What is pulse width and what value should I use to get great NMR spectra?
Pulse width is the length of time that radiofrequency (RF) energy irradiates a sample in nuclear magnetic resonance (NMR) spectroscopy. It is a critical NMR acquisition parameter, as RF energy causes the longitudinal magnetization to point in a different direction. So, what does this actually mean?
We say that RF energy rotates, tips, or flips the longitudinal magnetization. As a result, we need to measure pulse width in degrees; a parameter that is also known as the flip/tip angle. To become an NMR ninja, you must learn how to select a pulse width that changes the angle by 90°. This is because a flip angle of 90° produces the maximum measurable signal, which is essential for great NMR spectra.
The most important part is choosing the length of time for an RF pulse that produces the maximum signal. This depends on the nucleus type, pulse power, probe tuning, and the sample itself. How can you find a 90 degree pulse for your sample? Read on.
Calibration of 90° Pulse Width or Flip Angle
This calibration is straightforward: Excite the nuclei with different pulse width values and collect the responses. Signal intensity is proportional to the sine of the flip angle. Below is the representation of measured signals for various pulse widths/flip angles.
Notice that the null is easier to determine than the peak. For this reason, pulse width corresponding to 360° is measured first, and the 90° pulse width is calculated by dividing the result by 4.
I Calibrated My Pulse Width, but My NMR Signal is Weak
A 90° pulse width should give the maximum signal. However, if you find the signal is weak, it is highly likely that you determined the null for 180° instead of 360°.
If your calibrated pulse width is small, you likely calibrated based on the null corresponding to 180°. If that is the case, you are working with a 450 pulse width, which explains the weak signal. Here are a couple of quick experiments that can be done to verify the calibration.
1) Double the value of the pulse width you obtained from your experiment and re-acquire the signal. If the signal is higher, it means you have fixed it and gotten your 90° pulse width value without having to redo the pulse width calibration experiment.
2) Run the experiment with 3 times the value of the pulse-width (270° pulse width). If your signals from 90° and 270° are equal in intensity but opposite in phase you have the right 90°.