NMR sample tube having a varying outer diameter, NMR probe head having a varying inner diameter, NMR probe head assembly comprising a gas expansion nozzle, and temperature-control method

Abstract

The invention relates to an NMR probe head assembly 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, said NMR sample tube being arranged inside the central tube and comprising a bottom section having a closed lower axial end, a neck section having an open upper axial end, and a transition region between the bottom section and the neck section; a radio-frequency (RF) receiver coil system which is arranged radially about the central tube in the z-direction, the RF receiver coil system defining an RF-active axial region; and a temperature-control device for introducing a temperature-control gas in the z-direction into a flow channel provided between the central tube and the NMR sample tube; characterised in that the geometry of the flow channel is selected such that the flow channel forms a gas expansion nozzle. This makes it possible to avoid undesirable temperature gradients.

Description

[0001] NMR sample tubes with varying outer diameter, NMR sample head with varying inner diameter, NMR sample head arrangement with gas expansion nozzle, temperature control method

[0002] Background of the invention

[0003] The invention relates to an NMR sample tube with an elongated extension in a z-direction, a bottom section with a closed lower axial end and a neck section with an open upper axial end, wherein the NMR sample tube has a transition region between the bottom section and the neck section.

[0004] The invention also relates to an NM sample head with a central tube, a sample inlet and a high-frequency (HF) receiving coil system arranged radially around the central tube with respect to a z-direction, wherein the HF receiving coil system defines an HF-active axial area.

[0005] Furthermore, the invention relates to an NMR probe head arrangement comprising a central tube extending in a z-direction, an NMR probe tube oriented in the z-direction for receiving a liquid NMR sample substance, which is arranged inside the central tube and has a bottom section with a closed lower axial end, a neck section with an open upper axial end and a transition region between the bottom section and the neck section, a high-frequency (HF) receiving coil system arranged radially around the central tube in the z-direction, wherein the HF receiving coil system defines an HF-active axial region, and a temperature control device for introducing a temperature-controlling gas in the z-direction into a flow channel located between the central tube and the NMR probe tube.

[0006] An NMR probe head arrangement with temperature control device is known from [1].

[0007] Particularly in cryogenic NMR probes, NMR samples cool down through radiation exchange with the surrounding cryogenic coils. To counteract this, it is known to use a temperature control device to circulate warm gas (temperature control 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, which negatively affects the linewidth and lock of the NMR spectrum measured with this probe. [5]

[0008] [2] and [3] disclose temperature control systems for NMR sample tubes using a gas flow, whereby the gas is guided past the NMR sample tube in a meandering countercurrent pattern. However, this principle requires a great deal of space in the NMR sample head and is therefore not feasible for many applications.

[0009] From [1] a cryo-NMR sample head is known 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 insulating layer reduces the gradient by approximately 50%, but cannot eliminate it completely.

[0010] [4] discloses a specially shaped sample tube with which the improvement of the signal-to-noise ratio is to be achieved by reducing the influence of an electrically conductive sample substance on the NMR coils. Object of the invention

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

[0012] Description of the invention

[0013] This problem is solved according to the invention by an NMR sample tube according to claim 1, an NMR sample head according to claim 3, an NMR sample head arrangement according to claim 7 and a method according to claim 16.

[0014] In the NMR sample tube according to the invention, the outer diameter of the NMR sample tube changes continuously in the z-direction by 5% to 50%, preferably 20% to 50%, in the transition region, wherein the transition region has an axial extension along the z-direction of > 10 mm. Due to the geometry according to the invention, a gas expansion nozzle can be realized when used in an NMR probe head, 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.

[0015] A steady (continuous) change in the outer diameter of the NMR sample tube (i.e., the absence of abrupt changes in radius) is important for fluid dynamics reasons, as turbulence can otherwise occur. Likewise, the change in the outer diameter of the sample tube from the transition region to the bottom section and to the neck section should be steady.

[0016] 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. In particular, the longitudinal profile of the NMR sample tube can be conical, at least in sections, in the transition region.

[0017] 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 probe head arrangement described below, wherein the NMR sample tube is characterized in particular by a length-to-width ratio of 15 to 70 and by 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.

[0018] The outer diameter of the NMR sample tube decreases continuously in the z-direction during 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.

[0019] Preferably, the lower axial end of the NMR sample tube tapers to a rounded shape. In particular, the outer diameter of the NMR sample tube decreases in the opposite direction to the z-axis over a length of at least 10% of the bottom section. The outer diameter is thus reduced towards the closed end. This downward reduction of the outer diameter allows the sample tube to be inserted into existing centering devices and thus precisely aligned.

[0020] In the NMR probe head according to the invention, the inner diameter of the central tube changes continuously in the z-direction within the RF-active axial region. A continuous change in the inner diameter of the central tube of the NMR probe head (i.e., the absence of abrupt changes in radius) is important for fluid dynamics reasons, as turbulence can otherwise occur. The geometry according to the invention allows a gas expansion nozzle to be implemented when used with an NMR probe tube. This nozzle can influence the temperature of a gas flowing between the NMR probe tube and a central tube of the NMR probe head, and thus the temperature of an NMR sample substance located in the NMR probe tube.

[0021] The central tube has an annular cross-section with an outer diameter and an inner diameter and includes a sample inlet at one axial (upper) end for inserting the sample tube and a gas inlet at the opposite (lower) end for supplying a temperature-controlled gas. The central tube thus has a continuous opening through which gas can flow.

[0022] Preferably, the NMR probe head is suitable for use in an NMR probe head arrangement described below.

[0023] Preferably, the sample inlet is located at the top of the central tube, while the RF-active section is located at the bottom. A temperature control device can be arranged at the end opposite the sample inlet.

[0024] In a particularly preferred embodiment, the inner diameter of the central tube increases continuously in the z-direction within the RF-active axial region, with the z-direction pointing from the RF-active region towards the sample inlet. This is particularly advantageous when the NMR sample 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.

[0025] Alternatively or additionally, the wall thickness of the central tube in the RF-active axial region can be continuously changed, preferably continuously decreasing, 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.

[0026] 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 a cryogenic temperature, since the NMR sample substance, cooled by the cold coils of the RF receiver coil system, must be tempered. In the NMR probe head arrangement according to the invention, the geometry of the flow channel is selected such that it forms a gas expansion nozzle.

[0027] The flow channel is essentially annular and radially bounded by the central tube and the NMR sample tube, and optionally by a structure located 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 of the tempering gas introduced between the NMR sample tube and the central tube, thus allowing a desired temperature gradient (preferably around 0°C) to be established in the sample substance.

[0028] The central tube of the NMR probe head and the sample tube are preferably arranged coaxially.

[0029] The RF-active axial region is the area in which the NMR measurements take place. The RF-active axial region is preferably smaller than the length of the NMR sample tube.

[0030] 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 in the RF-active axial region increases continuously.

[0031] The change in cross-sectional area is achieved primarily through a continuous change in the radial dimension of the flow channel (ring width of the flow channel). This continuous change does not necessarily have to extend over the entire radius. Rather, there can 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 area (su). The change is preferably monotonic, meaning that the cross-sectional area (especially the radial dimension) either increases exclusively or decreases exclusively.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 in such a way that the cross-sectional area of ​​the flow channel varies in the z-direction in the RF-active axial region.

[0032] In a preferred embodiment, the cross-sectional area of ​​the typically annular flow channel is reduced in the lower region of the RF-active axial area and expanded in the upper region. This results in an increased flow velocity with a correspondingly lower pressure in the lower region with the reduced cross-sectional area, and a decreased flow velocity with a correspondingly increased pressure in the upper region with the expanded cross-sectional area. This is intended to selectively compensate for or adjust any existing temperature gradient in the liquid NMR sample. At the lower axial end of the NMR sample tube, the gas flow velocity v increases, while the gas pressure p necessarily decreases simultaneously (Bernoulli's Law: y + μ = 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 leads to a lower flow velocity, resulting in higher gas pressure and thus an increase in 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 takes place between the cold coils and the central tube, and 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 flowing tempering gas are cooled. With the NMR sample head arrangement according to the invention, the cooling of the gas flow due to the radiation from the sample substance and the heating of the gas flow due to the gas expansion nozzle can be compensated, thereby achieving a uniform temperature of the sample substance over the entire length of the RF-active region. 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 to 0.3 mm.This dimension of the flow channel enables the generation of a temperature gradient of more than 1°C in the temperature control gas itself and of more than 0.25°C in the liquid in the sample tube in the area of ​​the expansion nozzle, at flow rates of the temperature control gas typical in the application.

[0034] To avoid high frictional forces on the sample tube, turbulence of the temperature control 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 NM sample tube within the RF-active area to be between 0.3 mm and 2.5 mm. The RF-active area typically has a length of > 10 mm, preferably 20–21 mm. By changing the radial dimension of the flow channel within the aforementioned range and the associated change in cross-sectional area, a desired temperature change of the temperature control gas can be generated without creating turbulence in the flow channel.

[0035] Preferably, the transition region of the NMR sample tube is located 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 within the RF-active axial region, preferably over its entire axial length. Preferably, it decreases continuously. The gas expansion nozzle is thus realized by a tapering of the NMR sample tube. The inner diameter of the central tube is preferably constant within the RF-active region. This embodiment is particularly advantageous because it can be implemented using existing NMR sample heads.

[0036] In a particularly preferred embodiment, the NM sample tube is an NMR sample tube as described above.

[0037] In a particularly preferred embodiment, the NMR probe head is an NMR probe head as described above. The gas expansion nozzle is realized here by an expansion of the central tube. The outer diameter of the NMR probe tube is preferably constant in the RF-active region. In a special embodiment, the central tube and the NMR probe tube form the flow channel. Thus, the central tube and the NMR probe tube 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 probe tube are such that they form a gas expansion nozzle in the RF-active region.

[0038] If parts of a centering device protrude into the RF-active region, the geometry of the centering device must be taken into account. In a particular embodiment, a centering device is therefore provided with structural elements that project 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. Preferably, the centering device tapers in the z-direction within 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 within the RF-active region. This embodiment is particularly advantageous because it can be implemented using existing NMR sample heads and standard NMR sample tubes.

[0039] Preferably, the centering device is not electrically conductive.

[0040] Preferably, the centering device has a volume-specific susceptibility of less than le-8. The centering device can, for example, be made of a magnetically compensated alloy (susceptibility ~0).

[0041] In the inventive method for temperature control of a liquid sample substance in an NMR sample tube using a previously described NMR probe head arrangement, a temperature-controlling gas is introduced into the flow channel in the z-direction by means of a temperature control device. The gas inflow velocity, the geometry of the NMR sample tube, and the geometry of the central tube are coordinated such that a predetermined temperature gradient results in the sample substance contained in the NMR sample tube. The 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 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 sample head, the temperature gradient can also be finely adjusted for a fixed geometry.In this way, positive or negative temperature gradients can also be intentionally created in the NMR sample tube, for example to stimulate mixing of the liquids in the NMR sample tube by convection.

[0042] The inflow velocity can be regulated by controlling the gas flow rate of the temperature control device. Preferably, the NMR probe head arrangement includes means for adjusting the flow velocity of the incoming gas.

[0043] Preferably, the optimization of the width of the flow channel and / or the inflow velocity of the tempering gas is carried out using a Computational Fluid Dynamics (CFD) simulation.

[0044] In a preferred variant, the specified temperature gradient is less than 100 mK, preferably less than 50 mK.

[0045] Alternatively, a larger temperature gradient can be specified, for example to influence convection processes in the liquid NMR sample substance.

[0046] In a preferred variant, during the introduction of the tempering gas, a current temperature gradient in the sample substance located in the NMR sample tube is determined, and the inflow velocity of the gas is adjusted if the determined temperature gradient deviates from the specified temperature gradient by a fixed limit value.

[0047] Further advantages of the invention will become apparent from the description and the drawing. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention. Detailed description of the invention and drawing

[0048] Fig. 1 shows an NMR probe head arrangement according to the invention, in which a gas expansion nozzle is effected by an NMR probe tube according to the invention.

[0049] 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.

[0050] Fig. 3 shows an NMR probe head arrangement according to the invention, in which a gas expansion nozzle is effected by a centering device.

[0051] Fig. 4a-c show NMR sample tubes of various sizes according to the invention and a matching central tube of an NMR sample head.

[0052] Figures 1 to 3 show NMR probe head assemblies 1, 2, 3, each with an NMR sample tube 6, 7 oriented 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-closed base section 8 and a neck section 9 open at the top, as well as a transition region 10 between the base section 8 and the neck section 9. Each NMR probe head assembly has a central tube 4, 5 extending in a z-direction and a high-frequency (HF) receiving coil system 11 arranged radially around the central tube 4, 5, which together form an NMR probe head. The RF receiving coil system 11 defines an RF-active axial region 12 in which the NMR measurements take place, i.e., the RF-active region is to be understood as the axial region over which the coils of the RF receiving coil system 11 extend.In the examples shown in Figures 1 to 3, the transition region 10 of the NMR sample tube 6, 7 is located entirely within the RF-active region. This means that the NMR sample substance located in the transition region of the NMR sample tube 6, 7 is the portion of the sample substance that contributes to the measurement and for which a predetermined 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. A temperature control device is located on the underside of the central tube 4, 5. This device serves to supply 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.In all three embodiments 1, 2, 3, the geometry of the flow channel is selected 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 ring width B, and thus its cross-sectional area, increases in the z-direction, so that the tempering gas is heated from the bottom section 8 to the neck section 9, or cooling is prevented.

[0053] Fig. 1 shows an embodiment of the NMR sample head arrangement 1 according to the invention, 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 located in the RF-active region 12 of the RF receiver coil system 11, that changes continuously along the z-direction. In the case shown in Fig. 1, the outer diameter of the NMR sample tube 6 decreases towards the top. Thus, the outer diameter of the NMR sample tube 6 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 remains constant in the embodiment shown in Fig. 1, the gas expansion nozzle according to the invention is formed.

[0054] Figures 4a-c show various NMR sample tube-central tube combinations for the embodiment 1 depicted in Figure 1. For clarity, the RF receiving coil system has been omitted in Figures 4a-c. 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. 2 shows an embodiment of the NMR probe head arrangement 2 according to the invention, 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 located in the RF-active region 12 of the RF receiver coil system 11, that changes continuously along the z-direction. In the case shown in Fig. 2, the inner diameter of the central tube 5 increases upwards. Thus, the inner diameter of the central tube 5 varies in the RF-active region between a minimum inner diameter Dmin and a maximum inner diameter Dmax. In combination with the NMR probe tube 7, whose outer diameter remains constant in the embodiment shown in Fig. 2, the gas expansion nozzle according to the invention is formed.

[0056] Fig. 3 shows an embodiment of the NMR sample head arrangement 3 according to the invention, 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 project into the RF-active area. The structural elements 15, together with the central tube and the NMR sample tube, form a flow channel for the gas from the temperature control device 13. In the embodiment shown in Fig. 3, the structural elements 15 taper along the z-direction in the RF-active area. In combination with the NMR sample tube 7, whose outer diameter remains constant in the embodiment shown in Fig. 3, and the central tube 4, whose outer diameter also remains constant in the embodiment shown in Fig. 3, the gas expansion nozzle according to the invention is formed.

[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 need not extend over the entire circumference. For example, the centering device can comprise individual elongated structural elements 15 oriented axially 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 within the centering device (not shown).

[0058] The embodiments of the NMR probe head arrangement 1, 2, 3 shown in Figures 1-3 represent particularly preferred embodiments, since only one element (NMR probe 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 aforementioned elements (NMR probe tube, central tube, and centering device) have a varying geometry in the RF-active region 12. The crucial 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.

[0059] List of symbols

[0060] 1 NMR probe assembly with NMR probe tubes with varying outer diameter in the RF-active range

[0061] 2 NMR probe head arrangement with central tube with varying inner diameter in the RF-active range

[0062] 3 NMR probe head arrangement with centering device projecting into the RF range

[0063] 4 Central tubes with constant inner diameter in the RF-active range

[0064] 5 Central tubes with varying inner diameter in the RF-active range

[0065] 6 NMR sample tubes with varying outer diameters in the RF-active range

[0066] 7 NMR sample tubes with a constant outer diameter in the RF-active range

[0067] 8 Bottom section of the NMR sample tube

[0068] 9 Neck section of the NMR sample tube

[0069] 10 Transition region of the NMR sample tube

[0070] 11 RF receiving coil system

[0071] 12 RF-active area of ​​the RF receiving coil system

[0072] 13 Temperature control unit

[0073] 14 Sample inlet of the central tube

[0074] 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

[0075] Dmin minimum inner diameter of the central tube

[0076] 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); bibliography

[0077] [1] US 6 441 617 B2

[0078] [2] DE 10 2019 216 108 Al

[0079] [3] DE 10 2010 029 080 B4 [4] EP 1 795 910 p

[0080] [5] US 9 482 729 B2

Claims

Patent claims 1. NMR sample tubes (6, 7) with an elongated extension in a z-direction, a bottom section (8) with a closed lower axial end and a neck section (9) with an open upper axial end, wherein the NMR sample tube (7) has a transition region (10) between the bottom section (8) and the neck section (9), the outer diameter of the NMR sample tube in the transition region (10) changes continuously by 5% to 50% in the z-direction, the transition region (10) having an axial extension along the z-direction of > 10 mm, characterized in that the NMR sample tube is suitable for forming a gas expansion nozzle in a central tube of an NMR sample head, with which an inflowing gas is to be heated along the z-direction, and that the outer diameter of the NMR sample tube (6) in the transition region (10) decreases steadily in the z-direction, with 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 in that the lower axial end of the NMR sample tube (6, 7) tapers to a round shape.

3. NM sample head with a central tube (4, 5) having an inner diameter and an outer diameter, a sample inlet (14) and a high-frequency (HF) receiving coil system (11) arranged radially around the central tube (4, 5) with respect to a z-direction, wherein the NM sample head is suitable for forming a gas expansion nozzle with an NMR sample tube through which the temperature of a between the NMR sample tube and the central tube of the NMR sample head, 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 in that the inner diameter of the central tube (4, 5) in the RF-active axial region (12) changes continuously in the z-direction.

4. NM - Sample head according to claim 3, characterized in that the inner diameter of the central tube (5) in the HF-active axial area (12) increases continuously in the z-direction, wherein the z-direction is directed from the HF-active axial (12) area to the sample inlet (14).

5. NM - Sample head according to one of claims 3 to 4, characterized in that the wall thickness of the central tube (5) in the HF-active axial area (12), preferably over the entire axial length of the HF-active axial area (12), changes continuously in the z-direction, preferably decreases continuously.

6. NM sample head according to one of claims 3 to 5, characterized in that the NMR sample head is a cryo sample head in which the RF receiving coil system (11) can be cooled to cryogenic temperature.

7. NMR sample head arrangement (1, 2, 3) comprising a central tube (4, 5) extending in a z-direction, an NMR sample tube (6, 7) oriented 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 un- comprising an 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 high-frequency (HF) receiving coil system (11) arranged radially in the z-direction around the central tube (4, 5), wherein the HF receiving coil system (11) defines an HF-active axial region (12), and a temperature control device for introducing a temperature-controlling gas in the z-direction into a flow channel located between the central tube (4, 5) and the NMR sample tube (6, 7), characterized in that the geometry of the flow channel is selected such that it forms a gas expansion nozzle.

8. NMR probe head arrangement (1, 2, 3) according to claim 7, characterized in that the cross-sectional area of ​​the flow channel in the HF-active axial region (12), preferably over the entire axial length of the HF-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 in that the minimum radial dimension of the cross-sectional area of ​​the flow channel between the central tube (4, 5) and the NMR probe 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 in 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 probe tube (6, 7) within the RF-active area (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 in that the transition region (10) of the NMR probe tube (6) is arranged in the RF-active axial region (12) of the RF receiver coil system (11) and the outer diameter of the NMR probe 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 in that the NMR probe tube is an NMR probe tube according to one of claims 1 to 2.

13. NMR probe head arrangement (1, 2, 3) according to one of claims 7 to 12, characterized in 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 in that the central tube and the NMR probe tube form the flow channel.

15. NMR probe head arrangement according to one of claims 7 to 13, characterized in 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 probe tube (7) form the flow channel.

16. Method for temperature control of a liquid sample substance in an NMR sample tube (6, 7) using an NMR sample 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 such 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 in that the predetermined temperature gradient is less than 100 mK, preferably less than 50 mK.

18. Method according to one of claims 16 to 17, characterized in that during the introduction of the tempering gas, a current temperature gradient in the sample substance located in the NMR sample tube (6, 7) is determined, and that the inflow velocity of the gas is adjusted if the determined temperature gradient deviates from the predetermined temperature gradient by a fixed limit value.

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