Optimum data quality in many NMR experiments depends on properly adjusted pulse duration (or width). There is a linear relationship between pulse duration and magnetization flip angle so that once one knows the flip angle that results from a given pulse width one can then readily determine flip angles produced by other pulse widths. A typical value for the 90° pulse width is around 10 us. The 180° pulse width will be twice the value of the 90° pulse. One can observe all the pulse width values in an experiment by entering
dps
on the command line to graphically display the pulse sequence. To find the correct value for the 90° pulse width do the following:
Load a standard 1D parameter set. Set
pw=5
Collect and process a spectrum and adjust the spectrum phase so that all peaks are symmetric and all peak intensity is above the baseline (ie. do a normal phase adjustment)
Enter the following:
array (Enter pw for the parameter to be arrayed, 25 for the number of steps, 2 for the starting value, and 2 for the step increment)
d1=10
nt=1
go
After acquistion completion enter
wft
to process all the spectra in the array. Enter
dssh
to display all the spectra in the array. It's probably best to zoom in on a particular peak before doing this, otherwise the entire spectrum is shown as it varies with pulse width. The first place where the peaks are nulled out is the 180° pulse. Half of this value (i.e., where the peak intensity is maximized) is the value for the 90° pulse. Enter
da
to display the array of pw values. Observe the array of spectra and determine the array index for the first null (the array index is obtained by counting spectra from left to right). Look at the array of pulse width values and read the value associated with the array index of the first null, that value is the 180° pulse width. Divide the 180° pulse width by two to obtain the 90° pulse width.