Accurate and Precise External Calibration Enhances the Versatility of Quantitative NMR (qNMR)

Abstract

Quantitative ¹H nuclear magnetic resonance (qHNMR) is a highly regarded analytical methodology for purity determination as it balances metrological rigor, practicality, and versatility well. While ideal for intrinsically mass-limited samples, external calibration (EC) qHNMR is overshadowed by the prevalence of internal calibration and perceived rather than real practical limitations. To overcome this hurdle, this study applied the principle of reciprocity, certified reference materials (caffeine as analyte, dimethyl sulfone as calibrant), and a systematic evaluation of data acquisition workflows to extract key factors for the achievement of accuracy and precision in EC-qHNMR. Automatic calibration of the 90° pulse width (90 PW) formed the foundation for the principle of reciprocity and used optimized nutation experiments, showing good agreement with values derived from manual high-precision measurement of 360 PW. Employing the automatic 90 PW calibration, EC-qHNMR with automatic vs manual tuning and matching (T&M) yielded the certified purity value within 1% error. The timing of T&M (before vs after shimming) turned out to be critically important: sufficient time is required to achieve full-temperature equilibrium relative to thermal gradients in the air inside the probe and the sample. Achievable accuracy across different NMR solvents varies with differences in thermal conductivity and leads to 2% or greater errors. With matching solvents, the demonstrated accuracy of ∼1.0% underscores the feasibility of EC-qHNMR as a highly practical research tool.