High-temperature NMR MAS probe with optimized temperature gradient across sample rotor
A rotor housing assembly for NMR spectroscopy. An elongate rotor has a distal drive end, a proximal end and an internal sample space positioned along its length between the drive and proximal ends. The rotor is driveable about a rotation axis by a drive gas flow. A rotor housing has an interior space in which the rotor is at least partially received. At least one first heated gas flow inlet is positioned opposite the internal sample space, through which a first heated gas flow is controllably flowable into the interior space to heat it and the rotor. At least a pair of spaced apart second heated gas flow outlets are axially spaced from the first heated gas flow inlet to controllably convey a second heated gas flow to heat distal and proximal areas of the sample space to minimize a temperature gradient extending axially within the sample space.
1. A rotor housing assembly for holding and spinning a sample during NMR spectroscopy, comprising:
an elongate rotor having a distal drive end, a proximal end and an internal sample space positioned along a rotor length between the drive end and the proximal end, the rotor being driveable to rotate about a rotation axis by a drive gas flow;
a housing having an interior space in which at least a portion of the rotor is received;
at least one first heated gas flow inlet positioned in the housing and opposite the internal sample space of the rotor, wherein a first heated gas flow from a first gas source is controllably flowable through the first heated gas flow inlet and into the interior space of the housing to heat the interior space and the rotor; and
at least a pair of spaced apart second heated gas flow outlets that are axially spaced from the first heated gas flow inlet and positioned towards the distal drive and proximal ends of the rotor, respectively, wherein a second heated gas flow from a second gas source through the second heated gas flow outlets is controllable to heat distal and proximal areas of the sample space within the rotor to minimize a temperature gradient extending axially within the sample space.
2. The rotor housing assembly of claim 1 , further comprising proximal and distal air bearing areas in the housing positioned to receive air bearings for rotatably supporting the rotor, and wherein the second heated gas flow outlets are configured to direct the second heated gas flow towards the proximal and distal air bearing areas, respectively, wherein the second heated gas flow comprises a heated bearing gas flow.
3. The rotor housing assembly of claim 1 , wherein the first heated gas flow comprises a heated variable temperature (VT) gas flow.
4. The rotor housing assembly of claim 1 , further comprising a drive gas inlet positioned adjacent the drive end of the rotor, wherein the drive gas flow is controllable to flow through the drive gas inlet to impinge on the drive end and drive the rotor to rotate.
5. The rotor housing assembly of claim 4 , wherein the drive gas flow is unheated.
6. The rotor housing assembly of claim 4 , wherein the drive gas flow is heated.
7. The rotor housing assembly of claim 6 , wherein the drive gas flow is heated to a temperature of at least 200° C.
8. The rotor housing assembly of claim 1 , wherein the temperature gradient within the sample space is 3° C. or less from an area of the internal sample space opposite the first heated gas flow inlet extending axially toward either the distal or proximal area of the internal sample space.
9. The rotor housing assembly of claim 1 , wherein the first heated gas flow is heated to a temperature of up to 400° C.
10. The rotor housing assembly of claim 1 , wherein the second heated gas flow is heated to a temperature of up to 200° C.
11. The rotor housing assembly of claim 1 , wherein the first heated gas flow inlet is formed in a wall of the housing.
12. The rotor housing assembly of claim 1 , wherein a flow path for the second heated gas flow comprises an inlet passage formed in a wall of the housing, a junction with one distally extending passage and one proximally extending passage, the distally extending passage and the proximally extending passage each terminating at a respective one of the second heated gas flow outlets.
13. The rotor housing assembly of claim 12 , further comprising proximal and distal air bearings positioned to rotatably support the rotor, and wherein the second heated gas flow outlets are positioned adjacent the proximal and distal air bearings, respectively, and wherein the second heated gas flow comprises a heated bearing gas flow that supplies the proximal and distal air bearings and transfers heat from along the flow path and through the wall, and from the proximal and distal air bearings into the housing interior space.
14. The rotor housing assembly of claim 12 , further comprising a dewar with a heating coil connected to supply the second heated gas flow to the inlet passage in the wall of the housing assembly.
15. The rotor housing assembly of claim 1 , further comprising a surrounding shell having an exhaust opening, and wherein the first heated gas flow and the second heated gas flow are exhausted from the housing into the shell, further comprising a purge air flow pathway by which a purge air flow of cooling air is directed into the shell to mix with and cool exhausted first and second heated gas flows before they are exhausted from the shell.
16. The rotor housing assembly of claim 15 , wherein the shell comprises an annular space by which air from an electronics area is exhausted.
17. The rotor housing assembly of claim 1 , wherein the internal sample space of the rotor is capable of being pressurized up to 400 bar.
18. A method of operating an NMR probe at elevated temperatures, comprising:
flowing a drive flow gas to impinge upon a drive end of a rotor having a sample space to drive the rotor in rotation about an axis, the drive flow gas being fed through a passage in a housing positioned radially outwardly of the rotor;
flowing a first heated gas flow through an opening in the housing and towards the sample space of the rotor; and
flowing a second heated gas flow in opposite directions toward opposite ends of the rotor;
wherein the first heated gas flow and the second heated gas flow are controllable to reduce a temperature gradient extending axially along the sample space of the rotor.
19. The method of claim 18 , wherein the temperature gradient extending axially along the sample space is 3° C. or less.
20. The method of claim 19 , wherein the temperature gradient along the sample space of 3° C. or less is reached in five minutes or less of operation.
21. The method of claim 18 , wherein the second heated gas flow comprises a bearing gas flow directed to air bearing areas to operate air bearings at opposite ends of the rotor to support the rotor during rotation.
22. The method of claim 18 , comprising heating the first heated gas flow to an elevated temperature of up to 400° C.
23. The method of claim 18 , comprising heating the second heated gas flow to an elevated temperature of up to 400° C.
24. The method of claim 18 , further comprising heating the drive gas flow to assist in reducing the temperature gradient in the sample space.
25. A method of in situ NMR monitoring of a high temperature chemical process, comprising:
spinning a rotor having a sample in space which a sample is contained to conduct a MAS NMR operation,
heating the sample space within the rotor to a temperature above 250° C. with a first heated gas flow primarily directed at a first location of the rotor;
maintaining a temperature gradient within the sample space of 3° C. or less using heat applied to the rotor at a second location spaced away from the first location; and
monitoring in situ phase transitions occurring in the sample.
26. The method of claim 25 , wherein the sample space is pressurized.