Coherence Selection: Phase Cycling and Gradient Pulses

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Coherence Selection Phase Cycling andGradient
Pulses
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Purpose
Systematically vary the phases of RF pulses and
receiver in a pulsesequence to compensate for
imperfections cancel artefacts select for the
desired signals only
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A simple example
CYCLOPS Cancels imbalances between x and y
channel of the receiver system and eliminates the
Quad spike x, y, -x, -y are usually referred to
as 0 1 2 3 or 0 90 180 270
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Refocussing pulses
F rec x y y y -x y -y -y
EXORCYCLE Removes the effects of an imperfect
refocussing pulse The receiver only follows the
signal that has been properly inverted
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Difference Spectroscopy
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Multiple Quantum Spectroscopy
Coherences, of which transverse magnetization is
one example, can be classified according to a
coherence order, p, which is an integer taking
values 0, 1, 2 ... Single quantum coherence
has p 1, double has p 2 and so on
z-magnetization, "zz" terms and zero-quantum
coherence have p 0 Number of transverse terms
in a product operator if we consider a pulse
which causes a change in coherence order of Dp
then altering the phase of that pulse by an angle
f will result in the coherence acquiring a phase
label Dp f.
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Coherence Transfer Pathways
f1
f2
f3
f1
f2
f3
DQF-COSY
NOESY
The same pulsesequence is used for different
experiments Different coherence orders are
selected by a phasecycle
f1 f2 f3 receiver x IySy x -Iz-Sz x
-Iy-Sy x IySy -2IxSz-2SxIz 2IxSy-2SxIy 2IxSz2Sx
Iz x y -Ix-Sx y -Iz-Sz x -Iy-Sy
x -Ix-Sx -2IySz-2SyIz 2IySx2SyIx -2IzSx-2SzIx -
x -x -Iy-Sy -x -Iz-Sz x -Iy-Sy
x -Iy-Sy 2IxSz2SxIz 2IxSy-2SxIy 2IxSz2SxIz x
-y IxSx -y -Iz-Sz x -Iy-Sy
x IxSx 2IySz2SyIz 2IySx2SyIx -2IzSx-2SzIx -x
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Axial peak suppression
Peaks at co-ordinates F1 0 and normal F2
frequency i.e. magnetization which has not
evolved during t1 and has no frequency
label. Common sources z Magnetisation during
evolution period Iz is made observable by
subsequent pulses longitudinal relaxation during
t1 or pulse imperfection / miscalibration to
suppress axial peaks select the pathway Dp
1 on the first pulse with two-step cycle 0,
180 on the first pulse and the receiver Make
sure there is transverse magnetisation
Gives a total 8 step phasecycle when added to
the 4 step coherence selection for NOESY f1 x
,-x, y, -y, -x, x, -y, y f2 2x, 2y, 2(-x),
2(-y) f3 x Rec 4x, 4(-x)
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Heteronuclear Experimentse.g 13C HMQC
separate coherence orders are assigned to the I
and S spins. DpS 1 for the first S pulse is
desired /- together with the receiver, can be
combined with a phasecycle for the second S pulse
and EXORCYCLE for the 180.. 16 steps
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Problems with phasecycling
two major practical problems. The first is that
the need to complete the cycle imposes a minimum
time on the experiment. In two- and
higher-dimensional experiments this minimum
time can become excessively long, far longer than
would be needed to achieve the desired
signal-to-noise ratio. The second problem is
that phase cycling always relies on recording
all possible contributions and then cancelling
out the unwanted ones by combining subsequent
signals. It is a difference method. If the
spectrum has high dynamic range, or if
spectrometer stability is a problem, this
cancellation is less than perfect. Especially
when dealing with proton detected heteronuclear
experiments on natural abundance samples(1 13C),
or spectra with intense solvent resonances.
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Selection with pulsed field Gradients PFG
xy Magnetsation dephases under the influence of
a field gradient The Rate of dephasing is
proportional to the coherence order P (DQC twice
as fast as SQC, ZQC or z-Magnetisations does not
dephase) this is reversible and can be undone by
an appropriate PFG of opposite polarity by
applying gradient pulses of different strengths
or durations it is possible to refocus coherences
which have, for example, been changed from
single- to double-quantum by a pulse. Advantage
Not a difference method, selection in a single
scan, no need to complete a long and complex
phasecycle Disadvantage extra hardware required.
PFG probes etc
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Selection by refocusing The gradients must be
balanced
Selection by suppression, z-Filter The desired
signal is put along z and everything else is
purged with a gradient
The coherence selection is achieved with a
spoiler gradient during tm. Axial peak
suppression by phasecycling is still recommended,
though
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ge DQF COSY
Possible solution Incorporate gradients into echos
Problem Evolotion during gradients will give
phase distortions
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ge-HMQC
I shift evolution during G1 is refocussed by the
180 Only the S contribution needs to be
refocussed e.g for 13C HMQC G1/-2G2 Both
pathways are required for pure phase spectra
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ge-HSQC
gradient selection G1 /- gI/gS G2
zz filter Suppresses uncoupled magnetization
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Sensitivity enhancementse HSQC
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H2O suppressionWATERGATE
For H2O 0 deg rotation No refocussing Other spins
normal echo
Can be easily incorporated into echo or INEPT
elements
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The Full Montyct-ge HNCA