Independent certification of a suite of qNMR internal standards: ensuring metrological traceability in a wide range of applications

The Brazilian National Metrology Institute produced a suite of certified reference materials (CRMs) intended as internal standards (ISs) for quantitative nuclear magnetic resonance (qNMR). Being a ratio primary method, the use of qNMR in organic chemistry has already crossed the borders of research laboratories, despite the cost of instrumentation. The International Bureau of Weights and Measures (BIPM) proposed eight potential qNMR ISs. Four candidate materials were selected for their solubility in various solvents and distinct chemical shifts, making them suitable for qNMR analysis of diverse analytes. The certification process compared orthogonal primary methods such as mass balance, qNMR, freezing-point depression, and coulometry, to ensure independent value assignment. Different approaches were compared to assess batch homogeneity and stability. While directly comparing the main compound's chromatographic area proved to be a quick and fit-for-purpose approach, the determination of individual impurities provided lower uncertainties but required more laborious work. CRM batches of maleic acid, dimethyl sulfone, potassium hydrogen phthalate, and dimethyl terephthalate were delivered with over 999.8 mg g^-1 purity and uncertainty in the range of 0.6 to 3 mg g^-1 (k = 2). The literature shows certification procedures for qNMR ISs whose traceability chain is exclusively based on qNMR measurements. As opposed to that, the methodology presented here provides robust certified values assigned by methods independent of qNMR, in accordance with BIPM recommendations and less prone to qNMR biases. The CRMs developed in this work have already been used for SI-traceable purity evaluation of compounds such as drugs and pesticides, by laboratories in Brazil and abroad.