Solvent Suppression

Numerous solvent suppression methods have been developed. The more useful methods that have been implemented on the SSPPS spectrometers are described below.

Presaturation

Presaturation consists of applying a long, low power pulse during the relaxation delay at the frequency of the signal to be supressed. Such a pulse will saturate the unwanted resonance and greatly reduce the intensity of the signal.

Setup

Record a 1D 1H spectrum without solvent suppression ("rpar UCSD_PROTON", "gpro", "rga", "zg") and process the spectrum ("ft", "apk", "abs"). Zoom in on the peak you want to suppress so that you can see the top of the peak. Click on the carrier frequency definition icon (the bent red arrow near the top right) then select the top of the peak with the cursor. In the dialog box that pops up click the "O1" button. Note the value of "O1" that you just defined.

Read in a parameter set that uses presaturation, "rpar UCSD_PROTON_PRESAT" for example. Set the default pulse lengths and power levels ("gpro") and set the carrier frequency ("O1") to the value measured previously in the spectrum without solvent suppression. Optimize the receiver gain ("rga"), which should be much larger than for the spectrum without presaturation, and record your spectrum ("zg"). The spectrum is processed in the usual way ("ft", "apk", "abs").

Optimisation

Good lineshape is essential for efficient solvent suppression by presaturation. It is worth spending a a little extra time shimming to get good lineshape and thus suppress more of the unwanted signal.

Typically, the carrier frequency ("O1") needs to be adjusted slightly to obtain the best water suppression. The easiest way to do this is to use the "gs" mode to modify the carrier frequency while monitoring the fid or transformed spectrum. You may also want to adjust the presaturation power level ("PL9").

Jump-return

Presaturation is a robust and easy to use solvent suppression technique but suffers from the fact that signals from exchangeable protons, such as hydroxyls and amines, are attenuated as well as the solvent signal. The jump return technique avoids this drawback by generating a non-linear excitation profile where the peaks on one side of the supressed signal are positive and those on the other are negative. Signals close to the center of the spectrum are greatly reduced and pass through zero intensity at the carrier frequency. This technique is particularly useful for nucleic acids which have exchangeable signals far from the solvent.

Setup

Record a 1D 1H spectrum without solvent suppression ("rpar UCSD_PROTON", "gpro", "rga", "zg") and process the spectrum ("ft", "apk", "abs"). Note the frequency in hertz of the peak you want to suppress.

Read in the parameters for the jump-return experiment ("rpar UCSD_PROTON_JR"), update the pulse lengths and power levels ("gpro"), enter the frequency of the peak you want to suppress ("O1"), optimize the receiver gain ("rga") and acquire a spectrum ("zg"). Transform the spectrum ("ft") and phase it manually so that peaks on one side of the suppressed solvent signal are positive and those on the other are negative. The baseline is likely to be very distorted at the center.

Excitation sculpting

The presaturation and especially jump-return techniques can produce spectra with large baseline distortions. The excitation scultping method, which makes use of pulsed field gradients, generally produces much flatter baselines.

Setup

Record a 1D 1H spectrum without solvent suppression ("rpar UCSD_PROTON", "gpro", "rga", "zg") and process it ("ft", "apk", "abs"). Note the frequency in hertz of the peak you want to suppress.

Read in the parameters for an excitation sculpting experiment (e.g. "rpar UCSD_PROTON_ES"), update the pulse widths and power levels ("gpro"), set the carrier frequency to the value measured in the spectrum without solvent suppression ("O1"), optimize the receiver gain ("rga") and collect a spectrum ("zg"). The spectrum is processed in the usual way ("ft", "apk", "abs").

Optimization

The parameter that will have most affect on an excitation sculpting spectrum is the power level for the shaped pulses ("SP1"). Other parameters that can be optimized are the carrier frequency ("O1"), and the presaturation power level ("PL9", the excitation sculpting experiments implemented on the SSPPS spectrometers include a small amount of presaturation during the relaxation delay). These parameters are most easily modified by using "gs" mode.

Multiple solvent suppression

It is not uncommon to want to suppress more than one signal. Samples dissolved in methanol or mixed solvents can benefit from suppression of multiple resonances. Suppression of two solvent signals can be achieved with the excitation sculpting sequences implemented on the SSPPS spectrometers. Presaturation during the relaxation delay is used to suppress one signal while the excitation sculpting module supresses another.

Setup

Record a 1D 1H spectrum without solvent suppression ("rpar UCSD_PROTON", "gpro", "rga", "zg") and process it ("ft", "apk", "abs"). Note the frequency in hertz of the peak that you want to suppress that is closest to the center of the spectrum. Measure the distance in hertz from this peak to the other peak that you want to suppress.

Read in the parameters for an excitation sculpting experiment (e.g. "rpar UCSD_PROTON_ES") and update the pulse widths and power levels ("gpro"). Set the carrier frequency ("O1") to the value of the more central peak measured in the spectrum without solvent suppression. Set the offset of the shaped pulses ("SPOFFS1") to the value measured for the distance to the second peak. Note that the offset is negative for peaks upfield (smaller ppm) of the carrier frequency. Optimize the receiver gain ("rga") and acquire a spectrum ("zg"). The spectrum is processsed in the usual way ("ft", "apk", "abs").

Optimization

To optimize suppression of the two unwanted peaks use "gs" mode to adjust the presaturation power ("PL9") and the shaped pulse power ("SP1"). The presaturation power affects suppression of the central peak, while the shaped pulse power affects suppression of the other peak.

Suppressing more than two peaks

It is theroretically possible to suppress more than two resonances with the WET pulse sequence but in practice I have had limited success. If you want to try it please contact us.

Written by Brendan Duggan. Last modified 2015-Feb-2

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