Abstract:
An electron multiplier with a source for spontaneously generating electrons is used as an electron source for an ionization source in a mass spectrometer or the like. The electron multiplier can be a microchannel plate, in which case it produces a wide electron beam. The microchannel plate can be acid-leached to provide a surface for spontaneous generation of electrons, or the first strike surface can be coated with an alkali-containing material. The electron source can be tuned by providing an electrode for rejecting electrons having too high an energy and a grid for rejecting electrons having too low an energy.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to an ionization source for mass spectrometry or the like and to an electron source for such an ionization source. 
     DESCRIPTION OF RELATED ART 
     Analytical instrumentation is playing an expanded role in everyday life. Expanding applications include quality control, drug discovery, and medical diagnostics. 
     Mass spectrometers are one of the most useful analytical instruments in use today. These devices provide compositional and structural information of unknown materials. The use of these instruments was originally limited to highly educated, specialized chemists known as mass spectroscopists. However, with the advent of low-cost computers with large memory capacity, a new generation of computer-controlled instruments, intended to be used by non-chemists, is now available. The use of on-board library spectra matching and identification has now enabled a whole new generation of non-technical professionals to benefit from the capabilities of these instruments. User-friendly instruments continue to expand the use of mass spectrometry into non-traditional market areas; indeed, a well-known restaurant chain now uses a mass spectrometer to check the quality of seafood before it is served. 
     Mass spectrometer instruments include three basic parts: the ionization source, the mass filter, and the detector. The ionization source converts neutral molecules to be identified into ions. The mass filter separates the unknown ions from other ions in accordance with mass. The detector detects the ions by their mass and provides an amplified output signal, typically proportional to the abundance of the material present. 
     More specifically, the function of the ionization source is to apply a net charge to the neutral molecule which is to be identified. Ionization of a molecule is accomplished by adding electrons to the molecule or more often by subtracting electrons from the molecule. There are four common ionization techniques: electron impact, chemical ionization, field ionization, and photo ionization. 
     The efficiency with which molecules are ionized in mass spectrometers is low, typically under 1% for electron impact ionization. Low ionization efficiency directly translates to low instrument sensitivity. Currently all instruments utilizing electron impact ionization rely on a thin wire filament (similar to that found in a light bulb) heated to a high temperature to produce electrons. These filaments produce a narrow stream of electrons which upon collision with a molecule may produce an ion. 
     FIG. 1 illustrates the typical filament-based ionization source employed in a current mass spectrometer. The narrow beam produced by the filament provided a relatively small area in which ions can be produced. 
     As shown in FIG. 1, in typical filament-based ionization source  100 , the material to be ionized is introduced via gaseous sample inlet  102  to ionization space  104 . Cathodic filament electron emitter  106 , supplied with filament heater potential  108  and electron accelerating potential  110 , emits a beam of electrons along electron trajectory  112  to electron discharge anode  114 . The beam of electrons ionizes the material to produce ion beam  116  which exits ionization space  104  through ion exit port  118 . Ion beam  116  passes through extracting lens  120 , focusing lens  122  and accelerating lens  124  to the analyzer. 
     Conventional filament-based ionization sources have many performance disadvantages. They have a short life, typically 90 days. Heat generated by the filament can affect the chemistry of the molecule being identified. Filaments are unstable and require a warm-up period. Typical sources provide a narrow beam for collisions with molecules to be ionized. 
     Conventional electron multiplier fabrication and processing techniques are designed to reduce or eliminate spontaneous electron emission, which is seen as noise and a reduction in the signal-to-noise ratio of the device. Most manufacturing processes are designed with priority given to producing smooth emissive surfaces with high dielectric integrity and freedom from physical imperfections which may be conducive to field emission. Variations in the standard manufacturing processes, which induce microfractures, sharp edges, or other surface imperfections, are considered undesirable in conventional electron multiplier manufacture. An example of a conventional electron multiplier is the microchannel plate (MCP) of U.S. Pat. No. 4,978,885, issued Dec. 18, 1990, to White and Laprade. The patent to White et al teaches the importance of minimizing ion feedback to provide an MCP with quiet operation. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an ionization source which overcomes the above-noted disadvantages. 
     A further object of the invention is to provide a cold ionization source. 
     A further object of the invention is to provide an ionization source having a large emission area. 
     A further object of the invention is to provide an ionization source having a high-density, uniform emission pattern. 
     A further object of the invention is to provide an ionization source which is durable and which will not burn out. 
     A further object of the invention is to provide a low-maintenance ionization source which uses cold ionization and therefore does not require frequent cleaning. 
     A further object of the invention is to provide an ionization source which does not require a warm-up/stabilization time. 
     A further object of the invention is to provide an ionization source with fine emission level control. 
     A further object of the invention is to provide an electron source which makes such an ionization source possible by not requiring a primary source to initiate emission and by not being limited to operation in the unstable ion feedback mode. 
     To achieve these and other objects, the present invention is directed to an electron source comprising: a generating portion for spontaneously generating electrons; and an electron multiplying portion, receiving the electrons spontaneously generated by the generating portion, for multiplying the electrons to produce an electron beam. 
     The present invention is further directed to a method of making an electron source, the method comprising the following steps: (a) forming an electron multiplier, and (b) treating the electron multiplier so that at least a portion of the electron multiplier spontaneously generates electrons. 
     The invention is further directed to an ionization source comprising: an electron source including electron generating and multiplying portions for spontaneously generating electrons and multiplying the electrons to produce an electron beam; an ionization space disposed to receive the electron beam from the electron source, the ionization space having (i) an inlet for receiving a material to be ionized so that the material to be ionized passes through the electron beam to produce ions and (ii) an exit port for allowing the ions to exit the ionization space; and anode means, disposed in the ionization space, for discharging electrons in the electron beam from the ionization space once the electrons in the electron beam have passed through the ionization space. 
     Electron multipliers can be processed in such a way as to spontaneously produce a stream of electrons when a high voltage is applied. The use of an electron multiplier (such as a microchannel plate, discrete dynode, or single-channel electron multiplier) as the source of electrons would provide a much larger area of high current density in which to produce ions. In this fashion, significant increases in ionization efficiency can be realized. 
     In addition to producing a large area electron beam with high current density, an electron multiplier based electron source does not require the warm-up period and provides a longer useful lifetime. 
     The variations in standard manufacturing processes described above can be capitalized upon to produce noisy electron multipliers which can be used to produce a stream of electrons for electron impact ionization sources. Process variations suitable for producing spontaneous emission electron multipliers can be used in the invention, as will be described in greater detail below. The noisy electron multiplier is in contrast to the quiet electron multiplier of White et al. 
     The electron source according to the present invention is self-starting and does not require an external source to initiate or control the emission. In addition, such an electron source operates both in and out of the ion feedback mode, as is required for all analytical instrumentation operation, thus providing fine emission control. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be set forth in detail with reference to the drawings, in which: 
     FIG. 1 shows a filament-based ionization source according to the prior art; 
     FIG. 2 shows a first embodiment of the ionization source according to the present invention; 
     FIG. 3 shows a second embodiment of the ionization source for ensuring monoergic electron energy according to the present invention; 
     FIG. 4 shows a flow chart of a process used to produce a microchannel plate electron source for the ionization source of FIG. 2 or FIG. 3; 
     FIG. 5 shows a portion of another microchannel plate electron source for the ionization source of FIG. 2 or FIG. 3; 
     FIG. 6 shows an ionization source having a single-channel electron multiplier tube; 
     FIG. 7 shows an ionization source having discrete dynodes; 
     FIG. 8 shows two microchannel plates in a chevron configuration; 
     FIG. 9 shows two microchannel plates in a Z stack configuration; 
     FIG. 10 shows emission current from a chevron in ion runaway, the chevron configuration having a 5 μm pore size; 
     FIG. 11 shows the emission current in amperes as a function of applied voltage in kV for a single MCP, a chevron configuration, and a Z stack configuration; 
     FIGS. 12 and 13 show the emission current in amperes as a function of the applied voltage in kV in greater detail for the chevron configuration and the Z stack configuration, respectively; 
     FIG. 14 shows the ion source emission current as a function of backfill argon chamber pressure; 
     FIG. 15 shows the percentage of initial emission current over time for no feedback and argon at 5×10 −5 torr; and 
     FIG. 16 shows the emission current as a function of applied voltage for a Z stack configuration and a pore size of 5 μm after 144 hours of operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An ionization source in accordance with a first embodiment of the invention is shown in FIG.  2 . In this embodiment example, a specially processed microchannel plate is positioned at the beginning of an ionization cavity to serve as an electron source. The manner of special processing will be described in detail below. 
     As shown in FIG. 2, in ionization source  200 , the material to be ionized is introduced via gaseous sample inlet  202  to ionization space  204 . Microchannel plate (MCP)  206  used as an electron source has input and output faces  206 A,  206 B respectively. Microchannel plate electron source  206  is supplied with emission current supply voltage  208  between input face  206 A and output face  206 B. As a result, microchannel plate  206  emits a wide beam of electrons  215  along electron trajectory  212  to electron discharge anode  214 . The beam  215  of electrons ionizes the material  201  to produce ion beam  216  which exits ionization space  204  through ion exit port  218 . Ion beam  216  passes through extracting lens  220 , focusing lens  222  and accelerating lens  224  to the analyzer (not shown). 
     Application of a high voltage to the input surface  206 A of microchannel plate  206  by emission current supply voltage  208  initiates the emission of electrons from output surface  206 B of microchannel plate  206 . Varying the voltage across microchannel plate  206  varies the emission current of beam  215 ; the emission current is roughly proportional to the applied voltage. Other than application of the voltage, no special start-up step is required. 
     A series of grids  217  located adjacent to output surface  206 B of MCP  206  and connected to electron energy supply  210  is used to establish the impact energy of the electrons (typically 70 volts) and thus to establish the exit electron energy from MCP  206 . As the large-area electron beam  215  traverses the ionization space  204 , impact with the ambient molecules will result in the production of positive ions. Increasing the pressure within ionization space  204 , increasing the emission current, or doing both will increase ion production. 
     The ionization space is constructed as a closed, conductive chamber bounded by the MCP and the anode. Voltage  208  is connected between input side  206 A of MCP  206  and ionization space  204 . Voltage  212  is connected between grids  217  and ionization space  204 . 
     A second embodiment of an ionization source according to the present invention has a tunable electron source shown in FIG.  3 . Ionization source  300  in FIG. 3 is in many ways similar to ionization source  200  of FIG.  2 . That is, in ionization source  300 , the material  301  to be ionized is introduced via gaseous sample inlet  302  to ionization space  304 . MCP  306 , supplied with emission current supply voltage  308  and electron energy supply  310  voltage, emits a wide beam of electrons  315  along electron trajectory  312  to electron discharge anode  314 . The beam of electrons ionizes the material to produce ion beam  316  which exits ionization space  304  through ion exit port  318 . Ion beam  316  passes through extracting lens  320 , focusing lens  322  and accelerating lens  324  to the analyzer (not shown). 
     However, ionization source  300  also includes high-energy electron discriminator  330  including voltage source  331  and electrode  332  and low-energy electron discriminator  334  including voltage source  335 , grid  336  and case  337 . Electrons leaving microchannel plate  306  travel past electrode  332  of high-energy electron discriminator  330 . Those electrons whose energy is below a first level, determined by the voltage applied to electrode  332 , follow trajectories  336   a  toward ionization space  304 . Those electrons whose energy is above the first level follow trajectories  336   b  away from ionization space  304 . Low-energy electrons following trajectories  336   a  are incident on the grid  336 , which has a potential applied to it by the low-level energy discriminator  334  to form an energy barrier. Those electrons having energies above a second level determined by the grid. potential pass through grid  336  into ionization space  304 . Those electrons having energies below the second level do not pass through the grid  336 . 
     Thus, electrons whose energies lie in a band between the first and second levels enter ionization space  304 , while all others are rejected. When the potentials applied by electron discriminators  330  and  334  are varied, the band changes. Thus, the electron source in ionization source  300  is tunable. 
     Continuous electron multipliers such as MCP  206  or  306  can be made by various methods to emit electrons spontaneously when a high voltage is applied. Variations in the manufacturing processes which cause the emissive surface of the first strike conversion area (the area where, in conventional microchannel plates, the electron entering the channel would be incident and where the first electron multiplication would take place) to field emit free electrons will effectively transform the electron multiplier into an electron source. 
     Single-channel electron multipliers and microchannel plates can be made to perform as spontaneous electron sources by subjecting the in-process device to an extended acid leaching step prior to the hydrogen reduction operation. As shown in the flow chart of FIG. 4, step  404  of acid leaching is performed between step  402  of forming the microchannel plate and step  406  of hydrogen reduction, with steps  402  and  406  being performed through any suitable techniques such as those known in the art. Spontaneous electron emission may be obtained by these devices with any acid leaching process such as hydrochloric, acetic, nitric, or sulfuric acid solution of more than about 1% (by volume) for an exposure time exceeding about one minute, at a temperature above about 20 degrees centigrade. 
     A second method for converting electron multipliers into spontaneous electron sources, as shown in FIG. 5, is achieved by coating the known first strike conversion surface  504  of a channel  502  in microchannel plate  500  with any alkali-containing compound  506  which will serve to lower the work function of the surface sufficiently to cause spontaneous electron emission. This approach will work for discrete dynode emitters as well as spontaneous electron emitters. 
     Of course, the methods of FIGS. 4 and 5 are illustrative rather than limiting. Any technique for causing an electron multiplier to act as a spontaneous electron source can be used in the present invention. 
     According to the invention, the ionization source or microchannel plate herein disclosed can be replaced by a single-channel electron multiplier, a discrete dynode multiplier, or other electron multiplier. Any electron multiplier having a source of spontaneously emitted electrons will suffice. Another possible variation is to use not a single MCP, but multiple MCP&#39;s in series. The MCP&#39;s can be arranged in a known chevron or Z configuration. Only the first MCP in the series is required to provide spontaneous electron emission. 
     Some such variations are shown in FIGS. 6-9. These figures show schematic diagrams; those skilled in the art will appreciate that working embodiments may look different. For example, electron sources are shown as discrete for the sake of clarity, although they may be formed integrally with other components in manners such as those described above. Also, electrical connections are not shown, since those skilled in the art will readily understand where such electrical connections should be introduced. 
     FIG. 6 shows an ionization source  600  including an electron source  602  and a single-channel electron multiplier tube  604 . An electron e leaving the source  600  enters the tube  602  and hits a wall, releasing more electrons. This process is repeated many times, so that a single electron e gives rise to multiple electrons e′ emerging from the far end of the tube  602  from the source  600 . 
     FIG. 7 shows an ionization source  700  including an electron source  702  and discrete dynodes  704 ,  706 ,  708 , . . . An electron e leaving the source  700  hits a dynode  704 , which releases more electrons. This process is repeated for each of dynodes  706 ,  708 , and so on, so that a single electron e gives rise to multiple electrons e′ emerging from the last dynode. While only three dynodes are shown, those skilled in the art will know how many to provide for proper operation in any application. 
     FIGS. 8 and 9 show two configurations of two MCP&#39;s in series. FIG. 8 shows MCP&#39;s  802  and  804  in a chevron configuration, while FIG. 9 shows MCP&#39;s  902  and  904  in a Z stack configuration. Of course, if more than two MCP&#39;s are used, they may be in a chevron, Z stack, or mixed configuration. 
     Experimental results will now be set forth with reference to FIGS. 10-16. 
     FIG. 10 shows ion runaway for a chevron configuration with a 5 μm pore size. The emission current caused by ion runaway begins at a chevron voltage of 2 kV and exceeds 10 μA at less than 2.3 kV. 
     FIG. 11 shows the emission current in amperes as a function of applied voltage in kV for a single MCP, a chevron configuration, and a Z stack configuration, each having a 5 μm pore size. In all three configurations, the emission current varies roughly linearly with the applied voltage. FIGS. 12 and 13 show the emission current in amperes as a function of the applied voltage in kV in greater detail for the chevron configuration and the Z stack configuration, respectively. 
     FIG. 14 shows the ion source emission current as a function of backfill argon chamber pressure. FIG. 15 shows the percentage of initial emission current over time for no feedback and argon at 5×10 −5  torr. The emission current peaks at one day of operation and asympototically approaches a value of less than 20% of initial emission current. After 144 hours of operation, the emission current as a function of applied voltage for a Z stack configuration and a pore size of 5 μm is shown in FIG.  16 . 
     While two embodiments of the invention have been set forth in detail, those skilled in the art will recognize that other embodiments can be realized within the scope of the invention. In addition to the modifications of the electron source described above, the electron source can be tuned in any suitable way. Also, the material to be ionized can be introduced directly into the multiplier. The MCP electron source can be used in settings other than the ionization source; for example, a cathode-ray tube can be made thin (around 2 mm) by using an MCP as large as the desired display as the electron gun. Modifications described separately can be combined.