Characterisation of folic acid by mass balance versus quantitative NMR

Abstract

Two approaches were used to determine the mass fraction of a sample of folic acid, one indirect (mass balance) where the components other than folic acid were measured, and one direct, using quantitative nuclear magnetic resonance (qNMR). The procedures used for the mass balance were essentially those described in the European Pharmacopoeia monograph for folic acid. qNMR is a technique for quantifying the main component within an organic compound (Quantitative NMR (qNMR)—BIPM in https://www.bipm.org/en/bipm/chemistry/organic-analysis/qnmr/, Santosh and Raja in Trends Anal Chem 35:5–26, 2012), whereas mass balance quantifies its minor components. The results obtained with the two approaches are independent. In this study, their joint application generated information that was used in a more than complementary fashion. The mass balance characterisation, in particular the results obtained by liquid chromatography with ultraviolet detection (LC–UV), allowed to improve the accuracy of the qNMR result by subtracting the interfering contribution of a related structure impurity. On the other hand, qNMR enabled concluding that the non-uniform response of UV detection at a single wavelength was not critically biasing the LC–UV results. In the end, the mass fractions obtained by mass balance and qNMR were not that different. It is concluded that mass balance and qNMR may be not only complementary but also synergistic. It was also observed that access to monographs of the European Pharmacopoeia, when available, gives skilled analysts an important tool for improved substance characterisation.