 . The reflective properties of layer  805  approximately double electron-to-photon conversion capability of scintillator  800 , thus making practical the use of scintillator  800  for electro-optically isolating high post-acceleration voltages across detector assembly  100  from collector  900 , promoting high sensitivity to massive ions. 
   Referring again to  FIG. 5 , collector  900  includes a photomultiplier  905  which, responsive to the output photons of scintillator  800 , generates on the order of 5×10 6  electrons for every photon that strikes photomultiplier  905 . Collector  900  also includes a socket  910  into which photomultiplier is received. Photomultiplier  905  and socket  910  are electrically connected with pins (not shown) extending from photomultiplier  905  and received in electrical contacts (not shown) in socket  910  in a known manner. 
   An exemplary photomultiplier  905  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 may be 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  744  of MCP  740  a coating  742 . Coating  742  also extends into each channel  20  of MCP  740 . Coating  742  enhances the first strike conversion capability, or ability to convert ions into electrons, of MCP  740 . An exemplary coating  742  is magnesium oxide (MgO). Magnesium oxide has been found to provide superior secondary electron emissivity properties over other coatings, such as aluminum oxide. Coating  742  also may be tin oxide (SnO 2 ), quartz (SiO 2 ), barium flouride (BaF 2 ), rubidium tin (Rb 3 Sn), berrylium oxide (BeO) or diamond. 
   Referring to  FIG. 11 , in operation, detector assembly  100  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  740 , between ionization source output S o , which is at ground, and MCP input voltage P mi . Ions −i are attracted to MCP  740  by the net positive voltage bias with respect to MCP  740 . The voltage between ionization source S and MCP  740  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  740 , i.e. between MCP input (P mi =+10 kV) and MCP output (P mo =+11 kV), accelerates electrons −e, converted from ions −i, as discussed above, through MCP  740 . A net positive voltage, such as +2 kV, between MCP  740  and scintillator  800 , i.e. between MCP output (P mo =+11 kV) and scintillator input (P si =+13 kV), accelerates electrons −e from MCP  740  toward scintillator  800 . 
   Scintillator  800  converts electrons −e into photons P. Photons P are insensitive to electrical fields, therefore the voltage across scintillator  800  may drop to ground. Photons P strike collector  900 . 
   The photomultiplier (not shown in  FIG. 11 , but see  FIG. 5 ) of collector  900  converts photons P into electrons (not shown). A net positive voltage across collector  900 , such as +600 kV, from collector input (P co =−600 kV) to the grounded output, urges electrons through collector  900 . The electrons are summed into a charge pulse at the output C. 
   Referring to  FIG. 12 , detector assembly  100  is bi-polar in that detector assembly  100  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  740 . A net −10 kV voltage between ionization source S and MCP  740 , i.e. between ionization source output S o  and MCP input voltage P mi . Ions +i are attracted to MCP  740  by the net negative voltage bias with respect to MCP  740 . 
   A net positive potential, such as +1 kV, across MCP  740 , between MCP input voltage P mi  (e.g. −10 kV) and MCP output voltage P mo  (e.g. −9 kV), likewise accelerates electrons −e through MCP  740 . 
   Electrons −e from MCP  740  travel toward scintillator  800 , driven by a net positive voltage, such as +3 kV, between MCP  740  and scintillator  800 , i.e. between MCP output (P mo =9 kV) and scintillator input (P si =6 kV). 
   Scintillator  800  converts electrons −e into photons P. The output of scintillator  800  is grounded. 
   Photomultiplier (not shown in  FIG. 12 , but see  FIG. 5 ) in collector  900  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.