Abstract:
A novel NMR principal detector element of the present invention comprises a NMR-active inductor in a radio frequency (RF) resonance circuit, and mitigates the distortions in a static magnetic field caused by the introduction of a toroid cavity NMR detector. The RF resonance circuit includes a novel principal detector element, capacitor elements, and a sample chamber (the cavity). The novel principal detector element defines the NMR active portion of the inductance of the electronic resonance circuit. In one embodiment of the invention, the novel principal detector element includes two hemispheres connected by a central rod. The central rod can be of any length and diameter, and the hemispheres can be of any diameter. A typical embodiment of the invention would include a rod that is 2 cm long having a diameter of 1 mm, and hemispheres having a diameter of 5 mm.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of the filing date of co-pending U.S. Provisional Application No. 60/363,447 filed on Mar. 12, 2002. 
     
    
     CONTRACTUAL ORIGIN OF THE INVENTION  
       [0002] The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and Argonne National Laboratory. 
     
    
     
       FIELD OF THE INVENTION  
         [0003]    The present invention relates to novel NMR principal detector elements that improve the resolution and line shape of NMR spectra obtained using toroid cavity detector NMR probes.  
         DESCRIPTION OF THE RELATED ART  
         [0004]    It is critical to improve the magnetic field homogeneity in toroid cavity detector NMR probes in order to make them widely accepted and used in the analytical and biochemistry communities. We have invented a passively shimmed principal detector element for toroid cavity probes for the purpose of improving the magnetic field homogeneity in the volume of space occupied by the sample.  
           [0005]    The development of high-field Toroid Cavity Detectors (TCDs) for nuclear magnetic resonance (NMR) analyses of mass-limited biological samples is currently motivated by the promise of a substantial increase in sensitivity. However, the inhomogeneity of the external magnetic field (B 0 ) over the sample volume in a TCD is a problem that results in reduced sensitivity. The purpose of the present invention is to improve B 0  homogeneity in TCD NMR probes by introducing novel geometrical modifications to the principal detector element (central conductor).  
           [0006]    The geometrical interface between two materials with dissimilar volume magnetic susceptibilities is known to cause magnetic field distortions in the NMR-active space. In TCDs the magnetic field distortion problem is due to the exterior and interior interfaces of the cylindrical cavity. The exterior air-copper interface at the top and bottom of the cavity causes distortions in the magnetic field that can be eliminated by elongating the cavity. The interior copper-sample interface at the top and bottom of the cavity also causes distortions in the magnetic field in the sample volume, but elongating the cavity cannot eliminate these distortions. We have invented passively shimmed principal detector elements that reduce magnetic field distortions caused by the interior interfaces at the top and bottom of the cavity. Passive shimming takes advantage of materials with different volume susceptibilities and geometrical forms to homogenize the magnetic field in a prescribed volume. The more commonly used approach to homogenize magnetic fields is the active shimming method. Active shimming requires energized magnet coils that generate correction fields. The magnet coils that are included in commercial NMR instruments were designed to homogenize the sample volume in commercial (Helmholtz-style) NMR probes, and are not capable of correcting magnetic field distortions caused by TCD probes. Magnet coils specifically designed for TCD probes are not available. Alternatively, a combination of different materials arranged in the cylindrical geometry of the TCD affords a passive approach to eliminating magnetic field distortions. A principal detector element of novel geometric designs contained within a TCD which eliminates distortions in the magnetic field in the sample volume is the subject of the present invention.  
           [0007]    Three embodiments of a passively shimmed principal detector element in accordance with the present invention have been disclosed by the inventors and are as follows: (1) a cylindrical spool with surfaces that extend beyond the sample volume that is contained within a larger cylindrical cavity containing the sample, (2) susceptibility matching cylindrical plugs above and below the central conductor, and (3) a cylindrical central conductor with spheroid interfaces at both ends. These three devices use a combination of different materials and geometrical structures with cylindrical symmetry to improve the magnetic field homogeneity in TCD probes. We expect that a combination of active and passive shimming devices will be required to make the magnetic field homogeneity performance of TCD probes competitive with commercial NMR probes.  
           [0008]    Nuclear magnetic resonance (NMR) analysis is a powerful method by which to determine chemical structures and to examine reaction dynamics in a diversity of chemical and biochemical systems. In particular, NMR analyses of chemical and biochemical systems using toroid cavity NMR detector probes afford the possibility of detecting very small sample quantities not normally analyzable with conventional NMR detector probes that are commercially available.  
           [0009]    For example, U.S. Pat. No. 5,574,370, issued Nov. 12, 1996 to Woelk et al., discloses a toroid cavity detector (TCD) system for determining the spectral properties and spatial distance from a fixed axis for a sample using Nuclear Magnetic Resonance. The detection system consists of a toroid cavity with a central conductor oriented along the main axis of the toroidal cylinder and oriented parallel to a static uniform magnetic field, B 0 . A radio frequency (RF) signal is applied to the central conductor to produce a concentric magnetic field B 1  perpendicular to the central axis of the toroid and whose field strength varies as the inverse of the radial position in the toroid. (The position of the center axis of the toroid cavity is the location of the zero position for the radial dimension and axis.) The toroid cavity detection system can be used to encapsulate a sample, or the detection system can be perforated to allow a sample to flow into the detection device or to place the samples in specified sample tubes. Flexible capillary tubing can also be wound about the central conductor and used to transfer and contain a sample or a series of samples. The central conductor can also be coated to determine the spectral property of the coating and the coating thickness. The sample is then subjected to the respective magnetic fields and the responses measured to determine the desired properties.  
           [0010]    U.S. Pat. No. 6,046,592, issued Apr. 4, 2000 to Rathke et al., discloses a near-electrode imager for employing nuclear magnetic resonance imaging to provide in situ measurements of electrochemical properties of a sample as a function of distance from a working electrode, also know as the central conductor. The near-electrode imager uses the radio frequency field gradient within a cylindrical toroid cavity resonator to provide high-resolution nuclear magnetic resonance spectral information on electrolyte materials, electrode-electrolyte interphases, and electrode coating materials.  
           [0011]    U.S. Pat. No. 6,191,583, issued Feb. 20, 2001 to Gerald II et al., discloses a toroid cavity detector that includes an outer cylindrical housing through which extends a wire along the central axis of the cylindrical housing from a closed bottom portion to the closed top end of the cylindrical housing. In order to analyze a sample placed in the housing, the housing is placed in an externally applied static main homogeneous magnetic field (B 0 ). An RF current pulse is supplied through the wire such that an alternately energized and de-energized magnetic field (B 1 ) is produced in the toroid cavity. The B 1  field is oriented perpendicular to the B 0  field. Following the RF current pulse, the response of the sample to the applied B 0  field is detected and analyzed. In order to minimize the detrimental effect of probe ringing, the cylindrically shaped housing is elongated sufficiently in length so that the top and bottom portions are located in weaker, fringe areas of the static main magnetic B 0  field. In addition, a material that tends to lessen the effect of probe ringing is positioned along the top and bottom ends of the toroid cavity. In another embodiment, a plug is positioned adjacent the inside of the top and bottom ends of the toroid cavity so that the sample contained in the toroid cavity is maintained in the strongest and most homogeneous region of the static magnetic B 0  field.  
           [0012]    U.S. Pat. No. 6,469,507, issued Oct. 22, 2002 to Gerald II et al., discloses imaging apparatus that is used in a toroid cavity detector for nuclear magnetic resonance (NMR) analysis to hold samples relative to a principal detector element which is a flat metal conductor, the plane of which is parallel to the longitudinal axis of the toroid cavity. A sample is held adjacent to or in contact with the principal detector element so that the sample can be subjected to NMR analysis when a static main homogeneous magnetic field (B 0 ) produced by a NMR magnetic device is applied to the toroid cavity and an RF excitation signal pulse is supplied to the principal detector element so that an alternately energized and de-energized magnetic field (B 1 ) is produced in the sample and throughout the toroid cavity. The sample may be components of a coin cell battery which are mounted within the toroid cavity relative to the principal detector element by a non-conductive coin cell battery imager or a press assembly so that the components are hermetically sealed together and so that a direct current potential can be applied to the components. Alternately, a sample is positioned within an O-ring maintained relative to the principal detector element by a pair of glass plates that are disposed on opposite sides of the principal detector element and are compressed towards each other so that NMR analysis can be used when light is transmitted through the sample or to analyze a sample separated from the principal detector element by semi-permeable membranes.  
           [0013]    The subject matter of each of the aforementioned U.S. Pat. Nos. 5,574,370; 6,046,592; 6,191,583; and 6,469,507 is incorporated herein by reference.  
           [0014]    A principal object of the present invention is to provide a novel principal NMR detector element that allows recording of undistorted NMR spectra using a toroid cavity NMR detector probe.  
         SUMMARY OF THE INVENTION  
         [0015]    In brief, the novel principal NMR detector element of the present invention comprises the NMR-active inductance of a radio frequency (RF) resonance circuit. The RF resonance circuit includes a principal detector element, capacitor elements, and a cavity sample chamber. The principal detector element forms an inductor in the RF resonance circuit.  
           [0016]    In one embodiment of the invention, the principal NMR detector element is comprised of two metallic hemispheres connected together by a metallic central rod to form the general shape of a dumbbell. Attaching each hemisphere to an opposed end of the rod and connecting this combination to an RF circuit allows the combination to function as the inductor of the NMR RF circuit.  
           [0017]    More specifically, a cylindrical tube is fitted and extended over both hemispheres to form a gas-tight volume (the sample volume) bounded by the flat bottoms of the hemispheres and the outer surface of the rod. The sample fully or partially fills the sample volume. The geometry and material of the hemispheres are chosen and designed according to the arrangement of the fluid or fluids disposed within the sample volume so that a uniform static magnetic field fills the sample volume when the fluid(s) are surrounding the principal detector element.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The appended claims set forth those novel features which characterize the invention. However, the invention itself, as well as further objects and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which:  
         [0019]    [0019]FIG. 1 is a side elevation view shown partially in phantom of a NMR principal detector element in accordance with the principles of the present invention;  
         [0020]    [0020]FIG. 2 is a transverse sectional view of the NMR principal detector element shown in FIG. 1 taken along site line  2 - 2  therein;  
         [0021]    [0021]FIGS. 3 and 4 are side elevation views of two embodiments of a cylindrical tube for use in the NMR principal detector element of the present invention; and  
         [0022]    [0022]FIG. 5 is a side elevation view of a NMR principal detector element in accordance with the present invention containing a sample of liquid water wherein the detector element&#39;s outer tube and the surface tension of the water act to contain the water sample. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    With reference now to the drawings, a novel NMR principal detector element  100  in accordance with the present invention which takes advantage of various geometrical shapes and materials to reduce or eliminate distortions in a static magnetic field caused by the presence of materials used to contain the sample is shown in FIG. 1. NMR principal detector element  100  advantageously is used to record NMR spectra that exhibit minimal distortions to a theoretical resonance line shape. The NMR principal detector element  100  is fabricated from one piece of material or from several pieces of material and assembled as shown in FIG. 1. FIG. 1 is a side elevation view shown partially in phantom of the inventive NMR principal detector element  100 . A transverse sectional view of the NMR principal detector element  100  shown in FIG. 1 taken along site line  2 - 2  therein is shown in FIG. 2.  
         [0024]    In accordance with features of the invention, NMR principal detector element  100  makes possible NMR spectroscopy investigations of gases, fluids, liquids, semisolids, and solids. NMR data derived from NMR principal detector element  100  is unique in that it is characterized by greater sensitivity than conventional commercially available detectors. The level of detail of the composition of the sample is also greater because the sensitivity obtainable from NMR principal detector element  100  is greater than that available from existing devices.  
         [0025]    Enhanced signal sensitivity for samples that are in direct contact with the NMR principal detector element  100  has been observed. The inventive NMR principal detector element  100  includes a central rod  105  disposed on the longitudinal axis of the detector and first and second hemispheres  101  and  102  disposed on opposed ends of the central rod. The central rod  105 , lower portion of the first hemisphere  101 , and the second hemisphere  102  are shown in dotted line form. The sensitivity from NMR principal detector element  100  is radially distributed with the most sensitive region immediately adjacent to the surface of the detector&#39;s central rod  105 . An electrical conductor  110  is inserted in a threaded aperture  112  in an upper portion of the first hemisphere  101  and an electric conductor  116  is inserted in a threaded aperture  118  in a lower portion of the second hemisphere  102  for electrically coupling the NMR principal detector element  100  to an RF resonant circuit  114 .  
         [0026]    The first and second hemispheres  101 ,  102  of the NMR principal detector element  100  can each be fabricated from a single piece of material, or may be in the form of plural generally flat disks  106  arranged in a stacked manner. The first and second hemispheres  101  and  102  are respectively provided with threaded apertures  103  and  104  to threadably engage opposed ends of the central rod  105  for providing an RF current path. The disks can be made of metallic or non-metallic materials and may possess electrical conductivity properties for the purpose of conducting RF currents. In one embodiment, the disks are of uniform composition and are fabricated from metallic substances. In another embodiment, the disks are fabricated from non-metals or substances with poor electrical conductivity and are coated on their outer edge periphery with a good electrical conductor  107 . The coating can be of any thickness, but a preferred thickness is that of the RF skin depth for the conductive material and RF frequency for which the NMR principal detector element  100  is designed. The flat disks  106  can have virtually any shape, preferably are circular. The central rod  105  can have any shape, but a preferable shape is cylindrical. The central rod  105  can be made of metallic or non-metallic materials and may possess electrical conductivity properties for the purpose of conducting RF currents. In one embodiment, the central rod  105  is a solid metallic material. In another embodiment, the central rod  105  is fabricated from a non-conductive material and is coated on its exterior surface with a good electrical conductor. The coating can be of any thickness, but a preferred thickness is that of the RF skin depth for the conductive material and RF frequency for which the NMR principal detector element  100  is designed. The central rod  105  and the hemispheres  101  and  102  can also be fabricated from one piece of material. In one embodiment of NMR principal detector element  100 , the central rod  105  and the hemispheres  101  and  102  are fabricated from one piece of copper, silver, gold, or other metal. In another embodiment of NMR principal detector element  100 , the central rod  105  is fabricated from one piece of copper, silver, gold, or other metal and is provided with threaded apertures at both opposed ends, and threadably positioned in respective apertures that extend into the hemispheres  101  and  102 .  
         [0027]    Sample analyses by nuclear magnetic resonance spectroscopy and imaging is one of the most powerful approaches available for the determination of chemical structures and molecular dynamics in a diversity of chemical and physical systems. The NMR principal detector element  100  for acquiring undistorted NMR spectra of the present invention is a new and useful analytical device for non-invasive spectroscopic and imaging investigations of the molecular architecture and dynamics of gas, fluid, liquid, semisolid, and solid materials.  
         [0028]    As also shown in FIG. 1, NMR principal detector element  100  further includes a tube or sleeve structure, or containment vessel,  200  is for holding the sample to be analyzed in a compartment defined by the tube, the bases of the first and second hemispheres  101  and  102 , and the outer surface of central rod  105 . In one embodiment, tube  200  is in the form of a Teflon® sleeve with an outer diameter  201  of 0.5936″ and an inner diameter  202  of 0.3936″ as shown in the sectional view of FIG. 2. The sleeve&#39;s inner diameter  202  closely matches the diameter of the first and second hemispheres  101  and  102  so that a gas-tight fit is formed when sleeve  200  is positioned over the NMR principal detector element  100 .  
         [0029]    Sleeve  200  is comprised of conductive or non-conductive material. A sleeve  200  made of conductive materials is generally insulated in some manner so that the sleeve does not provide an electrical pathway between the first hemisphere  101  and the second hemisphere  102 . In some cases, such a pathway may exist provided that sleeve  200  is designed so that the conduction of RF currents by the sleeve does not render the NMR principal detector element  100  inoperative.  
         [0030]    The tube or sleeve designated  200  can be fabricated of multiple adjoining tubes of different materials. The tube or sleeve designated  200  can be fabricated of multiple concentric tubes of different materials. The tube or sleeve designated  200  can be fabricated of a combination of multiple concentric tubes and multiple adjoining tubes of different materials.  
         [0031]    The NMR principal detector element  100  can be formed by a selected one of various metals, alloys, and pure or doped semimetals. The principal detector element inductor  100  can be formed, for example, of gold, silver, gallium, platinum, and palladium.  
         [0032]    The tube or sleeve  200  can be formed by a selected one of various metals, alloys, and pure or doped semimetals. The tube or sleeve  200  can be formed, for example, of Teflon®, Ultem®, polymers, plastics, composites, ceramics, metals, gold, silver, gallium, platinum, and palladium.  
         [0033]    [0033]FIGS. 3 and 4 show an additional embodiment of the tube which could be used in the NMR principal detector element of the present invention. In the embodiment shown as FIG. 3, tube  300  is comprised of Teflon® and is 2.25″ in length and has two concentric sleeves  301  and  302  (shown in dotted line form) positioned in the interior of the tube. The outside of the tube  300  is shown in FIG. 4 and includes two concentric and movable outer sleeves  303  and  304  positioned at the exterior of the tube. Concentric sleeves  301  and  303  form a first capacitor, with variable capacitance according to the mutual overlap of both sleeves. Concentric sleeve  301  makes RF electrical contact with the NMR principal detector element. Concentric sleeves  302  and  304  form a second capacitor, with variable capacitance according to the mutual overlap of both sleeves. Concentric sleeve  302  makes RF electrical contact with the NMR principal detector element. These capacitors provide the required coupling between the NMR principal detector element and an external RF circuit (not shown) that is required to affect NMR analyses on samples. Electrical connections can be made between sleeves  301  and  302  and/or between sleeves  303  and  304  for the purpose of adjusting the resonance frequency of the NMR principal detector element.  
         [0034]    [0034]FIG. 5 is a side elevation view of a NMR principal detector element  400  in accordance with the present invention which includes cylindrical tube  402 . The NMR principal detector element  400  further includes a rod (not shown) and a pair of hemispheres (where only one hemisphere is shown as element  406  in the figure). An electrical conductor  408  is coupled to the hemisphere  406 . A sample of liquid water  404  is disposed substantially within the NMR principal detector element  400  and is disposed about its rod which is not shown for simplicity. The tube  402  and the surface tension of the water act to contain the water sample  404 . In the embodiment shown, the tube  402  does not completely contain the water sample  404 . This embodiment is useful for NMR investigations of the molecular structure and dynamics of water responsible for the surface tension at a water-air interface. In the preferred embodiment the tube  402  completely contains the sample and may extend beyond the total longitudinal length dimension of the NMR principal detector element  400 .  
         [0035]    While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.