A spatial editing of NMR spectra by use of heteronuclear multiple spin echoes

Abstract

NMR mu ltiple spin echoes (MSEs), observed when the Hahn spin-echo pulse sequence 90°-7-90’ (1) is applied to samples with strong equilibrium polarizations (for example, 3He or water) in a linear field gradient (2-d)) may also be seen in heteronuclear experiments. This allows a novel form of spatial editing of heteronucleus NMR spectra, restricting signals to those from heteronuclei spatially coincident with high proton concentrations. The sequence 90°-7-90° normally produces a single spin echo at time 7 following the second pulse. However, mu ltiple spin echoes, occurring at times nr (n = 2, 3, 4, . . . ) following the second pulse, are observed when the same sequence is applied to samples with large equilibrium magnetizations in a linear field gradient, G . The echoes arise from the partial cancellation of the applied field gradient by the dipolar field generated by the z component of the nuclear magnet ism (5). Heteronuclear mu ltiple spin echoes ( HMSEs ) can be observed at times n ( yn / y x )7 for signals of dilute heteronuclei X in samples with high proton concentrations if the heteronucleus signal is measured following a 90°-r-90’ sequence applied to protons. F igure 1 shows the first two of a train of such echoes obtained for a solution of Na2HP04 in HZ0 using the sequence (I):