z-Spectra of 23Na+ in stretched gels: Quantitative multiple quantum analysis

The 23Na NMR spectrum of NaCl in various stretched hydrogels displays a well-resolved triplet with the theoretically predicted relative intensities of the components of 3:4:3. Families of such spectra were obtained using partially-saturating radio-frequency (RF) radiation over a range of off-set frequencies; the resulting steady-state irradiation envelopes, or 'z-spectra', have the notable feature that marked suppression of the three peaks occurs when the irradiation is applied on any of them or exactly in the middle between the central peak and either of the two satellites. We present a quantum mechanical analysis that describes this phenomenon and show that it depends on double and triple quantum transitions. The physical-mathematical analysis is an extension of our quadrupolar case for HDO with 2H NMR. The experimental procedures and results have implications for enhancement of contrast in 23Na magnetic resonance imaging of heterogeneous systems using quadrupolar interactions.