Abstract:
An ion trap for mass spectrometric chemical analysis of ions is delineated. The ion trap includes a central electrode having an aperture; a pair of insulators, each having an aperture; a pair of end cap electrodes, each having an aperture; a first electronic signal source coupled to the central electrode; a second electronic signal source coupled to the end cap electrodes. The central electrode, insulators, and end cap electrodes are united in a sandwich construction where their respective apertures are coaxially aligned and symmetric about an axis to form a partially enclosed cavity having an effective radius r 0  and an effective length 2z 0 , wherein r 0  and/or z 0  are less than 1.0 mm, and a ratio z 0 /r 0  is greater than 0.83.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under contract DE-AC05-96OR22464, awarded by the United States Department of Energy to Lockheed Martin Energy Research Corporation, and the United States Government has certain rights in this invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to mass spectrometers, and more particularly to a submillimeter ion trap for mass spectrometric chemical analysis. 
     2. Description of the Related Art 
     Microfabricated devices for liquid-phase analysis have attracted much interest because of their ability to handle small quantities of sample and reagents, measurement speed and reproducibility, and the possibility of integration of several analytical operations on a monolithic substrate. Although the application of microfabricated devices to vapor-phase analysis was first demonstrated 20 years ago, further application of these devices has not been prolific due primarily to poor performance because of mass transfer issues. However, some low pressure analytical techniques, such as mass spectrometry, should be possible with microfabricated instrumentation. Recent reports of microfabricated electrospray ion sources for mass spectrometry make the possibility of miniature ion trap spectrometers especially attractive. 
     Ion traps of millimeter size and smaller have been used for storage and isolation of ions for optical spectroscopy, though not for mass spectrometry. The principal requirement for ion trap geometry is the presence of a quadrupole component of the radio frequency (RF) electric field. Conventional ion trap electrode constructions include hyperbolic electrodes, a sandwich of planar electrodes, and a single ring electrode. For more information concerning ion trap mass spectrometry, the three-volume treatise entitled: “Practical Aspects of Ion Trap Mass Spectrometry” by Raymond E. March et al. may be considered, and is incorporated herein by reference. 
     The smallest known quadrupole ion trap that has been evaluated for mass analysis or for isolation of ions of a narrow mass range was a hyperbolic trap with an r 0  value of 2.5 mm, as reported by R. E. Kaiser et al. in  Int. J. of Mass Spectrometry Ion Processes  106, 79 (1997). One problem with this and other small-scale ion traps used in mass spectrometry is their limited spectral resolution. For instance, existing small-scale ion traps typically do not provide useful mass spectral resolution below 1.0-2.0 AMUs (atomic mass units). Moreover, there is a demand for even smaller ion traps, (i.e., submillimeter with r 0  and/or zvalues less than 1.0 mm), for use in mass spectrometry, though ion traps of this size exacerbate the present limitations in mass spectral resolution. 
     Thus, there was a need for a submillimeter ion trap with improved spectral resolution in performing mass spectrometry. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a submillimeter ion trap for mass spectrometric chemical analysis. In the preferred embodiment, the ion trap is a submillimeter trap having a cavity with: 1) an effective length 2z 0  with z 0  less than 1.0 mm; 2) an effective radius r 0  less than 1.0 mm; and 3) a z 0 /r 0  ratio greater than 0.83. Testing demonstrates that a z 0 /r 0  ratio in this range improves mass spectral resolution from a prior limit of approximately 1.0-2.0 AMUs, down to 0.2 AMUs, the result of which is a smaller ion trap with improved mass spectral resolution. Employing smaller ion traps without sacrificing mass spectral resolution opens a wide variety of new applications for mass spectrometric chemical analysis. 
     The ion trap comprises: a central electrode having an aperture; a pair of insulators, each having an aperture; a pair of end cap electrodes, each having an aperture; a first electronic signal source coupled to the central electrode; and a second electronic signal source coupled to the end cap electrodes. In the preferred embodiment, the central electrode, insulators, and end cap electrodes are united in a sandwich construction where their respective apertures are coaxially aligned and symmetric about an axis to form a partially enclosed cavity having an effective radius r 0  and an effective length 2z 0 . Moreover, r 0  and/or z 0  are less than 1.0 mm, and the ratio z 0 /r 0  is greater than 0.83. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There are presently shown in the drawings embodiments which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown, wherein: 
     FIG. 1 is an exploded perspective view of an ion trap in accordance with the present invention. 
     FIG. 2 is system view employing the ion trap of FIG. 1 to perform mass spectrometric chemical analysis. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an ion trap  10  manufactured in accordance with the present invention. While ion trap  10  is shown as a cylindrical-type-geometry trap, the present invention may be incorporated into other known ion trap geometries. 
     A ring electrode  12  is formed by producing a centrally located hole of appropriate diameter in a stainless steel plate. Here, the hole&#39;s radius r 0  is 0.5 mm, so the diameter of the drilled hole in ring electrode  12  is 1.0 mm. The thickneess of ring electrode  12  is approximately 0.9 mm. 
     Planar end caps  14  and  16  comprise either stainless steel sheets or mesh. The end caps  14  and  16  include a centrally located recess of approximately 1.0 mm diameter, with the bottom surface of the recess having a hole of approximately 0.45 mm diameter. End caps  14  and  16  are separated from ring electrode  12  by insulators  18  and  20 , each of which include a centrally located hole of 1.0 mm diameter. Insulators  18  and  20  may comprise Teflon tape with opposing adhesive surfaces. 
     The holes in the ring electrode  12 , end caps  14  and  16 , and insulators  18  and  20  are produced using conventional machining techniques. However, the holes could be formed using other methods such as wet chemical etching, plasma etching, or laser machining. Moreover, the conductive materials employed for ring electrode  12 , and end caps  14  and  16  could be other than described above. For example, the conductive materials used could be various other metals, or doped semiconductor material. Similarly, Teflon tape need not necessarily be the material of choice for insulators  18  and  20 . Insulators  18  and  20  could be formed of other plastics, ceramics, or glasses including thin films of such materials on the conductive materials. 
     The centrally located holes in ring electrode  12 , end caps  14  and  16 , and insulators  18  and  20  are preferably coaxially and symmetrically aligned about a vertical axis (not shown), to permit laser access and ion ejection. When assembled into a sandwich construction, the interior surfaces of ion trap  10  form a generally tubular shape, and bound a partially enclosed cavity with a corresponding cylindrical shape. 
     The distance between lower surface  22  of upper end cap  14  and upper surface  24  of lower end cap  16  is 2z 0 , where z 0  is 0.5 mm. As previously mentioned, r 0  is approximately 0.5 mm. Thus, the ratio z 0 /r 0  is 1.0, which falls within a desired range which produces improved mass spectral resolution for ion trap  10  during mass spectrometry. A z 0 /r 0  ratio range which is greater than 0.83 is desirable, as testing shows it provides mass spectral resolution down to 0.2 AMUs, achieving a significant improvement over the art. 
     In the preferred embodiment, ion trap  10  is a submillimeter trap having a cavity with: 1) an effective length 2z 0  with z 0  less than 1.0 mm; 2) an effective radius r 0  less than 1.0 mm; and 3) a z 0 /r 0  ratio greater than 0.83. However, those with skill in the art will appreciate that a z 0  and/or an r 0  greater than or equal to 1.0 mm could be employed while maintaining a z 0 /r 0  ratio greater than 0.83. Similarly, those with skill in the art appreciate that various other changes may be made to ion trap  10 , such as substituting different conductive materials for ring electrode  12  and end caps  14  and  16 . Additionally, the cavity in ion trap  10  need not necessarily be centrally located. 
     FIG. 2 illustrates a system  26 , which includes ion trap  10 , for performing mass spectrometry. Ion trap  10  is conventionally mounted in a vacuum chamber  28  with a Channeltron electron multiplier detector  34 , manufactured by the Galileo Corp. of Sturbridge, Mass. Detector  34  is located near the central axis of ion trap  10  to detect the generated ions. A Nd:YAG laser source  30  produces a pulsed 266-nm harmonic (˜1 mJ/pulse, ˜5 ns duration, 10 Hz repetition rate) beam focussed by a 250 mm lens  32  through a window in vacuum chamber  28  to generate ions within ion trap  10 . Laser source  30  is a DCR laser made by Quanta Ray Corp. of Mountain View, Calif. A beam stop (not shown) made from copper tubing is placed near detector  34  to intercept laser light emerging from ion trap  10  to minimize ion generation and photoelectron emission external to trap  10  itself. Helium buffer gas at nominally 10 −3  Torr and a sample vapor may be introduced into the vacuum chamber  28  through needle valves (not shown). Ion trap  10  is operated in the mass-selective instability mode, with or without a supplementary dipole field for resonant enhancement of the ejection process. 
     To provide the radio frequency (RF) signal for ring electrode  12 , a conventional computer  36  provides control signals to amplitude modulator  38 , a DC345 device manufactured by Stanford Research Systems of Sunnyvale, Calif. A conventional frequency generator  40 , implemented with a DC345 device manufactured by Stanford Research Systems, receives signals from amplitude modulator  38 , and outputs the desired trapping voltage and ramp for mass scanning. The output signal from frequency generator  40  is then amplified by a 150 W power amplifier  42 , the 150A100A amplifier manufactured by Amplifier Research of Souderton, Pa., and is applied to ring electrode  12 . 
     When axial modulation is desired, a supplementary voltage from frequency generator  44 , a DC345 device manufactured by Stanford Research Systems, may be applied to end caps  14  and  16 . The output of frequency generator  44  is delivered to a conventional RF amplifier phase inverter  46  before delivery to end caps  14  and  16 . Alternatively, end caps  14  and  16  are grounded. The Channeltron detector&#39;s bias voltage, up to 1700 V, is supplied by DC power supply  48 , the BHK-2000-0  1 MG manufactured by Kepco Corp. of Flushing, N.Y. DC power supply  48  may be programmed so that the detector&#39;s bias voltage is reduced during the laser pulse to avoid detector preamplifier overload. 
     The output from detector  34  is amplified by current-to-voltage preamplifier  52 , an SR570 manufactured by Stanford Research Systems, with a gain of 50-200 nA V- −1  and stored on digital oscilloscope  50 , a TDS 420A manufactured by Tektronix Corp. of Wilsonville, Oreg. 
     The ion trap  10  described above was machined using conventional materials and methods, and may be produced with any suitable material and method of manufacture. Moreover, those skilled in the art understand that ion trap  10  may be manufactured into versions that could be integrated with other microscale instrumentation. 
     As described above, ions are generated with ion trap  10  by employing a laser ionization source  30 ; however, in an alternative embodiment, electron impact (EI) ionization may be employed. An El source can generate ions from atomic or molecular species that are difficult to ionize with laser pulses. 
     When employing an EI source, it is preferably located within the vacuum chamber  28 , which houses ion trap  10 . This permits the EI source, ion trap  10 , and detector  34  to be self-contained, and therefore, much smaller in overall size than when the external pulsed laser  30  is used. Employing this self-contained arrangement minimizes mass spectrometer size. The size of the ion trap  10  and the associated sampling and detecting components are compatible with micromachining capabilities. 
     Moreover, those skilled in the art appreciate that any ion production method that works with a laboratory instrument could be used with ion trap  10 . For example, electrospray ionization or matrix-assisted laser desorption/ionization (MALDI) could be used most notably for large molecules such as biomolecules. Chemical ionization and other forms of charge exchange are also suitable methods of sample ionization. 
     Additionally, the interior surface of ion trap  10  has been described as having a generally tubular shape, and bounding a partially enclosed cavity with a corresponding cylindrical shape. However, those skilled in the art understand that other conventional ion trap geometries could be employed while maintaining a submillimeter ion trap, as described, namely one having a z 0 /r 0  ratio greater than 0.83. In instances where other than cylindrical geometry is employed for ion trap  10 , an average effective r 0  could be used for z 0 /r 0  determination. Similarly, for various other ion trap geometries, an average effective length 2z 0  could be employed for ratio determination. 
     While the foregoing specification illustrates and describes the preferred embodiments of this invention, it is to be understood that the invention is not limited to the precise construction herein disclosed. The invention can be embodied in other specific forms without departing from the spirit or essential attributes. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.