Friday, November 28, 2008
The quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) sequence can be used to measure the NMR spectra quadrupolar I = n/2 nuclei in the solid state. This technique is essentially a T2 sequence where a series of echos are collected. The entire echo train represents the time domain data and is Fourier transformed to produce a frequency domain spectrum. The QCPMG spectrum consists of spikelets separated in frequency by the reciprocal of the time separation between the echos in the echo train. The intensity envelope of the spikelets mimics the static line shape. This is analogous to the rotational echoes in the FID's of MAS data and the associated spinning sidebands in the frequency domain MAS spectra. The QCPMG technique represents an improvement in sensitivity compared to a single conventional Hahn echo as the intensity is concentrated in the spikelets rather than spread across the entire frequency span of the spectrum. The figure below shows the pulse sequence, an echo train and a QCPMG 23Na spectrum of solid sodium sulfate at 4.7 Tesla. The 23Na Hahn echo spectrum is also ahown as a comparison to the QCPMG spectrum. The spectrum represents the central transition only. The satallite transitions are not visible.