INTERCHANGEABLE SAMPLE CELL FOR DNP-NMR MEASUREMENTS

Abstract

The invention relates to a sample cell for performing DNP-NMR measurements, for interchangeable use in an EPR microwave resonator, with the sample cell comprising a flat sample cavity for holding a liquid sample to be measured, wherein the flat sample cavity extends with a maximum length L and a maximum width W in a sample cavity plane, and extends with a maximum height H perpendicular to the sample cavity plane, with H≤ 1/15*L and H≤ 1/15*W, and an NMR coil wound around the flat sample cavity for generating an RF magnetic field B.sub.2, wherein a coil axis of the NMR coil about which the NMR coil is wound is oriented perpendicular to the sample cavity plane. The invention provides a sample cell which is easy to handle and improves the quality and the reproducibility of DNP-NMR measurements.

Claims

1. A sample cell for performing DNP-NMR measurements, for interchangeable use in an EPR microwave resonator, the sample cell comprising: a flat sample cavity for holding a liquid sample to be measured, wherein the flat sample cavity extends with a maximum length L and a maximum width W in a sample cavity plane, and extends with a maximum height H perpendicular to the sample cavity plane, with H≤ 1/15*L and H≤ 1/15*W, and an NMR coil wound around the flat sample cavity for generating an RF magnetic field B2, wherein a coil axis of the NMR coil about which the NMR coil is wound is oriented perpendicular to the sample cavity plane.

2. The sample cell according to claim 1, further comprising a first plate and a second plate, wherein the flat sample cavity is formed by a cavity recess in the first plate and/or a cavity recess in the second plate, and wherein the first plate and second plate are joined to each other, with the flat sample cavity enclosed between them.

3. The sample cell according to claim 2, wherein a coil recess is formed in the first plate and/or a coil recess is formed in the second plate, wherein the NMR coil is arranged in the coil recess of the first plate and/or the coil recess of the second plate.

4. The sample cell according to claim 3 wherein the NMR coil is enclosed between the first plate and the second plate.

5. The sample cell according to claim 3, wherein the flat sample cavity is formed only by the cavity recess of the first plate, and only the second plate forms the coil recess, in which the NMR coil is arranged.

6. The sample cell according to claim 1, wherein for an offset distance O.sub.d between a centre point P.sub.NMR of the NMR coil and a centre point P.sub.CS of the flat sample cavity, taken along the direction perpendicular of the sample cavity plane, O.sub.d≤0.5 mm.

7. The sample cell according to claim 1, wherein the maximum length L of the flat sample cavity is 3 mm≤L≤7 mm, the maximum width W of the flat sample cavity is 3 mm≤W≤7 mm, and the maximum height H of the flat sample cavity is 0.12 mm≤H≤0.36 mm.

8. An NMR probe for performing DNP-NMR measurements, for use in an EPR microwave resonator, the NMR probe comprising: a sample cell according to claim 1, a sample cell holder for reversible insertion of the sample cell, wherein the sample cell is inserted into the sample cell holder, and two sweeping coils for generating a sweeping magnetic field which is substantially parallel to the sample cavity plane in the area of the flat sample cavity of the inserted sample cell.

9. The NMR probe according to claim 8 wherein, with the sample cell inserted into the sample cell holder, the NMR probe forms at least one tempering channel for varying and/or keeping a temperature of the liquid sample to be measured.

10. The NMR probe according to claim 9 wherein, at least in a region of the flat sample cavity, the at least one tempering channel extends parallel to the flat sample cavity and adjacent to the sample cell.

11. The NMR probe according to claim 10, wherein the NMR probe comprises a dielectric sleeve that at least partially limits the at least one tempering channel.

12. The NMR probe according to claim 11 wherein an inner region of the dielectric sleeve is subdivided into two tempering channels with the sample cell inserted in the sample cell holder.

13. The NMR probe according to claim 8, wherein the two sweeping coils are rod-shaped.

14. The NMR probe according to claim 8, wherein the two sweeping coils run parallel to each other.

15. The NMR probe according to claim 8 wherein the two sweeping coils run parallel to the sample cavity plane.

16. A DNP-NMR measuring device comprising an EPR microwave resonator, an NMR probe according to claim 8, wherein the NMR probe is positioned at least partially inside the EPR microwave resonator such that the flat sample cavity and the NMR coil of the sample cell inserted into the sample cell holder are positioned inside the EPR microwave resonator, and a magnet arrangement for generating a B0 magnetic field which is parallel to the sweeping magnetic field and parallel to the sample cavity plane in the area of the flat sample cavity of the inserted sample cell.

17. The DNP-NMR measuring device according to claim 16, wherein the EPR microwave resonator is dimensioned and configured for a particular microwave mode, and wherein the sample cell is positioned such that a B1 microwave magnetic field of said particular microwave mode is oriented substantially perpendicularly to the B0 magnetic field in the area of the flat sample cavity, and the sample cavity plane is oriented along a plane of minimum electric field of said particular microwave mode, wherein the plane of minimum electric field of said particular microwave mode runs through the flat sample cavity.

18. The DNP-NMR measuring device according to claim 17, wherein the B1 microwave magnetic field of said particular microwave mode is oriented substantially parallel to the sample cavity plane in the area of the flat sample cavity.

19. The DNP-NMR measuring device according to claim 16, wherein the EPR microwave resonator is dimensioned and configured for microwave mode TE102.

20. The DNP-NMR measuring device according to claim 16, further comprising: a microwave source, a waveguide for guiding microwave radiation to the EPR microwave resonator, and an iris for coupling the microwave radiation from the waveguide into the EPR microwave resonator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] FIG. 1 shows an exploded view of an embodiment of a sample cell according to the invention, with two plates, a coil recess, a cavity recess and an NMR coil.

[0063] FIG. 2 shows an enlarged view of the first plate of FIG. 1 in the area of the cavity recess.

[0064] FIG. 3 shows a schematic longitudinal section of the sample cell from FIG. 1 in assembled form in the plane A there.

[0065] FIG. 4 shows a schematic, partially sectional drawing of an embodiment of an NMR probe according to the invention, with the sample cell of FIG. 1 inserted in a sample cell holder and two sweeping coils.

[0066] FIG. 5 shows a schematic cross-section of the NMR probe from FIG. 4 in the plane B there.

[0067] FIG. 6 shows a schematic, partially sectional drawing of an embodiment of an DNP-NMR measuring device according to the invention, with an EPR microwave resonator and the NMR probe of FIG. 4 inserted.

[0068] FIG. 7 shows a simplified schematic cross-section of the NMR probe from FIG. 4 in the plane B there, with the directions and approximate positions of the different magnetic fields that are used during DNP-NMR measurements.

DETAILED DESCRIPTION

[0069] FIG. 1 shows an exploded view of a sample cell 1 according to the invention. The sample cell 1 comprises a first plate 2, a second plate 3 and an NMR coil 4. The plates 2, 3 are basically rectangular and elongated in a long direction (here the vertical direction). In the embodiment shown, each of the two plates 2, 3 have a (maximum) length LS of 50 mm, a (maximum) width WS of 10 mm and a (maximum) height HS of 1 mm; note that the example dimensions of LS, WS and HS are typical for a 10 GHz EPR microwave resonator.

[0070] A cavity recess 5 is etched into the first plate 2 which forms a flat sample cavity 6; the cavity recess 5 is etched into the first plate 2 from the inner flat side facing the second plate 3. The flat sample cavity 6 is approximately cuboidally shaped with curved edges. Furthermore, two channel recesses 7 are etched into the first plate 2 which form two channels 8. The two channels 8 are positioned parallel and next to each other here. In the embodiment shown, the two channels 8 are shaped approximately cuboidally with a length of 20 mm, a width of 0.2 mm and a height of 0.2 mm. At an upper end 9 of the first plate 2, the channels 8 open into connections (inlet/outlet) for a liquid sample to be measured; the connections are simply called inlets here. First halves of the inlets 10a for the two channels 8 are formed in the first plate 2, wherein the two first halves of the inlets 10a are shaped semi-conical here. The two channels 8 are connected to the flat sample cavity 6 whereby the two channels 8 widen at the transition into the flat sample cavity 6.

[0071] A coil recess 11 is etched into the second plate 3; the coil recess 11 is etched into the second plate 3 from the inner flat side facing the first plate 2. The coil recess 11 here also comprises two contact channels 11a, 11b for contact lines 14a, 14b to the NMR coil 4, wherein the contact lines 14a, 14b lead from contacts 13a, 13b to the NMR coil 4. The NMR coil 4 can be arranged in the coil recess 11 of the second plate 3. The NMR coil 4 is arranged on the side of the second plate 3 that faces the flat sample cavity 6 of the first plate 2. The contacts 13a, 13b for the NMR coil 4 are arranged on the outer side of the second plate 3 facing away from the first plate 2 and the sample cavity 6, in respective contact recesses 12 prepared on said outer side of the second plate 3. In the example shown, the NMR coil 4 comprises two conductor loops that are wound in a pancake type fashion. In an alternative embodiment the NMR coil 4 may e.g., comprise more than two conductor loops or only one conductor loop (not shown).

[0072] The NMR coil 4 is wound around a coil axis 4a. The coil axis 4a is centrally arranged inside the NMR coil 4 or its wound conductor loops. The NMR coil 4 is bridged in a bridging area 15 for contacting an inner end of the NMR coil 4, so that the NMR coil 4 or its wound conductor loops can be basically flat. Typically, the NMR coil 4 is arranged in the coil recess 11 of the second plate 3 via sputtering (i.e., the NMR coil 4 is built up directly inside the coil recess 11); the same applies to the contact lines 14a, 14b in the contact channels 11a, 11b.

[0073] On an upper end 16 of the second plate 3 two second halves of inlets 10b are formed. The two second halves of inlets 10b are shaped semi-conical and form the counterpart to the two first halves of inlets 10a of the first plate 2. In an assembled form of the sample cell 1 the two semi-conical halves of inlets 10a and the two semi-conical halves of inlets 10b form two conical inlets or connections for inserting and/or removing a liquid sample.

[0074] When the two plates 2, 3 are joined to each other the cavity recess 5 that forms the flat sample cavity 6 is enclosed between and sealed by the two plates 2, 3. Only then it will be possible to fill a liquid sample via the channels 8 into the sample cell 1. The conical form of the inlets or connections formed by the semi-conical halves of inlets 10a, 10b simplifies filling the liquid sample into the sample cell 1 or removing the liquid sample from the sample cell 1. The small size of the channels 8 prevents impurities like dust from easily entering the flat sample cavity 6; in addition it minimizes a signal contribution from liquid sample that is not located inside the desired sample volume, i.e. in the flat sample cavity 6 (where the RF magnetic field is maximum and homogeneous). Furthermore, the small size of the channels 8 also prevents fast evaporation of the solvent of the liquid sample filled into the sample cell 1.

[0075] Further, in the joined form the NMR coil 4 is enclosed between the two plates 2, 3. Note that for electrical contacting, the (external) contacts 13 of the NMR coil 4 and a bridging contact 15a in the bridging area 15 for connecting the inner end of the NMR coil 4 with the contact line 14b and the contact 13b are arranged on the outer side of second plate 2 (i.e. on the side facing away from the first plate 2) here. The NMR coil 4, in particular its wound conductor loops, as well as the contact lines 14a, 14b are well protected from the environment. Thereby, the risk of damaging the NMR coil 4 or the contact lines 14a, 14b during sample preparation or during insertion of the sample cell 1 into a measurement device, and in particular into an NMR probe or a sample cell holder, is minimized.

[0076] An NMR coil cross-sectional area 17a, taken perpendicular to the NMR coil axis 4a, is confined by the NMR coil 4. Here, in the joined form of the sample cell 1, about 90% of the NMR coil cross-sectional area 17a is taken up by (or overlaps with) a sample cavity cross-sectional area 18 of the flat sample cavity 6, taken also perpendicular to the NMR coil axis 4a. Thereby, a high filling factor of the NMR coil 4 is achieved. Due to the high filling factor of the NMR coil 4 a high strength NMR signal can be achieved. In use, the NMR coil 4 that is wound around the flat sample cavity 6 generates an RF magnetic field B.sub.2, with the RF magnetic field B.sub.2 basically parallel to the NMR coil axis 4a in an area of the flat sample cavity 6 (see FIG. 7 below).

[0077] The recesses 5, 7, 11, 12 are made by etching. As etching method laser-assisted micromachining is preferably used because this method offers various possibilities of creating different 3D structures. Alternatively other etching methods like ion beam etching or dry etching may be used. As a material for the two plates 2, 3 fused silica is preferably used. Alternatively, it is also possible to use borosilicate. Both fused silica and borosilicate are possible wafer materials for laser-assisted micromachining. Note that the invention is not restricted with respect to possible manufacturing methods of the sample cell 1; in particular, it may be possible to use 3D printing for preparing a sample cell 1 or its plates 2, 3.

[0078] In alternative embodiments, the cavity recess 5 can be formed in two parts, with a part in the first plate 2 and a part in the second plate 3, or the cavity recess 5 can be formed only in the second plate 3 (not shown). Further the coil recess 11 can be formed in two parts, with a part in the second plate 3 and a part in the first plate 2, or the coil recess 11 can be formed only in the first plate 2 (not shown).

[0079] FIG. 2 depicts an enlarged view of the cavity recess 5 of the first plate 2 from the embodiment according to the invention of FIG. 1. The cavity recess 5 forms the flat sample cavity 6. The flat sample cavity 6 is connected to the two channels 8.

[0080] Here the flat sample cavity 6 is formed approximately cuboidally with curved edges. In the embodiment shown, the flat sample cavity 6 has a maximum length L of 5 mm, a maximum width W of 5 mm and a maximum height H of 0.2 mm. The volume of the flat sample cavity 6 taking into account the curved edges results in an effective sample volume of 4.8 μl here.

[0081] The flat sample cavity 6 basically extends along a sample cavity plane 19; the directions in which the maximum length L and the maximum width W extend are in parallel with the sample cavity plane 19, and the direction in which the maximum height H extends is perpendicular to the sample cavity plane 19. The sample cavity plane 19 is centred in the flat sample cavity 6 (see also FIG. 3).

[0082] Basically, with such a geometry of the flat sample cavity 6 and the sample cell 1, during a DNP-NMR measurement the flat sample cavity 6 can easily be positioned at and oriented parallel to a plane of zero electric field of the microwave radiation to avoid or at least minimize sample heating. At the same time the dimensions allow for an increased volume of the flat sample cavity 6.

[0083] In FIG. 3 a schematic longitudinal section of the sample cell 1 of FIG. 1 according to the invention in assembled form in the plane A there is shown. The sample cell 1 comprises the first plate 2 with the cavity recess 5 and the flat sample cavity 6, and the second plate 3 with the coil recess 11 and the NMR coil 4.

[0084] In FIG. 3, the dimensions LS and HS of the sample cell 1 and L and H of the flat sample cavity 6 are indicated again; note that for the purpose of better visibility the proportions of L to H and LS to HS are not to scale here.

[0085] In the first plate 2 the sample cavity plane 19 of the flat sample cavity 6 is shown as a dashed line. The sample cavity plane 19 is centrally arranged inside the flat sample cavity 6 with respect to a direction PD perpendicular to the sample cavity plane 19. As the height H of the flat sample cavity 6 is 0.2 mm this means that the sample cavity plane 19 is located at 0.1 mm in direction of H inside the flat sample cavity 6.

[0086] Arranged in the second plate 3, the NMR coil 4 comprises two conductor loops arranged in the coil recess 11; note that the coil recess 11 is wound in accordance with the course of the NMR coil 4, and accordingly has two loops, too. For simplicity, the NMR coil 4 is drawn with oval cross-section of the corresponding conductor. In the example shown, the NMR coil 4 (with its wound conductor loops) is arranged basically along an NMR coil plane 17. The NMR coil plane 17 is parallel to the sample cavity plane 19, and is located centrally with respect to the NMR coil along the NMR coil axis 4a.

[0087] The flat sample cavity 6 comprises a centre Point P.sub.CS. The centre point P.sub.CS is located in the centre of the sample cavity plane 19. Furthermore, the NMR coil 4 comprises a centre Point P.sub.NMR. The centre point P.sub.NMR is located in the centre of the NMR coil plane 17. An offset distance O.sub.d between the centre Point P.sub.CS and the centre point P.sub.NMR is about 0.12 mm here. Generally, the q factor of an ESR resonator using a sample cell as shown used during a DNP-NMR measurement may reach particularly high values when the offset distance O.sub.d is small. When a large q factor is achieved a better quality of the DNP-NMR measurements is achieved.

[0088] FIG. 4 shows a schematic drawing of an NMR probe 20 according to the invention, in a view basically with the NMR probe 20 cut in half. The sample cell 1 is shown as whole.

[0089] The NMR probe 20 comprises a sample cell holder 21 for inserting and removing the sample cell 1. Here, the sample cell 1 is shown in an inserted state in the sample cell holder 21.

[0090] The NMR probe 20 further comprises two sweeping coils 22. The sweeping coils 22 are arranged on both of the (flat) sides of the sample cell 1. The sweeping coils 22 run parallel to each other. Further, the sweeping coils 22 run parallel to the sample cavity plane of the flat sample cavity 6. In use the sweeping coils 22 generate a sweeping magnetic field. The sweeping magnetic field is parallel to the sample cavity plane in the area of the flat sample cavity 6 (see also FIG. 7).

[0091] In FIG. 5 a schematic cross-section of the NMR probe 20 according to the invention from FIG. 4 in the plane B there is shown. The NMR probe 20 comprises the sample cell 1, the sample cell holder 21 and the two sweeping coils 22. The sample cell 1 is shown schematically with the first plate 2 comprising the flat sample cavity 6 filled with a liquid sample 23 (dotted area) and the second plate 3 comprising the coil recess (not shown here for simplification) containing the NMR coil 4.

[0092] Here the sample cell holder 21 is formed with a dielectric sleeve 24. The dielectric sleeve 24 has an inner height HD of here 2 mm and an inner width WD of here 10 mm. An inner region 25 of the dielectric sleeve 24 extends with the inner height HD and the inner width WD and is shaped basically rectangular in cross-section with rounded corners. The maximum height HS of the sample cell 1 is 1 mm and the maximum width WS of the sample cell 1 is 10 mm in the example presented (see FIG. 1), which just fits into the dielectric sleeve 24.

[0093] With the sample cell 1 centrally inserted into the dielectric sleeve 24, two tempering channels 26 are formed. The two tempering channels 26 are limited by the sample cell 1 and the dielectric sleeve 24 in cross-section. Each of the channels 26 has a height HT of here 0.5 mm. The two tempering channels 26 extend parallel to the flat sample cavity 6 and adjacent to the sample cell 1.

[0094] The tempering channels 26 are used to vary and or to keep the temperature of the liquid sample 23 to be measured. In use, the temperature of the tempering channel 26 is controlled via a tempering fluid flow (for example a gas stream or a liquid). A temperature control system (not shown) may be used for controlling the temperature of the liquid sample 23 or the tempering fluid flow, respectively. When DNP-NMR measurements are performed the temperature of the liquid sample 23 to be measured is stabilized by the tempering fluid flow during microwave irradiation. Thereby, it is possible to produce comparable and reproducible results.

[0095] Alternatively, the sample cell holder 21 can be formed such that the sample cell 1 and the dielectric sleeve 24 form only one tempering channel 26 (not shown). Then preferably, the tempering channel 26 extends parallel to the flat sample cavity 6 on the side of the sample cell 1 closest to the flat sample cavity 6 (not shown) to ensure a good temperature control.

[0096] In the embodiment shown, the two sweeping coils 22 are rod-shaped with a circular cross-section. The sweeping coils 22 are of tube type. Coil wires (not shown) are included inside the sweeping coils 22. The sweeping coils 22 run parallel to the sample cavity plane and parallel to each other. Here, an offset O.sub.S of the sweeping coils 22 is 6.5 mm and a maximum outer diameter d.sub.out of the sweeping coils 22 is 1 mm. In use, with such an outer diameter d.sub.out a relatively high q factor can be achieved, as compared to larger outer diameters d.sub.out.

[0097] FIG. 6 depicts a schematic drawing of a DNP-NMR measuring device 27 (cut in half) according to the invention with an EPR microwave resonator 28 (cut in half) and the NMR probe 20 (cut in half) with the sample cell 1 inserted (and shown as whole and half transparent).

[0098] The DNP-NMR measuring device 27 further comprises a magnet arrangement (not shown, but compare FIG. 7). In use, the magnet arrangement generates a B.sub.0 magnetic field (see FIG. 7). The DNP-NMR measuring device 27 also comprises a microwave source 30 for generating microwave radiation and a waveguide 31 that guides the microwave radiation generated at the microwave source 30 to the EPR microwave resonator 28. Therefore, the microwave radiation is guided to an iris 32 and coupled into the EPR microwave resonator 28.

[0099] A resonant volume VR of the EPR microwave resonator 28 encloses a cubic volume here with a length LR of 22.7 mm, a width WR of 11.5 mm and a height HR of 41.2 mm (note that the nomenclature here sticks to the allocation of letters L, W and H as for the flat sample cavity 6, and R indicates the resonator). In use, the EPR microwave resonator 28 is operated in TE.sub.102 mode with a microwave radiation having a resonance frequency of about 10 GHz here. A B.sub.1 microwave magnetic field of the TE.sub.102 mode is oriented perpendicular to the B.sub.0 magnetic field in the area of the flat sample cavity 6 (see FIG. 7).

[0100] The NMR probe 20 is inserted into the EPR microwave resonator 28 and fastened with clamping elements 33. About half of the NMR probe 20 is located inside the EPR microwave resonator 28. The flat sample cavity 6 and the NMR coil 4 of the inserted sample cell 1 are positioned centrally (and completely) in the EPR microwave resonator 28. There the microwave magnetic field B.sub.1 peaks and the microwave electric field vanishes (i.e., is minimal) in the area of the flat sample cavity 6.

[0101] The strength of the B.sub.0 magnetic field generated by the magnet arrangement is here about 0.4 Tesla; note that a typical range for B.sub.0 values is 0.2 to 2 Tesla. So the DNP-NMR measuring device 27 is a low field DNP-NMR measuring device 27. The DNP-NMR measuring device 27 is of benchtop size.

[0102] FIG. 7 shows a simplified schematic cross-section of the NMR probe 20 from FIG. 4 in the plane B there with the directions and approximate positions, where applicable, of the different magnetic fields that are used, in particular generated and/or measured, during DNP-NMR measurements with the NMR probe 20 inserted into the EPR microwave resonator depicted in FIG. 6.

[0103] The NMR probe 20 comprises the sweeping coils 22, the sample cell holder 21 formed with a dielectric sleeve 24, the two tempering channels 26 and the sample cell 1. The sample cell 1 comprises the first plate 2 with the cavity recess for the flat sample cavity 6 and the second plate 3 with the coil recess for the NMR coil 4.

[0104] During DNP-NMR measurements four different magnetic fields are of relevance:

[0105] The B.sub.0 magnetic field is generated by the magnet arrangement 29 of the DNP-NMR measuring device. In the chosen embodiment, the magnet arrangement 29 comprises a permanent magnet and a ferromagnetic yoke encompassing the permanent magnet, with two pole shoes 29a, 29b attached to the yoke. In a gap between the pole shoes 29a, 29b, the sample cell 1 is located and exposed to the static B.sub.0 magnetic field established between the pole shoes 29a, 29b. In FIG. 7, only the two pole shoes 29a, 29b are partially shown. The B.sub.0 magnetic field is oriented parallel to the sample cavity plane 19 in the area of the flat sample cavity 6. The static B.sub.0 magnetic field (together with the sweeping magnetic field B.sub.S, if applicable) is used to split the degenerate energy states of the nuclear spins of the sample to be measured, and to split degenerate energy states of spins of paramagnetic electrons in the sample to be measured.

[0106] The sweeping magnetic field B.sub.S is generated by the sweeping coils 22 and is parallel to the B.sub.0 magnetic field in the area of the flat sample cavity 6, in particular in order to adjust the energy split of the paramagnetic electrons to the microwave frequency. The sweeping magnetic field B.sub.S is parallel to the sample cavity plane 19 in the area of the flat sample cavity 6.

[0107] The B.sub.1 microwave magnetic field of TE.sub.102 mode is generated by the microwave source. Inside the ESR resonator, a standing wave of microwave radiation is established, in accordance with said mode. The B.sub.1 microwave magnetic field has a maximum in the area of the flat sample cavity 6, and is oriented parallel to the sample cavity plane 19 in the area of the flat sample cavity 6. Due to the geometry of the flat sample cavity 6, in particular its small height H (see FIG. 2 or 3), the sample to be measured is excited homogeneously by the B.sub.1 microwave magnetic field. At the same time a plane of minimum electric field of TE.sub.102 mode runs through the flat sample cavity 6. Due to the small height H of the flat sample cavity 6 the absorption of microwave electrical energy is low and thereby unwanted heating of the sample to be measured is low, too. The B.sub.1 microwave magnetic field (which is in FIG. 7 perpendicular to the drawing plane) is oriented parallel to the sample cavity plane 19 in the area of the flat sample cavity 6 and perpendicular to the B.sub.0 magnetic field and the sweeping magnetic field B.sub.S in the area of the flat sample cavity 6.

[0108] The RF magnetic field B.sub.2 is generated and/or measured by the NMR coil 4. The B.sub.2 magnetic field is parallel to the NMR coil axis 4a, and further the B.sub.2 magnetic field is perpendicular to the flat sample cavity 6 or its sample cavity plane 19, respectively, in the area of the sample cavity 6. Furthermore, the B.sub.2 magnetic field is oriented perpendicular to the B.sub.0 magnetic field and the sweeping magnetic field B.sub.S. Finally, the B.sub.2 magnetic field is perpendicular to the B.sub.1 microwave magnetic field in the area of the flat sample cavity 6; in other words, the NMR coil plane is parallel to the B.sub.1 magnetic field, what minimizes interferences of the B.sub.1 magnetic field and the B.sub.2 magnetic field (or the NMR coil 4, respectively). The B.sub.2 magnetic field excites and detects the nuclear spin transitions in the sample to be measured.