GB2053481A

GB2053481A - NMR Spectrometers

Info

Publication number: GB2053481A
Application number: GB7923726A
Authority: GB
Original assignee: NEWPORT INSTR Ltd; Newport Instruments Ltd
Current assignee: NEWPORT INSTR Ltd; Newport Instruments Ltd
Priority date: 1979-07-06
Filing date: 1979-07-06
Publication date: 1981-02-04
Also published as: GB2053481B

Abstract

In a pulsed NMR spectrometer resonance signals are detected in phase-sensitive detector 9 and second quadrature phase-sensitive detector 10 gives an output used to obtain a signal to control the resonant frequency of receiver coil 2. Preferably the output of quadrature phase- sensitive detector 10 is switched to sample and hold circuit 13 immediately after the dead time after an rf. pulse. The output of circuit 13 is switched to integrator 15 during the recovery period before the next rf. pulse to control the tuning of the receiver coil. Phase-sensitive detector 10 can also provide a signal to stabilise the magnetic field. This signal is also switched to an additional sample and hold circuit after the dead time. The output of the additional sample and hold circuit is applied to an integrator during the recovery period. Additionally the angle of a 90 DEG transmitter pulse can be set up accurately by an initial sequence of two nominally 90 DEG pulses which produce a zero signal if they are of correct angle. Any residual signal is used to correct the pulse angle. <IMAGE>

Description

SPECIFICATION NMR Spectrometers This invention relates to NMR spectrometers, particularly pulsed NMR spectrometers.

NMR spectroscopy is widely used in chemical analysis and especially in the analysis of organic materials. An NMR spectrometer has a large magnet and a sample holder is positioned between the poles of the magnet. The sample holder is surrounded by a coil connected to a rf generator. A sample of material being analysed is placed in the sample holder and a rf signal of frequency f given by f = yHo/2n (where y is the gyromagnetic ratio for atomic nuclei in the sample being considered, usually protons, and Ho is the magnitude of the magnetic field) is applied to the coil a,nd causes precession of the nuclei in the sample. In a pulsed NMR spectrometer it is usual to subject the sample of rf pulses of duration T such that the nuclei precess through 900. For a rf field of magnitude Hl, T = 7r/2yHI.

Such pulses are known as 900 pulses. On termination of a pulse the nuclei relax to their original state and in doing so produce a free induction decay signal which is picked up by a receiver coil surrounding the sample. Since the magnitude of the decay signal is very much less than the magnitude of the pulse applied to the sample a waiting or "dead" time is required for the electronic circuits to recover before the signal is measured. The signal in the receiver coil contains information relating to the contents of the sample.

It is convenient to apply the signal picked up from the sample after the dead time to a phasesensitive detector, the reference input to the phase-sensitive detector being obtained from the oscillator driving the rf generator.

In practice errors may occur due to a number of factors. A significant source of error arises from the presence of the sample itself which alters the resonant frequency of the receiver coil. If the receiver coil is thrown off resonance in this way then phase errors occur in the picked up signal relative to the reference signal and the output of the phase-sensitive detector has a decreased amplitude. This phase error can be allowed for on initial setting up of an experiment by manual tuning of the receiver coil but repositioning the sample or changing it introduces errors which require re-calibration. Typically repositioning a sample can introduce a phase error of 50 and a variation over a range of about 150 may occur when changing a sample.

It is an object of the invention to provide an arrangement in which phase errors due to a receiver coil being off resonance are corrected.

According to the invention an NMR spectrometer includes transmitter coil means for energizing a sample with rf signals a rf generator connected to energise the transmitter coil means with rf pulses, tunable receiver coil means for picking up NMR signals from a sample, a first phase-sensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator, a second phasesensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator which are in phase quadrature with the reference signals applied to the first phasesensitive detector, and feedback means utilising the output of the second phase-sensitive detector to alter the frequency to which the receiver coil means is tuned in a manner to reduce errors in the output of the first phase-sensitive detector arising from incorrect tuning of the receiver coil means.

For a correctly tuned receiver coil and in the absence of other causes of phase error the signal input to the first phase-sensitive detector is exactly in phase with the reference input thereto and hence the signal input and reference input to the second phase-sensitive detector are exactly in phase quadrature. Thus the second or quadrature phase-sensitive detector provides a zero output. If at any time the output from the quadrature phasesensitive detector is not zero this indicates a departure from phase quadrature and is a measure, in sign and magnitude, of tuning error in the receiver coil. The output of the quadrature phase-sensitive detector can therefore be used in a feedback circuit to correct for this tuning error.

The feedback circuit can comprise means for measuring the output of the quadrature phasesensitive detector at an appropriate instant in time, preferably immediately after the end of the dead time when the signal in the receiver coil is at a maximum. To this end sample and hold circuit can be provided to which the output of the quadrature phase-sensitive detector is switched after the end of the dead time. The output of the sample and hold circuit can conveniently be used to control the magnitude of the capacitance of a variable capacitor connected across the receiver coil. To prevent residual errors the output of the sample and hold circuit, instead of being used directly, can be applied to an integrator for an interval of time during the recovery period and the output of the integrator used to determine the capacitance across the coil.

In addition to errors introduced by the receiver coil being off tune a further source of error is instability of the static magnetic field since it is difficult to maintain this field at a constant value over a period of time. A change in the magnitude of the static magnetic field will produce a change in the frequency of the free induction decay signal and for small such changes this will appear as a time varying phase change. For small values of dead time the phase change caused by drift in the magnetic field may not be appreciable but where the dead time is large, as sometimes occurs, the output of the quadrature phase-sensitive detector will include not only a component due to phase error caused by mistuning of the receiver coil but also a time varying component due to the change in the static magnetic field.

It is possible to compensate for the effect of the time varying component in contributing to phase error in the received signal. This may be done by measuring the output of the quadrature phase-sensitive detector a second time at a time interval after the first measurement and then taking a proportion of the second measured output and subtracting it from the first measured output. The resultant output signal so obtained is a measure of the tuning error of the receiver coil only.

Since the output of the quadrature phasesensitive detector contains information relating to changes in the static magnetic field (if such changes have indeed occurred) it is possible to utilise this output to alter the magnitude of the magnetic field in a manner to reduce errors in the output of the first phase-sensitive detector arising from such changes in the magnetic field.

Conveniently the second measured signal from the quadrature phase-sensitive detector is utilised for this purpose.

Furthermore since the said second measured signal includes a signal due to tuning error (if present) this latter component in the signal can be eliminated in the feedback signal used to control the magnetic field by subtracting the first measured signal from the second measured signal using the resulting signal to correct for field error.

Yet another source of error occurs if the rf pulses do not have an angle of exactly 900. This source of error can be corrected by providing means giving an initial sequence of two 900 pulses and utilising the resultant signal, which should be zero for pulses of the correct angle, as a pulse angle correction signal. The pulse angle correction signal is preferably obtained by shifting the input to the transmitter coil by 900 and measuring the resultant signal in the quadrature phase-sensitive detector. This correction step is carried out in an initial setting up procedure only whereas correction for tuning error and for magnetic field change is carried out continuously.

In order that the invention may be more fully understood reference will now be made to the accompanying drawings in which: Fig. 1 illustrates an embodiment of the invention in block diagramatic form, Fig. 2 shows waveforms explanatory of Fig 1, Fig. 3 illustrates a detail of a spectrometer embodying the invention, Fig. 4 shows a timing sequence for the arrangement of Fig 1, Fig. 5 shows an alternative timing sequence for the arrangement of Fig 1, Fig. 6 illustrates part of the spectrometer of Fig 1 but with additional features, Fig. 7 shows another feature of the invention, Fig. 8 shows a timing sequence for the arrangement of Fig 7, and Fig. 9 illustrates part of the spectrometer of Fig 1 but with additional features.

Referring now to Fig 1 there is shown therein a pulsed NMR spectrometer in diagramatic form. A sample holder 1 surrounded by a receiver coil 2 is placed between the poles 3 of a powerful magnet.

To provide a rf field in the sample an oscillator 4 applies a rf signal of an appropriate frequency to a power amplifier 6 through a switch 5. The output of amplifier 6 is fed to a transmitter coil 7 surrounding the sample. Switch 5 is periodically closed for controlled periods of time to provide 900 rf pulses to coil 7 with a recovery interval between each pulse. The resulting free induction decay signal generated in a sample on the termination of a pulse is picked up by receiver coil 2 and fed through an amplifier 8 to a first phasesensitive detector 9. The reference input to phase-sensitive detector 9 is obtained from oscillator 4. The output of phase-sensitive detector 9 is utilised in the analysis of the sample in known manner.

In addition to phase-sensitive detector 9 the spectrometer illustrated in Fig 1 is provided with a second phase-sensitive detector 10 the signal input to which is also obtained from amplifier 8 and the reference input to which is obtained from oscillator 4 through a 900 phase shift circuit 11.

In a correctly adjusted spectrometer the envelope of the output signal from the first phase-sensitive cietector 9 after a 900 pulse will have the form shown at (a) in Fig 2. It has an initial maximum value M and thereafter decays. The output from the second phase-sensitive detecter will remain at zero since the two inputs thereto are in phase quadrature. However in an incorrectly adjusted spectrometer the two signals to the second phase-sensitive detector will not be exactly in quadrature and the output of phase-sensitive detector 10 will appear as at (b) in Fig 2. This signal gives the phase error in both magnitude and sign.To utilise this signal for correction of the turning of coil 2 the output of the second phase sensitive detector 10 is fed through a switch 12 to a sample and hold circuit 1 3 the output of which is fed through a resistor R1 and switch 14 to an integrator 1 5.

The output of integrator 1 5 obtained at terminal 1 6 is utilised to determine the frequency to which coil 2 is tuned. This is shown more clearly in Fig 3. Coil 2 has connected across it two voltage controlled variable capacitance diodes 1 7 and 18 in series and terminal 16 is connected to the junction between these diodes. It is desirable to connect diodes 17 and 18 to the same points as the two protection diodes 19 and 20 which are provided to protect amplifier 8 from excessive rf voltages. This protection will thus also extend to diodes 17 and 18.

The circuit illustrated in Fig 1 and Fig 3 is operated by periodically closing switch 5 to provide 900 pulses. After a dead time on the termination of each pulse measurement of the free induction decay signal is made by detecting the signal in the first phase-sensitive detector. For correction of tuning errors switch 12 is closed for a short interval immediately after the dead time and switch 14 is closed for an interval during the recovery period. The timing of the switching sequences is shown in Fig 4 which illustrates a 900 pulse followed by a free induction decay output from amplifier 8 the envelope of which is shown at 21. Signal 21 is at a maximum immediately after the dead time and accordingly switch 12 closes for interval A immediately after the dead time.The magnitude of this signal is held in sample and hold circuit 13 and switch 14 is closed during interval C in the recovery period to apply this signal to integrator 1 5. Accordingly the output of integrator 1 5 as taken from terminal 1 6 will undergo an excursion during interval C with its direction and magnitude determined by the value of the error signal held in circuit 13. This change will alter the tuning of coil 2 in a direction to reduce the phase error. It is preferable to operate switch 1 4 during the recovery period so as not to interfere with a current measurement sequence and allow the correction to be completed before the next measurement sequence.

Where the dead time is short then the circuit described so far will correct for errors in tuning very readily. However a further source of phase error is drift in the value of the nominally static magnetic field provided by magnet 3. This drift produces a time varying phase change in the output of receiver coil 2 and the effect of this drift can be compensated for by the provision of certain additional components shown in Fig 1.

,These additional components are a switch 22 connected through a resistor to the output of the second phase-sensitive detector 10 and a further sample and hold circuit 23 followed by an inverter 24 the output of which is applied to switch 14 through a resistor R2. Switch 22 is operated during a time interval B as shown in Fig 5 which otherwise corresponds to Fig 4.

Where there is a time varying phase change in the output of coil 2 the output of the second phase-sensitive detector 10 will be a signal of initially increasing magnitude as the phase error increases. If the time between the 900 pulse and interval A is Ta and the time interval between the 900 pulse and switch period B is Tb then if Tb = rTa (where r is a ratio, say 3) then the output of circuit 23 contains the same contribution from tuning error as circuit 13 but r times the contribution from field error. Due to the presence of inverter 24 and if it is arranged that R2 = rR1 the contribution from field error will be cancelled out in the input to integrator 1 5 which will receive an input proportional to the tuning error only.The correction signal at terminal 1 6 will be reduced to a proportion (r-1 )/r of its value without the additional components, but this reduction can readily be allowed for.

Since a signal indicative of the magnetic field drift is available in the circuit thus far described and illustrated in Fig 1 and Fig 3 it is possible to use this signal to correct for magnetic field error in addition to the correction of the receiver coil for tuning error. A circuit corresponding to Fig 1 but with additional components for this purpose is shown in Fig 6 and like parts have like reference numerals to Fig 1. Fig 6 shows only that part of the circuit which follows the second phasesensitive detector 10.

In Fig 6 the output of sample and hold circuit 23 in addition to being supplied to inverter 24 is also supplied to a second integrator 25 through a resistor R3 and switch 26. The output of integrator 25, which is made available at terminal 27, is used to change the static magnetic field applied to the sample in accordance with the sign and magnitude of the signal at terminal 27.

The initial tuning error resulting from a change in a sample will be corrected by the circuit of Fig 1 and Fig 3 after a few cycles of measurement sequences followed by recovery periods. Any further tuning errors will occur very slowly and the correction circuit will continuously reduce this error to a very small or negligible value. The output of sample and hold circuit 23, which is a sample of the output of the quadrature phasesensitive detector 10 in interval B (Fig 5), will therefore normally consist of a signal relating to field error only. This signal is applied through switch 26 which is operated in interval C to generate a field control signal.

Immediately following a sample change when there may be an appreciable tuning error then the output of circuit 23 may not be entirely due to field error. In correcting for field error it is possible to compensate for tuning error by utilising the output of sample and hold circuit 1 3 and applying it through an inverter 28 and resistor R5 to switch 26. Resistor R5 is made equal to R3. With these additional components any tuning error is cancelled and integrator 25 will receive a fraction (r-1 )/r of the input proportional to the magnitude of the field error that it would have received without these additional components.

The additional components for eliminating tuning error from the field control signal will be of particular value where significant tuning errors are occuring as for example with rapid temperature changes of the sample or with moving or flowing samples. For stationary samples at constant temperature the disturbance in the field control due to tuning error will normally have disappeared within the first few cycles and correction of the input to integrator 25 may not be necessary. However an advantage in such cases is in the reduction of time taken for the correction of any initial field error.

The tuning control signal and the magnetic field control signal are both available throughout the measurement procedure and thus changes that take place during a measurement sequence are automatically corrected for as they occur.

The operation of switch 5 to provide 900 rf pulses and the operation of switches 12 and 14, and of switches 22 and 26 if provided, are all controlled by a programmer 30.

A further source of error in measurement arises when the rf pulses applied to the sample are not precise 900 pulses. To ensure that pulses of correct angle are applied an NMR spectrometer embodying the invention may include additional components which are used only in the initial setting up stage. For this purpose the spectrometer is subjected to an initial sequence of two successive nominally 900rf pulses. If the pulses have the correct angle then the resultant in-phase output of the first phase-sensitive detector 9 is zero. For pulse angle errors of 0 the output of phase-sensitive detector 9 is proportional to sin 20 and thus the output from phase-sensitive detector 9 can be used to correct the pulse angle. The additional components required are illustrated in Fig 7.They comprise a switch 30 followed by a sample and hold circuit 31 the output of which is applied through a further switch 32 to an integrator 33. The output of integrator 33 at terminal 34 is used to control pulse angle.

The operation of the components of Fig 7 is illustrated with reference to Fig 8. Two 900 pulses are applied and after a dead time the output from phase-sensitive detector 9 is measured. This should ideally be zero but if it is not it gives an indication of pulse angle error.

Switch 30 is operated in interval A when the output is at a maximum and the signal in circuit 31 is passed to integrator 33 during interval C some time later. The signal available at terminal 34 is used to correct the pulse angle, for example by altering the gain of the rf pulse amplifier 6 to change the pulse amplitude or by controlling the timing circuitry determining the period of closure of switch 5 so as to change the pulse length.

Since adjustment of the pulse angle as described with reference to Fig 7 and Fig 8 is not carried out simultaneously with control of the tuning of the receiver coil and control of the magnetic field but is a separate preliminary step a useful economy can be achieved by utilising circuit components already provided in connection with the arrangement shown in Fig 1 and Fig 6 and dispensing with the components described with reference to Fig 7. For this purpose certain additional components are required and these are illustrated in Fig 1 and Fig 9. They comprise a 900 phase shift circuit 35 and a switch 36 connected as an alternative path to amplifier 6 to the path through switch 5. In addition as shown in Fig 9 a further output is taken from sample and hold circuit 13 and applied through a resistor R4 and switch 37 to an integrator 38.The output of integrator 38 at terminal 39 is available for pulse angle control.

Fig 9, which is an alternative layout to that part of the circuit shown in Fig 6 and like parts have like reference numerals, illustrates all the components required for pulse angle control, for tuning control and for magnetic field control and all these control signals are obtained from the output of the quadrature phase-sensitive detector 9.

For obtaining a pulse angle control signal switch 36 is closed and two successive nominally 90b pulses are generated. Since the phase of these pulses are shifted by 900 with respect to the reference signals from oscillator 4 due to the use of the phase shift circuit 35 any error signal will appear at the output of the quadrature phasesensitive detector 10 and can be utilised in the circuit shown in Fig 9 in similar manner to the arrangement shown in Fig 7. When switch 37 is closed switches 14 and 26 remain open so that the outputs at terminals 16 and 27 remain constant. Programmer 30 controls the operation of all of the switches shown in Fig 9.

The circuits described herein are shown in diagramatic form only. Their detailed realisation, including arrangements for resetting the integrators will be apparent to those skilled in the art. The timing of the operation of the various switches may be changed from that described without departing from the invention.

Furthermore while simple analogue-type circuits have been shown the invention can be carried out by using digital techniques. Thus the outputs of the phase-sensitive detectors can be converted to digital form before being utilised to provide correction signals.

In the embodiment of the invention described above a transmitter coil 7 and a receiver coil 2 have been shown, the two coils being separate. It is known to combine the function of transmitting and receiving in a single coil and the invention is equally applicable to such single-coil spectrometers. Adequate protection needs to be provided for the variable capacitance diodes 1 7 and 1 8 or alternative tuning arrangements may be used, for example high voltage variable capacitance diodes or electrically controlled mechanically variable capacitors.

Claims

Claims

1. An NMR spectrometer including means for applying a static magnetic field to a region in which a sample can be positioned, transmitter coil means for energising the region with rf pulses, a rf generator connected to energise the transmitter coil means with rf pulses, tunable receiver coil means for picking up NMR signals from a sample in the region, a first phase-sensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator, a second phase-sensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator, a second phase-sensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator the signals to the second phase-sensitive detector being in relative phase quadrature compared with the signals applied to the first phase-sensitive detector, and feedback means utilising the output of the second phase-sensitive detector to provide a tuning control signal to alter the frequency to which the receiver coil means is tuned in a manner to reduce errors in the output of the first phase-sensitive detector arising from incorrect tuning of the receiver coil means.

2. The NMR spectrometer as claimed in claim 1 in which the feedback means includes means for measuring the output of the second phasesensitive detector at an appropriate instant in time after cessation of an rf pulse.

3. The NMR spectrometer as claimed in claim 2 in which a dead time is provided between cessation of an rf pulse and detection of NMR signals from the receiver coil and the output of the second phase sensitive detector is measured immediately after the said dead time.

4. The NMR spectrometer as claimed in claim 2 and claim 3 in which the feedback means includes a sample and hold circuit to which the output of the second phase-sensitive detector is applied.

5. The NMR spectrometer as claimed in claim 4 in which the output of the sample and hold circuit is applied to an integrator circuit the output of which comprises the tuning control signal.

6. The NMR spectrometer as claimed in claim 5 in which the output of the sample and hold circuit is applied to the integrator circuit during the recovery period between cessation of the detected signals and the next rf pulse.

7. The NMR spectrometer as claimed in any one of the preceding claims in which variable capacitor means is connected across the receiver coil means to alter the frequency to which the receiver coil means is tuned and the tuning control signal operates to control the magnitude of the capacitance of the said capacitor means.

8. The NMR spectrometer as claimed in any one of the preceding claims in which the output of the second phase-sensitive detector is measured twice with a time interval between the measurements and in which means are provided for subtracting a proportion of the second measured signal from the first measured signal to derive the tuning control signal.

9. The NMR spectrometer as claimed in claim 8 in which means are provided for subtracting the first measured signal from the second measured signal to derive a field control signal which is used to control the magnitude of the static magnetic field so as to tend to keep the said field at a constant value.

10. The NMR spectrometer as claimed in any one of the preceding claims in which means are provided for ensuring that the rf pulses applied to the transmitter coil means are 900 pulses, said means comprising means for applying an initial pulse sequence of two nominally 900 pulses to the transmitter coil means, means for detecting any resultant signal in the receiver coil means and means for correcting the angle of the rf pulses to tend to reduce the magnitude of said detected signal to zero.

11. The NMR spectrometer as claimed in claim 10 in which the said detection means comprises the first phase-sensitive detector.

12. The NMR spectrometer as claimed in claim 10 in which means are provided for enabling the phase of said initial pulse sequence to be shifted by 900 and the said detection means comprises the second phase-sensitive detector.

13. The NMR spectrometer as claimed in any one of the preceding claims in which the said transmitter coil means and receiver coil means are provided by a common coil.

14. An NMR spectrometer substantially as described with reference to Fig. 1 of the accompanying drawings.

1 5. An NMR spectrometer substantially as described with reference to Fig. 6 or Fig. 7 or Fig.

9 of the accompanying drawings.

New claims or amendments to claims filed on 3 July 1980 Superseded claim 1 New or Amended Claim:

1. An NMR spectrometer including means for applying a static magnetic field to a region in which is a sample can be positioned, transmitter coil means for energising the region with rf pulses, a rf generator connected to energise the transmitter coil means with rf pulses, tunable receiver coil means for picking up NMR signals from a sample in the region, a first phasesensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator, a second phase-sensitive detector supplied with NMR signals from the receiver coil means and reference signals from the generator, the signals to the second phase-sensitive detector being in relative phase quadrature compared with the signals applied to the first phase-sensitive detector, and feedback means utilising the output of the second phase-sensitive detector to provide a tuning control signal to alter the frequency to which the receiver coil means is tuned in a manner to reduce errors in the output of the first phase-sensitive detector arising from incorrect tuning of the receiver coil means.