Radiant energy – Ion generation – Field ionization type
Reexamination Certificate
2002-09-16
2003-10-14
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Ion generation
Field ionization type
C250S424000, C250S42300F, C250S282000
Reexamination Certificate
active
06633041
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of high-frequency multipole mass spectrometry and more particularly to a miniaturized mass spectrometer using a silicon chip field emitter array as the source of electrons for impact ionization of chemical species.
The number of applications for Quadrupole Mass Spectrometers (“QMS”) continues to increase. QMS instruments are used in the analysis of the environment for contaminants, medical testing and the development of new pharmaceuticals, energy research and biochemical analysis. Some QMS instruments are complex and relatively expensive research-grade instruments for biomedical and biochemical applications such as deducing the structure of proteins or the sequencing of DNA. On the opposite end of the spectrum, small, simple and inexpensive QMS devices are used as routine detectors for gas chromatography. Other types of QMS spectrometers are used by government agencies, for example in backpack portable instruments for in-situ analysis of hazardous chemicals in the environment, in mobile battlefield laboratories to warn of impending chemical or biological attack, or in enormous machines for the separation of atomic isotopes.
The ever-broadening range of applications of QMS spectrometers places ever-increasing demands on the performance of these devices. Unfortunately, existing technology has not always met current needs. New applications tend to require more specific, reliable mass analysis and more sensitive detection of ions having large mass-to-charge ratios which the current inventory of instruments cannot provide. However, this large, installed base of mass spectrometers represents a large and considerable capital investment in equipment and personnel that cannot readily be abandoned. Thus there is a desire to upgrade and improve the capabilities of existing instruments to meet the new demands.
Brief History of the Prior Art
The quadrupole mass filter is roughly 40 years old and is today widely used in a broad range of vacuum based instrumentation. Applications include sensitive leak detection, residual gas analysis, thermal desorption mass spectroscopy, molecular beam analysis, and detection in liquid and gas chromatography. Traditionally, these instruments have been very large laboratory devices owning principally to their need for very clean, very high rate vacuum systems, for high energy ionization sources and associated ion beam handling equipment, for sensitive detectors, and for heavy and sophisticated plumbing systems necessary to construct and house these instruments.
Since their development, multipole and in particular the QMS spectrometers have gained considerable scientific and commercial importance in many diverse fields ranging from chemical analysis to the establishment of highly precise atomic time standards. U.S. Pat. No. 2,939,952 entitled “Apparatus for Separating Charged Particles of Different Specific Charges” to Wolfgang Paul, et al., first describes the development of these devices. Many of the basic electrode configurations and anticipated uses for quadrupole mass filters are as shown or predicted in the '952 patent. Today, most of the analytical mass spectrometers in use are of the quadrupole type. The proliferation and wide acceptance of QMS spectrometers can be attributed to their simplicity, reliability, and low cost compared to other types of mass spectrometers.
U.S. Pat. Ser. No. 5,464,975 provides a brief recitation regarding conventional prior art QMS systems. These systems are shown to consists of the following components (see FIG.
1
): a sample inlet
1
; an ion source
2
for converting the sample into charged species of certain mass-to-charge (m/z) ratios; a quadrupole mass filter
3
(also called a quadrupole mass analyzer) that preferentially passes one m/z ratio at a time; and a detector
4
to detect the abundance of the transmitted charged particles. By scanning the RF and DC voltages applied to the quadrupole mass filter, a mass spectrum can be generated, showing signal intensity, correlated to relative abundance (in arbitrary units), versus the m/z ratio (in Dalton units).
In most current mass spectrometers, the ionizer section comprises what is commonly known as a hot filament, using essentially vacuum tube technology, or a radioactive source for producing a stream of electrons or other high energy charged particles. It is these high energy species which serve to ionize the analyte stream thereby ionizing some portion of the material within the stream and which are subsequently separated in the quadrupole mass filter.
However, hot filaments and radioactive sources exhibit a number of disadvantages. Environmental and health and hygiene concerns limit the latter sources to essentially fixed laboratory facilities. Furthermore, high energy sources, especially those producing heavy particles, are intended to produce dissociation fragments of the parent molecule. This includes hot filament electrodes which exhibit a tendency to damage delicate molecules under analysis. Hot filaments also exhibit a property known as “outgassing” wherein the operation of the filament not only produces electrons but also “boils” off metal atoms comprising the filament itself or absorbed or adsorbed species such as hydrogen, carbon monoxide/dioxide, and water, etc. This outgassing degrades the system cleanliness and reduces the high vacuum integrity of the system and, in turn, requires the use of large, rapid, and very expensive vacuum pumps in order to maintain operational pressures <10
−9
Torr.
The instant application describes a modification to the QMS spectrometer which overcomes some of the shortcomings of prior art devices. The instant invention improves upon the performance of the QMS and suggests a route to miniaturization, portability, and instrument operation at higher pressures; features which are beyond the present state-of-the-art. The improvements embodied in the instant application apply equally well to the closely related monopole and multipole mass spectrometers. The improvement in QMS systems disclosed and described in the present work comprises the use of a Field Emitter Array (“FEA”) as the ionizing source for producing of electrons for impact ionization in a QMS. In particular, the FEA described herein is manufactured by means similar to those used for silicon integrated circuit (“IC”) fabrication. Briefly, FEAs consist of a large number, (typically hundreds to tens of thousands) of sharp microscopic tips, each one of which is in close proximity to an electrode called a gate. Modest voltages (generally <100 Volts) applied between the gates and the emission tips produce a high concentration of field lines on these tips. This, in turn, causes electron emission via tunneling through the work function barrier of the material comprising the tips into the vacuum.
The ionizer section of the instant invention, therefore, uses a cold cathode FEA in place of the typical hot filament electron gun source. This arrangement retains the advantages of high electron fluxes and fragmentation patterns to distinguish between species having the same mass (such as CO and N
2
) while avoiding deleterious effects of the hot filament source. Furthermore, miniaturization affords redundancy in the electron source beyond the customary two filaments used in conventional mass spectrometers.
Earlier attempts at finding new and different ionization sources have been addressed in U.S. Pat. Nos. 3,852,595 and 4,988,869, both to Aberth, and in U.S. Pat. Ser. No. 5,278,510 to Baptist.
The first of these references describes a high voltage electrode comprising an array of sharp projections on a conductive substrate. In this approach, Aberth suggests producing a high electric field by impressing an electrical potential of about 3600V across the substrate and a control grid spaced apart from and parallel to the plane of the array of projections. Ionization occurs when a gas or otherwise entrained sample is passed through the electric field. In the second approach, Aberth su
Evans Timothy P.
Nguyen Kiet T.
Sandia National Laboratories
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