Phase Cycling and Gradient Pulses - The James Keeler Group

t 1
τ mix t 2
1
0
–1
If we group the first two pulses together they are required to achieve the
transformation ∆p = 0 and this leads to a four step cycle in which the pulses go
0 1 2 3 and the receiver remains fixed as 0 0 0 0. In this experiment axial
peaks arise due to z-magnetization recovering during the mixing time, and this
cycle will not suppress these contributions. Thus we need to add axial peak
suppression, which is conveniently done by adding the simple cycle 0 2 on the
first pulse and the receiver. The final 8 step cycle is 1st pulse: 0 1 2 3 2 3
0 1, 2nd pulse: 0 1 2 3 0 1 2 3, 3rd pulse fixed, receiver: 0 0 0 0 2 2
2 2.
An alternative is to cycle the last pulse to select the pathway ∆p = –1, giving
the cycle 0 1 2 3 for the pulse and 0 1 2 3 for the receiver. Once again, this
does not discriminate against z-magnetization which recovers during the mixing
time, so a two step phase cycle to select axial peaks needs to be added.
9.5.9.3 Heteronuclear Experiments
The phase cycling for most heteronuclear experiments tends to be rather trivial
in that the usual requirement is simply to select that component which has been
transferred from one nucleus to another. We have already seen in section 9.2.8
that this is achieved by a 0 2 phase cycle on one of the pulses causing the
transfer accompanied by the same on the receiver i.e. a difference experiment.
The choice of which pulse to cycle depends more on practical considerations
than with any fundamental theoretical considerations.
The pulse sequence for HMQC, along with the CTP, is shown below
I
∆
∆
t 2
S
1
p I 0
–1
1
p S 0
–1
t 1
Note that separate coherence orders are assigned to the I and S spins.
Observable signals on the I spin must have p I
= –1 and p S
= 0 (any other value
of p S
would correspond to a heteronuclear multiple quantum coherence). Given
this constraint, and the fact that the I spin 180° pulse simply inverts the sign of
p I
, the only possible pathway on the I spins is that shown.
The S spin coherence order only changes when pulses are applied to those
spins. The first 90° S spin pulse generates p S
= ±1, just as before. As by this
point p I
= +1, the resulting coherences have p S
= +1, p I
= –1 (heteronuclear
zero-quantum) and p S
= +1, p I
= +1 (heteronuclear double-quantum). The I spin
9–31