Transcript Coherence Selection: Phase Cycling and Gradient Pulses

Coherence Selection:
Phase Cycling and
Gradient Pulses
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
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
Refocussing pulses
F
x
y
-x
-y
rec
y
–y
y
-y
EXORCYCLE
Removes the effects of an imperfect refocussing pulse
The receiver only follows the signal that has been properly inverted
Difference Spectroscopy
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.
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: Iy+Sy
Iy+Sy
-2IxSz-2SxIz
x: -Iz-Sz
2IxSy-2SxIy
x: -Iy-Sy
2IxSz+2SxIz
x
x
y: -Ix-Sx
-Ix-Sx
-2IySz-2SyIz
y: -Iz-Sz
2IySx+2SyIx
x: -Iy-Sy
-2IzSx-2SzIx
x
-x
-x: -Iy-Sy
-Iy-Sy
+2IxSz+2SxIz
-x: -Iz-Sz
2IxSy-2SxIy
x: -Iy-Sy
2IxSz+2SxIz
x
x
-y: Ix+Sx
Ix+Sx
+2IySz+2SyIz
-y: -Iz-Sz
2IySx+2SyIx
x: -Iy-Sy
-2IzSx-2SzIx
x
-x
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 Gives a total 8 step phasecycle
when added to the 4 step
imperfection / miscalibration
coherence selection for NOESY
to suppress axial peaks:
f1: x ,-x, y, -y, -x, x, -y, y
select the pathway Dp = ±1
f2: 2x, 2y, 2(-x), 2(-y)
on the first pulse with two-step cycle
f3 :x
0°, 180° on the first pulse and the
Rec: 4x, 4(-x)
receiver
Make sure there is transverse
Heteronuclear Experiments
e.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
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.
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…
Selection by refocusing
The gradients must be balanced
Selection by suppression, zFilter
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
ge DQF COSY
Possible solution
Problem
Evolotion during
gradients will
give phase
distortions
Incorporate gradients into echos
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
ge-HSQC
zz filter
Suppresses uncoupled
magnetization
gradient selection
G1= +/- gI/gS G2
Sensitivity enhancement
se – HSQC
H2O suppression
WATERGATE
For H2O 0 deg rotation
No refocussing
Other spins
normal echo
Can be easily incorporated into
echo or INEPT elements
The ‘Full Monty’
ct-ge HNCA