Abstract:
An elongate sample volume matching an elongate region of uniform RF magnetic field established by a saddle coil, is approximated by at least one pair of cylindrical sample tubes in parallel orientation with the geometric axis of the saddle coil. The displacement of the two cylindrical tubes defines a direction transverse to the tube axes and this transverse direction is aligned parallel with the RF magnetic field of the saddle coil.

Description:
FIELD OF THE INVENTION 
     This work is in the field of nuclear magnetic resonance analysis and particularly relates to sample cells for such analysis. 
     BACKGROUND OF THE INVENTION 
     Magnetic resonance phenomena occurs in a magnetic environment that is controllably homogeneous: uniform in magnitude, direction and stable in time. This environment is known as the polarizing field and imparts to the chaotically directed nuclear spins, a preferred direction in space around which the variously directed nuclear spins precess. Much effort has been directed to the production and control of the polarizing field. 
     The resonant absorption of energy from an external source occurs through the agency of an RF magnetic field applied to the nuclei under study at an angle (preferably 90° to the polarizing field) through an RF resonator. Substantial effort has been expended to produce and control the homogeneity of this RF magnetic field associated with this resonator as experienced by and with the nuclei of the sample under study. The shape, material and motion of the vessel containing the (liquid) sample has been studied and taken as a subject for further advancement of the homogeneity of the environment of the sample. The present work concerns this latter area of sample shape for further innovation. 
     The RF resonator has been a fertile ground for development over many decades. The present work is intended for the case where the RF magnetic field associated with the resonator is in the plane normal to the polarizing field. The form of resonator for this arrangement is known as a saddle shaped coil and the present work is limited to operation with such saddle coils. It should be recognized herein that the role of the RF resonator is understood to encompass provision for signals corresponding to either or both excitation and de-excitation of the nuclei under study. 
     In conventional practice, liquid samples for investigation via NMR are presented in long cylindrical tubes along the axis of the RF saddle coil of the NMR probe. When the RF coil is saddle shaped, the direction of the RF magnetic field (within the volume defined by the coil) is transverse to the long axis of the cylindrical sample vessel.  FIG. 1   a  shows a schematic representation of the coil elements, the sample vessel and RF magnetic field in cross section for a conventional arrangement. This figure also shows the common arrangement of separate coaxially disposed coils. As shown, these coils produce respective RF fields on orthogonal directions in the plane transverse to their common axis. 
     An improvement to this conventional arrangement appears when the filling factor of the coil (the volume of the sample in respect of the interior volume defined by the coil) is optimized through allowing the inner dimensions of the RF coil to more closely approach the outer dimensions of the sample vessel. Moreover prior art recognized that the RF magnetic flux is substantially homogeneous within the inner confines of the RF coil and would be even more so were the cross section of the RF saddle coil to be deformed from conventional quasi-arc sectors (in cross section) of  FIG. 1   a , to planar segments and the sample vessel cross section similarly deformed to an elongate cross sectional shape (ellipsoidal or rectangular) in conformity with an elongate (ellipsoidal or rectangular) coil cross section. This geometry is intended to produce a greater degree of homogeneity in the RF magnetic field of the saddle coil and to yield an extended volume space wherein such homogeneity obtains. Such prior art is illustrated in  FIGS. 1   b  and  1   c  and more description appears in the U.S. Pat. Nos. 7,068,034 and 6,917,201, both assigned to Varian, Inc. 
     Lossy samples present a case of particular concern. Such samples exhibit a significant electrical conductivity and under the influence of RF electric fields there results RF currents, which contribute noise (for example, from the magnetic fields associated with these currents) and thus degrade the sensitivity of the NMR instrument. The geometrical region of a sample producing the greatest contribution to signal is that region of the highest RF magnetic field amplitude, which may be identified with a region proximate the saddle coil axis. As the sample cross section is increased, more sample may be included, but the influence of the RF E field will be more effective in producing noise in that portion of the space more remote from the central region. The signal to noise figure therefore suffers. For study of lossy samples, it has been common practice to present the sample in a 3 mm sample tube to minimize the effect of RF electric fields by confining the sample to close proximity to the RF coil axis. In such arrangements, sensitivity is inherently compromised by the dearth of sample volume. 
     The prior art sample vessel of rectangular cross section is difficult to manufacture to the uniform close tolerances necessary for application to high field/high resolution NMR analysis. By way of comparison, conventional cylindrical sample vessels of 3 mm O.D. are commercially offered with a concentricity specification of 0.0005 and a camber specification of 0.00025 over axial lengths of 8 inches (203.2 mm). To approach equivalent tolerances rectangular cross section vessels must be selected from a great number of units at considerable expense. Indeed, the specification of outer dimension and wall thickness for cylindrical tubes is more easily achieved than the specification of two outer dimensions and inner area for the prismatic tube. Accordingly, it is desired to obtain the benefit of the matching a transversely elongate sensitive volume (associated with a saddle coil) to an elongate sample volume in a reliably reproducible and inexpensive manner. 
     In another prior art sample cell (U.S. Pat. No. 5,552,709, assigned to Varian, Inc.), the same sample for analysis fills a plurality of separate sample holding structures for analysis. The multiple cells are intended to reduce the electrical current paths through lossy sample solutions. The array of closely packed sample vessels is uniform in cross sectional distribution and provides no benefit from alignment of the shape of a macro-sample in respect of the RF magnetic field. 
     SUMMARY OF THE INVENTION 
     An elongate cross section for a sample vessel, for use with a correspondingly elongate sensitive volume for NMR measurement, is achieved by parallel disposition of at least two (conventional) cylindrical vessels (NMR sample tubes). The axes of these cylindrical vessels are necessarily displaced transversely to their axes by a separation interval of minimum magnitude D, where D is the outer diameter of the (identical) cylinders in tangential contact (for a minimum separation interval). This separation interval defines an axis x, orthogonal to the z axis (parallel with the tube axes) and intersecting the x axis at the mean of the separation interval. The z axis is precisely aligned to coincide with a z axis of the RF saddle coil of an NMR probe. Such saddle coils exhibit an x′ axis, transverse to z, that defines the direction of an RF magnetic field associated with the saddle coil. These alignments and displacements are obtained in a sample holder assembly that further comprises an azimuthal reference that may take the form of a laterally projecting member providing an orientation reference for the x axis of the cylinder pair azimuthally in respect of a corresponding x′ axis of the RF saddle coil, so as to align the x axis and x′. 
     Briefly stated, it is a goal of this work to realize a sample volume (particularly well suited to lossy samples) realized from inexpensive components. In the plane of the RF magnetic field of a saddle coil this sample vessel assembly presents a sample cross section that is elongate in a dimension aligned with the RF magnetic field of that saddle coil. The elongate geometry is realized by a plurality of cylindrical NMR sample tubes (at least a pair) arrayed to provide a cross section having major and minor axes in the plane of the cross section. 
     Another embodiment of the sample vessel assembly adds an auxiliary pair of cylindrical sample tubes on an auxiliary separation interval orthogonal to the separation interval of first sample tube pair. The auxiliary tubes are preferably of diameter less than D in order to allow a minimal magnitude for the auxiliary separation interval and thus to bring these auxiliary sample tubes to an acceptable minimum selected distance from the assembly axis/RF coil axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  is a schematic representation of a conventional NMR probe and sample vessel. 
         FIG. 1   b  is an improvement to  FIG. 1   a  illustrating the field of the outer coil of a coaxial coil probe. 
         FIG. 1   c  shows the same as  FIG. 1   b  with excitation from the inner coil. 
         FIG. 2  shows the sample cell cross section of the present work. 
         FIG. 3   a  is one form of sample vessel holder for implementing the sample cell of  FIG. 2 . 
         FIG. 3   b  is a section A-A through a top portion of  FIG. 3   a.    
         FIG. 3   c  is a section through a bottom portion of the sample vessel assembly of  FIG. 3   a.    
         FIG. 4   a  is the anomeric proton portion of the sucrose spectrum for the present sample cell assembly. 
         FIG. 4   b  is the noise determination for the spectrum of  FIG. 4   a.    
         FIG. 4   c  is the anomeric proton portion of the sucrose spectrum for the prior art sample cell. 
         FIG. 4   d  is the noise determination for the data of  FIG. 4   c.    
         FIG. 5  is an alternative embodiment of this work. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3   a  shows a representative sample cell assembly for use with an NMR probe comprising a saddle coil  46  (here shown as one of the Alderman Grant variety) defining a sensitive volume  48  that is elongate in the plane transverse to the polarizing field. The sample cell assembly comprises a body  50  having an azimuthal reference  52  to obtain a selected azimuthal alignment of the body with respect to the saddle coil  46  of the NMR probe. Body  50  is characterized by axial geometry with body axis z coincident with the z axis of saddle coil  46 . A cap portion  51  of the body  50  determines the axial relationship of body  50  with the sensitive volume  48  of the NMR probe, not otherwise illustrated. Each sample tube is maintained in the body  50  by frictional engagement with the outer surface of respective O-ring(s)  57 A and  57 B, which are in turn secured to the top alignment plate  55 A through screws (not shown). The sample tubes are not otherwise constrained by passage through a bore through the interior of the body  50 . The top planar surface of the body  50  conveniently comprises a recessed portion  54  for receiving a top alignment plate  55 A, shown displaced from body  50  for clarity. The top alignment plate  55 A is relatively unconstrained by recessed portion  54 . As shown in  FIG. 3   b , NMR sample tubes  60 A and  60 B pass through apertures (also labeled  60 A and  60 B). In practice the apertures accommodating the tubes  60 A and  60 B may slightly overlap forming a dumbbell shape The opposite (bottom) planar end surface of the body  50  comprises a bore (not shown) for receiving a bottom alignment plate  55 B in a close fit (approximately 0.001 inch tolerance). The alignment plates (so termed without reference to relative thickness)  55 A and  55 B support a pair of conventional NMR sample tubes  60 A and  60 B in mutual parallel alignment with the axis z, at a selected displacement of the axes of the respective sample tubes  60 A and  60 B. It should be clear that this displacement has a minimum value of D where D is the outer diameter of the sample tube. For the arrangement described, the two sample tubes provide a cross section of sample at the sensitive volume as shown in the example of  FIG. 2 . At  FIG. 3   a , the dotted lines portray the sensitive volume  48  corresponding to an interior portion of the cross section of the saddle coil  46 , here shown as rectangular (not limited to such shape, but discussed here for experimental comparison below) and displaced for clarity. It should be understood that the “sensitive volume” is a term inclusive of tolerance for such degree of inhomogeneity of the field as may be theoretically appropriate or practically realizable. As shown in  FIG. 2 , the figure-8 shaped sample portion presents a section  48 ′ through the sensitive volume  48 . The sections  60 ′A and  60 ′B through the tubes  60 A and  60 B comprise the elongate dimension (e.g., x) of the section of the sensitive volume for the case of tangential contact of the tubes. The rectangular region shown in  FIGS. 1B and 1C  is a convenient reference for consideration of the sample volume cross section of this work and has been taken as a reference for experimental observation. A sample sectional area (neglecting sample tube wall thickness) of 2π(D/2) 2  is shown inscribed in a hypothetical section of the exemplary rectangular area of D×2D. It is observed that the dual sample tubes yield a 78% approximation to the area (and hence volume) for sample in a hypothetical sensitive volume of identical maximum dimensions. For wall thickness t, one observes that the loss of sample cross section area attributed to wall thickness is, to first order, 2π(D/2) 2 (4t/D). 
     The azimuthal reference  52  may be realized through a variety of means or through no “means” at all. There need only be established a known azimuthal relationship with the x axis (separation interval). In respect of engagement with the NMR probe, any appropriate means will suffice to establish alignment with the x′ axis of the saddle coil. For example, the body of the NMR probe can be adapted to include a notch to accept an azimuthal reference for the holder in the form of a radially protruding pin from the body  50  or bottom alignment plate  55 B. No such particular mechanical arrangement is required for this work: one of skill in the art recognizes that the holder may be re-oriented while observing the NMR resonance signal to obtain orientation at which the signal reaches maximum amplitude. 
     Alignment of the assembly proceeds through insertion of precisely machined tungsten rods through the top alignment plate  55 A and through the bottom plate  55 B and insertion of the body assembly into an appropriate jig establishing the coaxial relationship of body  50  and bottom plate  55 B. The degree of freedom afforded by the top plate  55 A in recessed portion  54  is then fixed by the jig. The dimension D for apertures (labeled  60 A and  60 B for convenience) incorporates an increment ε to accommodate diameter variation between different tubes which may be inserted into the assembly. For commercially available 3 mm tubes, ε is taken as 1×10 −3  inch. 
     After the sample holder assembly is assembled and alignment is procured the sample holder assembly is permanently secured and NMR sample tubes may be inserted and withdrawn as may be required. 
     A sample cell assembly was constructed following  FIGS. 2 and 3  for testing by comparison with a randomly selected rectangular cross section cell ( FIGS. 1   b  and  1   c ). The cylindrical NMR sample tubes were of 3 mm O.D. in tangential contact, that is axis to axis displacement of 3 mm. The inner diameter of the sample tubes employed for this prototype was 2.4 mm.  FIG. 4   a  is the spectral response of the anomeric proton of a sucrose sample (3.4 mg/ml in D 2 O). Expansion of the spectral range for this spectrum ( FIG. 4   b ) permits the measure of signal-to-noise over the range 200 Hz to 3200 Hz, yielding 188.5. The splitting of the anomeric proton resonance is exhibits a depth of 89% of the resonant peak. The half-height width of the lower split peak is 1.5 Hz. 
     Using the same probe, the same sample solution was presented for analysis in a sample cell of rectangular cross section having nominal outer cross sectional dimensions of 6 mm. by 3 mm and inner cross sectional area of 10.6 mm 2 . This reference rectangular (parallelipiped) cell was selected from a number of nominally identical cells by comparison of NMR spectral response of these cells. The corresponding results are shown in  FIGS. 4   c  and  4   d . The signal-to-noise parameter obtained is 188.3, essentially identical with the present work, but the depth of the split is found to be 78% of the resonance amplitude and the width at half amplitude for the lower peak is 1.66 Hz. Offering an actual sample volume 17% greater than the two tube sample of  FIG. 4   a , it is unexpected that a such significantly lower resolution results for the same signal-to-noise figure. 
     The difference in resolution for the same signal-to-noise figure is unexpected. The difference is attributed to dimensional irregularities of the prior art cell of rectangular cross section in comparison with the decidedly precise and reproducible characteristics of unselected cylindrical NMR sample tubes. It is important to recognize that a rectangular prism sample cell meeting the specifications of uniformity prescribed for a pair of cylindrical NMR sample tubes would have a cost ratio that can only be estimated as orders of magnitude. 
     It is noteworthy that the present work yields a filling factor significantly less than the rectangular, reference example. It is therefore quite unexpected that the same signal-to-noise ration is achieved with less sample and with an apparently non-optimum geometry. In this comparison, the prior art reference cell enjoyed a volume advantage factor of 1.17 over the sample volume for the dual tube composite cell of this work. Special thin wall NMR sample tubes, commercially available (from Wilmad Ltd., Buena, N.J.) would increase the inner diameter to 2.6 mm, providing additional volume (increased filling factor) with an attendant expected further increase in signal-to-noise parameter for the present work. 
     In the abstract sense, one might hypothesize an elongate array of NMR sample tubes geometrically limiting the sample to the desired region and capable of precise alignment with the RF magnetic field of the saddle coil. Practical considerations introduce limits on the volume given over to the material of the individual sample vessels at the expense of sample. Dimensional constraints are introduced by the NMR magnet bore and in consequence thereof, the space available for the NMR probe. It is with those considerations and the excellent specifications of commercially available NMR tubes, that two 3 mm NMR tubes are preferred in the assembly here described. 
     In another embodiment as shown in  FIG. 5 , an auxiliary pair of NMR sample tubes  70 A and  70 B are added to the assembly in mutual inter-axial displacement orthogonal to the inter-axial displacement of the axes of first sample tube pair  60 A and  60 B. The auxiliary pair of tubes are preferably of significantly smaller outer diameter to permit selection closer proximity to the central axis of the NMR coil. More precisely, the spatial separation between auxiliary tubes  70 A and  70 B must not be substantially equal to the separation interval for sample tubes  60 A and  60 B, because that would remove the elongate character here required. The auxiliary pair allow use of a chemical shift reference sample or other non-miscible reference, or any other sample. In one example, a low loss lock solvent might be employed. In the example shown the auxiliary pair  70 A and  70 B consist of 1.7 mm O.D. NMR tubes allowing about 44% of the volume of the (primary) tube pair. The user might then avoid dilution of the primary sample with labile deuterons in a deuterated solution at a slightly greater displacement from the NMR probe axis. It is observed that the auxiliary pair of tubes  70 A and  70 B may take on a diameter significantly greater than the more centrally disposed pair, thus establishing an elongate axis in the plane of the sample cross section. This creates a larger sample volume more distant from the RF coil axis and a smaller volume proximate that axis. Such arrangement would not be preferred for a lossy sample. 
     While this work has been described with reference to specific embodiments, the description is illustrative of the work and is not to be construed as limiting the scope of the work. Various modifications and changes may occur to those skilled in the art without departing from the true spirit and scope of the innovation as defined by the appended claims.