90 degree PW calibration
One of the most important parameters for obtaining ANY NMR spectrum is the correct pulsewidth for a 90 degree pulse at a given power level. We have learned in the lecture all of the ramifications of this parameter so now we can see how to experimentally determine it. On most modern (and well maintained) spectrometers this parameter does not change significantly provided the sample is not "lossy" and the probe is tuned properly. Assuming these two things to be true we can begin by noting the power level at which we wish to obtain the 90 degree pulse. In the case of the 300 MHz instrument we can use the default power of 57 dB and on the 500 MHz machine 60 dB.
1) Obtain a 1 scan spectrum of the doped Methyl Iodide sample with the automated 1H only procedure, display the spectrum and place the cursor over the central peak of the triplet as shown:
2) On the command line type nl movetof as shown:
On the 300 MHz instrument the tof (transmitter offset) will have been moved to a value of -854 which has the effect of placing the transmitter directly on the central peak of the triplet.
3) Click the Flags & Cond. Tab and deselect the autogain radio button as shown:
This step is neccessary as autogaining is not permitted in an arrayed experiment.
4) Create an array of the parameter pw by typing on the command line array as shown:
You will be prompted to input the parameter to be arrayed. Type pw on the command line and hit return. You will now be prompted to input the number of steps in the array as shown:
Enter 22 for the number of steps and hit return at which point you wil be prompted for the starting value for pw and the increment value as shown:
Enter 2 for each value and hit return. The parameter pw will then become an array of values as shown:
This indicates that 22 separate FID's will be acquired with the first having a pw = 2 microseconds and each subsequent FID will have a pw which is 2 microseconds greater up to a maximum of 44 microseconds. We are now set to begin.
5) Click the Acq & Obs Tab to review all of the parameters before clicking the green Start button as shown:
Halp! My imagers are broken!
Note that the box associated with the obs pulse is colored pink indicating that the parameter is arrayed. Once the Start button has been pushed the spectrometer will acquire, process and plot the array which should look as follows:
A more informative but less glitzy view is plotted and looks as follows:
From this display we can readily see the effect of increasing the pulsewidth as the peak goes first through a maximum (the 90 degree condition) to zero (the 180 degree condition) to inverted (the 270 degree condition) and back through zero (the 360degree condition). It is best to determine the 90 degree condition not as the first maximum but rather as 1/2 of the first null or 1/4 of the second null. A simple pulsewidth array contains a wealth of information about the probe and the relaxation behavior of the sample. Can you think of anything which could cause a 90 degree calibration to be erroneous? Is it better to measure a 90 degree pulse as 1/2 the 180 or as 1/4 of the 360?
6) Repeat the process (if time permits) by arraying pw around the 360 degree condition. Construct the array similar to the one shown below:
At which point does the signal go through the null? Is 1/4 of that value similar to 1/2 of the 180 value from the first array? Compare your value against the system value which is given by the number in the pi/2 pulse box on the Acq & Obs Tab.
7) If time permits you can calibrate the 90 degree condition for a different transmitter power level.
CAUTION anytime you are working with power levels be certain they are within acceptable ranges for the experiment in question.
Change the value in the power box on the Acq & Obs Tab from 57 to 55, effectively lowering the transmitter output by 2 dB. From the lecture on linear amplifier calculations can you tell what the new 90 degree pulse should be? Will it be shorter or longer? Try and determine the value.