Phase Cycling and Gradient Pulses - The James Keeler Group

lineshapes can be obtained. Also, both in generating the double quantum
coherence, and in reconverting it to observable magnetization, all possible
pathways have been retained. If we do not do this, signal intensity is lost.
One way of viewing this sequence is to group the first two pulses together
and view them as achieving the transformation 0 → ±2 i.e.
∆p = ±2. This is exactly the problem considered in section 9.5.5.2, where we
saw that a suitable four step cycle is for the first two pulses to go 0°, 90°, 180°,
270° and the receiver to go 0°, 180°, 0°, 180°. This unambiguously selects p =
±2 just before the last pulse, so phase cycling of the last pulse is not required
(see section 9.5.5.3).
An alternative view is to say that as only p = –1 is observable, selecting the
transformation ∆p = +1 and –3 on the last pulse will be equivalent to selecting p
= ±2 during the period just before the last pulse. Since the first pulse can only
generate p = ±1 (present during t 1
), the selection of ∆p = +1 and –3 on the last
pulse is sufficient to define the CTP completely.
A four step cycle to select ∆p = +1 involves the pulse going 0°, 90°, 180°,
270° and the receiver going 0°, 270°, 180°, 90°. As this cycle has four steps is
automatically also selects ∆p = –3, just as required.
The first of these cycles also selects ∆p = ±6 for the first two pulses i.e.
filtration through six-quantum coherence; normally, we can safely ignore the
possibility of such high-order coherences. The second of the cycles also selects
∆p = +5 and ∆p = –7 on the last pulse; again, these transfers involve such high
orders of multiple quantum that they can be ignored.
9.5.6 Axial peak suppression
Peaks are sometimes seen in two-dimensional spectra at co-ordinates F 1
= 0 and
F 2
= frequencies corresponding to the usual peaks in the spectrum. The
interpretation of the appearance of these peaks is that they arise from
magnetization which has not evolved during t 1
and so has not acquired a
frequency label.
A common source of axial peaks is magnetization which recovers due to
longitudinal relaxation during t 1
. Subsequent pulses make this magnetization
observable, but it has no frequency label and so appears at
F 1
= 0. Another source of axial peaks is when, due to pulse imperfections, not
all of the original equilibrium magnetization is moved into the transverse plane
by the first pulse. The residual longitudinal magnetization can be made
observable by subsequent pulses and hence give rise to axial peaks.
A simple way of suppressing axial peaks is to select the pathway ∆p = ±1 on
the first pulse; this ensures that all signals arise from the first pulse. A two-step
cycle in which the first pulse goes 0°, 180° and the receiver goes 0°, 180°
selects ∆p = ±1. It may be that the other phase cycling used in the sequence
will also reject axial peaks so that it is not necessary to add an explicit axial
peak suppression steps. Adding a two-step cycle for axial peak suppression
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