Fig. 4.

Signal-to-noise data of 10 mM sucrose, 3 mm NMR tube as a function of the NaCl concentration. S/N analysis of 1D ¹H spectra (left ordinate, ●) and HSQC trace (right ordinate, ■) for the anomeric proton (0.5 ppm noise region). The solid line depicts the predicted salt dependence for the 1D data based on Eq. (1). $\mathrm{S}/\mathrm{N}=\frac{1}{\sqrt{c{r}_{\mathrm{S}}^4[\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}]+R_{\mathrm{C}}(T_{\mathrm{C}}+T_{\mathrm{A}})}/f}$ where [NaCl] is the salt concentration [M] and f is a proportionality constant. The sample resistance is expressed as cr⁴[NaCl]. c incorporates several constants and $r_{\mathrm{S}}^4$ is the sample radius [mm]. The following values were obtained for the radical: $\sqrt{6.41\times {10}^{-7}{r}_{\mathrm{S}}^4[\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}]+7.43\times {10}^{-7}}$. If no salt is present the S/N is primarily determined by the second term in Eq. (1), while at 2 M NaCl it is assumed that the first term dominates, therefore the S/N curve may be approximated as k*[NaCl]^−0.5 where k = 792, for 3 mm tubes (dotted line). This fit provides a simple estimation of the S/N dependency at high salt concentrations. In either case, the conductivity is assumed to be proportional to the salt concentration which for high salt solutions is no longer accurate.