Purge Pulses and Spin Locking Pulses

Both spin locking pulses and purge pulses are very useful components of multipulse NMR experiments. Spin locking pulses are long pulses applied at the same phase as the transverse magnetization. While being applied, the magnetization is polarized along the static field of the spin locking pulse, B₁, in the rotating frame. The magnetization is therefore locked to the axis of the applied pulse in much the same way that an equilibrium magnetization vector is locked to the static magnetic field, B_o. Purge pulses are long pulses applied at a phase 90° from the transverse magnetization. While the pulse is being applied, the transverse magnetization precesses about the static field of the pulse, B₁, exactly like the way transverse magnetization precesses about B_o during a delay. If the pulse is long enough, the magnetization will dephase as a result of the B₁ inhomogeneity of the rf pulse and be lost.

A long high power pulse can behave as both a spin locking pulse and a purge pulse as demonstrated in the vector diagram below. Imagine a spectrum consisting of two singlets. If the transmitter is set to the frequency of one of the singlets and a 90°_x pulse is applied, both magnetization vectors are rotated to the -y axis. During a delay equal to one quarter of the reciprocal frequency difference between the singlets, the "on resonance" singlet will remain stationary while the "off resonance" singlet will rotate by 90° onto the x axis. If a long high power pulse is now applied along the y axis, it will behave as a spin locking pulse for the "on resonance" singlet and a purge pulse for the "off resonance" singlet. An example of this is shown in the figure below for a sample of methylene chloride and chloroform where the transmitter was set on the methylene chloride resonance. The top trace represents a simple one pulse measurement. The spectrum in the bottom trace was collected by applying a 90°_x pulse followed by a delay equal to one quarter of the reciprocal frequency difference between the methylene chloride and chloroform. A 1 msec y pulse was then applied at the same power level as the 90° pulse followed by detection. One can see that the resonance of methylene chloride is unaffected compared to the one pulse measurement while that of the chloroform has been completely suppressed.