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Monday, February 24, 2014

Determining 90° and 180° Soft Pulses

Many modern NMR pulse sequences (e.g. the 1D gradient selective NOESY experiment) depend on shaped pulses for selective excitation or inversion of specific resonances.  Since the width of the excitation profile of a shaped pulse is determined by its duration, the pulse duration is chosen by the user for the selectivity needed.  The longer the pulse, the higher the degree of selectivity.   Some spectrometer software will calculate the pulse duration based on a selected region in a spectrum containing the desired resonance for excitation.  The calculation depends on an initial pulse calibration.  The standard calibration method for hard pulses involves incrementing the pulse duration at a fixed power level.  The 90° pulse is at the first maximum and the 180° pulse is at the first null.  Since the duration of a the selective pulse is fixed by the desired selectivity, the 90° and 180° pulses must be found by varying the pulse power rather than the pulse duration.  The figure below shows the calibration for three different 50 msec shaped pulses on a Bruker AVANCE spectrometer.


An on-resonance water signal was observed as a function of pulse power using a selective one-pulse sequence.  The scale is in units of decibels of attenuation.  Maximum power is at -6 dB so the scale goes from low power on the left to higher power on the right.  The 90° and 180° pulses are indicated with arrows.  The intensity profiles are not sinusoidal due to the logarithmic dB scale.  The 90° and 180° pulses are separated by 6 dB of attenuation as expected.

7 comments:

AJAY said...

Dear Glen Sir,
Can you tell me what is child pulse and how it generate,what is the problem created by it?
Thanx

Glenn Facey said...

AJAY,

I am not familiar with a "child pulse".

Glenn

Michal said...

AJAY,

Did you mean "CHIRP" pulse?

Michal

ravikanth reddy said...

Dear Glen sir,
good evening, can you pl describe pulse width calibration for STD NMR

Glenn Facey said...

Ravikanth,

Set up a one-pulse measurement with the soft pulse you would like to use for your STD experiment and proceed as I have done in this post.

Glenn

Anonymous said...

Hi Glen,

Great blog. I have been looking through a number of different posts on information for selective saturation of a peak. I see here that you mention the pulse duration (pw) is set by the selectivity, so you have to change the power.

For example, I want to selectively saturate one peak that is 30 Hz from an adjacent peak, so does that mean the pw has to be 1/30 Hz = 33 ms? How does one choose the power desired power level?

Glenn Facey said...

Anonymous,

If you want to saturate one resonance which is close to another, I would just use a simple presaturation sequence (zgpr on a Bruker spectrometer) with the o1 set to the resonance you want to saturate. You could use for example a 2 second or more rectangular presaturarion pulse (very selective) and what ever power is necessary to saturate the peak. You can find the best power level by starting at very low power and slowly increasing it until you get satisfactory saturatipn.

Glenn