Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-01-22
2002-08-27
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S321000, C324S300000
Reexamination Certificate
active
06441617
ABSTRACT:
This application claims Paris Convention priority of DE 100 06 317.9 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 normally 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.
SUMMARY OF THE INVENTION
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 gradient in the z direction occurring during operation is considerably reduced without thereby impairing the NMR measurement. This object is achieved in accordance with the present invention in a both surprisingly simple and effective manner by providing at least one radiation shield between the RF receiver coil system and the room temperature pipe which radially surrounds the room temperature pipe and extends in the z direction and is composed of one or more materials oriented in the z direction which are almost completely transparent to RF fields, at least however have an absorption of <5%, preferably <1% for RF fields.
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. 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 and the room temperature pipe solely comprise materials which are oriented in the z direction. The axial orientation of the radiation shield material prevents their finite susceptibility from impairing the resolution of the NMR signals. On the other hand, the physical properties of the materials should be such as to effect as large a transparency as possible in the region of radio frequency radiation. In most cases, this material property has the associated disadvantage that reflection of lost heat back towards the measuring sample is not very high.
One embodiment of the inventive NMR probe head is particularly preferred, wherein N radiation shields are disposed in radial sequence between the RF receiver coil system and the room temperature pipe, wherein N≧2, preferably 5≦N≦25. The inventive plurality of radiation shields disposed one radially behind the other, allows formation of a substantial radiation barrier to considerably reduce heat loss from the measuring sample, even for materials which have no thermal radiation reflexivity at all (i.e. are “black”) as shown below in detail. Since the space available in the vacuum between the RF receiver coil system and the vacuum side of the room temperature pipe is quite limited, in practice, the number N of radiation shields which can be used in radial sequence is limited.
It is advantageous if the N radiation shields have at least a minimum separation from one another in the radial direction and do not contact each other or at the most contact at points or linearly to prevent direct heat conduction between the individual radiation shields in a radial direction which would lead to a thermal “short circuiting”. Occasional contact between the radiation shields is not a serious problem, in particular if the chosen material has very low heat conduction. As long as the individual contacting points or lines are sufficiently spaced apart from one another, the overall heat conduction between the radially disposed radiation shields can be essentially neglected for the purposes of the invention.
Since the separation between the RF receiver coil system and the measuring sample should not be increased in order to prevent reduction in the sensitivity of the NMR measurements, the individual radiation shields should be as thin as possible. The radiation shields should therefore have a radial thickness <0.1 mm, preferably <50 &mgr;m. One embodiment of the NMR probe head in accordance with the invention is particularly preferred, wherein the radiation shields are constructed from a material which reflects or at least absorbs radiation in a wavelength range of 10 &mgr;m≦&lgr;≦100 &mgr;m and which is transparent to radiation in a wavelength range of &lgr;>100 mm. The former wavelength range corresponds to heat radiation at a temperature of between approximately 20K to 300K which corresponds to the temperature difference between the measuring sample and the cooled NMR coils. The latter wavelength range corresponds to radiation of a frequency below 3 GHz, wherein the RF range which is important for NMR measurements is between several MHz and below approximately 1 GHz.
An optimum material which has practically no absorption losses in the considered RF range, and on the other hand is not transparent in the above-mentioned heat radiation range, is e
Bruker BioSpin AG
Lefkowitz Edward
Shrivastav Brij B.
Vincent Paul
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