Components: Relaxation Delay [d1]

While running repetitive transients (nt > 1, e.g.,) to obtain high signal-to-noise ratio in an NMR experiment, the time between each transient becomes important. This time, called the pulse repetition time (prt), also called recycle time should be long enough to allow complete relaxation of the nuclear spins between transients. Incomplete relaxation will lead to loss of signal and is one cause of inaccurate integrations. The pulse repetition time is given by the following equations.

prt = Pulse Width + Acquisition Time + Relaxation Delay or

prt = pw + at + d1

The relaxation time for a nucleus is called T₁ (longitudinal relaxation time) and if a 90 degree pulse is used to excite the spins (Figure II-2A), a pulse repetition time equal to or greater than 5xT₁ is required to have complete relaxation (see Figure II-5). If a pulse width less than 90 degree pulse width is used, the pulse repetition can be proportionally less. In the above equation, pw is in

microseconds whereas at and d1 are in seconds. Thus, pw can be ignored in the calculation of the pulse repetition time. In practice, on the Unity+300 and VXR-S NMR spectrometers, at will be a fixed parameter chosen for the resolution desired, and therefore, the user has the only option of setting the relaxation delay d1 to a proper value so that the pulse repetition time can be optimum.

For example, if T₁ is 0.5 sec, the optimum prt should be 2.5 sec for a 90 degree pulse used for excitation. The standard parameters for the Unity+300 and VXR-S 400 contain at set to 4.096 sec and 4.099 sec. Since these values are greater than 5xT₁, d1 could be set to 0 to meet the condition that prt be equal to or greater than 5xT_1. The standard parameters on the Unity+300 and VXR-S 400 NMR spectrometers for ¹H and ¹³C include 45 degree pulse width for excitation and the proper d1 to give a prt of 5.1 sec which will be optimum for most routine NMR runs, but exceptions are possible!