90° Pulse Calibration

A pulse is described by pw (pulse width) and tpwr (transmitter power). Many experiments need pw and tpwr to be set so that the pulse is a 90° pulse. A 90° pulse tips the equilibrium magnetization from the Z-axis by 90° to the XY-plane. The values of pw and tpwr that create a 90° pulse for the standard ethylbenzene sample are periodically updated and listed at the spectrometer. However, differences in samples and unexpected changes in the probe can change the values of pw and tpwr that give rise to a 90° pulse. Many experiments are designed to compensate for small pulse miscalibrations. However, large miscalibrations will attenuate spectrum intensities and can generate artifacts. It is recommended that you check the 90° pulse using the following procedure for all but the simplest experiments.

Determining the 1H 180° pulse width of the transmitter

  1. Using your sample or the standard sample, collect a 1-pulse experiment, with pw=2 and tpwr listed at the spectrometer. Type lb=0.5, set all other apodization functions to 'n', and process and phase the spectrum.
  2. Type gain='y'
  3. Set pw to your estimate for the 180° pulse width (twice the value of the 90° pulse width).
  4. Type ga vp=80 vs=90
  5. Determine if your value of pw is less than or greater than the 180° pulse width. Repeat steps 3 and 4 until you are satisfied that you have determined the 180° pulse width.
  6. Set pw to the 90° pulse width (half of the 180° pulse width), type gain='n', and type ga. Verify that the phase of the spectrum is OK. If the phase is incorrect, start again at step 1.

Determining the 13C 180° pulse width of the transmitter

  1. It is strongly recommended that you use the 13C labeled Ethanol sample for this calibration, since this sample has strong carbon signals that may be detected with a small number of transients (nt).
  2. Follow the steps for Determining the 1H 180° pulse width of the transmitter, using pw=6, lb=2.5 sw=19074.9 and tof=-3737.7 in step 1.

Determining the 1H 90° pulse width of the decoupler

  1. It is strongly recommended that you use the 90% CHCL3 in d6-acetone sample for this calibration, since this sample has strong carbon signals that may be detected with a small number of transients (nt).
  2. Click on main menu, setup, 13C Acetone to select carbon standard parameters.
  3. Type ppcal to set up the pp calibration experiment.
  4. Adjust the following parameters: pp=0
    pplvl=49 WARNING!!! Do NOT set pplvl larger than 49!
    d1=3.0
    d2=0.003185 for the acetone standard. Set to 1/(2*JCH)
    pw=13C 90° pulse width
    p1=13C 180° pulse width
    dm='yny'
    dmm='wcw'
    dof =-426.0
    nt=4 for the acetone standard
    gain='n'
  5. Type ga and phase the spectrum.
  6. Type gain='y'
  7. Set pp to your estimate of the decoupler 90° pulse width
  8. Type ga
  9. Repeat steps 6 and 7 until the intensity of the chloroform peak is minimum (zero intensity is ideal).
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Last updated: April 2nd, 1998
URL: http://nmr.chem.indiana.edu/NMRguide/operation/90calib.html
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