Phase Cycling and Gradient Pulses - The James Keeler Group

1 0 0 0 0
2 90 90 180 0
3 180 180 0 0
4 270 270 180 0
We can play one more trick with this phase cycle. As the third pulse is required
to achieve the transformation ∆p = –3 or +1 we can alter its phase by 180° and
compensate for this by shifting the receiver by 180° also. Doing this for steps 2
and 4 only gives
step 1st pulse 2nd pulse 3rd pulse receiver
1 0 0 0 0
2 90 90 0 180
3 180 180 0 0
4 270 270 0 180
This is exactly the cycle proposed in section 9.5.5.4.
9.5.9 Further examples
In this section we will use a shorthand to indicate the phases of the pulses and
the receiver. Rather than specifying the phase in degrees, the phases are
expressed as multiples of 90°. So, EXORCYCLE becomes 0 1 2 3 for the
180° pulse and 0 2 0 2 for the receiver.
9.5.9.1 Double quantum spectroscopy
A simple sequence for double quantum spectroscopy is shown below
τ
τ
t 1
t 2
9–30
2
1
0
–1
–2
Note that both pathways with p = ±1 during the spin echo and with p = ±2
during t 1
are retained. There are a number of possible phase cycles for this
experiment and, not surprisingly, they are essentially the same as those for DQF
COSY. If we regard the first three pulses as a unit, then they are required to
achieve the overall transformation ∆p = ±2, which is the same as that for the
first two pulses in the DQF COSY sequence. Thus the same cycle can be used
with these three pulses going 0 1 2 3 and the receiver going 0 2 0 2.
Alternatively the final pulse can be cycled 0 1 2 3 with the receiver going 0 3
2 1, as in section 9.5.5.4.
Both of these phase cycles can be extended by EXORCYCLE phase cycling
of the 180° pulse, resulting in a total of 16 steps.
9.5.9.2 NOESY
The pulse sequence for NOESY (with retention of absorption mode lineshapes)
is shown below