Electric lamp and discharge devices – Photosensitive – Photomultiplier
Reexamination Certificate
1998-01-09
2001-05-29
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Photomultiplier
C313S532000, C313S534000, C313S535000, C313S536000
Reexamination Certificate
active
06239549
ABSTRACT:
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 vari
Burle Technologies, Inc.
Dykema Gossett PLLC
Patel Vip
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