The selection process starts with a cross-polarization step from 1
H to 13C to enhance sensitivity (point (a) in Fig. 1) followed by a
z-filter and a selective excitation step on carbon-13, consisting in
a non-selective 90 ‘‘flip-back” pulse followed by a selective 90
pulse applied at the chemical shift of interest. This allows us to
only select, for example, magnetization present in the (rigid and
mobile) EC domains of the sample (Fig. 1, panel (b)). It should be
noted that with this experiment one is limited to samples where
at least one of the different components has at least one carbon
signal which does not overlap, which can be used for selective exci-
tation. However, it might be possible to use other filter elements
prior to selective excitation to obtain non-overlapping signals in
the carbon spectrum albeit at the cost of further reducing the sen-
sitivity of the experiment. In the case of, for example, a controlled
release pellet sample with API, whose 13C NMR signals overlap
with those of EC, a T1-filter element (saturation pulse train and
short recovery) could be inserted at the beginning of the sequence,
since APIs are usually crystalline leading to longer spin–lattice
relaxation times than cellul