90pulse

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90-Degree Pulse Width Calibration

(A PDF version of this handout is available from the Handouts page or by clicking Here)

Many NMR experiments rely on 90 degree proton pulses for optimal performance (e.g. HSQC, NOESY). The default values that are determined by the facility may not be optimal for your sample because of differences in solvent, salt concentration, etc. Therefore, you will need to determine its value when you want to maximize the performance of certain experiments. This page will guide you through the procedure for calibrating a 90 degree pulse. Typically, you will look for the 360 pulse, which is a null spectrum, and then divide that value by 4 to get the 90.

picture of a 90degree pulse width dcalibration experiment

Figure 1. Example of a pulse width calibration experiment. The pulse width was arrayed from 2 microsecs to 40 microsecs in steps of 2. Positive maxima occur at 90 and 450 degrees. The spectra with almost no signal are the 180 and 360 degree pulse spectra. The 360-degree spectrum would be used to calculate the 90-degree pulse width.

Note: Bold text represent boxes you should click. italic text represent text you should type and hit RETURN.

Preliminaries:
- Acquire a single scan proton spectrum (nt=1): transform and phase. If S/N is poor, you can increase the number of scans (nt).
- Expand around any well resolved peak(s).
- Type d1=5. This will set the recycle delay to 5 seconds. Complete relaxation between scans is necessary to get accurate results.
Arraying pw to find 360-degree pulse:
- Typically, you will look for the 360-degree pulse and then divide the value by 4. The 360-degree pulse will give a null peak. The total area (integral) should be zero. This is more accurate than directly looking for the 90-degree pulse.
- Type pw90?. This will give the default 90 degree pulse and is a good place to start.
- Type array. A set of questions will come up and must be answered.
- Question#1: Parameter to be arrayed:
  - Type pw
- Question#2: Number of increments:
  - This is the total number of spectra or data points you want. Typically, type 10.
- Question#3: Enter Starting Value:
  - Start with (4*pw90 - 1). For a pw90 of 10, I would enter the value from (4*10-1) or 39.
- Quesiton#4: Enter Array increment:
  - Type 0.5. This will increase the pw by 0.5 microseconds between each array element.
- Type pw[1]=3. This will replace the first increment with the value of 3, which will be positive.
- Type gain='y'
- Type go
- Type ai
- Type wft dssh. If the spectra are too small to see, type vp=vp*2 dssh. Repeat command, if necessary. The first spectrum (leftmost spectrum) should be positive. If it is not positive, you should:
  - Type ds(1)
  - Type f full. Phase the spectrum so that the peaks are positive (aph usually works). Expand around well-resolved peaks.
- Type dssh dssl. Look for the spectrum with the minimum signal and note its number. If all the spectra are positive, the starting value for pw was too large. Rerun the array with a lower starting pw. Thus, in this case, I would start with 37 instead of 39. If, on the other hand, the second peak on are negative, the starting value of pw was too small. Start with a larger value.

Last Updated: February 17, 2009 - WebMaster
URL: http://www2.chemistry.msu.edu/facilities/nmr/90pulse.html

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