Signal Suppression

Three methods exist for suppressing unwanted signals. Presaturation of a signal excites a small region of the sample for a relatively long time, reducing the intensities of the net magnetization of signals in the region. If nuclei are exchanging between two environments that give rise to two different signals, and if one of these signals is presaturated and reduced in intensity, then the other signal will also be reduced. For instance, if presaturation is used to reduce the water signal in a protein sample, the amide proton signals will also be reduced, since the amide protons exchange with the water proton.

Binomial signal suppression experiments use non-uniform excitation of the spectrum to reduce the signal intensities of small spectral regions. The higher-order binomial experiments better suppress peaks at the expense of a more rolling baseline. The left half and right half of the spectrum differ in phase by 180°, and the spectrum may require lots of first-order phase correction. Suppression will occur at multiplets of the offset frequency. The 1331 binomial experiment is recommended for 1D signal suppressions.

If the T1 relaxation rates of the interesting and unwanted signals differ by at least 200%, then the unwanted signal can be suppressed by collecting data when the unwanted signal is relaxed and the interesting signals are excited. See also Measuring Relaxation Rates.

Presaturation of a signal:

1. Acquire and process a 1-pulse experiment.

2. Record the value of tof. Move the cursor near the signal to be presaturated and type nl movetof. Set dof to the new value of tof. Set tof to its origional value.

3. Adjust the following parameters:

dof=frequency to be presaturated. May be different from tof.

dpwr=5 to 49 WARNING!!! Do NOT set dpwr greater than 49!

dm='y'

dmm='c'

homo='y'

3. Acquire and process the experiment.

4. Repeat steps 2 and 3 until the signal is suppressed. Adjust dof or dpwr.

Binomial suppression:

1. Acquire and process a 1-pulse experiment.

2. Type binom

3. Adjust the following parameters:

pw=90° pulse width (p1=p2=p3=p4=0)

seq=1331

offset: For ON-resonance suppression (suppression at the center of the spectrum), set offset to the difference (in hertz) between the center of the spectrum and frequency at which maximum excitation occurs. For OFF-resonance suppression (suppression not a spectrum center, center is where maximum excitation occurs), set offset to -(difference between center and signal to be suppressed).

4. Acquire and process the experiment.

5. Adjust tof (on-resonance suppression) or offset (off-resonance suppression) to set the spectrometer frequency exactly on the signal to be suppressed.

Suppression based upon different realxation rates:

1. If the unwanted signals relax faster than the interesting signals:

A. Acquire and process a 1D spectrum.

B. Set alfa to 3 times the T1 of the unwanted signals, pw=90° pulse.

C. Acquire and process the experiment.

D. Adjust alfa for maximum suppression while minimizing attenuation of interesting signals.

2. If the unwanted signals relax slower than the interesting signals:

A. Acquire and process a 1D spectrum.

B. Set pw=90° pulse

p1=90° pulse

d2=3*T1 of interesting signals

C. Acquire and process the experiment.

D. Adjust d2 for maximum suppression while minimizing attenuation of interesting signals.