Pulse Sequence Extras
This is a collection of miscellaneous items relating to pulse NMR.
B1 field strength is given by 1/(4 •pw90).
Note: dmf=1/pw90 ; Varian convention
Frequency Selective PulsesDesigning rectangular pulses to excite at one frequency and not another.
- The B1 field strength required for a selective 90° = delta v / Sqrt(15) where delta v is the difference between the excitation frequency and the null in Hz. For example to excite at Calpha and not at the carbonyl C's, the difference is 118 ppm (i.e. 176 - 58 ppm) or 118 * 201 = 23718 Hz on the 800, which would require a B1 field strength of 6127 Hz.
- The B1 field strength required for a selective 180° = delta v/ Sqrt(3). This requires a field strength of 13700 to flip C-alphas and not the CO's. Such a field strength requires a 90° pulse of 18.2 usec. Determine the field strength based on the requirement for a 90° then double for a selective 180°pulse.
- Because the power can only be set at integral values, it will be impossible to set the field strength precisely where needed. Use a reasonable guess for the power, then use pwco90x to calibrate the pulse, then set power and pwco90x to the best pair of values. For example, consider that we need a field strength corresponding to a 40.8 usec 90°pulse. At 52 dB we obtain a 90°pulse of 43.4 usec, while at 53 dB we get a 38.6 usec 90°. We choose 43.4 at 52 as the closest to what we need.
This is a standard Vnmrj tool used to view magnetization changes during a pulse, especially Mz (for 90° or 180°), or to view the excitation profile of a pulse.
Demonstration of how to use Pulsetool to determine the power of a selective rectangular pulse ("soft pulse").
- Type' pulsetool' in a terminal window.
- Specify the pulse shape by typing the name of the shape in the pulse window (e.g. hard.RF or seduce.RF)
- Set pw to some value (e.g. 0.04 for 40 usec pulse). Press Simulation button. Start with Mz =1. Place pulse on carrier with Freq = 0. Select B1 field to display Mz as a function of B1. Inspect plot to find best B1 to achieve desired pulse, 6.127 kHz (i.e. Mz = 0 for a 90, or Mz = -1 for a 180).
- Change Bloch simulator to Freq mode. Set B1max to the determined value (e.g. 6.127 kHz) Start Freq = -35 kHz to stop Freq = 35 kHz. Press Go. Z=0 at carrier and 1 at 23 kHz.
Basics of Obtaining Flat BaselinesGeneral users should not normally need to do this. It should already be set for you.
- Type dsp = 'y'. Check that oversampling is working by typing "oversamp?".
- Obtain a 1D spectrum. Phase by adjusting only the zero order phase correction, i.e p0.
- Type “calfa” to calculate the alfa delay time.
- rof2 should be 2.0 and alfa should be 6.0