Abstract:
An NMR sample, frozen in a tube and having achieved a specified higher degree of polarization for an NMR experiment, is rapidly heated and melted before it loses a significant portion of the achieved polarization and still retains 10% or more. The heating may be achieved by passing an electric current, or currents, through a heating wire, or wires, provided to the tube, or by placing the tube inside a furnace provided with an electric radiator. NMR experiments with high sensitivity can be carried out with such a sample still retaining a high level of polarization.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method of rapidly heating a NMR sample and an apparatus for using such a method. More particularly, this invention relates to a method and an apparatus for heating an NMR sample to room temperature or above from an initial temperature which is sufficiently low for polarizing the sample to a desired degree within a time comparable to its thermal relaxation time. 
     There is a class of experiments in which it is desirable to rapidly heat a NMR sample from a very low temperature to room temperature or above. The degree of polarization normally achieved by a NMR sample is inversely proportional to the absolute temperature and proportional to the magnetic field strength. For example, the thermal polarization of a sample at a temperature of 3° K is 100 times greater than the polarization produced near room temperature, i.e. about 300° K. If the sample can be quickly warmed from 3° K to 300° K within a time period comparable to its thermal relaxation time, a high resolution NMR spectrum can be achieved with greatly enhanced sensitivity. To achieve the same sensitivity at room temperature would require a time period roughly 10 4  times longer. 
     The solids effect can be achieved in solids containing electron radicals, i.e., unpaired electrons that are coupled to nearby nuclei. (See, for example, R. A. Wind et al., “Applications of Dynamic Polarization in  13 C NMR in Solids”, Progress in NMR Spectroscopy, Vol. 17, pp 33-67, 1985. In particular, see Sec. 2.3, The Solid State Effect). RF or microwave irradiation of the solid near the frequencies ν e +ν n  or ν e −ν n  causes simultaneous electron and nuclear spin flips, where ν e  is the electron Larmor frequency and ν n  is the nuclear Larmor frequency. The population redistribution results in an enhanced nuclear polarization. The enhancement can be substantial, approaching the ratio of γ e /γ n . For proton nuclei this ratio is approximately 650 and 3,400 for  13 C. 
     By combining the two effects, i.e., achieving a high thermal polarizationation at low temperatures, using the solid effect to further enhance the nuclear polarization while at the low temperature, one can achieve a substantial nuclear polarization. By quickly warming the sample much of this polarization can be maintained even after the sample melts, enabling one to achieve very high sensitivity NMR experiments upon liquid samples. The nuclear relaxation time T 1  is a measure of how long this excess nuclear polarization time lasts. At low temperatures, near the initial temperature of the sample when it was polarized, the relaxation time is typically very long, several minutes to hours. As the sample melts, the relaxation time becomes much shorter, perhaps in the range of a second or less to tens of seconds. After the sample has been polarized to achieve an initial polarization P 0 , the heating process is started. As the sample is heated it begins to lose some of its polarization. After a time τ, the remaining excess polarization, P, is given by the approximate formula: 
     
       
           P=P   0  exp−∫ 0   τ   dt/T   1 ( t )} 
       
     
     Using standard NMR procedures the relaxation time T 1  of the sample can be measured for various temperatures. The sample temperature can also be measured for various pre-selected heating times. Thus for a given experimental relaxation time, T 1 , can be expressed as a function of time, T 1 =T 1 (t), enabling one to integrate the expression above and obtain an estimate of the excess polarization. Typically the heating rate should be sufficiently rapid that ratio of excess polarization P to the initial polarization P 0  be down not more that a factor of 20, therefore P/P 0 &gt;0.05. 
     In a typical experiment, the analyte (material to be analyzed by NMR) is dissolved in a mixture of 40:60 solution of water/glycerol with the free radical 4-amino TEMPO as the source of electron polarization. (See C. T. Farrar et al., “High-Frequency Dynamic Nuclear Polarization in the Nuclear Rotating Frame”, J. Magn. Resonance, Vol. 144, pp 134-141, 2000). As pointed out in this reference, other dynamic nuclear polarization techniques may also be used to polarize samples at low temperature. 
     The microwave sources used to produce the microwave transitions range from low cost solid state oscillators such as an impact diode or Gunn oscillators to high power gyrotron oscillator tubes such as manufactured by CPI. 
     Golman and Ardenkjaer-Larsen have suggested the method of quickly warming the sample by adding a hot solvent. This method, however, has the effect of diluting the sample further and does not permit repeated experiments with the same sample unless it is purified between successive spectral runs. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a method and an apparatus for using the solids effect to polarize a sample while it is very cold and then rapidly heating it to a liquid state near room temperature within a time sufficiently short to prevent substantial loss of polarization during the heating process so as to enable narrow line liquids NMR data to be taken while a high degree of polarization remains. 
     It is another object of this invention to provide such method and apparatus which will not dilute the sample being heated such that experiments can be repeated many times on the same sample. 
     In one embodiment of this invention, the sample is rapidly heated by radiation. This is achieved by rapidly moving the pre-polarized sample to the center of a furnace tube. Analysis shows that a 5 mm sample tube of water (4.2 mm ID) could be heated from 1° K to 303° K (30° C.) in about 14 seconds for a furnace tube operating at 1000° K (1273° C.). After a proper amount of heat is absorbed, the sample could be dropped into the NMR probe for analysis. 
     An alternative heating method involved using a sample tube with embedded heater wires that produce heat by passing a current through them. The heater section of the sample tube could be located at one end of the sample tube. With the sample initially at the upper end of the sample tube so that after the sample is heated and melted, the sample would collect at the lower end of the tube free of the heater wires. The melted sample is now placed in the NMR spectrometer for analysis. 
     The processes described above can be carried out automatically after the sample containing the free radical and solvent is once loaded into the sample tube and sealed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a heater sample tube which is a part of an apparatus embodying this invention; 
     FIG. 2 is a schematic drawing showing an apparatus and a method embodying this invention, representing the heating unit of FIG. 1 only symbolically for convenience; 
     FIG. 3 is a schematic drawing of a heater sample tube structure embodying this invention with heat supplied by an external heating element in thermal contact with the sample tube; 
     FIG. 4 is a schematic drawing of an alternative heater sample tube structure embodying this invention with heat supplied by an external heating element in thermal contact with the sample tube; 
     FIG. 5 is a schematic drawing of still another heater sample tube structure embodying this invention with heat supplied by an external heating element providing radiative heat to the sample material; 
     FIG. 6 is a schematic drawing of the heater sample tube structure of FIG. 5 being used in an alternative method embodying this invention; 
     FIG. 7 is a schematic drawing of a sample tube heating apparatus embodying this invention; and 
     FIG. 8 is a schematic drawing for another sample tube heating apparatus embodying this invention. 
     Throughout herein, components which are equivalent or at least alike may be indicated by the same numerals and may not necessarily be explained repetitively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described next by way of an example. A sample to be analyzed is added to a mixture of water and glycerol which also contains a free radical. FIG. 1 shows a special heater sample tube  10  embodying this invention into which this mixture is then placed. The sample tube  10  is characterized not only as having a closed bottom and a sealable top but also as containing therein a heating unit  12 . The heating unit  12  typically comprises an array of heating wires  13  connected mutually in parallel such that they are all driven in parallel, thereby limiting so the voltage drop across the heating wires  13 . Numeral  15  indicates metal posts penetrating the side of the sample tube  10  for supporting the wires  13 . The heating wires  13  are selected so that their electrical resistance is sufficiently low and that little or no electrolysis of the sample material will take place. Alternatively, they may be coated with or contained in an electrically insulating material that has a high thermal conductivity such as very small sapphire tubes. The wires  13  of the heating unit  12  extend along the tube  10  and through about one third of its length from one end. 
     As the aforementioned mixture is put into the tube  10  held vertically upward (with the top above the bottom), say, to make it a little less than half full, the heating unit  12  is completely immersed in the mixture, and then the top of the tube  10  is sealed, as schematically shown at Position A in FIG.  2 . (For the simplicity of presentation, the heating unit  12  is drawn simply as a single resistor in FIG. 2 in order to show only whether the tube  10  is right-side up or upside-down.) While at Position A, the sample inside the tube  10  is frozen such that it will stay at the bottom part of the tube  10  when the tube  10  is inverted, as shown at Position B in FIG.  2 . 
     The inverted tube  10  containing the frozen sample at its bottom is moved from Position B to Position C inside a magnetic field established for producing the so-called dynamic nuclear polarization (DNP) effect, a magnet providing this magnetic field being indicated by numeral  50 . At Position C, the sample, now being in a DNP region, is further cooled as a microwave field is applied to produce the DNP effect, a microwave radiator being indicated by numeral  52 . The frequency of the microwave magnetic field is set equal to the difference between the electron resonance frequency and the resonance frequency of the nuclei to be polarized. In some applications, the microwave frequency is set to be equal to the electron resonance frequency, and in other applications the microwave frequency is set to equal the sum of the electron and the nuclear resonance frequencies. 
     After the high Boltzmann polarization of the electrons is transferred to the nuclei and the sample has become highly polarized, the sample is moved back to Position B where power is applied from a power source  54  to drive the wires  13  of the heating unit  12 . Sufficient power is applied to sufficiently quickly melt the sample and to bring the liquid to a desired temperature within a short enough time with respect to the relaxation time of the nuclei, as discussed above. Since the tube  10  is kept in the inverted orientation, the melted liquid pools at the lower end (i.e., at the sealed top) of the tube  10 . 
     The sample in the melted state is now quickly moved to Position D provided with an NMR probe coil  56  inside the magnetic field established by the magnet  50 . Desired NMR experiments are carried out at Position D, with the tube  10  kept in the inverted orientation, until a large portion of the sample polarization is lost and the sample needs to be polarized again. 
     For repolarizing the sample, the sample is moved back to Position A and the process described above is repeated. After all the desired data have been taken, the sample may be manually removed in preparation of another experiment. 
     Although the method according to this invention was described above as if each step were to be carried out manually, an automatic sample handling system may be provided for carrying out the steps described above with reference to FIG.  2 . 
     The implantation of the heater wires directly into the sample material provides a very rapid method of heating the entire sample in a uniform way. This is particularly advantageous when using large samples of 10 mm or more. For small sample tubes (particularly 3 mm or less), it is often sufficient to provide the heating by thermal contact through the sample tube wall by thermal radiation from a heating surface which does not make direct contact with the sample tube wall. 
     In FIG. 3, which shows a heater sample tube structure, a heating element  113  is affixed onto a sample tube  110  such that heat flows directly from the heating element  113  through a thin support structure  115  to the outer surface of the sample tube  110 . The heating element  113  forms a serpentine structure with vertical extent sufficient to heat the entire length of the sample material. The support structure  115  extends completely around the sample tube  113 , thereby providing heat along the entire length and circumference of the sample material contained in the sample tube  110 . The purpose of the support structure  115  is to support heating wires of the heating element  113 . 
     In FIG. 4, which shows another heater sample tube structure, a heating element  114  in the shape of a spiral coil is wound on a sample tube  110 , affixed in place by means of a support structure  116 . Alternatively, the heating element  114  may be wound directly on the sample tube  110 . The heating element  114  may comprise a non-ferromagnetic conductor such as tantalum or tungsten, and may be in the form of round wires or thin flat wires. The sample tube  110  may be made of materials such as quartz, alumina, or sapphire. Sapphire has the advantage of providing extremely high thermal conductivity at low temperatures. Alumina has intermediate thermal conductivity, and quartz provides a lower thermal conductivity material at a lower cost. 
     FIG. 5 shows still another heater sample tube structure with a sample tube  210  completely detached from a furnace  200 , which contains an electrical heated radiator  220  with embedded electric heating element  225  and surrounded by a thermal radiation shield  230 . During a sample heating process, the sample material  211  is in the furnace  200 . The sample tube  210  is arranged such that the sample material  211  is centered within the walls of the thermal radiator  220 . The radiator  220  is electrically heated to a high temperature, typically between 600° C. and 1500° C. 
     Thermal radiation from the radiator  200  is incident and absorbed by the sample, rapidly heating it. For the most part, the thermal radiation penetrates the sample tube walls and imparts heat directly to the sample material. As the sample melts, it pools in the lower part of the tube  210 . The time required to heat the sample is estimated from the Stefan-Boltzmann equation t=0.5 hρd/(εσT 4 ) where t is the time (in seconds) to heat the sample from its initial temperature to the desired final temperature, h is the total required heat in cal/gram of the sample, ρ is the density of the sample in g/cm 3 , d is the inner diameter of the sample cell in cm, ε is the emissivity of the sample, and σ is the Stefan-Boltzmann constant (=1.35×10 −12  (cal/sec)/K 4  cm 2 ). 
     In operation, the heater-sample tube configuration of FIGS. 3,  4  and  5  may be used in the same manner as described above with reference to FIG.  1  and illustrated with the aid of FIG.  2 . 
     An alternative mode of using the system shown in FIG. 5 is to keep the liquid and sold in the same region as the sample is melting. As shown in FIG. 6, a frozen sample  211  is contained in the lower region of the sample tube  210  as it is placed in the furnace  200 . As the sample  211  is heated, the liquid and solid remain in contact until the entire sample  211  is melted. This makes for a somewhat simpler system for moving the sample between the various positions in the various steps of the experiment, as will be illustrated in FIG.  7 . 
     FIG. 7 schematically illustrates a method of automatically cycling a sample  211  by moving through various positions. After the sample  211  is loaded into a sample tube  210  and the sample tube  210  is sealed, it is attached to one end of a wire or cord  250 , the other end of the wire or cord  250  being wound on a spool  240 . The spool  240  is turned by a stepping motor controlled by a computer (not shown), enabling the sample  211  to be moved to various positions required for carrying out various steps of the process to be described below. First, the sample  211  is lowered to Position A inside a cooling chamber  230 . After the sample  211  becomes frozen and cooled to a desired starting temperature, it is raised to Position C where a microwave irradiator  52 , which may comprise a wire or a microwave cavity, applies microwave radiation to the sample, thereby achieving the DNP. In some experiments, a radio frequency field may also be applied at this point. Additional cooling may be applied by an applicator (not shown) while the sample is being irradiated by microwaves. After the sample  211  is polarized, it is moved to Position B where the sample  211  is melted and brought to the temperature desired for NMR analysis. Power supply  54  supplies power to a furnace  200 . The sample  211  is then quickly lowered to Position D where the desired NMR experiments are carried out with the aid of a coil  56 . When the sample polarization becomes too small to be useful, the sample  21  is lowered to Position A again and the steps of the process described above are repeated. 
     FIG. 8 shows another embodiment of the invention different from the one described above with reference to FIG. 7 in that the cooling chamber  230  is brought inside the strong field of the magnet  50  and combined with the microwave irradiator  52  in the same region (indicated as Position A in FIG. 8) and also in that the furnace  200  containing the thermal radiator  220  is also brought into the field of the magnet  50 . A thin metal or dielectric rod  252  or the wire or cord,  250  of FIG. 7 is used to support and move the sample to the various positions in place. One end of rod  252  is fixed to sample tube  210  and the other end fixed to the rack  244  of rack and pinion gear  242 . The rack and pinion gear translates the linear motion required to move the sample to a rotary motion of pinion  246 . Pinion  246  is coupled to a stepping motor, or a motor and a position encoder, not shown, to indicate and control the position of sample  210 . The embodiment shown in FIG. 8 is preferable because it is advantageous to provide the rapid heating within the magnetic field since the sample polarization then decays toward the value determined by the magnetic field strength of the magnet  50  rather than by the earth&#39;s magnetic field strength. For the sake of clarity, the power source  54  of FIG. 7 is not shown in FIG.  8 . 
     The invention has been described above with referenced to only a limited number of examples, but they are not intended to limit the scope of the invention. For example, in some cases it may be desirable to enclose the regions traversed by the sample and the sample positioning mechanisms within a vacuum tight enclosure, permitting a controlled atmosphere surrounding the sample tube. Many modifications and variations are possible within the scope of the invention, and the disclosure is intended to be interpreted broadly. In particular, where the heating of an NMR sample is said to be achieved rapidly, a time period of less than 30 seconds, and preferably less than 15 seconds, is intended and where a frozen sample is described as retaining a substantial amount of its achieved polarization while being heated by a method of this invention, the term “substantial amount” is intended to be understood as being 10% or more. This is because one of the main advantages to be gained by this invention is an increase in sensitivity in NMR experiments. Even if the gain in sensitivity is 10% of the maximum sensitivity gain achievable according to this invention, the time required for the experiment will be reduced by a factor of the order of 100, and it is indeed a significant advantage that would overcome the additional cost and complexity required to practice the present invention.