90° Pulse Calibration

A pulse is described by pw (pulse width) and tpwr (transmitter power). Many experiments (especially 2D experiments) need pw and tpwr to be set so that the pulse is precisely 90°. 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 toluene in CDCl3 sample are periodically updated and listed at the spectrometer. However, differences in samples, solvents 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 90° pulse width of the transmitter

1. Using your sample, collect a 1-pulse experiment, with pw equal to the 90° pulse width (pw90) 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. Type array then answer the prompts:
1. Parameter to be arrayed: pw
2. Number of steps in array: 4
3. Starting value: (whatever pw90 is)
4. Array increment: (whatever pw90 is)
4. Type ai ga and hit Enter
5. Type vp=80 vs=90 dssh
6. Determine if your value of the last pw is less than or greater than the 360° pulse width. You should see 4 spectra, the first one (90°) should point up, the second (180°) should be weak and dispersive (but possible slightly positive), the third (270°) should point down, and the last (360°) should again be weak and dispersive.  If the last one is still pointing down, your pw is too short, if it's pointing up, pw was too long.  Repeat steps 3, 4, and 5 until you are satisfied that you have determined the correct 360° pulse width.
7. Set pw to the 90° pulse width (1/4 of the 360° pulse width), type gain='n', and type ga. Verify that the phase of the spectrum can be adjusted correctly. If it cannot, start again at step 1.
   

Determining the ¹³C 360° 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 360° pulse width of the transmitter, using 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*J_CH)
   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).