NMR sample tubes with varying outer diameters, NMR probe heads with varying inner diameters, NMR probe head arrangements with gas expansion nozzles, temperature control methods

Abstract

An NMR probe head arrangement comprising a central tube extending in a z-direction, an NMR sample tube oriented in the z-direction for receiving a liquid NMR sample substance, which is arranged inside the central tube and has a base section with a closed lower axial end, a neck section with an open upper axial end and a transition region between the base section and the neck section, a radio frequency (RF) receiver coil system arranged radially in the z-direction around the central tube, wherein the RF receiver coil system defines an RF-active axial region, and a temperature control device for introducing a temperature-controlling gas in the z-direction into a flow channel present between the central tube and the NMR sample tube, is characterized in that the geometry of the flow channel is selected such that it forms a gas expansion nozzle. This allows disruptive temperature gradients to be avoided.

Description

Background of the invention

[0001] The invention relates to an NMR sample tube having an elongated extension in a z-direction, a bottom portion with a closed lower axial end and a neck portion with an open upper axial end, wherein the NMR sample tube has a transition region between the bottom portion and the neck portion.

[0002] The invention also relates to an NMR probe head having a central tube, a sample inlet and a radio frequency (RF) receiving coil system arranged radially around the central tube with respect to a z-direction, wherein the RF receiving coil system defines an RF-active axial region.

[0003] Furthermore, the invention relates to an NMR probe head arrangement comprising a central tube extending in a z-direction, an NMR sample tube oriented in the z-direction for receiving a liquid NMR sample substance, which is arranged inside the central tube and has a base section with a closed lower axial end, a neck section with an open upper axial end and a transition region between the base section and the neck section, a radio frequency (RF) receiving coil system arranged radially in the z-direction around the central tube, wherein the RF receiving coil system defines an RF-active axial region, and a temperature control device for introducing a temperature-controlling gas in the z-direction into a flow channel present between the central tube and the NMR sample tube.

[0004] An NMR probe head arrangement with a temperature control device is known from [1].

[0005] Particularly in cryogenic NMR probeheads, NMR sample substances cool down through radiation exchange with the surrounding cryogenic coils. To counteract this, it is known to use a temperature control device to flow warm gas (tempering gas) around the NMR sample tube from below. However, this gas heats the lower axial end (bottom) of the NMR sample tube more than the upper axial end, where the sample inlet is usually located. This creates a temperature gradient in the NMR sample substance, which negatively influences the linewidth of the NMR spectrum and lock measured with this probehead. [5]

[0006] [2] and [3] disclose temperature control systems for NMR sample tubes using a gas flow, where the gas is meandered past the NMR sample tube in a countercurrent flow pattern. However, this principle requires a lot of space in the NMR probe head and is therefore not feasible for many applications.

[0007] A cryogenic NMR probe head is known from [1], in which the central tube is insulated by a layer of numerous glass fibers, which is intended to attenuate the radiation exchange between the NMR sample substance and the coils. This insulation layer reduces the gradient by approximately 50%, but cannot completely eliminate it.

[0008] [4] discloses a specially shaped sample tube designed to improve the signal-to-noise ratio by reducing the influence of an electrically conductive sample substance on the NMR coils.

[0009] [6] discloses a vessel for high-temperature EPR (electron paramagnetic resonance) measurements. The vessel has a lower part that flares upwards and an upper part that borders the flare but has a smaller diameter than the flared end of the lower part. Thus, the vessel has an edge where the diameter of the vessel changes abruptly, causing turbulence to form at the edge.

[0010] [7] discloses an MR probe head with a central tube block with three elongated recesses. The middle recess is open only on one side and serves to accommodate a sample vial, which is to be cooled via the walls of the central tube block. The two outer recesses are continuous and serve to conduct a cooling fluid to cool the walls of the central tube block (and ultimately the sample in the sample vial). Object of the invention

[0011] The object of the invention is to provide an NMR probe head assembly and components (NMR sample tube and NMR probe head) of an NMR probe head assembly that can avoid a disruptive temperature gradient. It is also an object of the invention to provide a method by which a liquid sample substance in an NMR sample tube can be tempered with a desired temperature gradient. Description of the invention

[0012] This object is achieved according to the invention by an NMR sample tube according to patent claim 1, an NMR probe head according to patent claim 3, an NMR probe head arrangement according to patent claim 7 and a method according to patent claim 16.

[0013] In the NMR sample tube according to the invention, the outer diameter of the NMR sample tube changes continuously in the transition region by 5% to 50%, preferably 20% to 50%, in the z-direction, wherein the transition region has an axial extension along the z-direction of > 10 mm. The geometry according to the invention, when used in an NMR probe head, allows a gas expansion nozzle to be realized, through which the temperature of a gas flowing between the NMR sample tube and a central tube of the NMR probe head, and thus the temperature of an NMR sample substance located in the NMR sample tube, can be influenced.

[0014] Preferably, the axial extent of the transition region, in which the outer diameter of the NMR sample tube changes continuously in the z-direction, is 20 - 21 mm.

[0015] In particular, the longitudinal profile of the NMR sample tube can be conical at least in sections in the transition region.

[0016] To maximize the quality of NMR measurements, the NMR sample tube is preferably made of a material with low electrical conductivity and low dielectric losses. Preferably, the NMR sample tube is suitable for use in an NMR probehead assembly described below, with an NMR sample tube being particularly characterized by a length-to-width ratio of 15 to 70 and being made of a homogeneous material. Preferably, the maximum diameter of the NMR sample tube is 3-11 mm, and the length of the NMR sample tube is 150-220 mm.

[0017] According to the invention, the outer diameter of the NMR sample tube decreases continuously in the z-direction in the transition region. This is particularly advantageous when the NMR sample tube is used to form an expansion nozzle in a central tube of an NMR probe head, with which an inflowing gas is to be heated along the z-direction.

[0018] Preferably, the lower axial end of the NMR sample tube is rounded. Specifically, the outer diameter of the NMR sample tube decreases counter to the z-direction over a length of at least 10% of the bottom section. The outer diameter is thus reduced toward the closed end. The reduction of the outer diameter toward the bottom allows the sample tube to be inserted into existing centering devices and thus precisely aligned.

[0019] In the NMR probe head according to the invention, the inner diameter of the central tube changes continuously in the z-direction in the RF-active axial region. The geometry according to the invention, when used with an NMR sample tube, allows for the creation of a gas expansion nozzle, which can influence the temperature of a gas flowing between the NMR sample tube and a central tube of the NMR probe head, and thus the temperature of an NMR sample substance located in the NMR sample tube.

[0020] Preferably, the NMR probe head is suitable for use in an NMR probe head arrangement described below.

[0021] Preferably, the sample inlet is located at the top of the central tube, while the RF-active region is located at the bottom. A temperature control device can be arranged at the end opposite the sample inlet.

[0022] In a particularly preferred embodiment, the inner diameter of the central tube increases continuously in the z-direction in the RF-active axial region, with the z-direction extending from the RF-active region to the sample inlet. This is particularly advantageous when the NMR probe head is used to form an expansion nozzle with an NMR sample tube, and a gas flowing in from below is to be heated along the z-direction.

[0023] Alternatively or additionally, the wall thickness of the central tube in the RF-active axial region can be provided to continuously change, preferably continuously decrease, in the z-direction, preferably over the entire axial length of the RF-active axial region. Preferably, the outer diameter of the central tube remains constant.

[0024] The advantages of the invention are particularly evident when the NMR probe head is a cryogenic probe head in which the RF receiver coil system can be cooled to cryogenic temperature, since in this case the NMR sample substance cooled by the cold coils of the RF receiver coil system must be tempered.

[0025] In the NMR probe head arrangement according to the invention, the geometry of the flow channel is selected so that it forms a gas expansion nozzle.

[0026] The flow channel is essentially annular and is radially bounded by the central tube and the NMR sample tube, and possibly by a structure arranged within the central tube (e.g., a centering device). The gas expansion nozzle formed by the geometries of the central tube and the NMR sample tube can influence the temperature in the temperature-regulating gas introduced between the NMR sample tube and the central tube, allowing a desired temperature gradient (preferably of approximately 0°C) to be set in the sample substance.

[0027] The central tube of the NMR probe head and the sample tube are preferably arranged coaxially.

[0028] The RF-active axial region is the region where the NMR measurements take place. The RF-active axial region is preferably smaller than the length of the NMR sample tube.

[0029] In a particularly preferred embodiment, the cross-sectional area of the flow channel in the RF-active axial region changes continuously in the z-direction, preferably over the entire axial length of the RF-active axial region. Preferably, the radial dimension of the flow channel increases continuously in the RF-active axial region.

[0030] The change in the cross-sectional area is achieved, in particular, by a continuous change in the radial dimension of the flow channel (ring width of the flow channel). With this continuous change, the increase or decrease in the radial dimension of the flow channel does not necessarily have to extend over the entire radius. Rather, there may be radial sections where the radial dimension of the flow channel remains constant in the z-direction, as long as there are at least some radial sections where the radial dimension of the flow channel changes continuously in the z-direction. This can be the case, for example, if parts of a centering device protrude into the RF-active region (see below).

[0031] The change preferably occurs monotonically, i.e., the cross-sectional area (especially the radial dimension) either exclusively increases or exclusively decreases. The geometry of the central tube and / or the geometry of the NMR sample tube and / or the geometry of a centering device are thus coordinated such that, in the RF-active axial region, the cross-sectional area of the flow channel varies in the z-direction.

[0032] In a preferred embodiment, the cross-sectional area of the generally annular flow channel is reduced in the lower region of the RF-active axial region and expanded in the upper region thereof, so that the tempering gas flowing through it achieves an increased flow velocity with a correspondingly reduced pressure in the lower region with a reduced cross-sectional area and a reduced flow velocity with a correspondingly increased pressure in the upper region with an expanded cross-sectional area, in order to specifically compensate or adjust an existing temperature gradient in the liquid NMR sample substance. At the lower axial end of the NMR sample tube, the flow velocity v of the gas flow increases, whereby the gas pressure p necessarily decreases at the same time (Bernoulli's law: v22+pρ=const. ). A reduced gas pressure p directly results in a proportionally reduced gas temperature T (ideal gas law: pV = nRT). Similarly, increasing the width of the flow channel at the upper axial end of the NMR sample tube results in a lower flow velocity, which leads to higher gas pressure and thus to an increase in the gas temperature at the upper axial end of the NMR sample tube.

[0033] Particularly in the case of a cryogenic RF receiver coil system, an exchange of infrared radiation occurs between the cold coils and the central tube, and a convective heat transfer occurs between the central tube and the sample tube. Thus, energy flows away from the sample substance, and the sample substance and the passing temperature-regulating gas are cooled. With the NMR probe head arrangement according to the invention, the cooling of the gas stream due to the radiation from the sample substance and the heating of the gas stream due to the gas expansion nozzle can be compensated, thus achieving a uniform temperature of the sample substance across the entire length of the RF-active region.

[0034] Preferably, the minimum radial dimension of the cross-sectional area of the flow channel between the central tube and the NMR sample tube (minimum cross-sectional width of the flow channel) is 0.15 mm - 0.3 mm. This dimension of the flow channel enables the generation of a temperature gradient of more than 1°C in the tempering gas itself and of more than 0.25°C in the liquid in the sample tube in the area of the expansion nozzle, at the flow velocities of the tempering gas typically used in the application.

[0035] To avoid high frictional forces on the sample tube, turbulence in the tempering gas, and areas with excessive flow velocities, it is advantageous for the change in the radial dimension of the cross-sectional area of the flow channel between the central tube and the NMR sample tube within the RF-active region to be between 0.3 mm and 2.5 mm. The RF-active region is typically > 10 mm long, preferably 20-21 mm. By changing the radial dimension of the flow channel to the extent specified above and the associated change in the cross-sectional area, a desired temperature change of the tempering gas can be generated without creating turbulence in the flow channel.

[0036] Preferably, the transition region of the NMR sample tube is arranged in the RF-active axial region of the RF receiver coil system, and the outer diameter of the NMR sample tube changes continuously in the z-direction in the RF-active axial region, preferably over the entire axial length of the RF-active axial region. It preferably decreases continuously. The gas expansion nozzle is thus realized here by tapering the NMR sample tube. The inner diameter of the central tube is preferably constant in the RF-active region. This embodiment is particularly advantageous because it can be implemented with existing NMR probe heads.

[0037] In a particularly preferred embodiment, the NMR sample tube is an NMR sample tube as described above.

[0038] In a particularly preferred embodiment, the NMR probe head is an NMR probe head as described above. The gas expansion nozzle is realized by widening the central tube. The outer diameter of the NMR sample tube is preferably constant in the RF-active region.

[0039] In a special embodiment, the central tube and the NMR sample tube form the flow channel. The central tube and the NMR sample tube thus act together as a gas expansion nozzle, at least in the RF-active axial region. In other words, the geometry of the central tube and the geometry of the NMR sample tube are coordinated to form a gas expansion nozzle in the RF-active region.

[0040] If parts of a centering device extend into the RF-active region, the geometry of the centering device must be taken into account. In a special embodiment, a centering device is provided with structural elements that extend into the RF-active axial region and, together with the central tube and the NMR sample tube, form the flow channel. Here, the central tube, the NMR sample tube, and the structural elements act together as a gas expansion nozzle, at least in the RF-active axial region. The centering device preferably tapers in the z-direction in the RF-active region. The outer diameter of the NMR sample tube, the inner diameter of the central tube, and the wall thickness of the central tube then preferably remain constant in the RF-active region. This embodiment is particularly advantageous because it can be implemented with existing NMR probe heads and standard NMR sample tubes.

[0041] Preferably, the centering device is not electrically conductive.

[0042] Preferably, the centering device has a volume-specific susceptibility of less than 1e-8. The centering device can, for example, be made of a magnetically compensated alloy (susceptibility ~0).

[0043] In the method according to the invention for tempering a liquid sample substance in an NMR sample tube by means of a previously described NMR probe head arrangement, a tempering gas is introduced into the flow channel in the z-direction by means of a tempering device, and the inflow velocity of the gas, the geometry of the NMR sample tube and the geometry of the central tube are coordinated in such a way that a predetermined temperature gradient results in a sample substance located in the NMR sample tube.

[0044] Gas expansion in the RF-active axial region creates a temperature difference between the lower and upper parts of the NMR sample tube. This temperature difference can then be dimensioned such that the temperature gradient existing in the NMR sample tube prior to gas expansion is eliminated. By changing the gas flow through the NMR probe head, the temperature gradient can also be finely tuned for a fixed geometry. This also allows intentional positive or negative temperature gradients to be created in the NMR sample tube, for example, to stimulate mixing of the liquids in the NMR sample tube through convection.

[0045] The inflow velocity can be regulated by controlling the gas flow rate of the temperature control device. The NMR probe head assembly preferably includes means for adjusting the flow velocity of the inflowing gas.

[0046] Preferably, the width of the flow channel and / or the inflow velocity of the tempering gas is optimized using a Computational Fluid Dynamics (CFD) simulation.

[0047] In a preferred variant, the predetermined temperature gradient is less than 100 mK, preferably less than 50 mK.

[0048] Alternatively, a larger temperature gradient can be specified, for example to influence convection processes in the liquid NMR sample substance.

[0049] In a preferred variant, a current temperature gradient in the sample substance in the NMR sample tube is determined during the introduction of the tempering gas, and the inflow velocity of the gas is adjusted if the determined temperature gradient deviates from the specified temperature gradient by a set limit value.

[0050] Further advantages of the invention will become apparent from the description and the drawings. The embodiments shown and described are not intended to be exhaustive, but rather serve as examples for describing the invention. Detailed description of the invention and drawing Fig. 1 shows an NMR probe head arrangement according to the invention in which a gas expansion nozzle is effected by an NMR sample tube according to the invention. Fig. 2 shows an NMR probe head arrangement according to the invention, in which a gas expansion nozzle is effected by an NMR probe head according to the invention. Fig. Figure 3 shows an NMR probe head arrangement according to the invention in which a gas expansion nozzle is effected by a centering device. Fig. 4a-c show NMR sample tubes of various sizes according to the invention and a matching central tube of an NMR probe head.

[0051] Fig. 1 to Fig. 3 show NMR probe head assemblies 1, 2, 3, each with an NMR sample tube 6, 7 aligned in a z-direction for receiving a liquid NMR sample substance. The NMR sample tube 6, 7 is arranged inside the central tube 4, 5 and has a bottom section 8 closed at the bottom and a neck section 9 open at the top, as well as a transition region 10 between the bottom section 8 and the neck section 9. The NMR probe head assemblies each have a central tube 4, 5 extending in a z-direction and a radio-frequency (RF) receiver coil system 11 arranged radially around the central tube 4, 5, which together form an NMR probe head. The RF receiver coil system 11 defines an RF-active axial region 12 in which the NMR measurements take place. The transition region 10 of the NMR sample tube 6, 7 is in the Fig. 1 to Fig. In the examples shown in Figure 3, the NMR sample substance is arranged entirely in the RF-active region, i.e., the NMR sample substance arranged in the transition region of the NMR sample tube 6, 7 is the part of the sample substance that contributes to the measurement and for which a specified temperature gradient is to be achieved. However, it is also possible for the transition region to extend beyond the RF-active region without negatively affecting the measurement result.

[0052] Located on the underside of the central tube 4, 5 is a temperature control device that supplies a temperature control gas into an intermediate space (flow channel) between the central tube 4, 5 and the NMR sample tube 6, 7. On the side of the central tube 4, 5 opposite the temperature control device 13, the central tube 4, 5 has a sample inlet 14 through which the NMR sample tube 6, 7 can be inserted into the central tube 4, 5. The geometry of the flow channel is selected in all three embodiments 1, 2, 3 such that the flow channel forms a gas expansion nozzle. In the embodiments shown in the figures, the flow channel is designed such that its annular width B, and thus the cross-sectional area, increases in the z-direction, so that the temperature-controlling gas is heated from the base section 8 to the neck section 9 or cooling is prevented.

[0053] Fig. Figure 1 shows an embodiment of the inventive NMR probe head assembly 1, in which the gas expansion nozzle is realized by the geometry of the NMR sample tube 6. For this purpose, the NMR sample tube 6 has an outer diameter in the transition region 10, which is arranged in the RF-active region 12 of the RF receiver coil system 11, which changes continuously along the z-direction. Fig. 1, the outer diameter of the NMR sample tube 6 decreases upwards. The outer diameter of the NMR sample tube 6 thus varies in the RF-active region 12 between a maximum outer diameter dmax and a minimum outer diameter dmin. In combination with the central tube 4, whose inner diameter in the Fig. 1 remains constant, the gas expansion nozzle according to the invention results.

[0054] In Fig. 4a-c are different NMR sample tube-central tube combinations for the Fig. 1 shown embodiment 1. For clarity, the RF receiver coil system has been shown in the Fig. 4a-c omitted. The Fig. The embodiments shown in Figures 4a-c differ in the inner diameters D of the central tube 4 and the maximum outer diameters dmax and minimum outer diameters dmin of the NMR sample tube 6.

[0055] Fig. Figure 2 shows an embodiment of the inventive NMR probe head assembly 2, in which the gas expansion nozzle is realized by the geometry of the central tube 5. For this purpose, the central tube 5 has an inner diameter in the transition region 10, which is arranged in the RF-active region 12 of the RF receiver coil system 11, which changes continuously along the z-direction. Fig. In the case shown in Figure 2, the inner diameter of the central tube 5 increases towards the top. The inner diameter of the central tube 5 therefore varies in the RF-active region between a minimum inner diameter Dmin and a maximum inner diameter Dmax. In combination with the NMR sample tube 7, whose outer diameter in the Fig. 2 remains constant, the gas expansion nozzle according to the invention results.

[0056] Fig. Figure 3 shows an embodiment of the inventive NMR probe head assembly 3, in which the gas expansion nozzle is realized by the geometry of a centering device with structural elements 15. For this purpose, the structural elements 15 of the centering device protrude into the RF-active region. The structural elements 15, together with the central tube and the NMR sample tube, form a flow channel for the gas from the tempering device 13. In the Fig. 3, the structural elements 15 taper along the z-direction in the RF-active region. In combination with the NMR sample tube 7, whose outer diameter is in the Fig. 3 shown embodiment remains constant and the central tube 4, whose outer diameter in the embodiment shown in Fig. 3 also remains constant, the gas expansion nozzle according to the invention results.

[0057] The wall thickness (radial extent) of the structural elements 15 of the centering device arranged on a circumference can vary along the circumference. In particular, the structural elements 15 do not have to extend over the entire circumference. For example, the centering device can comprise individual elongated structural elements 15 that are axially oriented in the direction of the z-axis (e.g., wedge-shaped or rib-shaped structural elements). Alternatively or additionally, the centering device can contain capillaries through which the gas can flow (not shown). The centering device can also have ribs or fins that, for example, run spirally in the centering device (not shown).

[0058] The Fig.The embodiments of the inventive NMR probe head arrangement 1, 2, 3 shown in Figures 1-3 represent particularly preferred embodiments, since in each case only one element (NMR sample tube 6 or NMR probe head 5 or structural elements 15) has a varying geometry in the RF-active region 12. However, embodiments are also conceivable in which several of the above-mentioned elements (NMR sample tube, central tube, and centering device) have a varying geometry in the RF-active region 12. The decisive factor is that the passage area for the gas, i.e., the cross section through which the gas can flow, changes in the RF-active region. List of reference symbols 1 NMR probe head arrangement with NMR sample tubes with varying outer diameter in the RF active range 2 NMR probe head arrangement with central tube with varying inner diameter in the RF active range 3 NMR probe head arrangement with centering device extending into the RF range 4 Central tube with constant inner diameter in the RF active area 5 Central tube with inner diameter varying in the HF active range 6 NMR sample tubes with outer diameters varying in the RF active range 7 NMR sample tubes with constant outer diameter in the RF active range 8 Bottom section of the NMR sample tube 9 Neck section of the NMR sample tube 10 Transition region of the NMR sample tube 11 RF receiving coil system 12 RF active area of the RF receiving coil system 13 Tempering device 14 Sample inlet of the central tube 15 Structural elements of the centering device B Ring width of the flow channel dmin minimum outer diameter of the NMR sample tube dmax maximum outer diameter of the NMR sample tube Dmin minimum inner diameter of the central tube Dmax maximum inner diameter of the central tube z Direction along which the central tube and the NMR sample tube are aligned (from the bottom section to the neck section of the NMR sample tube) Literature list [1] US 6 441 617 B2 [2] DE 10 2019 216 108 A1 [3] DE 10 2010 029 080 B4 [4] EP 1 795 910 B1 [5] US 9 482 729 B2 [6] ATS Life Sciences Wilmad, “High Temperature Dewar for Bruker ER4114HT Cavity” [7] DE 10 2006 046 888 A1

Claims

[1] NMR sample tubes (6, 7) having an elongated extension in a z-direction, a bottom portion (8) with a closed lower axial end and a neck portion (9) with an open upper axial end, wherein the NMR sample tube (7) has a transition region (10) between the bottom portion (8) and the neck portion (9), the outer diameter of the NMR sample tube in the transition region (10) changes continuously by 5% - 50% in the z-direction, whereby the transition region (10) has an axial extension along the z-direction of > 10 mm, characterized by , that the NMR sample tube is suitable for forming a gas expansion nozzle in a central tube of an NMR probe head, with which an inflowing gas is to be heated along the z-direction, that the outer diameter of the NMR sample tube (6) decreases continuously in the z-direction in the transition region (10), the z-direction running from the bottom section to the neck section of the NMR sample tube. [2] NMR sample tube (6, 7) according to claim 1, characterized by that the lower axial end of the NMR sample tube (6, 7) is rounded. [3] NMR probe head with a central tube (4, 5) with an inner diameter and an outer diameter, a sample inlet (14) and a radio frequency (RF) receiving coil system (11) arranged radially around the central tube (4, 5) with respect to a z-direction, wherein the NMR probe head is suitable for forming a gas expansion nozzle with an NMR sample tube, through which the temperature of a gas flowing between the NMR sample tube and the central tube of the NMR probe head and thus the temperature of an NMR sample substance located in the NMR sample tube can be influenced, wherein the RF receiving coil system (11) defines an RF-active axial region (12), characterized by that the inner diameter of the central tube (4, 5) in the RF-active axial area (12) changes continuously in the z-direction. [4] NMR probe head according to claim 3, characterized by that the inner diameter of the central tube (5) in the RF-active axial region (12) increases continuously in the z-direction, wherein the z-direction is directed from the RF-active axial (12) region to the sample inlet (14). [5] NMR probe head according to one of claims 3 to 4, characterized by that the wall thickness of the central tube (5) in the RF-active axial region (12), preferably over the entire axial length of the RF-active axial region (12), changes continuously in the z-direction, preferably decreases continuously. [6] NMR probe head according to one of claims 3 to 5, characterized bythat the NMR probe head is a cryogenic probe head in which the RF receiving coil system (11) can be cooled to cryogenic temperature. [7] NMR probe assembly (1, 2, 3) comprising a central tube (4, 5) extending in a z-direction, an NMR sample tube (6, 7) aligned in the z-direction for receiving a liquid NMR sample substance, which is arranged inside the central tube (4, 5) and has a bottom section (8) with a closed lower axial end, a neck section (9) with an open upper axial end and a transition region (10) between the bottom section (8) and the neck section (9), a radio frequency (RF) receiving coil system (11) arranged radially in the z-direction around the central tube (4, 5), wherein the RF receiving coil system (11) defines an RF-active axial region (12), and a temperature control device for introducing a temperature-controlling gas in the z-direction into a flow channel between the central tube (4, 5) and the NMR sample tube (6, 7), characterized by , that the geometry of the flow channel is selected so that it forms a gas expansion nozzle. [8] NMR probe head arrangement (1, 2, 3) according to claim 7, characterized by that the cross-sectional area of the flow channel in the RF-active axial region (12), preferably over the entire axial length of the RF-active axial region (12), changes continuously in the z-direction, preferably increases continuously. [9] NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 8, characterized by that the minimum radial dimension of the cross-sectional area of the flow channel between the central tube (4, 5) and the NMR sample tube (6, 7) is 0.15 mm - 0.3 mm. [10] NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 9 characterized by that the change in the radial dimension of the cross-sectional area of the flow channel between the central tube (4, 5) and the NMR sample tube (6, 7) within the RF-active region (12) is between 0.3 mm and 2.5 mm. [11] NMR probe head arrangement (1) according to one of claims 7 to 10, characterized by that the transition region (10) of the NMR sample tube (6) is arranged in the RF-active axial region (12) of the RF receiving coil system (11) and the outer diameter of the NMR sample tube (6) in the RF-active axial region (12), preferably over the entire axial length of the RF-active axial region (12), changes continuously in the z-direction, preferably decreases continuously. [12] NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 11, characterized by that the NMR sample tube is an NMR sample tube according to one of claims 1 to 3. [13] NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 12, characterized by that the NMR probe head is an NMR probe head according to one of claims 4 to 7. [14] NMR probe head arrangement (1, 2) according to one of claims 7 to 13, characterized by that the central tube and the NMR sample tube form the flow channel. [15] NMR probe head arrangement according to one of claims 7 to 13, characterized by that a centering device is provided with structural elements (15) which project into the RF-active axial region (12) and which, together with the central tube (4) and the NMR sample tube (7), form the flow channel. [16] Method for tempering a liquid sample substance in an NMR sample tube (6, 7) by means of an NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 15, wherein a tempering gas is introduced into the flow channel in the z-direction by means of a tempering device (13), and wherein the inflow velocity of the gas, geometry of the NMR sample tube (6, 7) and geometry of the central tube (4, 5) are coordinated in such a way that a predetermined temperature gradient results in a sample substance located in the NMR sample tube (6, 7). [17] Method according to claim 16, characterized by that the specified temperature gradient is less than 100 mK, preferably less than 50 mK. [18] Method according to one of claims 16 to 17, characterized by that during the introduction of the tempering gas, a current temperature gradient is determined in the sample substance in the NMR sample tube (6, 7), and that the inflow velocity of the gas is adjusted if the determined temperature gradient deviates from the predetermined temperature gradient by a set limit value.

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