Development and validation of a suite of standards for the purity assignment of organic compounds by quantitative NMR spectroscopy

The evaluation of seven internal standard reference materials (ISRMs) to act as a ‘universal’ SI-traceable calibrator suite for organic compound purity determination by quantitative nuclear magnetic resonance (qNMR) spectroscopy is described. The set of compounds demonstrated to constitute such a suite are: potassium hydrogen phthalate (KHP), maleic acid (MA), 3,5-bis-trifluoromethyl benzoic acid (BTFMBA), dimethyl sulfone (DMSO₂), dimethyl terephthalate (DMTP), 1,4-bis-trimethlsilyl benzene (BTMSB or BTMSB-d ₄) and perdeuterated sodium 3-trimethylsilyl-1-propanesulfonate (DSS-d ₆). The compounds were selected such that at least one ISRM should be suitable for use as the internal standard for the qNMR purity assignment of an organic compound soluble in a given deuterated solvent. They allow for the selection for use as the internal reference for quantitative integration from a set of simple, sharp NMR signals dispersed over the proton chemical shift range.

Optimized conditions for acquiring qNMR spectra were developed and are described, as well as the results of an extensive series of studies validating the use of the ISRM suite to assign mass fraction values in four representative solvents (D₂O, DMSO-d ₆, CD₃OD and CDCl₃). Proper use and application of these ISRMs result in standard uncertainties in the assigned values of the analyte of interest of the order of 1 mg g⁻¹ in optimal cases. These materials are of particular interest for the mass fraction purity determinations by qNMR of organic compound reference materials required as analyte specific calibrators to underpin the SI-traceability of the results for routine laboratory analysis based on techniques such as gas and liquid chromatography.