Abstract:
A pressurizable holder for a sample to be examined by NMR, comprises a pressure retaining nonmagnetic tube surrounding a radio-frequency coil which in turn surrounds a space for the sample. The pressure retaining tube is formed of (i) nonmetallic electrically insulating material such as a ceramic or (ii) nonmetallic electrically insulating matrix material reinforced with electrically insulating filaments such as glass fiber, or (iii) non-metallic electrically insulating matrix material reinforced with electrically conductive filaments configured so that conductivity is anisotropic. There is good coil filling factor without constraint on wall thickness of the pressure retaining tube. Avoidance of isotropically conductive material inhibits eddy currents when an NMR spectrometer&#39;s magnetic field gradient coils are switched on and off. The tube resists hoop stress from internal pressure. Longitudinal stress is resisted by structure connecting end pieces at the ends of the pressure retaining tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to British Application no. 1210808.0 filed Jun. 19, 2012, which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    One technology for examining properties of a solid or liquid sample is nuclear magnetic resonance (NMR) also referred to as magnetic resonance imaging (MRI). There are circumstances where a sample is examined under pressure, thus requiring a sample holder which can contain the sample under pressure while it is in the magnetic field of an NMR spectrometer. 
         [0003]    A category of samples for which examination under pressure may be required is solid and also liquid samples collected below ground. When drilling through underground rock, it is common practice to drill around a cylinder of rock which is subsequently brought to the surface as a sample, habitually referred to as a rock core. Once brought to the surface, rock cores may be subjected to various measurements and tests. Tests which have been carried out include examination by nuclear magnetic resonance (NMR) also referred to as magnetic resonance imaging (MRI) which entails placing the core within a magnetic field and using one or more radio-frequency coils to apply radio-frequency energy to the core and receive radio-frequency signals from it. 
         [0004]    The subterranean rock formations from which such cores are taken are of course at high pressure and it can be desirable to carry out NMR measurements while the sample is under pressure. Liquid samples brought to the surface may also be subjected to NMR measurements whilst still under pressure. 
         [0005]    Design of a sample holder which can retain pressure and which can be placed in the magnetic field of an NMR spectrometer needs to address several issues, including mechanical construction for containing pressure, choice of materials to enable the core to be exposed to both magnetic field and radio-frequency, and positioning of the core holder in relation to the functional components (i.e., magnets and coils) of an NMR spectrometer. 
         [0006]    A pressurizable core holder which is available from ErgoTech Ltd, Conwy, Wales uses a tube of glass fiber reinforced composite to contain a core under pressure. Compressive force applied to the ends of the tube opposes stress longitudinally relative to the pressurized tube. The radio-frequency coil and the magnets of an NMR spectrometer are positioned outside the tube in spaces between the tube and tie rods connecting the structural parts which apply compressive force to the ends of the tube. In this arrangement, where the magnets fit between the tube and the tie rods, the tie rods are spaced apart in the direction of the magnetic field and the spacing between them has to be greater than the distance between the magnets. 
         [0007]    Another sample holder intended for a sample under pressure is illustrated in US published application 2011/0050223. It has a non-magnetic metal tube around the sample to retain pressure. The radio-frequency coil required for NMR is located inside this tube, with the sample placed inside the coil. The metal tube and the end pieces screwed into it provide sufficient strength to resist both radially outward stress (hoop stress) and also longitudinal stress resulting from the internal pressure. 
       SUMMARY 
       [0008]    This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed. 
         [0009]    One aspect of the subject matter disclosed in this application is a pressurizable holder for a sample to be examined by NMR, comprising a pressure retaining tube formed of material which is non-magnetic and does not provide isotropic electrical conductivity, with this pressure retaining tube surrounding one or more radio-frequency coils which in turn surround a space for the sample. 
         [0010]    A tube which does not provide isotropic electrical conductivity may be electrically insulating or may provide conductivity which is anisotropic. Both possibilities will inhibit the induction of eddy currents when there is a change in the magnetic field to which the sample holder is subjected, as happens when gradient coils which impose a magnetic field gradient are switched on and off. 
         [0011]    The pressure retaining tube may be formed of non-metallic material and may be electrically non-conductive. Employing a non-metallic electrically insulating material for the pressure retaining tube can prevent induction of unwanted eddy currents in the material of the tube when gradient coils in the NMR spectrometer are switched on and off to superimpose a temporary magnetic field gradient on the main magnetic field of the spectrometer. Locating the radio-frequency coil(s) inside the pressure retaining tube is beneficial, because the sample can occupy a large portion of the cross-sectional area within the coil. This is referred to as a “high coil filling factor” and gives a better signal-to-noise ratio than arrangements with the radio-frequency coil(s) outside a pressure retaining tube. Non-metals are usually avoided when strength is required, but with the coil(s) on the inside, the pressure-retaining tube can be constructed with substantial wall thickness, and thus be able to contain internal pressure, yet without reducing the coil filling factor. 
         [0012]    A range of nonmetallic materials may be used for the pressure retaining tube. One possibility is a ceramic material which may be homogenous. Another possibility is an electrically insulating composite comprising an electrically insulating matrix material reinforced with electrically insulating filaments. The matrix may be ceramic or may be an organic polymer and possibilities for insulating filaments include glass fiber. 
         [0013]    Another possibility within the present disclosure is that the tube comprises a composite material comprising an electrically insulating matrix material reinforced with conductive filaments such as carbon fibers. Carbon fibers provide good mechanical strength but are electrically conductive along their length, which makes them unacceptable inside a radio-frequency coil. However, this is not an issue when, as disclosed here, the pressure retaining tube surrounds the radio-frequency coil. Going further, if carbon fibers are oriented in a common direction so that they extend generally side by side, even though some may cross at a small angle, the there will be greater conductivity along the fibers than transversely across them, because conduction transverse to the fibers requires a pathway which crosses from one fiber to another. The conductivity of the carbon fibers is anisotropic with the consequence that the carbon fiber reinforced composite inhibits eddy currents when gradient coils are switched on and off. Some limited conductivity transverse to the fiber direction may be acceptable because even this will reduce eddy currents compared to a material which is electrically isotropic and conductive in all directions. 
         [0014]    A carbon fiber reinforced composite may comprise an organic polymer as matrix material strengthened by the carbon fibers. Carbon fibers may be the sole reinforcing fibers or may be employed jointly with other fibers such as glass fibers or fibers of poly-paraphenylene terephthalamide—marked under the trade name Kevlar. 
         [0015]    It is possible that all carbon fibers are in a common orientation, such as circumferential relative to the tube axis. However, it is also possible that carbon fibers could be arranged in some other way and still be able to resist flow of eddy currents. A possibility is to provide carbon fibers in a plurality of layers within the wall of the pressure retaining tube with different fiber orientations in the layers and spacing between the layers so that carbon fibers in one layer do not make good electrical contact with differently oriented carbon fibers in another layer. 
         [0016]    It is also conceivable that metallic fibers could be incorporated into a composite material, provided these were oriented and spaced so that electrical conductivity was anisotropic, that is to say conductive along the fibers but not in the transverse direction through contact between fibers. 
         [0017]    The sample contained in a sample holder as above may possibly be a liquid, or may be a solid, such as a rock core which may be a porous rock with liquid in its pores. 
         [0018]    In a further aspect of the present disclosure, a pressurizable holder for a liquid sample to be examined by NMR comprises a pressure retaining tube, which may metallic or or may be non-metallic, surrounding one or more radio-frequency coils which in turn surround a non-metallic tube to contain the liquid sample. A space between the pressure retaining tube and the sample-containing tube may be pressurizable so that the tube for the sample does not need to be dimensioned to resist stress. The tube for the liquid sample may contain a movable piston to allow liquid under pressure to make a controlled entry against resistance applied by the piston. Such an arrangement can provide a good coil filling factor within the radio-frequency coil while also providing physical containment of the liquid sample. The tube which contains the liquid sample may be part of a non-metallic vessel or it may have its ends closed by separate parts which may be metallic. Here also a composite material may be used as a material for the pressure retaining tube and the composite may contain carbon fibers as discussed above. 
         [0019]    Another aspect of the disclosed subject matter of this application is an NMR spectrometer providing magnetic field with a removable sample holder as above placed in the magnetic field such that the axis of the pressure retaining tube is transverse to the magnetic field and with the radio-frequency coil(s) of the sample holder connected to the spectrometer, so that application of a radio-frequency signal to a coil by the spectrometer induces magnetic resonance of nuclei in a sample within the sample holder. 
         [0020]    In a further aspect, the disclosed subject matter provides a method of examining a sample by NMR comprising placing a sample in a sample holder as above, placing the sample holder in an NMR spectrometer such that the axis of the pressure retaining tube is transverse to the magnetic field, connecting the radio-frequency coil(s) of the sample holder to the spectrometer, applying a radio-frequency signal to a coil within the sample holder to induce magnetic resonance of nuclei in the sample, and using the same or another coil within the sample holder to receive radio-frequency emissions from the sample. 
         [0021]    A sample holder as discussed above may be provided with structural parts to contain pressure longitudinally relative to the tube, these structural parts comprising pieces to apply force longitudinally at the ends of tube and connecting structure extending between these pieces outside the tube. 
         [0022]    The disclosed subject matter also includes an NMR spectrometer providing a magnetic field between a pair of pole pieces spaced from each other and a removable sample holder to be placed in the magnetic field, wherein the sample holder comprises a pressure retaining tube to contain the sample, a pair of end pieces to apply force at the ends of the tube and so retain pressure longitudinally within the tube and least two tie members connecting the end pieces to each other and extending alongside the tube, configured such that the NMR spectrometer accommodates the removable sample holder in the magnetic field between the pole pieces with the tie members extending at either side of the tube at positions spaced laterally, relative to the magnetic field, from the tube axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic cross-section showing basic parts of a sample holder on line I-I of  FIG. 2 ; 
           [0024]      FIG. 2  is a cross sectional view on line II-II of  FIG. 1 , with the sample holder in the magnetic field of an NMR spectrometer; 
           [0025]      FIG. 3  is partly in section on line III-III of  FIG. 2 , with the sample holder in the magnetic field of an NMR spectrometer; 
           [0026]      FIG. 4  is a cross section through the pressure-retaining tube, showing an example of the orientation of reinforcing fibers; 
           [0027]      FIG. 5  is a cross section of an example sample holder with provision for liquid flow; 
           [0028]      FIG. 6  is a schematic cross-section of another example sample holder; 
           [0029]      FIG. 7  is a schematic cross-section of a further example sample holder; 
           [0030]      FIG. 8  is a schematic cross-section of an example sample holder able to receive liquid under pressure; and 
           [0031]      FIG. 9  is a schematic cross-section of a second example of sample holder able to receive liquid under pressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The sample holder shown in  FIGS. 1 to 3  has a non-metallic pressure retaining tube  10  with substantial wall thickness, indicated by double headed arrow  11 , so as to withstand hoop stress, i.e., radially outward pressure from within. Longitudinal stress from internal pressure is restrained by end pieces  12 ,  14  which are connected together by a four tie rods  16  which are in tension. Thus the tube  10  is not required to provide longitudinal strength. Both end pieces  12 ,  14  may be removable and sealed to the tube by O-rings as shown or the tube  10  could be permanently attached to one of the end pieces. 
         [0033]    Fluid to pressurize the interior space within the tube  10  is admitted through an inlet  18  which can be closed by a valve  19 . A radio-frequency coil  20 , which is generally cylindrical is located within the tube  10  and is close to its inside wall. The coil  20  is shown schematically as a single helical solenoid coil but it may be wound with multiple layers. It is possible that there could be more one coil, for example one coil as an emitter and one as an antenna with the two coils wound one on top of the other. It is also possible that other configurations of coil(s) could be employed such as a pair of saddle coils with a space for the sample between them. A cylindrical rock core  26  which is to be examined by NMR fits within the interior of the coil  20 . 
         [0034]    Electrical connections  22  to the coil  20  are led out through pressure tight seals  24  in the end piece  12 . Devices for electrical feed through a pressure barrier are known. One possibility is single pin feed thru connectors available from Kemlon Products Inc., Pearland, Tex. 
         [0035]      FIGS. 2 and 3  show the sample holder in position within an NMR spectrometer. This spectrometer has a pair of disc shaped permanent magnets  31 ,  32  facing each other but spaced apart so that a magnetic field extends in the direction indicated by the arrow B 0  in  FIG. 2 . Both permanent magnets  31 ,  32  may be made of rare earth compounds to give a high magnetic field. Specifically, they may possibly be neodymium iron boron (NdFeB) magnets which can be manufactured in the desired shapes or assembled from smaller blocks. 
         [0036]    Gradient coils  34  are positioned adjacent the magnets  31 ,  32 . When these gradient coils  34  are energised, a magnetic field with a gradient along the length of these coils, i.e., as indicated by arrow B 1  is superimposed on the static field B 0 . This field gradient is proportional to the current in the coils  34  and its magnitude can thus be controlled. 
         [0037]    The pressure retaining tube  10  of the sample holder is positioned in the magnetic field between the gradient coils  34  and parallel to them. As best seen from  FIG. 2 , the tie rods at either side of the tube are then located at positions which are spaced laterally from the axis of tube  10 . 
         [0038]      FIG. 2  shows that the geometry can be compact. The four tie rods  16 , which are indicated as  16 . 1  to  16 . 4  in  FIG. 2 , are at a radial distance from the axis of tube  10  which is approximately the same as the distance from the tube axis to the magnets  31 ,  32 . The tie rods  16 . 1  and  16 . 2  are spaced from the axis of the tube  10  and from the tie rods  16 . 3  and  16 . 4  in directions transverse to the magnetic field B 0 . The spacing between the tie rods  16 . 1  and  16 . 3  is equal to spacing between rods  16 . 2  and  16 . 4 , and is approximately 1.25 times the external diameter of the tube  10 . It is thus less than double, moreover less than 1.5 times, the external diameter of the tube  10 . The spacing in the direction of the magnetic field B 0  between the tie rods  16 . 1  and  16 . 2  is equal to spacing between rods  16 . 3  and  16 . 4  and is less than the spacing between the magnets  31 ,  32 . Moreover, it is less than the diameter of the tube  10 . 
         [0039]    As can be appreciated from  FIGS. 1-3 , the various parts are dimensioned such that a core  26  of standard diameter fills most of the available cross-section within the coil  20 . Thus the coil filling factor is high. It is possible for the pressure retaining tube  10  to have substantial wall thickness because coil filling factor is independent of this thickness. Consequently a number of non-metallic materials can be used for the tube  10 . One possibility is an inorganic ceramic material. Another possibility is a fiber-reinforced composite in which elongate fibers are bound in a matrix material which may be an inorganic ceramic or may be an organic polymer. Glass reinforced polymer (GRP) and carbon fiber reinforced polymer (CFRP) are examples of composite materials. An organic polymer matrix of a composite material may be any of a number of polymers including epoxy resin and polyetherether ketone (PEEK) which is well established as an engineering plastic. 
         [0040]      FIG. 4  is a cross section through a pressure retaining tube  10  which is a composite reinforced with carbon fiber. Carbon fibers are conductive, but can be used in the wall of tube  10  because it is outside the coil  20 . The carbon fibers  36 ,  37  are shown with exaggerated thickness for the purpose of explanation. The carbon fibers within the wall of the tube  10  extend circumferentially around the central axis of the tube  10  but extend only slightly, if at all, in the axial direction of the tube. In this orientation the fibers are best placed to withstand hoop stress. Fibers may extend around the axis for a full circle of 360 degrees or more, as illustrated by the fibers  36  or they may extend as arcs of less than 360 degrees, as illustrated by the fibers  37 . A layer  38  of glass fibers is included within the thick wall of the tube  10 . These insulating fibers extend axially (the direction indicated by double headed arrow  39  in  FIG. 1 ) to increase strength of the tube  10  in its longitudinal direction. This axial strengthening is not required for resisting internal pressure, but makes the tube  10  more robust during handling and assembly of the sample holder. 
         [0041]    Because the conductive carbon fibers are oriented in a common, circumferential direction, the electrical conductivity within the wall of the tube  10  is anisotropic, with much less conductivity in the axial direction than circumferentially around the tube axis. Consequently, turning the gradient coils  34  on and off will not generate eddy currents to the same extent as in a material which is fully conductive in all directions because the electrical resistivity transverse to the fibers&#39; orientation, i.e., in the axial direction, will inhibit current flow and thus inhibit circulation of eddy currents. 
         [0042]    It has been reported in the literature that conductivity in the direction of carbon fibers may be greater than conductivity transverse to the fibers by a factor which is dependent on composition and manufacturing procedures, but this factor may be 100 or considerably more. Conductivity transverse to fibers is dependent on the volume fraction of carbon fibers in the composite. If carbon fibers lying in a common orientation are no more than about 30 to 40% by volume of the composite, conductivity transverse to the fibers may be very small. See for instance Pratap et al., IEEE Transactions on Magnetics, Vol. 32, March 1996, pp. 437-444 at page 438. Park et al., Smart materials and Structures, Vol. 16, 2007, pp. 57-66 at page 61 measured conductivities along and transverse to fibers at a number of volume fractions, and even at a volume fraction of 55 to 60% carbon fiber oriented in a common direction the conductivity along the fibers was several orders of magnitude greater than conductivity transverse to them. 
         [0043]    There are a number of ways to arrange reinforcing fibers in the wall of tube  10  to incorporate carbon fiber and achieve anisotropic conductivity. Carbon fibers may be used throughout the wall of tube  10  or in layers within the wall of tube  10  where the only reinforcing fibers are carbon fibers in a common orientation. Alternatively carbon fibers might be mixed with non-conducting fibers such as glass or Kevlar, for example using sufficient non conducting fibers that the volume fraction of carbon fibers is below 50% of the composite but the total of all fibers was a volume fraction above 50%, such as 55 to 60% of the composite 
         [0044]    If glass fibers are included, they may be confined to a layer such as layer  38  or may be distributed more generally. They may extend axially as described or predominantly axially but also a layer of glass fibers could be a mat of randomly oriented fibers. Carbon fibers could be arranged in layers such that the fibers in each layer are in a common orientation, but with different orientations in different layers so that anisotropic conductivity in one layer is in a different direction from that in another, provided conductivity between layers is low. Thus with circumferentially oriented carbon fibers as shown in  FIG. 4  it would be possible to incorporate carbon fibers extending axially, provided they were electrically insulated from the circumferential fibers, such as in a separate layer akin to the layer  38 . 
         [0045]    Another possibility for the arrangement of carbon fibers would for example be a layer of carbon fibers extending as a right handed helix, overlaid with an insulating layer  38 , which in turn is overlaid with carbon fibers in a left handed helix. 
         [0046]      FIG. 5  shows an arrangement in which a number of features are the same as in  FIGS. 1 to 3 , and are indicated with the same reference numerals. In this embodiment, the cylindrical surface of the core  26  is enclosed by an elastomeric sleeve  44 . This sleeve  44  is urged against the cylindrical surface of the core  26  by a fluid introduced along line  46  to fill the space between the sleeve  44  and the inside wall of tube  10 . This fluid may be a perfluorocarbon so that the fluid does not contain hydrogen atoms and does not give any signal when NMR is used to examine resonance of hydrogen nuclei. Consequently, the end faces of the core  26  are exposed but the cylindrical surface of the core is sealed by the elastomeric sleeve  54 . Electrical connections  48  to the coil  20  extend radially through the wall of tube  10 , either extending through feed through connectors or sealingly embedded in the material of the tube wall. 
         [0047]    Fluid to pressurize the interior of the tube  10  can be introduced along passages  50  and  52  which extend through the end members  12  and  14 . It is also possible to flow fluid linearly through the core  26 , using passage  50  as an inlet for fluid under pressure while maintaining a slightly lower pressure in passage  52 . At least one spacer piece  64 , made of non-magnetic and electrically insulating material is used to keep the core  26  in position. Such a spacer piece may be made of a porous material or may incorporate apertures, to allow flow out of the end face of the core  26  to reach the passage  52 . 
         [0048]      FIG. 6  shows a variation of the arrangement of  FIGS. 1 to 3 . The coil  20  is embedded within the material of the tube  10  close to its internal face. Electrical connections  58  to the coil  20  are led out at the ends of tube  10 . With such an arrangement the material of the tube  10  which is positioned inwardly from the coil  20  could contribute to the NMR signal obtained. For example, if the intention is to observe resonance of hydrogen atoms in the core  26 , hydrogen atoms (if any) in the thin layer of tube  10  which is positioned inwardly from the coil  20  make a small and constant contribution to the NMR signals. One possibility for avoiding this is to make the tube  10  from an inorganic ceramic material or a composite which is a fiber reinforced inorganic ceramic. Such a material may have no hydrogen atoms in its composition and therefore provide no NMR signal when the resonance of hydrogen nuclei is being examined. 
         [0049]      FIG. 7  shows a further possible arrangement. The pressure retaining tube  10  is again made of non-magnetic material, dimensioned to withstand hoop stress. It is closed at one end by an end member  62  which is generally similar to the end members  12 ,  14  shown in  FIG. 1 . The other end of the tube  10  is closed by a piston  64  which is a sliding fit within tube  10  and sealed to it by  0 -ring  66 . The piston  64  also slides within a cylinder  68  supported by yoke  70  which in turn is connected to the end member  62  by tie rods  16 . Thus, longitudinal stress from pressure within the tube  10  is retained by the tie rods  16  which are in tension, connecting the end member  62  to the yoke  70 . The space within the tube  10  can be pressurized by supplying hydraulic oil through an inlet, not shown, to the interior  72  of cylinder  68  so as to urge the piston  64  into the tube  10 . 
         [0050]    A radio-frequency coil  20  is positioned within the tube  10 . A rock core  26  can be received within the space within the coil  20 . In this embodiment, the cylindrical surface of the core  26  is enclosed by an elastomeric sleeve  44  which, as in  FIG. 5 , is urged against the cylindrical surface of the core  26  by a fluid introduced along line  46 . This fluid may be a perfluorocarbon so that the fluid does not contain hydrogen atoms. Consequently, the end faces of the core  26  are exposed to fluid in the tube  10  but the cylindrical surface of the core is sealed by the elastomeric sleeve  44 . Tubes  74  and  52  allow the interior of tube  10  to be filled with fluid. If desired, fluid can be made to flow linearly through the core  26 . For this fluid is supplied along tube  74  to enter one end face of the core and fluid leaving through the other end face of the core leaves through tube  52 . As in  FIG. 5 , a spacer  60  is provided between the core  26  and the end piece  62 . 
         [0051]    As in  FIGS. 1 to 3  the core  26  fills most of the cross-section within the coil  20 . The core holder with the core  26  located within the coil  20  can be placed between the magnets of an NMR spectrometer in the same manner as is shown by  FIGS. 2 and 3 . 
         [0052]      FIG. 8  shows a sample holder for a liquid sample which is required to be kept under pressure. The general arrangement has similarity to that in  FIGS. 1 to 3 . A pressure retaining tube  10  contains hoop stress, while longitudinal stress is contained by end members  12 ,  14  connected by tie rods  16  which are in tension. There is a radio-frequency coil  20  within the tube  10 . 
         [0053]    Inside the radio-frequency coil there is a cylindrical vessel  80  made of an inorganic ceramic material. This is non-magnetic and electrically insulating. A suitable material is magnesia stabilized zirconia. Dynamic-Ceramic Ltd of Crewe, UK supply this under the name Technox 500 as a raw material for fabricating ceramic articles. This vessel  80  is constructed as a rigid vessel able to sustain its own shape and provide a container for liquid, but with thin walls so that it does not take up an excessive amount of the cross section within the coil  20 . The generally annular cavity  81  between the vessel  80  and the tube  10 , which contains the coil  20 , is pressurized with a fluid admitted through inlet  82 . This fluid may be a perfluorocarbon so that it does not contain hydrogen atoms. The cavity  81  is pressurized to a pressure which is that same as, or close to, pressure inside the vessel  80 . By balancing pressure inside and outside the vessel  80 , it does not need to be constructed as a pressure vessel with thick walls. 
         [0054]    In order to admit a sample fluid under pressure, the vessel  80  encloses an internal floating piston  84 . The sample fluid is admitted under pressure along line  86  to the chamber  87  at one side of the piston  84  while the chamber  88  at the other side of floating piston  84  is pressurized with fluid supplied along line  89 . This fluid may be the same as that supplied through inlet  82  to space  81 . The pressure in chamber  88  is reduced to slightly less than the pressure of the incoming sample entering through inlet  86 , so that the incoming sample slowly drives the piston  84  along the vessel  80 , expelling fluid along line  89  until vessel  80  is filled with the sample fluid. The cavity  81  may be connected to the vessel  80  at one side or other of the piston  84  so that pressure in the vessel is communicated to the cavity  81  and thus pressure in the cavity is automatically balanced with a pressure in the vessel  80 . A connection to the cavity  81  may allow communication of pressure to cavity  81  without permitting fluid to flow from the vessel  80  into the cavity  81 . Thus sample liquid may be excluded from cavity  81 . 
         [0055]      FIG. 9  illustrates an arrangement which is similar in principle (and equivalent parts are indicated with the same reference numerals) but has a tube  90  (which is made of magnesia stabilized zirconia) closed by the end members  12 ,  14  to receive the liquid sample. This tube  90  is again a rigid tube able to sustain its own shape and enclose liquid. It is surrounded by a pressure retaining tube  10  with the coil  20  in a cavity which is pressurized with perfluorocarbon fluid via line  92  which connects to line  89 . Thus the cavity outside tube  90  is at the pressure of (and is filled by) the perfluorocarbon fluid which is expelled from tube  90  as the sample fluid enters and drives the floating piston  84  along the tube  90 . 
         [0056]    Within the tube  90  there are end pieces  94 ,  95  which occupy space at each end of the interior of tube  90 . When the sample has been introduced through inlet  86  and the floating piston  84  has been driven fully along the tube  90  so that it abuts end piece  95  as shown, the volume occupied by the sample is the space  96  within a middle part of the length of coil  20 . Consequently the liquid sample is spaced from any distortions of the radio-frequency field near the ends of the coil  20 . 
         [0057]    The purpose of using NMR to examine a liquid sample may be to observe diffusion within the liquid, for which a magnetic field gradient provided by gradient coils  34  will be employed. Consequently, in  FIGS. 8 and 9  the pressure retaining tube  10  may be made of non-metallic and electrically insulating material such as the ceramic and composite materials already mentioned. In the event that the NMR examination of a liquid sample does not require a magnetic field gradient, so that gradient coils  34  will not be present, or will not be operated if present, the pressure retaining tube  10  of these embodiments could be made of a non-magnetic metal such as stainless steel. 
         [0058]    It will be appreciated that the example embodiments described in detail above can be modified and varied within the scope of the concepts which they exemplify. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.