Increased 29Si NMR sensitivity in glasses with a Carr–Purcell–Meiboom–Gill echotrain

The use of a Carr–Purcell–Meiboom–Gill (CPMG) echotrain to increase the 29Si NMR sensitivity in glasses was investigated. The echo intensity decay follows a stretched exponential behavior M(t) = M0 exp[−(t/t2)β] with values for the exponent β in the range of 0.41–0.65. The signal to noise in the spectra can be increased by a factor of up to 4 by taking the weighted sum of the spectra obtained from the individual echoes. However, differential T2 relaxation for the different Qn species is observed, with a shorter relaxation time for Q4 than Q3. Thus, summing of the echoes leads to distorted spectral intensities and quantitative information can no longer be obtained from the spectra. To circumvent the problem with differential T2 relaxation, an alternative approach is developed in which the spectral intensity for each chemical shift value is determined from the stretched exponential fit to its echo decay. With this approach, the sensitivity can be increased by a factor of up to 2.4, while quantitative information can still be obtained from the spectra. The increased sensitivity permitted the detection of five-coordinated silicon in a potassium silicate glass with natural 29Si abundance at ambient pressure.