Comparison of inversion‐recovery methods for measuring longitudinal relaxation rates

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Abstract
The most common experiment for determining T1 is the inversion‐recovery sequence: 180° ‐ τ −90°‐acquire. The advantage is a large range of peak intensity change (theoretically −1 to +1), and the penalty is the requirement for collecting many long‐τ spectra to establish precisely the equilibrium magnetization value, M(∞). A modification of inversion‐recovery: 180° ‐ τ −90° ‐acquire (+) ‐ wait −90° ‐ acquire(−) ‐ wait, offers the same range of intensity change (2 to 0) but relaxes the requirement for many long‐τ spectra, since M(∞) = 0. A more fundamental advantage is a reduction in the number of adjustable parameters in the fit, thereby making the detection and analysis of nonexponential relaxation more straightforward than for traditional inversion‐recovery data. Nonexponential behavior is fairly common in solid‐state NMR, and this “exponential decay to zero” approach is useful in cross‐polarization and single‐pulse modes.