rd  - 1962 - ENC Conference

SPIN-ECHO MEASUREMENT OF NMR RELAXATION TIMES
by
R. S. Codrington
Ridgefield Instrument Group
Ridgefield, Connecticut
In steady-state NMR studies the parameters of primary importance,
apart from the signal amplitude, are the linewidth of the resonance
and the frequency at which the resonance occurs in a given magnetic
field. It is wel~known, however, that in order to obtain meaning-
ful steady-state NMR spectra, the modulation frequency and sweep rate
must meet certain conditions determined by the relaxation times T 1
and T 2. These relaxation times, respectively called the spin-lattice
and spin-spin times are defined by the Bloch equations. 1 In Fig. i,
possible methods of measuring these times with steady-state equip-
ment are shown. For both measurements, it is assumed that the steady-
state apparatus has been set to observe the peak of the absorption
curve. Inherent in the Bloch formulation is the assumption that when
a sample is placed in a steady-state apparatus, the signal will in-
crease exponentially with a characteristic time T I. The assumption
is also made that if the r.f. driving voltage is suddenly shut off
while a signal is being observed, the signal will decay exponentially
with a characteristic time T2. The simple assumptions of Bloch are
valid for NMR spectra of most liquids but they do not always apply to
the NMR spectra of complex liquids, semi-solids and solids in which
T2 decays with nonexponential character may be observed. The tech-
niques of measuring T1 and T 2 described in Fig. 1 would only be use-
ful for a restricted range of relaxation times. A direct measure-
ment of T 1 and T 2 over a much wider range can be achieved with spin-
echo techniques which are based on the same principles as the tech-
niques of Fig. 1 but which utilize pulsed rather than steady-state
apparatus.
A great deal of information about the internal structure of materials
can be obtained from a detailed analysis of NMR spectra. Such
analysis usually includes studies of the linewidth or second moment
versus temperature and the behavior of T1 versus temperature. Al-
though in principle the steady-state apparatus can provide all the
information required for a complete NMR analysis, it is often found
to be easier and quicker to complement the information provided by
the steady-state apparatus with spin-echo data. An example of the
use of spin-echo in aiding the interpretation of steady-state NMR
signals is given in Fig. 2. In this figure the derivative absorp-
tion curves from two polyurethane samples are compared with the T 2
decay curves. In the steady-state presentation, the signal from