Suppressing Undesired Signals
When working with a non-deuterated solvent, it is common to suppress its signal during acquisition, either by saturation or by a pulse sequence that does not excite it. The residual signal can still be a cause of problems. Correcting its phase may be impossible; its tails can alter the baseline; nearby peaks can be difficult to recognize.
This command operates after acquisition, during processing. It does not modify the experiment itself.
You can delete any peak, not only solvent peaks. The algorithm provided by iNMR performs a natural kind of suppression. The suppression is intentionally conservative. Only the frequency point corresponding to the peak maximum is removed, while the surrounding region is smoothed gradually rather than flattened. The result is so realistic that only close inspection can reveal the trick.
How to Suppress Peaks:
- To suppress the solvent peak at the transmitter frequency:
- Start from time domain. If you have already transformed the spectrum, choose File > Reload.
- Choose Process > Suppress.
- iNMR automatically inserts the transmitter frequency. Just click Suppress.
- To suppress a generic peak:
- Start from frequency domain. If you haven't transformed the spectrum yet, perform a Fourier Transform first.
- Right-click the unwanted peak and choose Suppress from the menu that pops up.
- You can suppress up to 4 different peaks in the same way. Only four suppression frequencies can be active at the same time. If you specify a fifth frequency, the fourth peak reappears.
- This simplified method removes the frequency of the peak maximum. To delete a generic frequency, you have to type it, in the dialog Process > Suppress, before performing the FT.
- To make the suppression milder or stronger:
- Choose File > Reload and Process > Suppress.
- As a rule, begin with the highest selectivity factor. It allows an effective suppression of the unwanted peaks, while preserving the rest of the spectrum almost intact.
- When the unwanted peak is broad, decrease the selectivity factor. You'll get a stronger suppression.
- To remove all suppression frequencies:
- Choose File > Reload and Process > Suppress.
- Empty the four cells.
- Click Suppress.
The suppression is never completely selective. The signals near the suppression point can be severely attenuated and their integrals are not reliable.
In the case of a 2-D or 3-D spectrum, you can only suppress the peaks along the direct dimension.
It is also possible to substitute a piece of the spectrum, with zeroes or with a fragment taken from another spectrum, using the console commands copy() and paste() (not to be confused with the namesake commands in the Edit menu).
Related Topics
Subtracting a Spectrum from Another One
Measuring the Area by Fitting to a Model