CPMG is a NMR technique commonly employed to enhance the S/N of an NMR spectrum
CPMG is a very useful technique for SSNMR spectroscopists. It uses a train of \(\pi\) pulses to refocus inhomogeneous broadening of the nuclear spins. This is very beneficial and can be used to enhance S/N, measure diffusion, measure T2 processes, and reduce experimental time. CPMG was initally developed by Carr and Purcell, at Rutgers and Harvard, respectivley. Their work was added on to by Meiboom and Gill, resulting in a pulse sequence bearing each of their names.
Like the Hahn echo, a \(\pi\) pulse is placed after the last pulse in the NMR experiment, which refocuses the spins leading to echo formation. For CPMG echo train acquisition, M \(\pi\) pulses are applied every 2ntr (if the sample is spinning), resulting in M echoes. The number of echoes which can be acquired is directly related to T2 processes. Neglecting pulse imperfections, the echo tops will diminish in intensity due to coherence losses between spins, which is homogenous T2, as the \(\pi) pulses refocus the inhomogeneous T2 due to the varying magnetic field experienced by the sample.
Figure 15. CPMG Pulse Sequence
After acquisition, the issue becomes the processing of the data. There are two schools of thoughts in this. First, some people directly FT the data. This leads to a spiklet pattern in which the spikes are due to the echoes which are spaced at 1/2ntr. The spikelets map out the NMR spectrum. There are some debate as to whether these spikelet patterns are arbitrarily increasing the signal to noise. The other school of though is to stack the echoes on top of one another. This then adds all the signals together and one spikelet-less spectrum is obtained with the S/N increased by the number of echoes obtained.