Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-01-22
2002-10-15
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S320000, C324S322000
Reexamination Certificate
active
06466019
ABSTRACT:
This application claims Paris Convention priority of DE 100 06 324.1 filed Feb. 12, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns an NMR (=nuclear magnetic resonance) probe head comprising an RF (=radio frequency) receiver coil system, which can be cooled down to cryogenic temperatures, and a room temperature pipe extending in a z direction for receiving a sample tube containing a sample substance to be examined through NMR measurements.
A cooled NMR probe head of this type is e.g. known from U.S. Pat. No. 5,247,256.
The probe head is installed in a magnet, for generating a highly homogeneous static B
0
field, and comprises RF receiver coils disposed about a z axis which are cooled down during operation to temperatures of approximately 10 to 25 K by means of suitable heat exchangers and heat conducting elements to improve the signal-to-noise-ratio of the received NMR signal during the measurement. The RF receiver coils are in an evacuated region for heat insulation reasons which is formed essentially by a usually metallic casing of the probe head which is penetrated by a room temperature pipe disposed cylindrically about the z axis for receiving a sample tube. To permit passage of the RF signals from the sample to the RF receiver coils, the otherwise metallic room temperature pipe is replaced in the axial region of the coils by an RF permeable inner pipe, in most cases a glass pipe, which is connected to the metallic parts of the room temperature pipe in a vacuum-tight fashion.
After insertion of the sample tube into the room temperature pipe from the bottom, it is substantially maintained at a desired temperature (usually approximately 300K) using warm air flowing from below through the room temperature pipe to control the temperature of the sample substance. This, however, causes the associated problem that the measuring sample “feels” the considerably cooler surroundings of the NMR resonator, cooled down to 10 to 25 K, and radially radiates heat in this direction. This lost heat must be continuously replenished by the surging warm tempering air flow to ensure that the measuring sample remains essentially at the desired temperature. In consequence, an axial and radial temperature gradient is produced in the measuring sample which strongly impairs the NMR measurement. It is therefore the underlying purpose of the present invention to provide a cooled NMR probe head comprising the above-mentioned features wherein the temperature gradients occurring during operation are considerably reduced with as simple as possible technical means and without thereby impairing the NMR measurement.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the present invention in a both surprisingly simple and effective manner by providing a centering device for centering the sample tube in its measuring position about the axis of the room temperature pipe.
In addition to exchangeable sample tubes, the NMR probe heads in accordance with the invention also include so-called flow-through heads wherein the sample tube remains fixedly installed and the fluid to be examined is introduced through a thin conduit on the one side (bottom) and is guided out on the other side (top). Probe heads of this type may be used in continuous passage and also in a flow and stop mode (for an extended measuring period). These probe heads are used for rapid introduction of the sample as well as for an important analysis step following a liquid chromatography separating cell. The former are called flow-through probe heads, the latter LC-NMR coupling. Probe heads of this type are also referred to as LC heads (liquid chromatography, in particular also HPLC High Pressure Liquid Chromatography). Probe heads of this type can particularly profit from cryotechnology and also from the modifications in accordance with the invention. The transverse temperature gradients, extending radially with respect to the z axis, which can occur during operation of a cooled NMR probe head and which are substantially responsible for the instabilities in the spectrum and also in the lock system result from integration of the local axial temperature gradients along the z direction, from the bottom of the sample to the observed point. The local axial temperature gradients are given by the product of the heat loss per unit surface and the local reciprocal value of the mass flow of tempering gas. This mass flow distribution depends on the asymmetry and the angular deviation of the axis of the sample tube from the z axis of the temperature pipe. Since this asymmetry appears as a factor in the overall product, even small axial displacements or inclinations of the measuring sample within the room temperature pipe have a large influence on the tempering flow. Therefore, the proposed centering device can have a considerable effect with regard to a reduction in the temperature gradients in the xy plane and an improvement of the quality of the NMR signals in the cooled probe head.
In a further development which is particularly easy to realize, the centering device comprises one or more spacers disposed between the room temperature pipe and the sample tube and symmetrically distributed about the z axis of the room temperature pipe.
These spacers may be disposed in the area of the bottom of the sample tube in its measuring position and/or in the area of the feed opening of the room temperature pipe on the side of the room temperature pipe facing the sample tube.
Alternatively, the spacers may extend over the entire axial length of the RF receiver coil system thus effecting as precise a centering of the sample tube as possible in the room temperature pipe.
In advantageous embodiments of the NMR probe head in accordance with the invention, several, preferably between 3 and 8, in particular 6 spacers are symmetrically distributed about the z axis of the room temperature pipe. This configuration has given the best results to date.
One further development is also advantageous, wherein the spacers consist of strips of elastic material extending in the direction of the z axis which are rigidly connected to the room temperature pipe at their ends facing away from the sample glass in its measuring position and whose ends facing the sample glass in its measuring position have a bead which is bulged towards the sample glass and whose free leg seats on the room temperature pipe. The centering device of this embodiment is particularly simple and inexpensive to produce and can be easily retrofitted in existing NMR probe heads.
To prevent disturbance of the NMR measurements, the spacers should be produced from a material which is transparent to RF radiation and possibly also magnetically compensated.
In a preferred further development, the spacers consist of sheet metal strips having a thickness of approximately 100 &mgr;m and a width transverse to the z axis of approximately 0.5 mm to 2 mm, preferably approximately 1 mm.
A particularly preferred embodiment of the NMR probe head in accordance with the invention provides for radiation shields disposed between the RF receiver coil system and the room temperature pipe which surround the room temperature pipe in a radial direction, extend in the z direction and are made of one or more materials oriented in the z direction which are almost completely transparent to RF fields or at least have an absorption of <5%, preferably <1% for RF fields.
Although cryotechnology has used radiation shields for some time to curtail heat radiation losses, this procedure is not directly applicable for a cooled NMR probe head since the normally metallic radiation shields, which reflect heat radiation, either completely block or at least strongly impair propagation of RF fields from the measuring sample to the RF receiver coils such that the incoming NMR signals are at least highly attenuated, distorted or completely unusable.
In accordance with the inventive solution, the radiation shields provided in the vacuum between the RF coils
Bruker BioSpin AG
Lefkowitz Edward
Vargas Dixomara
Vincent Paul
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