Pulse width calibration

Calibration of 1H Carrier Frequency (tof)

To achieve the best water suppression tof has to be centered on the H2O resonance. To optimize tof:

1. Load a presat or water dataset and set the following parameters:
   nt=1 ss=2 satmode='y' ssfilter='n' av
2. Acquire a 1D spectrum with ga and expand the region around the H2O signal
3. Array the parameter satfrq around the current value of tof; a range of +/- 10 Hz with a step of 1-2 Hz will suffice
4. Acquire the arrayed spectra. Process and display with wft dssh dssl. The intensity of the water signal should fall, reach a minimum and then rise again. The value of satfreq at the minimum is the correct tof
5. Update the probefile with the new tof value, if necessary

<img src="%ATTACHURLPATH%/tof1.jpg" alt="tof1.jpg" width='556' height='231' /> 
 

It is important to use a single increment (nt=1), since phase cycling will likely produce additional water suppression and obscure the optimum satfreq. The solvent subtraction filter should obviously be disabled (ssfilter='n'). Absolute-value mode (av) is better here since you don't have to phase the spectrum.

The water signal profile can be indicative of shimming quality: good shimming will result in a symmetric profile, while bad shimming will lead to an asymmetric profile.

It may be tempting to run a small tip angle 1D and simply take the water peak maximum as the tof value. This method usually yields a different value and worse water suppression than the procedure above.

1H 90° Pulse Width Calibration

The 1H 90° pulse width is very sensitive to the salt content of the sample. Below are two methods for manual calibration.

1H 90° Pulse Width from 1D

To accurately calibrate 1H pw90 with a 1H 1D experiment you have to find the correct 1H pw360 and then take pw90 as pw360/4. Follow these steps:

1. Load a presat or water dataset and set the following parameters:
   nt=1 ss=2 satmode='y' ssfilter='n' ph
2. Acquire a 1D spectrum with ga and phase it to pure absorption. Expand the region of interest (either amide or aliphatic), excluding the water line
3. Array the parameter pw around the expected value of 4*pw90
4. Acquire the arrayed spectra. Process and display with wft dssh dssl. The intensity should be negative for pw less than pw360, go through zero at pw360 and become positive at pw greater than pw360.
5. Record a spectrum with pw set to pw180. If pw360 is determined correctly the result should be near zero.
6. Update the probefile with the new pw90 value, if necessary

Arraying pw directly with the aim of observing maximum signal at pw90 will not yield the true 90° pulse width. Instead it will determine the so-called "Ernst angle", which depends on the recycle delay d1 and the T1 relaxation time of the sample.

However, when working with an unknown sample it is a good idea to use the Ernst angle pulse as an estimate pw90 and then use it to properly set the array range in the pw360 method.

1H 90° Pulse Width from [15N, 1H]- or [13C, 1H]-HSQC

You can also calibrate 1H pw90 with an HSQC dataset (gNhsqc, gNfhsqc, gChsqc or gCfhsqc)

1. Load an HSQC dataset. If using gChsqc select the proper type: aliphatic, aromatic, etc.
2. Set nt=2 ss=4 ni=1 phase=1 calH=1.0
3. Acquire the first increment, process it with FT and phase to pure absorption. Expand the region of interest.
4. Array pw from 0 in steps of 1 us
5. Acquire arrayed spectra and process them with wft dssh dssl. The intensity maximum will correspond to pw90.
6. Update the probefile with the new pw value, if necessary

You can also set calH=2.0 - in this case the correct pw90 will correspond to zero intensity.

15N and 13C 90° Pulse Width Calibration

1. Load a [15N, 1H]-HSQC dataset (gNhsqc or gNfhsqc)
2. Set nt=2 ss=4 ni=1 phase=1 calN=1.0
3. Acquire the first increment, process it with FT and phase to pure absorption. Expand the region of interest.
4. Array pwN from around the expected 90° pulse width
5. Acquire arrayed spectra and process them with wft dssh dssl. The intensity maximum will correspond to the true 90° pulse width.
6. Update the probefile with the new pwN value, if necessary

You can also set calN=2.0 - in this case the correct pwN will correspond to zero intensity.

13C 90° Pulse Width Calibration

1. Load a [13C, 1H]-HSQC dataset (gChsqc or gCfhsqc). Select the proper type: aliphatic, aromatic, etc.
2. Set nt=2 ss=4 ni=1 phase=1 calC=1.0
3. Acquire the first increment, process it with FT and phase to pure absorption. Expand the region of interest.
4. Array pwC from around the expected 90° pulse width
5. Acquire arrayed spectra and process them with wft dssh dssl. The intensity maximum will correspond to he true 90° pulse width.
6. Update the probefile with the new pwC value, if necessary

You can also set calC=2.0 - in this case the correct pwC will correspond to zero intensity.

If looking at the methyl region you may want to change dof to point to 25 ppm instead of default 35 ppm.

You can use two macros cof and nof to set dof and dof2 precisely to a given ppm position. The syntax is like cof(H1 center in ppm, desired C13 center in ppm). For example, cof(4.73,35) if one wants to set the center of C15 as 35ppm. The same holds true for nof. cof and nof are stored in the vnmrsys/maclib.

15N and 13C 90° pulse widths are much less sensitive to salt concentration than 1H 90° pulse width.

Longer than usual pulse widths may indicate a poorly tuned and matched channel.

If the calibrated pulse widths are consistently calibrated long for different samples, it indicates a deteriorating probe.

-- Main.AlexEletski - 25 Feb 2008