U.S. patent number 3,792,346 [Application Number 05/308,212] was granted by the patent office on 1974-02-12 for proton-enhanced nuclear induction spectroscopy.
This patent grant is currently assigned to Massachusetts Institute of Technology. Invention is credited to Michael G. Gibby, Alexander Pines, John S. Waugh.
United States Patent |
3,792,346 |
Gibby , et al. |
February 12, 1974 |
PROTON-ENHANCED NUCLEAR INDUCTION SPECTROSCOPY
Abstract
A method is described for the detection of the natural magnetic
and/or electric quadrupole reasonance frequencies of isotopically
rare or chemically dilute nuclei in the presence of at least one
abundant nuclear spin species. The free induction decay of the
dilute nuclei is directly detected after an applied r.f. field at
the Larmor frequency of the dilute nuclei is removed. A high
resolution spectrum of the dilute nuclei is obtained by applying an
r.f. field at the Larmor frequency of the abundant spin system
during the detection interval.
Inventors: |
Gibby; Michael G. (Arlington,
VA), Pines; Alexander (Berkeley, CA), Waugh; John S.
(Lincoln, MA) |
Assignee: |
Massachusetts Institute of
Technology (Cambridge, MA)
|
Family
ID: |
23193028 |
Appl.
No.: |
05/308,212 |
Filed: |
November 20, 1972 |
Current U.S.
Class: |
324/314 |
Current CPC
Class: |
G01R
33/4608 (20130101); G01R 33/62 (20130101) |
Current International
Class: |
G01R
33/62 (20060101); G01R 33/46 (20060101); G01R
33/44 (20060101); G01n 027/78 () |
Field of
Search: |
;324/.5R,.5A,.5AC |
Other References
A Abragam and G. R. Proctor - Physical Review - Vol. 109 - 1958 -
p. 1441. .
N. Bloembergen and P. Sorokin - Physical Review - Vol. 110 - 1958 -
p. 865. .
L. Sarles and R. Cotts - Physical Review- Vol. 111 - 1958 - p. 853.
.
S. Hartmann and E. Hahn - Physical Review - Vol. 128 - 1962 - p.
2042. .
M. Goldman and A. Landesman - Physical Review - Vol. 132 - 1963 -
p. 610. .
F. Lurie and C. Slichter - Physical Review - Vol. 133 - 1964 - p. A
1108. .
A. Abragam and W. Proctor - Comptes Rendas des Academie des
Sciences - Vol. 246 - p. 1258 - 1958. .
M. Mehring, A. Pines, W. Rhim and J. Waugh - Journal of Chemical
Physics - Vol. 54 - 1971 - p. 3239..
|
Primary Examiner: Lynch; Michael J.
Attorney, Agent or Firm: Smith, Jr.; Arthur A. Santa; Martin
M.
Claims
What is claimed is:
1. A method for improving the signal-to-noise ratio and resolution
of the NMR spectrum of a dilute nuclear spin system, the S-spins,
in a solid in the presence of a more concentrated nuclear spin
system, the I-spins comprising the steps of
1. transfering nuclear polarization from the I-spins to the S-spins
and,
2. then applying radiofrequency fields in the neighborhood of the
I-spin Larmor frequency to reduce the broadening of the S-spin
resonance which would otherwise be caused by said I-spins, and
3. detecting the nuclear induction radiation signal of the S-spins
obtained from the S-spin polarization acquired via the I-spins.
2. The method of claim 1 in which step 1 is accomplished by an
adaptation of rotating frame, double-resonance procedures,
comprising
polarizing said sample ofI-spins of I-spins spin-lattice processes,
cooling the I-spins in their rotating frame,
establishing contact between I and S spin system by turning on an
r.f. field, H.sub.1 (S), near the Larmor frequency of S-spins,
thereby acquiring said S-spin polarization along H.sub.1 (S).
3. The method of claim 1 in which step 1 is accomplished by "solid
effect" techniques, comprising
polarizing said system of I-spins by spin-lattice processes,
irradiating the sample with an r.f. field at the sum or difference
of the I-spins and S-spins Larmor frequencies, thereby acquiring
said S-spins polarization along the laboratory Zeeman field.
4. The method of claim 1 in which step 1 is accomplished by a
rotating frame version of the "solid effect" technique,
comprising
polarizing said sample I-spins by spin-lattice processes,
cooling the I-spins in their rotating frame by spin-locking said
I-spins along an r.f. field, H.sub.1 (I),
simultaneously applying an r.f. field, H.sub.1 (S), to the S-spins
and an audio frequency magnetic field parallel to the laboratory
Zeeman field direction with a frequency of .gamma..sub.I H.sub.1
(I) + .gamma..sub.s H.sub.1 (S) or .gamma..sub.I H.sub.1 (I) -
.gamma..sub.s H.sub.1 (S), thereby acquiring said S-spins
polarization along H.sub.1 (S).
5. The method of claim 1 in which step 1 is accomplished by mixing
the I and S spin systems in weak magnetic fields, comprising
polarizing said sample of I-spins by spin-lattice processes in a
large laboratory magnetic field,
removing the sample to a weak magnetic field thereby enabling I-S
mixing,
returning the sample to the large magnetic field thereby acquiring
S-spins polarization along the laboratory Zeeman field.
6. The method of claim 1 in which step 2 is accomplished by
continuous wave irradiation of the sample near the I-spin Larmor
frequency.
7. The method of claim 1 in which step 2 is accomplished by pulsed
irradiation near the I-spin Larmor frequency comprising
a train of 180.degree. pulses spaced in time of the order of the
shorter of the I-spin or S-spin spin-spin relaxation times.
8. The method of claim 1 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
9. The method of claim 2 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
10. The method of claim 3 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
11. The method of claim 4 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
12. The method of claim 5 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
13. The method of claim 6 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
14. The method of claim 7 comprising, in addition,
acquiring said S-spin polarization a plurality of times,
detecting said S-spins free induction decay after each polarization
to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal from a single I-spin polarization.
15. The method of claim 1 comprising, in addition,
Fourier transforming of said averaged S-spins signal to provide the
frequency spectrum of the S-spins.
16. The method of claim 1 comprising, in addition,
applying said I-spins field excitation a plurality of times
corresponding to the number of S-spins field excitations,
said I-spins excitation being gradually applied to avoid destroying
the I-spin order and of substantially full magnitude before said
S-spin excitation is removed,
said I-spins excitations being gradually reduced to substantially
zero amplitude after said free induction decay signal is detected
and before said S-spin excitation is applied again, the amplitude
of said I-spins excitation being sufficient to remove the dipolar
coupling of the I-spins.
17. The method of claim 2 comprising, in addition,
applying said I-spins field excitation a plurality of times
corresponding to the number of S-spins field excitations,
said I-spins excitation being gradually applied to avoid destroying
the I-spin order and of substantially full magnitude before said
S-spin excitation is removed,
said I-spins excitations being gradually reduced to substantially
zero amplitude after said free induction decay signal is detected
and before said S-spin excitation is applied again, the amplitude
of said I-spins excitation being sufficient to remove the dipolar
coupling of the I-spins.
18. The method of claim 1 comprising, in addition,
maintaining said I-spin r.f. magnetic field during the time said
S-spin r.f. magnetic field is applied, and
adiabatically reducing said I-spin field to zero before said S-spin
field is abruptly turned off.
19. The method of claim 1 comprising, in addition,
maintaining said I-spin r.f. magnetic field during the time said
S-spin r.f. magnetic field is applied, and
adiabatically reducing said I-spin field to zero before said S-spin
field is abruptly turned off.
20. The method of claim 1 comprising, in addition,
adiabatically reducing the amplitude of the I-spins r.f. magnetic
field when said S-spins magnetic field excitation is applied,
adiabatically increasing the amplitude of the I-spins r.f. magnetic
field prior to turning off said S-spins field excitation,
maintaining said increased amplitude of said I-spins r.f. magnetic
field during the time said S-spins free induction decay signal is
being detected in order to remove the dipolar coupling of the
I-spins,
adiabatically reducing said I-spins r.f. magnetic field to zero
after said S-spins signals are not being detected,
applying said S-spins field excitation a plurality of times,
the amplitude of said I-spins field excitation during said
plurality of S-spin excitation fields being at the amplitudes
recited previously during and between said S-spin excitation fields
with adiabatic changes in said amplitudes of the I-spin fields.
21. The method of claim 2 comprising, in addition,
adiabatically reducing the amplitude of the I-spins r.f. magnetic
field when said S-spins magnetic field excitation is applied,
adiabatically increasing the amplitude of the I-spins r.f. magnetic
field prior to turning off said S-spins field excitation,
maintaining said increased amplitude of said I-spins r.f. magnetic
field during the time said S-spins free induction decay signal is
being detected in order to remove the dipolar coupling of the
I-spins,
adiabatically reducing said I-spins r.f. magnetic field to zero
after said S-spins signals are not being detected,
applying said S-spins field excitation a plurality of times,
the amplitude of said I-spins field excitation during said
plurality of S-spin excitation fields being at the amplitudes
recited previously during and between said S-spin excitation fields
with adiabatic changes in said amplitudes of the I-spin fields.
22. The method of claim 20 wherein said I-spin magnetic field
amplitude and the S-spin magnetic field amplitude during the time
said S-spins field is applied satisfy the Hartmann-Hahn condition,
.gamma..sub.I H.sub.1 (I) = .gamma..sub.s H.sub.1 (S), for a time
sufficient to most rapidly establish spin thermodynamic
equilibrium.
23. The method of claim 21 wherein said I-spin magnetic field
amplitude and the S-spin magnetic field amplitude during the time
said S-spins field is applied satisfy the Hartmann-Hahn condition,
.gamma..sub.I H.sub.1 (I) = .gamma..sub.s H.sub.1 (S), for a time
sufficient to most rapidly establish spin thermodynamic
equilibrium.
24. The method of claim 1 comprising, in addition,
applying the sequence of radio frequency magnetic fields at the
Larmor frequency of the I-spins by spin-locking by applying a
90.degree. pulse followed by a 90.degree. phase shift in the
frequency of the radio frequency carrier which is continuously
applied,
applying and removing said S-spins excitation a plurality of
times,
detecting said S-spins free induction decay field after each
removal to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal.
25. The method of claim 2 comprising, in addition,
applying the sequence of radio frequency magnetic fields at the
Larmor frequency of the I-spins by spin-locking by applying a
90.degree. pulse followed by a 90-degree phase shift in the
frequency of the radio frequency carrier which is continuously
applied, which
applying and removing said S-spins excitation a plurality of
times,
detecting said S-spins free induction decay field after each
removal to provide a plurality of signals,
averaging the S-spin signals to provide an averaged S-spin nuclear
induction signal.
26. The method of claim 1 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins
comprises
spin-locking by applying a 90.degree. pulse followed by a
90.degree. phase shift in the frequency carrier.
27. The method of claim 2 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins
comprises
spin-locking by applying a 90.degree. pulse followed by a
90.degree. phase shift in the frequency carrier.
28. The method of claim 1 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins
comprises
spin-locking followed by adiabatic demagnetization by applying a
90.degree. pulse followed by a 90.degree.phase shift in the radio
frequency carrier followed by a gradual shutting off of the carrier
in a time long compared to 1/.gamma..sub.1 H.sub.1.
29. The method of claim 2 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins
comprises
spin-locking followed by adiabatic demagnetization by applying a
90.degree. pulse followed by a 90.degree. phase shift in the radio
frequency carrier followed by a gradual shutting off of the carrier
in a time long compared to 1/.gamma..sub.1 H.sub.1.
30. The method of claim 1 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins comprises
pulsed preparation of the dipolar state by
applying a first 90.degree. pulse followed by a
45.degree.pulse,
said 45.degree. pulse follows the 90.degree. pulse by a time on the
order of T.sub.2 and is phase shifted by 90.degree..
31. The method of claim 2 in which the sequence of radio frequency
magnetic fields at the Larmor frequency of the I-spins comprises
pulsed preparation of the dipolar state by applying a first
90.degree. pulse followed by a 45.degree.pulse, said 45.degree.
pulse follows the 90.degree. pulse by a time on the order of
T.sub.2 and is phase shifted by 90 degrees.
Description
This invention was made in the course of work supported in part by
a grant from the National Institutes of Health and a grant from the
National Science Foundation.
This invention relates to a procedure for the enhanced detection of
the natural magnetic dipole and/or electric quadrupole resonance
frequencies, the Larmor frequencies, of isotopically rare or
chemically dilute nuclei (to be called S-spins) in the presence of
at least one abundant nuclear spin species (to be called I-spins).
The method is applicable to solids or solid-like material (polymer
suspensions, for example) in which the S-spins are spin-spin
coupled to I-spins possessing, in turn, a homonuclear spin-spin
coupling.
The technique employs established ideas of double resonance in a
novel fashion to obtain an enhanced nuclear induction signal of the
rare (S) spin system. The signal-to-noise advantage of this method
facilitates the rapid acquisition of NMR (nuclear magnetic
resonance) data, making feasible the study of very dilute systems
or extremely small samples. The method is general and applicable to
the study of both broadline and high resolution magnetic resonance
phenomenon. The "high resolution"NMR is accomplished by strong
radio frequency (r.f.) irradiation at the Larmor frequency of the
I-spins during the recording of S-spin induction signal. The
irradiation removes I-spin dipolar interactions as they affect the
S-spin signal. The normal broadline signal is obtained in the
absence of the I-spin decoupling field. The "high resolution"
version enhances the usefulness of the technique as a chemical tool
since the strong I-S dipolar interactions typically obscure the
information of chemical interest, i.e., chemical shielding
parameters.
The enhancement method exploits the abundant spins I (N.sub.I is
the abundance, .gamma..sub.I is the gyromagnetic ratio) as a giant
reservoir of nuclear polarization to be transferred to the rare
S-spins (N.sub.S is the abundance, .gamma..sub.S is the
gyromagnetic ratio) bit by bit in a time short compared to any
spin-lattice relaxation times of the I system. The I-spin
polarization is obtained by placing the sample in the strong
laboratory field, H.sub.o, and waiting for a time on the order of
the spin-lattice relaxation time. The acquired S-spin polarization
is observed as a transient induction decay which is recorded and
stored for later processing. The Fourier transform of this time
domain transient will yield the "normal" absorption (frequency)
spectrum characteristic of the spin system. The gain in power
signal-to-noise of this technique over conventional Fourier
transform NMR goes as
(N.sub.I /N.sub.s)(.gamma..sub.I /.gamma..sub.S).sup.2
Although the method has been discussed in terms of the magnetic
resonance of nuclei, the electric quadrupole analog is just as
feasible.
Some pertinent references to the prior art are as follows: S.R.
Hartmann and E.L. Hahn, Phys.Rev., 128, 2042 (1962); F.Lurie and
C.P. Slichter, Phys.Rev., 133, A1108 (1964); L.R. Sarles and R.M.
Cotts, Phys, Rev., 111, 853 (1958); N. Bloembergen and P.P.
Sorokin, Phys.Rev., 110, 865 (1958); M.Goldman and A.Landesman,
Phys.Rev., 132= 610 (1963); A.Abragam and W.G.Proctor, C.R.Acad.
Sci., 246, 1258 (1958); M.Mehring, A.Pines, W-K.Rhim and J.S.
Waugh, J.Chem. Phys., 54, 3239 (1971); A.Abragam and W.G. Proctor,
Phys.Rev., 109, 1441 (1958).
The objects and featuresoof the invention will become more apparent
from the following detailed description when read in conjunction
with the following figures:
FIGS. 1(a), 1(b), and 1(c) show sequences of radio frequency
magnetic fields at the Larmor frequency of the I-spins for cooling
the I-spins by spin locking, spin-locking followed by adiabatic
demagnetization in the rotating frame, and pulse preparation of the
ordered state, respectively.
FIGS. 2(a) and 2(b) show pulse sequences for the I-spins and
S-spins irradiation respectively, which provides a broadline
undecoupled spectrum for the S-spins.
FIGS. 2(b) and 2(c ) show pulse sequences for the S-spins and
I-spins irradiation respectively, which speeds up cross
polarization of the S-spins and provides a broadline spectrum for
the S-spins.
FIGS. 3(a) and 3(b) show pulse sequences for the I-spins and
S-spins radio frequency fields, respectively, which provides a high
resolution spectrum of the S-spins.
FIG. 4(a) shows the broadline spectrum obtained by Fourier
transformation of a single free induction decay of natural
abundance .sup.13 C in adamantane.
FIG. 4(b) shows the broadline spectrum of (a), but with the
enhancement provided by 50 single shot contacts of the cross
polarization method.
FIG. 4(c) shows the spectrum with decoupling and cross
polarization. It is the result of one proton polarization and 14
I-S contacts.
FIG. 5 is a block diagram of an apparatus suitable to the
invention.
The Invention
The method involves a sequence of steps, some of which are
familiar, standard NMR techniques combined in an innovative manner
and coupled with an original mode of detection. This section will
outline the steps of the method and provide several of many
alternative means of performing the invention.
Step 1: The I-spin system (abundant) is allowed to acquire a large
degree of polarization. This may be accomplished by placing the
sample of material, for example, an organic compound with abundant
protons .sup.1 H and dilute in .sup.13 C in a strong magnetic field
and waiting for several spin-lattice relaxation times of the
protons.
Step 2: The I-spin system is strongly cooled in the spin
temperature sense in a frame of reference rotating at the Larmor
frequency, .omega..sub.o, of the I-spins. (.omega..sub.o =
.gamma..sub.I H.sub.o, where .omega..sub.o is the angular frequency
of precession, .gamma..sub.I is the gyromagentic ratio of the I
nuclear spins, and H.sub.o is the magnitude of the static
laboratory field.) This higher ordered, non-equilibrium state will
serve as a source of magnetic order made available to the S-spin
system (dilute) by means of Step 3.
This cooling process may be effected by one of several traditional
means, i.e., spin-locking, spin-locking followed by adiabatic
demagnetization in the rotating frame (ADRF), or by a pulsed
preparation of the ordered state. FIGS. 1(a), 1(b) and 1(c) show
the sequence of radio frequency magnetic fields at the Larmor
frequency of the I-spins for these three methods.
a. Spin-locking may be performed by applying a 90.degree.-pulse on
the I-spins followed by 90.degree. phase shift in the radio
frequency (r.f.) carrier and continuous irradiation with the latter
phase. The r.f. excitations are provided by a coil whose axis is
perpendicular to the Zeeman field. While the r.f. carrier produces
an oscillating magnetic field in the coil, it is only one of the
circularly polarized components which results in the magnetic
transition of importance. The net effect of spin-locking is to take
a magnetization, originally parallel to the laboratory Zeeman
field, and lock it along a relatively weaker field, H.sub.1 (I),
rotating at the Larmor frequency. This ordered spin state appears,
in the rotating frame, to have acquired a temperature (T.sub.RF)
equal to [H.sub.1 (I)/H.sub.o ]T.sub.L for the case H.sub.1
(I)>>H.sub.L, where T.sub.L was the original spin-temperature
of the I system in the laboratory frame and H.sub.L is a measure of
the strength of the local dipolar field. For typical values of
H.sub.1 (I) and H.sub.o, T.sub.RF can be extremely low,
.about.10.sup.-.sup.3 T.sub.L. Adiabatic fast passage stopping on
resonance would accomplish the same as spin-locking.
b. Spin-locking followed by adiabatic demagnetization is another
means of preparing the desired ordered state and has the particular
advantage that there is no continuous irradiation of the I-spins.
It is accomplished by a gradual shutting-off of the r.f. carrier in
a time long compared to the precession period of the I-spins in the
field H.sub.L, i.e., 1/.gamma..sub.1 H.sub.L. FIG. 1(b)
demonstrates this technique.
c. The pulsed preparation of the dipolar state (J.Jeener and
P.Brockaert, Phys.Rev., 157, 232 (1967)) is an additional means of
effecting the cooling, but is less efficient than (a) or (b), i.e.,
a degree of the initial spin order is lost (about .about. 50
percent). This method does have the dual advantage of no continuous
I-spin irradiation without the need for ADRF apparatus. As shown in
FIG. 1(c), two pulses are required: the first, a 90.degree. pulse,
and the second a 45.degree. pulse. The second follows the first by
a time on the order of T.sub.2 and is phase shifted by 90.degree.
as in FIG. 1(c).
These are but three of numerous means of cooling the I-spin
system.
In the figures 90 degrees indicates the length of duration of the X
pulse, i.e., the time it takes the I-spin magnetization to nutate
about the X direction in the rotating frame from the Z direction
along the magnetic field to the X-Y plane perpendicular to the Z
direction. The Y portion of the r.f. excitation corresponds to a
90.degree. phase shift in the r.f. carrier and is the Y direction
in the rotating frame.
Step 3: After having cooled the I-spin system by some suitable
method, the nuclear species (S) of interest is brought into thermal
contact with the I-spin system. This is accomplished by turning on
a strong r.f. field at the Larmor frequency of the S-spins.
Establishing this thermal contact between the two spin reservoirs
results in an ordering of the S-spin system with the corresponding
growth of S magnetization along the direction of the rotating
component, H.sub.1 (S) of the r.f. field. As with the r.f. fields
applied to the I-spin system, these excitations are achieved from a
coil transverse to the laboratory static magnetic field. Once the
S-spin magnetization is acquired via the I-spin system, one of
several modes of operation can be used depending upon what
information is desired.
a. The normal, undecoupled free induction decay of the S-spins is
observed by abruptly (in a time short compared to T.sub.2) turning
off H.sub.1 (S). The transient, precessing magnetization, M.sub.S,
induces a detectable voltage in the same coil which generated
H.sub.1 (S). In the absence of a strong decoupling field on the
I-spins, the signal is the time domain equivalent of the broadline
(frequency) spectrum obtained by more traditional means.
Multiple contacts between the S-spin and I-spin systems can be made
with the intent of improving the signal-to-noise ratio. After each
I-S contact the nuclear induction signal is recorded and stored,
preferably, in digital form for averaging with later accumulations.
The contacts continue until the I-spin reservoir heats up and
further accumulation begins to degrade the signal-to-noise ratio.
In this manner it is possible to make many I-S contacts with one
I-spin polarization giving a substantial enhancement in
signal-to-noise.
FIG. 2 shows the sequence of events pertaining to an undecoupled,
multiple contact version of the method. The cooling process is
accomplished by spin-locking followed by adiabatic demagnetization
in the rotating frame, FIG. 2(a). Pulsing-on an r.f. field H.sub.1
(S) at the Larmor frequency of the S-spins is sufficient to
establish thermal contact between the I- and S-spin reservoirs.
This is shown in FIG. 2(b): The signal is acquired during the
sampling window which shows the nuclear signal as it might appear
at the output of the phase detector. To speed up the cross
polarization, it may be advantageous to turn on an additional r.f.
field at Larmor frequency of the I-spins. As demonstrated in FIG.
2(c), the turning on and off of H.sub.1 (I) is done adiabatically
so as not to destroy the magnetic order of the I-spins.
b) The greatest utility of the method for chemical or biological
application will perhaps be the "high resolution"adaptation of the
single or multiple contact procedure. By "high resolution" is meant
the removal of the static dipole-dipole interactions of the I-spin
system as they affect the dynamical behavior of the S-spins. The
suppression of these interactions is brought about by application
of intense r.f. irradiation at I-spin Larmor frequency during the
observation of the S-spin free induction decay.
FIG. 3 shows one of many possible solutions to the experimental
problem. The I-spins, FIG. 3(a), as before are spin-locked in the
usual fashion. Subsequently, the r.f. field is reduced
adiabatically to allow rapid thermal mixing between the two spin
systems and then raised again to its original strength to decouple
the I-spins from the S-spins during the free induction decay of the
S-spins. After sufficient signal has been acquired, the r.f. field
is shut off adiabatically with the intent of allowing the probe to
cool somewhat before the next contact. Typically, one might need a
larger H.sub.1 (I) to decouple the I-spins than mix them
efficiently with the S-spins. The signal, FIG. 3(b) recorded during
the decoupling process, free of I-spin dipolar coupling, might
contain the interesting chemical information which might otherwise
be obscured, i.e., chemical shifts of the S-spin system. The signal
from each contact might be stored in an averager and later fed to a
computer for Fourier transformation to obtain the more readily
analyzable frequency spectrum.
In the event that the spin-lattice relaxation time in the rotating
frame is too short to allow multiple contacts, the single contact
version will still yield a substantial signal-to-noise enhancement.
For a single contact the polarization enhancement over the usual
equilibrium value equals .gamma..sub.I /.gamma..sub.S. For I being
protons and S being .sup.13 C or .sup.15 N this factor is .about.4
and .about.10, respectively. Even for the single contact method
large signal-to-noise increases are feasible.
c. The S-spin magnetization obtained in a single I-S contact can be
utilized also for spin-lattice relaxation studies for
signal-to-noise enhancements. The acquired S magnetization is
restored anti-parallel to the Zeeman magnetic field by some
suitable method like adiabatic fact passage or 90.degree. pulse
shifted in phase by 90.degree. from the original S-spin r.f. field.
The decay of this magnetization back to its equilibrium value is
monitored in the well-known fashion, i.e., by applying a 90.degree.
pulse sometime later and recording the transient signal.
An example of the application of method (a) and (b) is shown in
FIG. 4. FIG. 4(a) is the broadline spectrum of natural abundance
C.sup.13 in adamantane obtained by Fourier transformation of a
single free induction decay on a fully equilibrated sample. FIG.
4(b) is the broadline spectra obtained by 50 I-S single contacts
(Method a). FIG. 4(c) is the high resolution, multiple contact
(Method b) experiment and the result of a single proton
polarization. The two lines show clearly the resolution of the two
types of carbon lines characteristic of adamantane. The spectra of
FIG. 4(a) and 4(c) were acquired in a nearly equivalent amount of
time and demonstrate dramatically the power of the invention for
chemical inquiry.
The effect of Steps 2 and 3 is to transfer nuclear polarization
from the I-spins to the S-spins. The rotating frame, double
resonance process is not the only means of accomplishing this. A
potentially useful method would be the "solid effect". R.F.
irradiation at the sum and differences of the two spins
(.omega..sub.s + .omega..sub.I and .omega..sub.I - .omega..sub.s)
would provide efficient transfer of I-spin polarization to the S
system. The resulting S-spin polarization would be parallel to the
laboratory field. A 90.degree. pulse on the S system would provide
the nuclear induction signal. Similarly, the rotating frame analog
of the "solid effect" experiment is feasible. To effect the
transfer of polarization, the I-spins may be "spin-locked" with an
r.f. field of amplitude H.sub.1 (I) which is followed with field
H.sub.1 (S) on the S-spins and an audio frequency magnetic field
parallel to the laboratory Zeeman field with a frequency equal to
.gamma..sub.I H.sub.1 (I) + .gamma..sub.s H.sub.1 (S) or
.gamma..sub.I H.sub.1 (I) - .gamma..sub.s H.sub.1 (S). The S-spin
magnetization is along H.sub.1 (S) and may be observed in the usual
manner by abruptly shutting off H.sub.1 (S).
An additional means of transferring I-spin polarization is to allow
thermal mixing of the I and S system in weak laboratory magnetic
fields. The sample is first brought to equilibrium with lattice in
a large Zeeman field. The sample is removed from the large magnetic
field to a weak field and returned again to the original field in a
time short compared to the spin-lattice relaxation times but long
enough to allow mixing of the I-S systems in the weak field. The
S-spin free induction decay is acquired in the large laboratory
Zeeman field upon applying a 90.degree. pulse to the S-spins.
The Apparatus
A diagram of a double resonance spectrometer capable of performing
the enhancement experiments is given in FIG. 5.
The basic operation of the instrument requires inter alia the logic
generator which provides the logic levels for opening and closing
mixer gates and pulses to trigger the sampling circuits and signal
averager.
The spectrometer provides two stable frequencies sources 51 and 52:
one, 51, at the Larmor frequency of the I-spins and the second, 52,
for S-spins. These frequencies may be derived from crystal
oscillators suitably thermostated for time stability. .omega..sub.I
is split by a power divider 53 into two channels. One channel
passes through a delay line 54 to give the r.f. carrier a 90-degree
phase shift relative to the second channel. As mentioned above the
two r.f. phases can be used for the spin-locking process. Both
channels of .omega..sub.I encounter mixer gates 55, 56 which are
slaved either directly (for pulse output) or indirectly (for
adiabatic variation of r.f. amplitude) through the ADRF circuit to
the logic generator 58. RF passing through the mixer gates 55, 56
goes through a channel combiner 59 and is amplified strongly by a
power transmitter 60. When .omega..sub.I reaches the probe 61, it
will have acquired sufficient intensity to decouple or mix the
I-spins.
Like the r.f. for the I-spins, .omega..sub.s is split into two
channels at power divider 62. One channel of .omega..sub.s provides
a reference signal to the phase detector 63. The delay line 64
enables the experimeter to pick the particular reference phase he
desires. As with .omega..sub.I, .omega..sub.s encounters a mixer
gate which again is under the control of the logic generator 58.
When .omega..sub.s is pulsed on, it gets amplified on its way to
the probe by a second high power transmitter 66. The duplexer 67
protects the receiver 68 from the intense r.f. irradiation of the
S-spin transmitter 66. During the reception mode the duplexer
switches (in the absence of large .omega..sub.s) to enable
detection of signal voltage from the probe. The low-noise receiver
68 amplifies the weak signal and transmits it to the phase detector
63. The NMR signal coming from the phase detector is in the
audio-frequency range. It is filtered, sampled discretely in time,
and converted to digital form in the A/D converter 69. Each point
in time is stored in its corresponding memory channel of the signal
averager 70 to be averaged with subsequent accumulations. After a
signal has been acquired with the requisite signal-to-noise ratio,
it is transferred to a computer 71 for Fourier transformation to
provide the more easily interpretable frequency spectrum .
* * * * *