Refining Universal Procedures for Ammonium Quantification via Rapid ¹H NMR Analysis for Dinitrogen Reduction Studies

As research on sustainable ammonia synthesis via electrochemical and photochemical N2 reduction progresses to include a wider variety of aqueous and aprotic electrolytes, 1H NMR spectroscopy is increasingly adopted as a means for ammonium quantification. However, this method is highly sensitive to experimental parameters, as demonstrated herein using a highly versatile and robust NMR pulse program. We demonstrate the sensitivity of the measurement to the final pH of the analyzed solution and identify a [H+] concentration range enabling robust quantification. We compare direct quantification versus calibration approaches to show that the former is highly sensitive to spin relaxation effects and identify the latter as the most reliable approach. This method, when optimized, enables direct, rapid quantification of both 14NH4+ and 15NH4+ within 12–22 min. The limit of detection of 5–10 μM, depending on the solvent, which meets the needs of current electrochemical and photochemical N2 reduction research.