Fourier transform NMR spectroscopy employing a phase modulated rf carrier

• US 3,968,424 A
• Filed: 08/01/1974
• Issued: 07/06/1976
• Est. Priority Date: 08/01/1974
• Status: Expired due to Term

• Current Assignee: Varian Associates
• Original Assignee: Varian Associates
• Inventors: Ernst, Richard R.
• Primary Examiner(s): Rolinec, R. V.
• Assistant Examiner(s): Tokar, Michael J.
• Application Number:
  US05/494,017
• Time in Patent Office: 705 Days
• Field of Search: 324/.5 A, 324/.5 AC
• US Class Current: 324/310
• CPC Class Codes: G01R 33/3607 RF waveform generators, e.g. frequency generators, amplitude-, frequency- or phase modulators or shifters, pulse programmers, digital to analog converters for the RF signal, means for filtering or attenuating of the RF signal
• CPC Class Codes: G01R 33/46 NMR spectroscopy

Abstract

A nuclear magnetic resonance Fourier transform spectrometer includes a pulse modulated rf transmitter for delivering a train of radio frequency pulses having a pulse width pulse repetition rate to generate Fourier side bands to cover the spectral lines of the sample to be analyzed for exciting resonance of different resonance lines, if any, within the sample. Resonance of the sample is scanned in between successive transmitted pulses to obtain a multiplicity of successive time displaced components which are sampled and stored in a multiplicity of memory channels of a computer for time averaging thereof. The time averaged signal components are periodically read out and Fourier transformed to obtain a time averaged spectrum of the sample under analysis. Anomalous phase and intensity effects in the detected spectral lines are avoided by periodically shifting the phase of the pulse modulated carrier signal from one phase to another of at least three different relative phases in accordance with a predetermined sequence.

Claims

1. In a magnetic resonance spectrometer:

means for generating a band of radio frequency energy for irradiation of a sample of matter to be analyzed, such band of energy having a sufficient bandwidth to cover the spectral lines of the sample to be analyzed for simultaneously exciting radio frequency magnetic resonance of different resonance spectral lines, if any, within the sample;

means for detecting the excited resonance of the sample to obtain a composite resonance signal therefrom for analysis of the sample;

means for repetitively time scanning, sampling and storing a multiplicity of successive time displaced components of the composite resonance signal in a multiplicity of respective channels;

means for reading out the signal components stored in such channels; and

said means for generating the band of radio frequency energy for irradiation of the sample including, means for generating a radio frequency carrier signal, and means for periodically shifting the phase of said carrier signal from one phase to another of at least three different relative phases in accordance with a predetermined sequence.

2. The apparatus of claim 1 wherein said phase shifting means for shifting the phase of the carrier energy includes, a plurality of delay means one for each of said phase relations relative to a reference phase relation, and means for sequentially switching respective ones of said delay means in circuit between said carrier signal generating means and the sample to be irradiated.

3. The apparatus of claim 1 wherein said phase shifting means for shifting the phase of the carrier energy includes, a source of radio frequency energy at a number n times the frequency of said carrier signal energy to be generated, means for dividing the radio frequency energy output of said source by the number n to derive said radio frequency carrier signal, and delay means interposed in circuit between said source of radio frequency energy and said divider means for variably delaying the radio frequency energy to said divider means by a variable number of periods of the radio frequency energy to be divided to derive variable phase shifts of the generated carrier signal.

4. The apparatus of claim 1 including pulse modulator means for pulsing the intensity of the carrier signal energy to produce a train of radio frequency pulses for generating bands of Fourier components of radio frequency energy for irradiation of the sample.

5. The apparatus of claim 4 wherein said means for repetitively time scanning, sampling and storing a multiplicity of successive time displaced components of the composite resonance signal in a multiplicity of respective channels includes means for scanning the detected composite resonance signal during the period between successive pulses of the carrier signal energy, and storing and adding respective time displaced composite resonance signal components to those of previous scans to obtain time averaged data in separate respective channels of a memory, and means for Fourier transforming the read out time averaged signal components from the time domain into the frequency domain to obtain time average spectral data concerning the sample under analysis.

6. A nuclear magnetic resonance spectrometer for producing nuclear magnetic resonance spectrum from a sample under analysis comprising means for producing a unidirectional polarizing magnetic field enveloping said sample, means for applying a plurality of time-spaced pulses of driving radio frequency magnetic field to said sample for an experiment time T to produce a series of successive free precession signals from said sample responsive to said driving pulses, means for demodulating said free precession signals to obtain a series of free induction decay envelope signals, means for time-averaging discrete segments of said decay signals over the series of successive signals, wherein said means for applying a plurality of time-spaced pulses of driving radio frequency magnetic field includes means for providing said pulses in N_i different phases relative to a reference phase where i is an integer greater than 2, means for dividing said experiment time T into i approximately equal time increments and means for arranging said time-spaced pulses in i different sets.

7. The spectrometer of claim 6 wherein said means for applying a plurality of time-spaced pulses includes means for selecting the sequence of said time-spaced pulses such that said first set is applied during said first time period, said second set is applied in said second time period and so on until said i set is applied in said i time period.

8. The spectrometer of claim 6 wherein said means for arranging said time-spaced pulses in i different sets includes means for generating the sets such that

9. The spectrometer of claim 8 wherein i equals 4, such that

10. In a method for obtaining magnetic resonance of a sample under analysis including the steps of:

generating a band of radio frequency energy;

irradiating a sample of matter to be analyzed with the band of radio frequency energy, such energy having a sufficient bandwidth to cover the spectral lines of the sample to be analyzed for simultaneously exciting radio frequency magnetic resonance of different resonance spectral lines, if any, within the sample;

detecting the excited resonance of the sample to obtain a composite resonance signal therefrom for analysis of the sample;

repetitively time scanning, sampling and storing a multiplicity of successive time displaced components of the composite resonance signal in a multiplicity of respective channels;

reading out the signal components stored in such channels; and

wherein the step of generating the band of radio frequency energy for irradiation of the sample includes the step of, generating a radio frequency carrier signal, and shifting the phase of the carrier signal from one phase to another of at least three different relative phases in accordance with a predetermined sequence.

11. The method of claim 10 wherein the step of generating a band of radio frequency energy for irradiation of the sample to be analyzed includes the step of pulse modulating the intensity of the carrier signal to produce a train of radio frequency pulses such train of pulses having Fourier side band components of radio frequency energy which are utilized for irradiation of the sample.

12. The method of claim 11 wherein the step of repetitively time scanning, sampling and storing a multiplicity of successive time displaced components includes, the step of time scanning the detected composite resonance signal during the period between successive pulses of the carrier signal energy, and storing and adding respective sampled time displaced composite resonance signal components to those of previous scans to obtain time averaged data in separate respective channels of a memory, and Fourier transforming the read out time averaged signal components from the time domain into the frequency domain to obtain time averaged spectral data concerning the sample under analysis.

13. The method of obtaining magnetic resonance spectrum of a sample under analysis including the steps of:

producing a unidirectional polarizing magnetic field enveloping said sample,

applying a plurality of time-spaced impulses of driving radio frequency magnetic field to said sample for an experiment time T to produce a series of successive free precession signals from said sample responsive to said driving pulses,

demodulating said free precession signals to obtain a series of free induction decay envelope signals,

time-averaging discrete segments of said decay signals over the series of successive signals,

wherein said step of applying said plurality of time-spaced pulses includes providing said pulses in N_i different phases relative to a reference where i is an integer greater than 2, dividing said experiment time T into i equal time increments and arranging said time-spaced pulses in i different sets.

14. The method of claim 13 wherein the step of applying a plurality of time-spaced includes selecting the sequence of said time-spaced pulses such that said first set is applied during said first time period, said second set is applied during said second time period, and so on until said i^th set is applied in said i^th time period.

15. The method of claim 14 wherein the step of arranging said time-spaced pulses in i different set includes generating the sets such that

16. The method of claim 14 wherein i = 4.