The WATERGATE (WATER suppression by GrAdient Tailored Excitation) scheme is one of the most popular methods to suppress the intense solvent resonance when ¹H spectra are recorded in H₂O solutions. This scheme is based on a single symmetrical spin-echo that combines selective 180º pulse and two symmetrical PFGs (see WATERGATE block).

Easy implementation on AVANCE spectrometer equipped with pulsed field gradients (PFGs).

The principle of WATERGATE ( 92JB661 and 93JMRA241-102 ) is to use a selective spin echo where a pair of identical PFGs surrounds a selective 180º pulse. The first PFG dephases all the resonances that have been excited by the first 90º pulse. Then, a 180º selective pulse reverse the sense of precession of all the spins of interest but not that of water. The second PFG refocuses the protons of interest whereas the water resonance dephases even further. The selective inversion element can be:A

• A symmetrical 3-9-19 pulse sequence 3a-t-9a-t-19a-t-19a-t-9a-t-3a, with 26a=180º degree. Additional suppression appears at different sidebands (1/t). Recently, a five-pulses element has been demonstrated to give improved selectivity under the same experimental conditions ( 98JMR125-132 ).
• A simple and very efficient method utilizes a non-selective 180º_x pulse flanked by two rectangular selective 90º_-x pulses, which compensate for the 180º rotation at the solvent frequency. Careful calibration of the duration and the phase relationship of the selective pulses are required.
• A water flip-back approach in which the initial hard 90º pulse is substituted by a 90º hard pulse followed by a selective 90º pulse on the water signal in order to restore the water magnetization to its equilibrium position at the start of the acquisition time.

The WATERGATE scheme is widely used for solvent suppression purposes in many multidimensional NMR experiments. It is usually incorporated as a building block just prior to acquisition.

WATERGATE experiments are usually acquired in a fully automated way. Minor changes from a predefined parameter set are required. The most important parameters to optimize are the interpulse delay in the DANTE sequences and the duration and the power level of the pulses involved in the inversion element.

For further details on practical implementation of water presaturation on AVANCE spectrometers see Tutorial: WATERGATE scheme

A conventional spectrum is obtained from which the intense solvent resonance has been efficiently suppresses in order to see smaller signals.

Solvent suppression schemes