rd  - 1962 - ENC Conference

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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 o<strong>rd</strong>er 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 increase 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 techniques of measuring T1 and T 2 described in Fig. 1 would only be use- ful for a restricted range of relaxation times. A direct measurement 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 techniques 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 absorption curves from two polyurethane samples are compared with the T 2 decay curves. In the steady-state presentation, the signal from
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rd  - 1962 - ENC Conference

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