Nuclear Spin Singlet Order Selection by Adiabatically Ramped RF Fields

We describe an NMR method to generate singlet order in spin pairs from longitudinal spin magnetization and suppress residual background signals. This method can also be used for generating and observing long-lived spin states. A singlet order selection (SOS) filter is proposed, which allows us to find signals of the spin pair of interest buried in a crowded NMR spectrum. Likewise, SOS filtering enables proton NMR measurements in H₂O without pulse sequences for solvent suppression. We demonstrate that the method works perfectly for both weakly and strongly coupled spin pairs. Furthermore, it can be combined with standard NMR pulse sequences: in this way, T₁- and T₂-relaxation times for spin pairs of interest can be measured. The power of the SOS-filter is demonstrated by relaxation studies in biomolecules.