Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-08-02
2001-04-17
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S300000
Reexamination Certificate
active
06218835
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems and methods for performing nuclear magnetic resonance (NMR) measurements, and in particular to systems and methods for reducing thermal equilibration times for NMR samples.
BACKGROUND OF THE INVENTION
Many NMR applications involve performing measurements at elevated temperatures. Such measurements typically require heating the samples of interest from ambient temperature to an elevated temperature of interest. In a common design approach, the sample of interest is heated by flowing a hot gas over a sample container positioned in an NMR probe. For descriptions of NMR systems and probes using gas-flow heating see for example U.S. Pat. Nos. 5,530,353,5,408,181, and 5,192,910. The gas-flow heating method may require minutes of waiting between measurements in order to allow the samples to come to thermal equilibrium at the desired elevated temperature. The waiting periods often limit system throughput.
In U.S. Pat. No. 5,300,888, Webster et al. describe an NMR probe including a non-inductive heating winding encapsulated within the probe wall. The temperature of a sample within the probe is presumably controlled by resistively heating the winding. A probe including a heating winding is also described by Webster et al. in the article “High Temperature
1
H NMR Probe,”
Rev. Sci. Instrum
. 50(3):390-391 (1979).
In the article “High-Temperature NMR using Inductive Heating,”
Rev. Sci. Instrum
. 61(1):77-80 (1990), Maresch et al. describe heating an NMR sample by applying RF energy to a metal-coated sample tube. The applied RF energy heats the metal coating of the sample tube, which in turn heats the sample.
SUMMARY OF THE INVENTION
The present invention provides NMR methods and devices for accelerating the thermal equilibration of NMR samples. A sample of interest is inserted into a nuclear magnetic resonance probe of an NMR spectrometer. The sample is held in a conventional electrically insulative sample holder. Typically but not necessarily, the probe is initially at a higher temperature than the sample. A set of heating radio-frequency pulses is applied to the sample, for accelerating the thermal equilibration of the sample at a desired measurement temperature. The sample is dielectrically lossy, and is heated directly through its interaction with the heating pulses. After the sample attains a suitable thermal equilibrium, a set of measurement pulses is applied to the sample, and the NMR response of the sample to the measurement pulses is measured.
In order to facilitate a rapid heating rate for the sample, the heating pulse frequencies, powers, durations, and interpulse spacings are chosen such that the heating pulses deposit into the sample more thermal energy per unit time than the measurement pulses. For heating pulses of the same frequency as the measurement pulses, the time-averaged power of the heating pulses is higher than the time-averaged power of the measurement pulses.
In the absence of heating pulses, the sample can generally be heated through two mechanisms: thermal contact with its environment, and direct interactions with applied measurement pulses. The first mechanism is typically measurement-independent, while the second mechanism is measurement-dependent. High-power RF heating pulses can be used to accelerate the thermal equilibration of the sample in the presence of either or both measurement-independent and measurement-dependent heating.
In one embodiment, the probe is heated by establishing thermal communication between the probe and a measurement-independent external heating device. For example, the sample holder within the probe can be heated by running a hot gas over its walls. High-power RF heating pulses can then be applied to accelerate the heating of the sample from the ambient temperature to a pre-measurement probe temperature. The pre-measurement probe temperature is the temperature of the probe prior to the application of the measurement pulses. The difference between the initial (ambient) sample temperature and the pre-measurement probe temperature can range up to hundreds of ° C.
High-power RF heating pulses can also be applied to accelerate the heating of the sample from the pre-measurement probe temperature to an equilibrium measurement temperature. The difference between the pre-measurement probe temperature and the equilibrium measurement temperature can range from negligible to a few ° C. A plurality of heating pulses or pulse subsequences of decreasing power can be used to minimize the time required for thermal equilibration, while preventing overshooting of the equilibrium temperature. Each pulse subsequence can be set to be proportional to the set of measurement pulses. Using heating subsequences that are proportional to the set of measurement pulses serves to automatically control or optimize the energy deposited into the sample by the heating pulses, for various measurement pulse sequences.
The heating pulse amplitudes, durations, and interpulse spacings are preferably chosen so as to minimize the time required for adequate thermal equilibration of the sample. The amplitudes, durations and interpulse spacings of the set of heating pulses are preferably predetermined, for example according to empirical calibration data taken for a given sample and desired temperature change. Alternatively, real-time NMR spectral data indicative of the sample temperature are used to dynamically control the heating pulses. The spectral data can be used to determine whether the sample has attained a suitable thermal equilibrium. Subsequent heating pulses are then controlled according to whether the sample has attained thermal equilibrium.
The heating and measurement pulses can be applied using one or more of the RF coils. The heating and measurement pulses are preferably offset in frequency, for mitigating the magnetic (non-thermal) effect of the heating pulses on the spins of interest in the sample.
The present invention further provides an NMR apparatus comprising a sample holder for holding an NMR sample, a set of coils inductively coupled to the sample holder, and control and measurement electronics electrically connected to the set of coils. The set of coils comprises one or more coils. The control electronics control the set of coils to apply to the sample the heating and measurement RF pulses. The measurement electronics measure the NMR response of the sample to the measurement pulses. The apparatus can further comprise a heating device in thermal communication with the sample holder, for heating the sample. The heating device preferably comprises a gas pump and heater in thermal communication with the sample through a hot gas flow.
In one embodiment, the control electronics are connected and responsive to the measurement electronics, for dynamically controlling the heating pulses according to NMR spectral data indicative of the sample temperature or of whether the sample has attained a suitable thermal equilibrium.
Further provided is an NMR apparatus comprising RF heating means for applying the heating RF pulses to the sample, RF measurement pulse application means for applying the measurement RF pulses, and measurement means for measuring the NMR response of the sample to the measurement pulses. The RF heating means and the measurement pulse application means include RF control electronics for applying the heating and measurement pulses. The measurement means include detection/acquisition electronics for acquiring time-domain waveforms of sample responses to applied measurement pulses.
REFERENCES:
patent: 4365199 (1982-12-01), McNair
patent: 4489275 (1984-12-01), Sancier et al.
patent: 5192910 (1993-03-01), Hepp et al.
patent: 5300888 (1994-04-01), Webster et al.
patent: 5408181 (1995-04-01), Dechene et al.
patent: 5530353 (1996-06-01), Blanz
Article by Webster et al., entitled “High Temperature1H NMR Probe,” published inRev.Sci.Instrum.in Mar. 1979, 50(3), pp. 390-391.
Article by Maresch et al., entitled “High-temperature NMR Using Inductive Heating,” published inRev.Sci.Instrum.in Jan. 199
Arana Louis
Bella Fishman Andrei Popovici
Varian Inc.
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