Magnetically compensated NMR rotor

Abstract

An NMR rotor comprises a receptacle for inserting a sample container into a homogeneous region of an NMR magnetic field with flux density B, the field vector of which in the homogeneous region extends in the vertical direction along a z-axis. The rotor passes through regions with inhomogeneous magnetic field components and a flux density gradient dB/dz when the sample container is introduced. The rotor includes at least two different materials, one with diamagnetic properties and another with non-diamagnetic properties. The different materials are arranged to be geometrically distributed in the rotor so that the magnetic force on the rotor under the effect of a product t of magnetic flux density B and flux density gradient dB/dz, the magnitude of which exceeds 1400 T.sup.2/m, either acts in the same direction as the weight force of the rotor or is smaller in magnitude than the weight force of the rotor.

Claims

1. An NMR rotor comprising: a receptacle for introducing a sample container having a sample substance for NMR measurements into a homogeneous region of an NMR magnetic field with flux density B, a field vector of which, in the homogeneous region, extends in a vertical direction along a z-axis, the NMR rotor passing through regions with inhomogeneous magnetic field components and at least one flux density gradient dB/dz when the sample container is introduced into the homogeneous region of the magnetic field, wherein the NMR rotor is composed of at least two different materials, of which one material has diamagnetic properties and another material has non-diamagnetic properties, and wherein the different materials are arranged so as to be geometrically distributed in or on the NMR rotor in such a way that the magnetic force on the NMR rotor under the effect of a product ϕ.sub.z of magnetic flux density B and flux density gradient dB/dz, the magnitude of which exceeds the value of 1400 T.sup.2/m, either acts in the same direction as the weight force of the NMR rotor or is smaller in magnitude than the weight force of the NMR rotor.

2. The NMR rotor according to claim 1, wherein the product ϕ.sub.z is given by ${\Phi }_z=\frac{1}{V_{Rotor}}.Math.{\int }_{V_{Rotor}}B.Math.\frac{dB\left(z\right)}{dz}.Math.\mathrm{dV}$ where |ϕ.sub.z|>1400 T.sup.2/m, and where V.sub.Rotor represents the volume of the NMR rotor and dV represents an infinitesimal volume element of the NMR rotor.

3. The NMR rotor according to claim 1, wherein at least one non-diamagnetic material of the NMR rotor has paramagnetic properties.

4. The NMR rotor according to claim 1, wherein at least one diamagnetic and at least one non-diamagnetic material are arranged in or on the NMR rotor such that the following applies in the NMR magnetic field with flux density B of a high-field NMR magnet at a location of the NMR rotor: ${\chi }_{dia}.Math.{\int }_{V_{\mathrm{dia}}}\frac{dB}{dr}.Math.\mathrm{dV}+{\chi }_{non-dia}.Math.{\int }_{V_{\mathrm{non}-\mathrm{dia}}}\frac{dB}{dr}.Math.\mathrm{dV}<0$ where: X.sub.dia represents a magnetic susceptibility of the diamagnetic material, X.sub.non-dia represents a magnetic susceptibility of the non-diamagnetic material, V.sub.dia represents a volume of the diamagnetic material in the NMR rotor, V.sub.non-dia represents a volume of the non-diamagnetic material in the NMR rotor, dB/dr represents a gradient of the flux density of the NMR magnetic field in a volume of the rotor in a radial direction with respect to the z-axis and dV represents an infinitesimal volume element of the NMR rotor.

5. The NMR rotor according to claim 1, wherein at least one of the materials of the NMR rotor is electrically conductive, and is arranged in the NMR rotor in such a way that it does not completely enclose the receptacle.

6. The NMR rotor according to claim 1, further comprising a rotor axis in the center of the receptacle, which axis is oriented along the z-axis when the sample container is introduced into the homogeneous region of the magnetic field, wherein the NMR rotor is constructed mirror-symmetrically with respect to a plane of symmetry containing the rotor axis and/or rotationally symmetrically with respect to the rotor axis, and wherein the diamagnetic material and the non-diamagnetic material is distributed mirror-symmetrically and/or rotationally symmetrically about the rotor axis in the NMR rotor.

7. The NMR rotor according to claim 6, wherein one of the materials of the NMR rotor is formed as at least two rod-shaped parts and is arranged in a base material of the NMR rotor in parallel bores extending in parallel with the rotor axis.

8. The NMR rotor according to claim 1, wherein the non-diamagnetic and the diamagnetic material of the NMR rotor are at least partially mixed.

9. The NMR rotor according to claim 1, wherein the materials of the NMR rotor having different magnetic properties are at least partially applied in one or more layers.

10. The NMR rotor according to claim 1, wherein the NMR rotor is predominantly composed of one of the at least two materials having different magnetic properties.

11. The NMR rotor according to claim 10, wherein the NMR rotor is composed of one of the at least two materials having different magnetic properties in a proportion between 90 and 95% by volume, and is composed of the other material in a proportion of from 5 to 10% by volume.

12. The NMR rotor according to claim 1, wherein the diamagnetic material contains plastics material and/or ceramic material.

13. The NMR rotor according to claim 1, wherein the non-diamagnetic material contains titanium, aluminum or platinum.

14. The NMR rotor according to claim 1, wherein a geometric distribution of the materials in or on the NMR rotor is configured such that a magnetic force on the NMR rotor is between 0.02 N and 0.2 N.

15. A method for the design and production of the NMR rotor according to claim 1, comprising: determining a magnetic susceptibility of a base material of the NMR rotor; determining a magnetic compensation of the rotor by a further material having different magnetic properties on the basis of the determined magnetic susceptibility of the base material; and arranging the geometrical distribution of the base material and the further material in the rotor so as to provide said magnetic force on the NMR rotor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated in the drawings and is explained in more detail with reference to embodiments. In the drawings:

(2) FIG. 1a is a schematic vertical sectional view of a first embodiment of the NMR rotor according to the invention with a casing part consisting of diamagnetic material and a cylindrical insert consisting of non-diamagnetic material, with a schematic half-section through a plane perpendicular to the axis being shown in the lower region of the figure;

(3) FIG. 1b is similar to FIG. 1a, but with a second embodiment of the NMR rotor according to the invention, in which bores which extend in parallel with the axis and are intended for inserting rods consisting of non-diamagnetic material are provided in a casing part consisting of diamagnetic material;

(4) FIG. 2 is a schematic vertical section through a detail of an NMR spectrometer according to the prior art with an NMR sampling head carrying a sample holder with an NMR rotor and an inserted sample tube in the measuring position;

(5) FIG. 3 is a schematic vertical section through an NMR spectrometer—for the sake of clarity without the sampling head—and, to the right, in the z-direction, shows the typical curve of the product ϕ.sub.z of magnetic flux density B and flux density gradient dB/dz for a magnet system with a field strength<1.1 GHz (solid line) and for an ultrahigh-field magnet with a field strength≥1.1 GHz (dotted line);

(6) FIG. 4 shows a typical curve of the product φ.sub.r of magnetic flux density B and flux density gradient dB/dr in the radial direction (perpendicular to the z-axis) with a radius r=10 mm for a magnet system with a field strength<1.1 GHz (dotted line) and for a ultrahigh-field magnet with a field strength≥1.1 GHz (solid line); and

(7) FIG. 5 shows a table with specific numerical values for a typical embodiment of an NMR rotor.

DETAILED DESCRIPTION

(8) FIGS. 1a and 1b of the drawings each show, in schematic views, preferred embodiments of the NMR rotor according the invention, and how it can also be incorporated, with these modifications, in the prior art NMR spectrometer shown in detail in FIG. 2.

(9) Such an NMR system, usually an NMR spectrometer, having an NMR magnet system 7 for generating a homogeneous static magnetic field with flux density B along a z-axis 5 comprises, in the measurement mode, a sampling head 6 that has an RF transmission and reception coil system 4 and an opening extending in the z-direction for receiving an NMR sample container 1 (usually in the form of a sample tube) which contains a sample substance to be examined by means of NMR measurement during operation. The NMR sample container 1 having the sample substance—not specifically shown in the drawings for the sake of clarity—is held by a sample container with an NMR rotor 2 (“spinner”) and is introduced (“injected”) into the NMR sampling head 6 together with a sample holder for the NMR measurement and is ejected (“ejected”) together therewith again after the measurement. When the sample holder is introduced into the homogeneous region of the magnetic field, it passes through regions with inhomogeneous magnetic field components and at least one flux density gradient dB/dz. In the NMR measurement mode, the NMR rotor 2, which is usually rotatable, is supported on an NMR stator 3 (“turbine”) or on an air flow generated by the NMR stator 3 or a compressed air device, with at least the part of the sample substance on which the NMR measurement is to be carried out being located in the homogeneous region of the magnetic field.

(10) The NMR rotor 2 according to the invention is distinguished from the known NMR rotors according to the prior art in that it is composed of at least two different materials, of which one material has diamagnetic properties and another material has non-diamagnetic, preferably paramagnetic properties. According to the invention, the different materials are arranged so as to be geometrically distributed in or on the NMR rotor 2 in such a way that the magnetic force on the NMR rotor 2 under the effect of a product t of magnetic flux density B and flux density gradient dB/dz, the magnitude of which exceeds the value of 1400 T.sup.2/m, either acts in the same direction as the weight force of the NMR rotor 2, or is smaller in magnitude than the weight force of the NMR rotor 2.

(11) FIG. 1a shows an embodiment of the NMR rotor 2 according to the invention that is particularly easy to produce. Said rotor comprises a casing part 2a consisting of diamagnetic material and an insert 2b consisting of non-diamagnetic material, which insert is cylindrical in the present embodiment. In this embodiment, the insert 2b comprises the receptacle 2c for a sample container.

(12) Another preferred embodiment of the NMR rotor 2 according to the invention is shown in FIG. 1b. Here, bores which extend in parallel with the rotor axis (and thus in parallel with the z-axis in the measurement mode) and are intended for inserting rod-shaped inserts 2b consisting of non-diamagnetic material are provided in a casing part 2a consisting of diamagnetic material. In this embodiment the non-diamagnetic material of the rod-shaped inserts 2b can also be electrically conductive, since this material is arranged in the NMR rotor 2 in such a way that it does not completely enclose the receptacle for the sample tube, and so no eddy current can flow in the plane perpendicular to the z-axis.

(13) FIG. 3 shows, on the left-hand side, a vertical section through an NMR spectrometer. On the right, the typical curve in the z-direction of the product t of magnetic flux density B and flux density gradient dB/dz is shown, namely for a magnet system with a field strength<1.1 GHz (solid line) and for an ultrahigh-field magnet with a field strength1.1 GHz (dotted line). Thus, the relevant ϕ.sub.z value can be assigned to the relevant z-position in the spectrometer.

(14) During the NMR measurement, the part of the sample substance to be measured is in the sample tube (not shown here for reasons of clarity) in a measurement volume of the NMR arrangement. This corresponds to the naturally relatively short z-section with the product ϕ.sub.z=B.Math.dB/dz running flat on the z-axis, i.e., the homogeneous magnetic field region in which the NMR measurement takes place. Above (and of course below) the product ϕ.sub.z has a curve with considerably higher values. Due to this inhomogeneous field region with, in some cases, very high magnetic forces acting on the NMR rotor, the sample holder having the measurement sample must be introduced before the measurement and ejected again after the measurement. Even during the actual measurement, the NMR rotor is at least partially in the inhomogeneous field region.

(15) FIG. 4 shows, in the z-direction at a distance of 10 mm from the z-axis, a typical curve of the product ϕ.sub.r of magnetic flux density B and flux density gradient dB/dr in the radial direction perpendicular to the z-axis for a magnet system with a field strength<1.1 GHz (dotted line) and for an ultrahigh-field magnet with a field strength≥1.1 GHz (solid line).

(16) It can be seen that for magnets>1.1 GHz there may be a region with the opposite sign of B.Math.dB/dr. If paramagnetic rotor material is used in this region, this material experiences a self-centering force. Apart from the sign of B.Math.dB/dr at the location of the NMR rotor and the ratio of the diamagnetic to paramagnetic volume fraction and the susceptibilities of the materials present, the resulting radial force on the NMR rotor also depends on the radial distribution of the materials in the NMR rotor.

(17) In the case of a layered structure of the NMR rotor according to the invention, the following specific numerical examples can be given for the layer thicknesses to be expected: If it is assumed that the NMR rotor has a volume of 10 cm.sup.3, this would roughly correspond to a cylinder having a length of 50 mm, an outer diameter of 16 mm, and a central bore (for the NMR sample tube) of 3 mm. If 0.77 cm.sup.3 of this were to consist of paramagnetic material, this would mean that the outermost 0.6 mm would have to consist of paramagnetic material. This would result in a diamagnetic cylinder with a 3 mm bore and an outside diameter of 15.4 mm. A layer with a thickness of 0.6 mm of paramagnetic material would then be applied (for example in an embodiment according to FIG. 1a).

(18) If an embodiment with rods (for example according to FIG. 1b) is decided on and, for example, provides eight rods, each would have to have a volume of 0.09625 cm.sup.3. With a rod length of 5 cm, this corresponds to a diameter of approximately 1.55 mm per rod.

(19) What is significant for the NMR rotor according to the invention is the fact that, in a product ϕ=B.Math.dB/dz, said rotor experiences a magnetic force of which the magnitude, if directed against the weight force, is less than its weight force. The sample container and the sample substance can be neglected in this consideration.

(20) The table in FIG. 5 shows typical numerical values of the weight force of a sample container (sample tube), a sample substance (water) and a spinner (here made of PCTFE). Volume1 and Volume2 are used to calculate the effective volume of the hollow cylindrical sample tube and spinner. It is clear from the calculation that the spinner makes the significant contribution to the weight force.

(21) To a good approximation, the magnetic field gradient is constant over the volume of the NMR rotor. The requirement of the correct geometric arrangement arises from the fact that the product ϕ.sub.z=B.Math.dB/dz nevertheless varies greatly and the two ends of the NMR rotor are each exposed to very different magnetic forces.

(22) In practice, it is beneficial for the user if a plurality of different NMR rotors, for example with different distributions of diamagnetic and non-diamagnetic materials, are supplied (usually together with an NMR spectrometer). The user of the spectrometer can then select a particularly suitable NMR rotor for each sample substance (e.g., aqueous solution/non-aqueous solution).

(23) The main element of the NMR rotor is preferably made of diamagnetic material. Depending on the specific measuring arrangement, a variety of different non-diamagnetic inserts, preferably in cylindrical form with corresponding outer diameters, can be individually and optimally selected to fit the receptacle bore in the diamagnetic “base spinner”. In this way, fluctuations in the susceptibility of the base material can be ideally and individually balanced.

(24) The features of all the above-described embodiments of the invention may also be combined with one another at least in most cases.