Improved NMR-based diffusion measurements for inorganic ions

Abstract

Advances in probe hardware that can produce strong field gradients have expanded the applicability of NMR-based diffusion measurements for various inorganic ions (e.g., Na⁺, K⁺, Li⁺, Mg²⁺, Cl⁻, and SO₄²⁻), thereby facilitating a wide range of chemical research. However, as we demonstrate in this paper, the commonly used NMR pulse sequences for diffusion measurements are not optimally suited for inorganic ions. For many inorganic ions, their NMR-active nuclei are quadrupolar nuclei with small gyromagnetic ratios and rapid longitudinal relaxation. These properties inherently reduce sensitivity, particularly when magnetization must be stored along the longitudinal axis, as required in stimulated-echo schemes. Strong gradients also demand efficient suppression of eddy current effects. Using ³⁹K and ²⁵Mg diffusion NMR experiments on DNA solutions, we demonstrate that the bipolar-pair (BPP) spin-echo method provides more than a 2-fold improvement in sensitivity compared to the BPP stimulated-echo method. While both methods yield consistent diffusion coefficients and achieve equally effective suppression of eddy currents, the BPP spin-echo method provides notably improved precision due to its higher signal-to-noise ratio in the NMR spectra. When strong gradients are available, the BPP spin-echo method is a more robust and sensitive option for diffusion NMR of inorganic ions, reducing the measurement time by a factor of 4 compared to the stimulated-echo method.