JP5939484B2 - NMR probe device - Google Patents

Description

The present invention is in the nuclear magnetic resonance (NMR) measurements in the field of NMR probe apparatus, NMR sample tubes and pulsed NMR measurement method.

For example, as is well known in Non-Patent Document 1, NMR measurement is performed by measuring the interaction between a nucleus of a sample placed in a static magnetic field (B0) and a high-frequency magnetic field (B1), thereby being a sample to be measured. This is done in order to obtain information on the structure etc. B1 is applied in a direction perpendicular to B0. Measurement is performed by applying a high-frequency magnetic field generated by a measuring coil to the sample and observing the response. The sample is stored in a sample container called a sample tube, and is set in a space inside the measurement coil.
The NMR probe apparatus includes a measurement coil, a resonator, a matching unit, and the like, and is used to perform NMR measurement by setting a sample at a predetermined position near the center of a static magnetic field. The NMR probe device is mechanically connected to the magnetic field generator, and at the same time, electrically connected to the NMR spectrometer, and NMR measurement is performed.

In general, in solid state NMR measurement using a solid sample as a measurement object, it is necessary to apply a high-intensity high-frequency magnetic field to the sample. Therefore, an efficient solenoid coil is used as a measurement coil.
FIG. 1 shows a structure in the vicinity of a measurement coil in a probe apparatus generally used in solid-state wide NMR measurement, which is one of such solid-state NMR measurements. The coordinate system employs an XYZ orthogonal coordinate system with the center of the static magnetic field B0 as the origin, and the direction of the static magnetic field B0 is the Z axis. Note that components not specifically described are not shown.

The solenoid type measurement coil 10 is installed such that its center is the origin (center of the static magnetic field B0). The central axis of the measuring coil 10 is set on the XY plane orthogonal to the static magnetic field B0. In this case, the central axis is particularly set on the X axis. The central axis of the measuring coil is the central axis of the approximately cylindrical magnetic field generated inside the measuring coil when a current is passed through the measuring coil. In the case of a solenoid type coil, the shape of the coil is cylindrical. It is the central axis of the cylinder when expressed approximately by. The high-frequency magnetic field B1 generated by the measuring coil 10 is directed in the direction along the central axis of the measuring coil, that is, the X-axis direction. The cylindrical sample tube housing 30 is installed coaxially with the measurement coil 10, and the sample is stored in a space inside the sample tube housing 30 so that the center thereof is located at the origin. The sample tube housing 30 has an opening in the axial direction, and the opening is sealed with a lid 40.

The magnitude of the static magnetic field used is generally 1 to 25 T, and a superconducting magnet is generally used as the magnetic field generator. The frequency of the high-frequency magnetic field varies depending on the magnitude of the static magnetic field and the nuclide to be measured, but is generally 10 to 1000 MHz.

The pulsed NMR method is also called a Fourier transform type NMR (FT-NMR) method, and is known as a method for obtaining a highly accurate NMR spectrum. The NMR spectrum is a diagram showing the intensity (vertical axis) of the NMR signal with respect to the frequency (horizontal axis).

The NMR measurement in the pulsed NMR measurement is generally performed as follows. First, a high frequency magnetic field having a frequency substantially equal to the resonance frequency (Larmor frequency) of the nuclear spin of the atom to be measured is applied in a pulsed manner. Such a pulsed high-frequency magnetic field is called a pulsed high-frequency magnetic field or an excitation pulse. The pulsed high-frequency magnetic field is a high-frequency magnetic field having a pulse-like intensity (or having a pulse-like envelope) that has a substantially constant wave height and can be emitted for a short time. The application of the pulse high frequency magnetic field is performed via the measurement coil 10. Next, a high frequency magnetic field generated from the sample and having a frequency substantially equal to the resonance frequency is observed. The frequency band to be measured is approximately 1/10000 (10 ppm) to 1/500 (2000 ppm) with respect to the resonance frequency, and is set according to the purpose of the measurement. This high frequency magnetic field is observed through the measurement coil 10. The high-frequency magnetic field observed at this time is called free induction decay (FID). An NMR spectrum can be obtained by Fourier transforming the magnitude of the FID signal observed along the time axis.

The excitation intensity of the sample with respect to the nuclear spin is represented by the product of the intensity (magnitude) of the excitation pulse to be applied and the pulse width (time length). The intensity of the obtained FID behaves sinusoidally with respect to the excitation intensity. The excitation pulse that provides the maximum intensity FID is called a 90-degree pulse.
The excitation pulse to be applied has a bandwidth on the order of the reciprocal of its pulse width, and can excite a nuclear spin having a resonance frequency of that bandwidth. Therefore, when it is desired to obtain a broad spectrum, that is, to excite a wide range of spins, the width of the excitation pulse must be reduced. In order to obtain a sufficient excitation intensity with a narrow pulse width, the intensity of the excitation pulse must be increased. In other words, if the intensity of the excitation pulse is large, the pulse width of the 90-degree pulse becomes narrower in inverse proportion to the magnitude of the excitation pulse, and a wider band measurement can be performed.

In general, solid NMR often has a broad spectrum compared to solution NMR. For this reason, in solid-state NMR measurement, a high-intensity excitation pulse is often required, and in general, the maximum intensity of the excitation pulse that can be applied is preferably as large as possible.

The measurement coil generally forms part of a resonator composed of a capacitor and an inductor. The resonator is in electrical communication with the outside through a matching unit. In principle, the strength of the high-frequency magnetic field is proportional to the square root of the magnitude of the high-frequency power input from the outside to the resonator. A high voltage is generated inside the resonator, particularly at both ends of the coil, in proportion to the square root of the input power. Generally, in a probe apparatus for solid-state NMR, this voltage reaches a high voltage of 1000 volts or more at the maximum load. As the input power is increased, electrical discharge occurs inside the resonator with a certain threshold as a boundary, and no more power acts effectively. That is, in a state where discharge is occurring in the NMR probe internal device, the input power does not contribute to the generation of the high frequency magnetic field. In order to generate a high-frequency magnetic field with a coil, a high-frequency high current must flow through the coil. However, since the coil has an inductance, a high voltage must be applied to both ends of the coil. The voltage required to obtain a specific high frequency magnetic field strength increases in proportion to the inductance of the measuring coil.

As described above, in pulse NMR measurement, due to discharge in the NMR probe apparatus and the like, the intensity of the excitation pulse has an upper limit specific to the NMR probe apparatus. Therefore, there is a lower limit of 90 degree pulse width corresponding to the upper limit of the intensity of the excitation pulse, which is an obstacle to wideband measurement.

The demand for a high-intensity excitation pulse is not limited to the example described above, but also exists in other NMR measurement methods. For example, in solid MQMAS (multi-quantum magic angle spinning) measurement for a quadrupole nucleus, the spectral quality is expressed as a function of the quadrupole coupling constant, the strength of the static magnetic field B0, and the strength of the high-frequency magnetic field B1, and the static magnetic field B0 The higher the high frequency magnetic field B1, the better.

  In general, in NMR measurement, the larger the static magnetic field, the better the sensitivity and resolution. Since the resonance frequency of the nuclear spin is proportional to the static magnetic field, the resonance frequency increases as the strength of the static magnetic field increases. In order to generate B1 of the same size with the same measurement coil, the higher the frequency, the greater the voltage applied to both ends of the coil, so that discharge or the like is more likely to occur.

In recent years, as a source of the static magnetic field B0, an NMR superconducting magnet that generates a strong magnetic field exceeding 21 T has been put into practical use. Measurement has come to be done. Along with this, the problem that the intensity of the high-frequency magnetic field B1 is insufficient in the existing NMR probe apparatus and the benefits of the strong magnetic field cannot be fully obtained has become remarkable.

  As one of methods for applying a high-intensity high-frequency magnetic field to a sample, use of a microcoil as disclosed in Patent Document 1 has been proposed. A microcoil is a coil with a remarkably small diameter. If the outer shape of the coil is reduced, the inductance is also reduced at the same time, so that the voltage required to obtain the same high-frequency magnetic field may be reduced. Such a method is very effective in light of the measurement principle, but has a drawback that a probe device dedicated to a microcoil must be prepared. Since a small coil is used, the volume that can be used for measurement inside the coil is also small. For this reason, such a dedicated probe device has the disadvantages that its application is remarkably limited and its versatility is poor.

Special table 2011-501196

Laboratory Chemistry Lecture 8 "NMR / ESR" 5th edition, The Chemical Society of Japan, (Maruzen, 2006)

An object of the present invention is to provide means for applying a higher-frequency high-frequency magnetic field to a sample using a measurement coil having the same shape as that of an existing NMR probe apparatus in NMR measurement.

As described above, the present invention uses a measurement coil having the same shape as that of an existing one, and thus can be applied to an existing NMR probe apparatus and is versatile. In other words, the NMR measurement method of the present invention can also be implemented by adding the components of the present invention to an existing probe apparatus. Further, the NMR probe apparatus of the present invention can be used in the same manner as an existing NMR probe apparatus by removing the constituent elements unique to the present invention. For this reason, the existing technique regarding NMR measurement can be utilized effectively.

A high-frequency magnetic field shield is installed inside the measuring coil that generates a high-frequency magnetic field, and the outer peripheral surface is made of an electrical conductor (electrically good conductor) with high electrical conductivity and shields the penetration of the high-frequency magnetic field into the inside. To do. When the high-frequency magnetic field shield is cut along a plane perpendicular to the central axis while moving along the central axis of the measurement coil, the high-frequency magnetic field shield has a uniform cross section over at least the entire length of the measurement coil and is measured. It is desirable to have a rotationally symmetric shape with respect to the central axis of the coil. By having a uniform cross section, the strength of the high-frequency magnetic field is constant in the section. Further, by the rotation symmetric shape, before Symbol cross section, the uniformity of the RF magnetic field is improved. The sample is set in a space between the measurement coil and the outer peripheral surface of the high-frequency magnetic field shield.

The outer peripheral surface of the high-frequency magnetic field shield forms a closed curve surrounding a certain area in a cross section cut by a cut surface perpendicular to the axis of the coil. The outer peripheral surface of the high-frequency magnetic field shield is a surface composed of a set of such closed curves. When the measuring coil generates a high-frequency magnetic field, a current is induced on the outer peripheral surface (closed curve in the cross section). This current has a magnitude that cancels the magnetic field that would otherwise be generated. Due to such an effect, the high frequency magnetic field is shielded, and the high frequency magnetic field cannot enter the inside of the high frequency shield. In general, a high frequency electromagnetic field cannot penetrate into an electrical conductor, which is widely known as the skin effect.

If the current flowing through the measurement coil is constant, the total magnetic flux generated inside the measurement coil is constant, but the magnetic flux concentrates in the remaining space because it cannot penetrate into the high frequency magnetic field shield. Therefore, the magnetic flux density of the high frequency magnetic field in the remaining space increases. The remaining space where the magnetic flux density increases is a space between the inner surface of the measurement coil and the outer peripheral surface of the high-frequency magnetic field shield. In the present invention, the sample is set in such a space where the magnetic flux density of the high-frequency magnetic field is increased. The magnitude of the increase in magnetic field strength is proportional to the cross-sectional area inside the measurement coil divided by the difference between the cross-sectional area inside the measurement coil and the cross-sectional area of the high-frequency magnetic field shield. For this reason, the cross-sectional area inside the measurement coil and the cross-sectional area of the high-frequency magnetic field shield are at least over the range in which the sample inside the measurement coil is set when the cut surface is moved along the central axis of the measurement coil. It is desirable to be uniform. Moreover, it is desirable that the cross-sectional shape of the high-frequency magnetic field shield be rotationally symmetric with respect to the central axis of the measurement coil.

The inside of the high-frequency magnetic field shield may be a good conductor or an insulator, but it is required not to disturb the static magnetic field B0. If the static magnetic field B0 is not uniform, the linearity of the NMR spectrum deteriorates. In order to adjust the permeability to the surrounding environment (air) and reduce the disturbance of the static magnetic field B0, the high-frequency magnetic field shield is made of a thin cylindrical casing, and a thin film made of a good electric conductor is formed on the outer peripheral surface. The interior is preferably a cavity. The cross-sectional shapes of the outer peripheral surface and the inner surface of the cylinder are preferably circular when one high-frequency magnetic field shield is used. The space inside the high-frequency magnetic field shield configured in this way can be filled with the sample to be measured or a material having the same magnetic permeability as the sample. Is more desirable. The sample set in the internal space is not observed by NMR itself and is not directly related to the NMR measurement, but causes a change in the permeability of the high-frequency magnetic field shield, thereby causing a static magnetic field. This can contribute to the improvement of the linearity of the NMR spectrum. Thus, the uniformity of the static magnetic field can be improved by setting an appropriate second sample in the space inside the high-frequency magnetic field shield according to the purpose. Moreover, it is desirable to select the materials of the high-frequency magnetic field shield case and the thin film on the surface so as to cancel each other's magnetization by combining diamagnetism and paramagnetism whose susceptibility is opposite.

A plurality of high-frequency magnetic field shields can be installed inside the measurement coil. In this case, a plurality of high-frequency magnetic field shields are arranged with a certain gap between them, and as a whole, they are arranged so as to be rotationally symmetric with respect to the central axis of the measuring coil with a certain gap between them. It is desirable to do. The reason why the constant interval is provided is to prevent a resistance from being generated on the conductor surface due to the proximity effect, thereby reducing the Q of the system. The proximity effect is an effect in which, when a high-frequency current flows through adjacent conductors, mutual currents interfere with each other, resulting in an apparent increase in conductor resistance. In order to apply a uniform high-frequency magnetic field to the sample, it is desirable to set the sample in a shape that is rotationally symmetric with respect to the central axis of the measurement coil.

When a plurality of high-frequency magnetic field shields are installed inside the measurement coil, a cylindrical space for setting a sample is provided near the central axis of the measurement coil, and the plurality of high-frequency magnetic field shields are spaced apart from each other at a fixed interval. It is good to install so as to surround the space. When the sample is a powder or the like, it is difficult to fill the thin cylindrical space with the sample. On the other hand, if the space for storing the sample is a cylindrical shape, the filling operation is easy.

The present invention can also be applied to the magic angle spinning (MAS) method. In this case, if the thin film made of the electrical conductor on the outer periphery of the high-frequency magnetic field shield is thick, the rotation of the MAS is adversely affected by the electromagnetic interaction between the rotation of the MAS and the static magnetic field. For this reason, it is desirable to set the thickness of the thin film to 3 times or less of the skin depth.

Thus, in the pulse NMR measurement method of the present invention, the high-frequency magnetic field shield whose outer peripheral surface is made of an electric conductor is installed inside the measurement coil, and is inside the measurement coil and the high-frequency magnetic field shield. A sample is set in the outer space, and a high frequency magnetic field having a frequency approximately equal to the resonance frequency of the nuclear spin of the atom to be measured is applied in a pulsed manner. In the NMR measurement method, the measurement is performed in a frequency band of approximately 1/10000 (10 ppm) to 1/500 (2000 ppm) with respect to the resonance frequency, and an NMR spectrum is obtained from the observed high-frequency magnetic field. A high-frequency high current is passed through the coil, and the cross-sectional area inside the measurement coil Features that increase the magnetic field strength in proportion to a value obtained by dividing the difference of the product.
In the pulse NMR measurement method of the present invention, preferably, when the high-frequency magnetic field shield is moved along the central axis of the measurement coil and cut along a plane perpendicular to the central axis, at least over the entire length of the measurement coil. In addition, the high-frequency magnetic field shield has a uniform cross section and is preferably arranged at the center of the measurement coil.

The NMR probe apparatus of the present invention is an NMR probe apparatus having a measurement coil and a high-frequency magnetic field shield that is installed inside the measurement coil and whose outer peripheral surface is made of an electric conductor, the high-frequency magnetic field shield being transparent. The thin film of the good electric conductor is formed on the surface of a material having a low magnetic susceptibility, and a sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield.
  The NMR probe apparatus of the present invention is an NMR probe apparatus having a measurement coil and a high-frequency magnetic field shield having an outer peripheral surface made of an electric conductor, which is installed inside the measurement coil. Consists of a thin cylindrical casing, a thin film made of a good electrical conductor is formed on the outer peripheral surface, and a sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield It is characterized by.
  In the NMR probe apparatus according to the present invention, preferably, the housing of the high-frequency magnetic field shield and the material of the thin film on the surface combine diamagnetism and paramagnetism whose susceptibility is opposite to each other and cancel each other's magnetization. It is good to be selected as follows.
  In the NMR probe apparatus of the present invention, the thin film preferably has a thickness d that is at least twice the skin depth.
  Here, the skin depth is obtained by the following equation, where μ is the permeability of the conductor, σ is the conductivity, and f is the frequency of the high-frequency magnetic field: π is the circumference.
    [Skin depth] = 1 / √ (π f μσ).
  In the NMR probe apparatus of the present invention, preferably, the thin film has a thickness d that is at least three times the skin depth.
The NMR probe apparatus according to claim 6.
  In the NMR probe apparatus of the present invention, the cylindrical housing is preferably made of a non-electric conductor.

Further, according to one aspect of the NMR probe apparatus of the present invention, the cross-sectional shape when the outer peripheral surface is cut by a cutting plane perpendicular to the central axis of the measurement coil is moved along the central axis. It should be uniform over at least the entire length of the measuring coil.
  Further, one aspect of the NMR probe apparatus of the present invention is that, when the high-frequency magnetic field shield is moved along the central axis of the measurement coil and cut along a plane perpendicular to the central axis, at least the entire length of the measurement coil. It is preferable that the high-frequency magnetic field shield has a uniform cross section and is arranged at the center of the measurement coil.

Also, one aspect of the NMR probe apparatus of the present invention is characterized in that the plurality of the high-frequency magnetic field shields surround the cylindrical space provided on the central axis of the measurement coil so that the central axis of the measurement coil It is preferable that the sample is set in the columnar space with a rotationally symmetric arrangement with a certain interval therebetween.

Further, one aspect of the NMR probe apparatus of the present invention is configured such that a plurality of the high-frequency magnetic field shields sandwich a plate-like space provided in the vicinity of the center axis of the measurement coil with respect to the center axis of the measurement coil. It is preferable that the sample is set in the plate-like space by being arranged in a rotationally symmetrical manner and spaced apart from each other.
Further, the NMR probe apparatus of the present invention is an NMR probe apparatus used for pulsed NMR measurement, and has a measurement coil and a high-frequency magnetic field shield having an outer peripheral surface made of an electric conductor, which is installed inside the measurement coil. A sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield, the high-frequency magnetic field shield is disposed at the center of the measurement coil, and the high-frequency magnetic field shield is the measurement When cut along a plane perpendicular to the central axis while moving along the central axis of the coil, it has a uniform cross section at least over the entire length of the measuring coil.

Further, the NMR sample tube of the present invention is an NMR sample tube having a sample tube housing and a high-frequency magnetic field shield that is arranged coaxially with the sample tube housing and has an outer peripheral surface made of an electric conductor , The high-frequency magnetic field shield is formed by forming a thin film of the good electric conductor on the surface of a material having a low magnetic permeability, and a sample is set in a space inside the sample tube housing and outside the high-frequency magnetic field shield. with that, the sample tube housing, wherein the measuring coil to be disposed coaxially provided at the center of the measuring coil.

Since it became possible to apply a high-intensity measurement excitation pulse, the NMR measurement range was expanded.

Perspective view of the vicinity of the measuring coil in the prior art Longitudinal sectional view and transverse sectional view in the vicinity of the measuring coil in the prior art Longitudinal sectional view and transverse sectional view in the vicinity of the measuring coil in Example 1 Sectional drawing of the high frequency magnetic field shield in Example 1 Longitudinal sectional view and transverse sectional view in the vicinity of the measuring coil in Comparative Example 1 Excitation pulse width dependence of NMR signal in Example 1 Excitation pulse width dependence of NMR signal in Comparative Example 1 Longitudinal sectional view and transverse sectional view in the vicinity of the measuring coil in Example 2 Longitudinal sectional view and transverse sectional view in the vicinity of the measuring coil in Example 3

Prior to the detailed description of the present invention, the prior art will be described more specifically. FIG. 2 is a cross-sectional view showing a further schematic view of the vicinity of a measurement coil of a solid-state wide-band NMR probe using a normal solenoid type measurement coil as shown in FIG. The solenoid type measuring coil 10 is approximately represented by a cylinder. In addition, the support mechanism for each component is not shown as in FIG.
When a current flows through the measurement coil, a substantially uniform magnetic field is generated in the cylindrical space inside the measurement coil. Such a solenoid coil approximates its shape as a cylinder, and the current flows along the circumference of the cylinder. Can be approximated by the current flowing uniformly.

The cylindrical sample tube housing 30 is coaxially disposed at the center of the solenoid-type measurement coil 10 so that the centers of the two generally coincide. The direction of the static magnetic field B0 is the Z axis, and the direction of the measurement coil is the X axis. The high frequency magnetic field B1 is applied parallel to the axis of the measuring coil. Set the X, Y, and Z axes in the normal Cartesian coordinate system. The central axis of the measurement coil may generally be in the XY plane, but here it is arranged on the X axis.
2 (a) and 2 (b) are a longitudinal sectional view and a transverse sectional view, respectively, and FIG. 2 (b) is a sectional view of the surface indicated by B on (a), and is indicated by A on (b). A sectional view of the surface is (a).

The sample is set in a sample space 50 provided inside the sample tube housing 30. The sample space 50 is arranged so that the centers of both the measurement coil 10 and the sample space 50 are substantially coincident. A sample (object to be measured) to be measured is set inside the sample space 50.

The measurement coil 10 is connected to a high-frequency electric circuit, a high-frequency high voltage is applied to both ends of the measurement coil 10, and a high-frequency large current flows through the measurement coil 10 accordingly. In general, the measuring coil forms part of a resonator. A high-frequency magnetic field B <b> 1 is generated in a direction parallel to the axis (X axis) of the measurement coil 10 due to the current flowing through the measurement coil 10.

FIG. 2C shows a diagram in which a structure transparent to a high-frequency magnetic field is erased in the cross-sectional view. For example, the sample tube and the sample are almost transparent to the high frequency magnetic field, and the absorption and reflection of the high frequency magnetic field by these is about 1/100 or less and can be ignored. In this case, the high frequency magnetic field is generated uniformly throughout the measuring coil 10. That is, the high frequency magnetic field space 20 is the entire inside of the measuring coil 10, and its cross section is circular as shown in FIG.

<Example 1>
FIG. 3 shows an embodiment of the first embodiment of the present invention. The high-frequency magnetic field shield 60 has a cylindrical shape, and is arranged on the same axis as the measurement coil 10 so that the centers of the two generally coincide. The outer peripheral surface of the high-frequency magnetic field shield 60 is made of oxygen-free copper which is a good electrical conductor. The material of the outer peripheral surface of the high-frequency magnetic field shield 60 may be a good electrical conductor, and gold, silver, aluminum, etc. can be used in addition to copper.

The sample is set in a cylindrical sample space 50a surrounded by the inner surface of the sample tube housing 30a, the outer peripheral surface of the high-frequency magnetic field shield 60, and the lid 40a.

An arbitrary sample can be stored in the inside 62 of the high-frequency magnetic shield independently of the sample to be measured. The lid 40a 'is used when any sample is stored in the inside 62 of the high-frequency magnetic field shield, and can be omitted if this is empty.

The operating principle of the high-frequency magnetic shield is equivalent to the fact that high-frequency electromagnetic waves cannot enter the inside of the conductor due to the so-called skin effect, and therefore the conductivity of the conductor surface is important. If the electrical resistance to the high-frequency current on the conductor surface is large, the Q of the resonator decreases, which adversely affects measurement sensitivity and the like. In addition to the conductivity inherent to the conductor, surface roughness and the like are effective for the conductivity of the conductor surface, so care must be taken. The outer peripheral surface of the high-frequency magnetic field shield 60 desirably has a smooth surface made of a good electrical conductor.

Since the outer peripheral surface of the high frequency magnetic field shield 60 only needs to be a good electric conductor, the high frequency magnetic field shield 60 is configured as a thin film of a good electric conductor formed on an arbitrary material. FIG. 4 shows a cross section of the high frequency magnetic field shield formed as described above. A copper thin film 63 that is a good electrical conductor is formed on the surface of a casing 61 made of cylindrical ceramics. Since the material used for NMR measurement should have a low magnetic permeability, it is desirable to form a thin film of a good electric conductor on the surface of a material having a low magnetic permeability. By making the internal space 62 hollow, the magnetic permeability of the entire high-frequency magnetic field shield can be brought close to the vacuum magnetic permeability.

The thickness d of the thin film needs to be at least twice the skin depth. The skin depth is obtained by 1 / √ (π f μσ), where π is the circumference and π is the frequency of the high-frequency magnetic field, where μ is the conductivity of the conductor and σ is the frequency of the high-frequency magnetic field. Here, the symbol √ represents a square root. The skin depth is a depth at which the electromagnetic field penetrating the metal is 1 / e (where e is a natural logarithm base e to 2.72). When the conductor is copper and the temperature is around room temperature, the skin depth is 6.6 μm when the measurement frequency is 100 MHz and 2.1 μm when the measurement frequency is 1 GHz. If the thickness d of the electric conductor on the surface of the shield is twice the skin depth, it is shielded by about 3/4, and if it is more than 3 times, it is shielded by 19/20 or more. The effect becomes larger as the shielding ratio increases, and if this ratio is approximately 3/4 or more, a significant effect is obtained. Thus, the thickness d needs to be at least twice, preferably 3 times or more the skin depth determined from the material of the conductor and the frequency used for measurement. If the measurement frequency is about 900 MHz, it can be said that d is 7 μm or more.

The high frequency magnetic field cannot penetrate into the shield 60. Therefore, the high frequency magnetic field space 20a is limited to a small cross section as shown in FIG. If the current flowing through the coil is the same, the total magnetic flux generated inside the coil is the same, but it is pushed into a small cross section, so the magnetic flux density is increased. Thus, the magnetic flux density of the high frequency magnetic field is inversely proportional to the cross-sectional area of the high frequency magnetic field space.

In order to verify the performance of the configuration of Example 1, a device having a configuration as shown in FIG. FIG. 5 shows the structure of the apparatus of Comparative Example 1. The PTFE tube 70 is a cylinder made of polytetrafluoroethylene (PTFE), which is an electrical insulator, and the shape, installation method, and the like are the same as those of the high-frequency magnetic field shield 60 in the first embodiment. In this case, the high-frequency magnetic field space is as shown in FIG. 5 (c), which is the same as FIG. 2 (c), which is an example of the prior art.

6 and 7 show the excitation pulse width dependence of the intensity of the NMR signal measured using the configurations of Example 1 and Comparative Example 1, respectively. Specifically, when the NMR spectrum is obtained by the single pulse method, the excitation pulse width is measured while being changed every 1 μs, and the peak portion of the obtained signal is extracted and written according to the pulse width series. is there. In the figure, the horizontal axis represents the pulse width, and the vertical axis represents the signal intensity. The inner diameter of the measurement coil was 4.8 mm, the inner diameter of the sample tube was 3.5 mm, the outer diameters of the high-frequency magnetic field shield 60 and the PTFE tube 70 were 3.3 mm, and the measurement magnetic field was 21.6 T. The measurement sample was ethanol, the measurement nucleus was 1H (proton), and the measurement frequency was approximately 930 MHz. The amplitude of the high-frequency signal input to the probe device is constant. The spectral resolution is not good because the resolution of the apparatus has not been sufficiently adjusted. Here, the pulse width dependence of the signal intensity will be discussed. Since the excitation intensity is amplitude × pulse width, in this experimental result, the excitation intensity is proportional to the pulse width.

In general, in this type of experiment, the signal intensity behaves in a sinusoidal manner with respect to the pulse width, and the pulse width when the signal intensity is maximized is referred to as a 90-degree pulse. Also, a point where the amplitude first changes from positive to negative while passing 0 is called a 180-degree pulse, and a point where the amplitude changes from negative to positive while passing 0 is called a 360-degree pulse. The width of the 90 degree pulse is approximately 1/4 of the width of the 360 degree pulse. The smaller the width of the 90-degree pulse, the greater the intensity of the irradiated high-frequency magnetic field. The strength of the high frequency magnetic field is inversely proportional to the width of the 90 degree pulse.

From FIG. 6, the width of the 90-degree pulse in Example 1 is read as 2.8 (± 0.2) μs. From FIG. 7, the width of the 90-degree pulse in Comparative Example 1 is read as 6.5 (± 0.2) μs. From this, it can be said that the ratio of the magnetic field strength in Example 1 and Comparative Example 1 is 2.3 times. Thus, in Example 1, it turned out that the intensity | strength of a high frequency magnetic field is increasing remarkably.

In addition, the magnetic field strength ratio between Example 1 and Comparative Example 1 was significantly larger than the value 1.9 calculated from both shapes. This is considered to be due to the fact that the sample is effectively irradiated with the high-frequency magnetic field because the sample is in close contact with the conductor of the high-frequency magnetic field shield in Example 1. In general, in the cross section perpendicular to the axis of an actual solenoid coil, the magnetic field strength is greater in the vicinity of the coil winding than in the center of the coil.

Since the current flowing through the coil flows in a linear coil arranged spatially discretely, the magnetic field strength is spatially non-uniform. On the other hand, the current that is secondarily induced and flows on the conductor surface on the outer peripheral surface of the high-frequency magnetic field shield spreads and flows over the entire surface, thereby improving the uniformity of the generated magnetic field. For this reason, according to the present invention, the linearity and resolution of the spectrum are improved.

Other than the solid wide NMR probe, for example, a solid magic angle spinning (MAS) probe is theoretically equivalent. Even if the shape of the measuring coil is a saddle-type coil, a scroll-type coil, or the like, the present invention can be similarly implemented by paying attention to the shape of the generated high-frequency magnetic field. In principle, the sample to be measured may be any of gas, liquid, and solid.
In the case of the MAS probe, the axis of the measurement coil intersects with the XY plane at an angle of approximately 35.3 degrees. In this case, the component projected on the XY plane of the magnetic field generated by the measurement coil effectively works as B1. Become.

<Example 2>
The shape of the high-frequency magnetic field shield is not limited to the cylindrical shape as in the first embodiment. It suffices to have a cross section that eliminates the intrusion of a high-frequency magnetic field and to set the sample in the remaining space.
As shown in FIG. 8, the high-frequency magnetic field shield 60b according to the present embodiment is configured by arranging six parts (high-frequency magnetic field shields) so as to be rotationally symmetric. The cross section of each part has a shape in which a portion near a center of an arc is cut out and a side is cut out from a fan shape having a central angle of 60 degrees. By cutting out the vicinity of the center of the arc, each component comes into contact with a cylindrical space for storing a sample provided near the center axis. Further, they are arranged apart from each other by the amount of the side cut off. That is, the high-frequency magnetic field shield 60b according to the present embodiment has a shape obtained when the cylinder is divided into six equal parts with a certain cutting width by a plane passing through the central axis. Here, the cutting white refers to a width that is scraped off in parallel with the surface along with the cutting, and corresponds to the width of the blade when a blade such as a grindstone or a saw is used for cutting.
The surface of these parts is smoothly formed of a good electric conductor and satisfies the constituent requirements as a high-frequency magnetic field shield as a single part. The space between the sides is filled with an epoxy resin that is an electrical insulator. The high frequency magnetic field space 20b in this case is shown in FIG. The sample is set inside a substantially cylindrical sample space 50b provided near the center.
It is desirable that the arc portion has a shape along a certain distance from the measurement coil. With such a shape, the cross-sectional area of the high-frequency magnetic field shield can be made as large as possible, and the cross-sectional area of the high-frequency magnetic field space 20b can be made as small as possible.

The configuration of the present embodiment has a feature that the sample can be easily set when the sample is powdery because the sample space is cylindrical.
In this embodiment, the number of high-frequency magnetic field shields is six, and they are arranged in six rotational symmetry. However, the selection of this number is arbitrary, and any natural number n of 2 or more can be selected. can do. The higher the number n, the higher the uniformity of the high-frequency magnetic field is expected. However, if the number n is set too large, the strength of the high-frequency magnetic field obtained will decrease. This is because when a plurality of high-frequency magnetic field shields are installed, a certain gap must be provided between them in order to avoid a decrease in efficiency due to the proximity effect. Therefore, the number of high-frequency magnetic field shields is preferably 12 or less, and more preferably 3 to 8.

<Example 3>
In this embodiment, as shown in FIG. 9, two high-frequency magnetic field shields 60c sandwich a thin plate-shaped rectangular parallelepiped sample space 50c provided on the central axis of the measuring coil 10 from the thickness direction. As described above, they are installed at a predetermined interval. The sample is set in a plate-like sample space 50c.
As shown in FIG. 9B, the cross-sectional shape of the high-frequency magnetic field shield 60c is a combination of an arc and a chord, and the arc portion is shaped so as to follow a certain distance from the measurement coil. It is desirable that With this shape, the cross-sectional area of the high-frequency magnetic field space 20c can be effectively reduced while securing a sample space necessary for measurement.
The high-frequency magnetic field shield 60c of the present embodiment has a shape obtained when a cylinder is divided into two by a plane passing through the central axis thereof with a constant cutting margin.

The configuration of the present embodiment is particularly effective when the sample is plate-shaped, is a thin film laminated on a substrate, or is film-shaped. For such a specific sample, it may not always be necessary to install a lid in the longitudinal (X-axis) direction, so the lid is not shown in FIG. When the sample is general such as liquid or powder, a lid may be used as in the other embodiments.

The present invention can be used to improve the sample filling rate in addition to increasing the strength of the high-frequency magnetic field. The sample filling rate is the ratio of the volume of the sample to the volume of the high frequency magnetic field space. Since the measurement sensitivity is approximately proportional to the sample filling rate, the measurement sensitivity decreases as the sample filling rate decreases.
If there is only a small amount of sample or the sample has a special shape and a large gap is left inside the measurement coil, it is inevitable that the sample filling rate will decrease.

When only a small amount of sample can be obtained, the sample may be filled into a thin tube and the measurement may be performed according to the configuration of Example 2. Since the high-frequency magnetic field space is limited to a small space including a cylindrical space near the center as shown in FIG. 8C, the sample filling rate is relatively improved, and highly sensitive measurement can be performed. .

When a thin plate-like sample is measured using a solenoid-type measuring coil, the filling rate is small and the sensitivity is inevitably lowered. This is clear when the cross-sectional shapes of the sample and the measuring coil are compared.
Thus, when measuring a thin plate-shaped sample, it is good to measure by the structure of Example 3. Since the high-frequency magnetic field space is limited to a small space including a plate-like space as shown in FIG. 9C, the sample filling rate is relatively improved and highly sensitive measurement can be performed.

10 Measurement coils 20, 20a, 20b, 20c Spaces 30, 30b, 30c in which high-frequency magnetic fields are generated Sample tube housings 40, 40a, 40b Lids 50, 50a, 50b, 50c Sample spaces 60, 60b, 60c High-frequency magnetic field shield 61 High-frequency magnetic field shield housing 62 High-frequency magnetic field shield interior 63 High-frequency magnetic field shield surface 70 PTFE tube d Thickness A Symbol B for specifying cutting plane B0 Symbol for specifying cutting plane Static magnetic field B1 High-frequency magnetic field X X-axis , Central axis of measurement coil Y Y axis Z Z axis, direction of static magnetic field

Claims (16)

A high frequency magnetic field shield whose outer peripheral surface is made of an electric conductor is installed inside the measurement coil,
A sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield,
A high frequency magnetic field having a frequency approximately equal to the resonance frequency of the nuclear spin of the atom to be measured is applied in pulses,
A high-frequency magnetic field generated from the sample and having a frequency substantially equal to the resonance frequency, and measured in a frequency band between approximately 1/100000 (10 ppm) and 1/500 (2000 ppm) with respect to the resonance frequency;
In the NMR measurement method for obtaining the NMR spectrum from the observed high frequency magnetic field,
While flowing a high frequency high current through the measurement coil,
A pulse NMR measurement method, wherein the magnetic field strength is increased in proportion to a value obtained by dividing the cross-sectional area inside the measurement coil by the difference between the cross-sectional area inside the measurement coil and the cross-sectional area of the high-frequency magnetic field shield .   When the high-frequency magnetic field shield is cut along a plane perpendicular to the central axis while moving along the central axis of the measurement coil, it has a uniform cross section at least over the entire length of the measurement coil,
  The pulse NMR measurement method according to claim 1, wherein the high-frequency magnetic field shield is disposed at a center of the measurement coil. A measuring coil;
The installed inside the measuring coil, the outer peripheral surface is an NMR probe device having a high-frequency magnetic field shielding unit made of electrically conductive material,
The high-frequency magnetic field shield is formed by forming a thin film of the good electric conductor on the surface of a material having a low magnetic permeability,
An NMR probe apparatus characterized in that a sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield. A measuring coil;
The installed inside the measuring coil, the outer peripheral surface is an NMR probe device having a high-frequency magnetic field shielding unit made of electrically conductive material,
The high-frequency magnetic field shield consists of a thin cylindrical casing, and a thin film made of a good electrical conductor is formed on the outer peripheral surface.
An NMR probe apparatus characterized in that a sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield.   The casing of the high-frequency magnetic field shield and the material of the thin film on the surface are selected to combine diamagnetism and paramagnetism whose susceptibility is opposite to each other and cancel each other's magnetization. Item 5. The NMR probe device according to Item 4.   The NMR probe apparatus according to any one of claims 3 to 5, wherein the thin film has a thickness d of at least twice the skin depth.
  Here, the skin depth is obtained by the following equation, where μ is the permeability of the conductor, σ is the conductivity, and f is the frequency of the high-frequency magnetic field: π is the circumference.
    [Skin depth] = 1 / √ (π f μσ).   The NMR probe apparatus according to claim 6, wherein the thin film has a thickness d of at least three times the skin depth.   The NMR probe apparatus according to claim 4, wherein the cylindrical casing is made of a non-electric conductor. The NMR probe apparatus according to claim 3 , wherein
The cross-sectional shape of the outer peripheral surface when cut along a cut surface perpendicular to the central axis of the measurement coil is uniform over at least the entire length of the measurement coil when the cut surface is moved along the central axis. An NMR probe apparatus characterized by being.   The NMR probe apparatus according to claim 3, wherein
  When the high-frequency magnetic field shield is cut along a plane perpendicular to the central axis while moving along the central axis of the measurement coil, it has a uniform cross section at least over the entire length of the measurement coil,
  The high-frequency magnetic field shield is arranged at the center of the measurement coil, and the NMR probe apparatus. The NMR probe apparatus according to any one of claims 3 to 9 ,
A plurality of the high-frequency magnetic field shields are arranged in a rotationally symmetrical manner with respect to the central axis of the measurement coil so as to surround a cylindrical space provided on the central axis of the measurement coil, and are spaced apart from each other at a constant interval. Installed across the
An NMR probe apparatus characterized in that a sample is set in the cylindrical space.   The plurality of high-frequency magnetic field shields have a shape obtained when a cylinder is equally divided by a plane passing through its central axis with a fixed cutting width.
  The cross-section of each high-frequency magnetic field shield consists of a shape that is cut out from the fan shape near the center of the arc and scraped off the sides.
  The notch provided in the vicinity of the center of the fan-shaped arc is in contact with the cylindrical space, and the parts are arranged apart from each other by the side cut off. 11. The NMR probe apparatus according to 11. The NMR probe apparatus according to any one of claims 3 to 9 ,
A plurality of the high-frequency magnetic field shields are rotationally symmetric with respect to the central axis of the measurement coil so as to sandwich a plate-like space provided in the vicinity of the central axis of the measurement coil, and are spaced apart from each other at a constant interval. An NMR probe apparatus, wherein a sample is set in the plate-like space.   The plurality of high-frequency magnetic field shields have a cross-sectional shape that is a combination of a circular arc and a chord, and the circular arc portion has a shape along a certain distance from the measurement coil. The NMR probe apparatus according to claim 13. An NMR probe apparatus used for pulsed NMR measurement,
A measuring coil;
A high frequency magnetic field shield having an outer peripheral surface made of an electric conductor, which is installed inside the measurement coil ,
A sample is set in a space inside the measurement coil and outside the high-frequency magnetic field shield,
The high-frequency magnetic field shield is disposed at the center of the measurement coil;
When the high-frequency magnetic field shield is cut along a plane perpendicular to the central axis while moving along the central axis of the measurement coil, it has a uniform cross section at least over the entire length of the measurement coil. NMR probe device. A sample tube housing;
An NMR sample tube that is arranged coaxially with the sample tube housing and has a high-frequency magnetic field shield whose outer peripheral surface is made of an electrical conductor ,
The high-frequency magnetic field shield is formed by forming a thin film of the good electric conductor on the surface of a material having a low magnetic permeability,
A sample is set in a space inside the sample tube housing and outside the high-frequency magnetic field shield ,
An NMR sample tube , wherein the measurement coil is provided so that the sample tube housing is coaxially disposed at the center of the measurement coil .