Octad of standards for qNMR purity measurements
The BIPM in collaboration with NIM (China) and AIST/NMIJ (Japan) has published a new guidance document for the use of benzoic acid as an internal standard for the purity assignment of organic compounds by quantitative nuclear magnetic resonance (qNMR). This reference document together with seven others provides guidance for the use of a universal suite of eight higher order SI-traceable primary standards, the Octad, for the purity assignment of essentially any organic compound using qNMR in any solvent.
Benzoic acid has historically been used extensively as an internal standard in qNMR due initially to the ready availability of highly pure and stable acidimetric standards, which could be assigned mass fraction content property values. Recently, SI-traceable benzoic acid reference materials, specifically for use as qNMR internal standards, have been developed. This guidance document is intended to describe specific properties of benzoic acid and considerations that are relevant for performing accurate, SI-traceable, qNMR measurements. Representative examples are given of its use in different solvents. These examples include the sources and magnitude of the combined standard uncertainty that can be expected for highly accurate purity assignments of organic substances for use as primary reference materials for the measurement of analytes important to the food, clinical and environmental sectors among others.