¹H 90 degree pulse width calibration

Due to incomplete relaxation of the spins with a short recycle delay, the signal often does not maximize with a 90 degree flip angle. Additionally, in a sample with a strong solvent signal (such as water), a single 90 degree pulse typically causes the receiver to overflow. It is common to calibrate a 180 or 360 degree pulse and back-calculate the 90 degree pulse width pw (pw=pw180/2 or pw=pw360/4).

1H pulse width is often most sensitive to solvent and temperature while the X-channel nucleus is not as much. Regardless, the 1H channel (or X-channel if used) must be tuned for shortest pulse or for the use of the default setting in a pulse width sensitive experiment.

Detailed 1H pulse width calibration procedure

In the following we assume the power level desired is tpwr=57. Change it accordingly if desired.

• Good 1H tuning is required for short 1H pulse. Make sure the 1H channel is well tuned.
• In a typical 1H one-pulse sequence (s2pul with p1=0), set tpwr=57 d1=3 at=1 lb=1 nt=1 gain=30 (reduce if overflows). Collect a 1H spectrum. Phase the spectrum. Expand around a strong peak near the center of the spectrum
• Type array
• Enter pw, initial value: 0, number of array elements: 30, stepsize 2. This sets up an arrayed experiment using pw from 0 to 60 usec with a 2 usec stepsize. Type go.
• As the experiment runs, type wft dssh dssl to process and display arrayed spectra
• Change vp (i.e. vp=50 orvp=vp+50) to move the base of the spectra up to the middle of the window.
• Change vs (i.e. vs=vs*0.5 or vs=vs*2) to rescale the peaks to fit window.
• Type wft dssh dssl to redisplay array after vs or vp change. The arrayed spectra should look like a sine-wave, as the spins goes from 0, 90, 180 to 360 degree rotation. Focus on where the peaks goes from negative to positive. The transition point with minimal signal signal (called a null) corresponds to the 360 degree pulse.
• Once the array passes through the 360 degree transition, you can stop the experiment by typing aa.
• Type da and Click Process->Text Output to see the array elements. See which pw value roughly corresponds to the 360 degree pulse.
• Type array again to re-array pw with a smaller stepsize: 20 steps, starting with a value a few usec below the rough 360 value found above. Recollect the arrayed data and find the 360 degree pulse width (pw360)
• Calculate the 90 degree pulse width: pw=pw360/4. This is the 90 degree pulse width at tpwr=57.

Quick Calibration if good estimate is known

Often you could simply array pw around an estimated value with a smaller number of steps and stepsize. Read procedure above for processing, display and calculation.

• Assuming pw ~ 9 at tpwr=57, pw360 should be ~ 36.
• Set tpwr=57 d1=3 at=1 lb=1 nt=1 gain=30 (reduce if overflows). Collect a 1H spectrum. Phase the spectrum. Expand around a strong peak near the center of the spectrum
• Array pw starting at ~32, with 20 steps and 0.5 stepsize. Look for the 360 degree null signal point where the peaks go from negative to positive. Calculate pw=pw360/4.

You can also type: pw=9,35,36,37 to set up a pw array for quick checking of pw360.

Example

Array pw, starting from 2 with 16 steps and a stepsize of 2. Estimated pw360 ~ 32, giving pw ~ 8.

H.Zhou updated Oct 2010