1. Acquire a Simple 1D Spectrum of Water without suppression to determine an initial o1 estimate
Purpose:
Identify the exact frequency of the water resonance and set the initial o1 value.
Steps:
- Create a new experiment and load standard 1D parameters:
- Use ‘new’ or ‘edc’ then ‘rpar PROTON’ to load a standard 1D 1H experiment.
- Check the box to set solvent, and choose the appropriate solvent, typically H2O+D2O.
- Select the radio button to Execute ‘getprosol’
- Prepare your sample: Insert into the magnet, lock, shim, and tune as usual.
- Set acquisition parameters:
- Excitation pulse width (us): p1 1
- Number of scans: ns 1
- Number of dummy scans: ds 0
- Receiver gain: rg 1
- Acquire the spectrum:
- Type: zg
- Process the data:
- Type: efp (Fourier transform)
- Type: apk (automatic phasing)
- Type: abs (automatic baseline correction)
- zoom in to the water peak (around 4.7 ppm)
Set o1 value:
- Move the cursor to the water peak center.
- Note the frequency in Hz (not ppm).
- Set o1 to this value: Type o1 and enter the frequency in Hz.
- Alternatively, use the “Set RF from cursor” function in the TopSpin GUI to automatically set o1 to the cursor position. The button to access this function looks like a downward pointing jagged arrow with no line under it. You can also access this function from the TopSpin command line with .seto123
2. Run a Water-Suppressed Experiment to fine-tune o1
Purpose:
Optimize o1 to minimize the water signal using a water suppression pulse sequence.
Steps:
- Create a new experiment an load standard 1D parameters for water suppression:
- Use ‘new’ or ‘edc’ then ‘rpar ZGPR’.
- Check the box to set solvent, and choose the appropriate solvent, typically H2O+D2O.
- Select the radio button to Execute ‘getprosol’
- set o1 to the value determined previously
- Number of scans: ns 1
- Number of dummy scans: ds 0
- Receiver gain: rg 1
- Check presaturation parameters:
- Power level: pl19 (start with 55 dB, don’t go below 40 dB. Remember dB is attenuation so lower dB means higher power).
- Presaturation delay: d1 (typically 2 s. Do not exceed 2 s on a Cryoprobe).
- Acquire spectrum:
- Type: zg
Method A: Using gs Mode
- Enter gs mode: Type gs in TopSpin.
- Select the Offset tab from the left panel.
- Adjust o1: Use the slider or enter values to adjust o1.
- Start with a precision of 1 and then go to a precision of 0.1
- Choose either the FID icon or the processed spectrum icon at the top.
- Monitor the water signal in real time; adjust o1 to minimize the integral of the FID or the water peak.
- Save and exit gs mode when finished.
Method B: Using popt
- Zoom in on the water peak
- Set the boundaries: Type dpl1
- Load the popt editor: Type popt
- Choose the Delete parameter button from the bottom of the window if the entry is not blank in the Group column.
- Choose the Add parameter button from the bottom of the window if no entries are available in the table.
- Populate the table:
- Optimize: Step by step
- Parameter: o1
- Optimum: MAGMIN
- VARMOD: LIN
- STARTVAL: subtract 10 from the o1 obtained from the water peak position above
- ENDVAL: add 10 to the o1 obtained from the water peak position above
- INC: 2
- Click the Start optimize button
- A dialog may ask “popt.protocol exists; Append or Overwrite? [a|y]. Choose y
- A dialog will show the number of experiments (eg 11) and the total experiment time (eg 33 sec) and ask if you wish to continue. Choose y if the length of time is what you expect.
- A dialog may ask if you wish to overwrite PROCNO 999. Choose no and select another procno.
- Inspect the results to identify the experiment with the best water suppression.
- Repeat popt again with a narrower range of o1 values and a smaller increment for more precision.
Updated 7/3/2025 KE
