Coupling Supercritical Fluid Chromatography with Internal Standard Corrected Quantitative NMR for Rapid and Accurate Purity Assessment

Purity assessment is critical for the quality control of active pharmaceutical ingredients (APIs). However, there is a lack of rapid and accurate purity assessment methods that operate independently of target-specific certified reference materials (CRMs). Herein, we proposed a traceable quantification method, named SFC-ISC-qNMR, for purity assessment and quantitative determination of low-purity APIs, which was accomplished by tracing the test solution to the reference solution via supercritical fluid chromatography (SFC) collection with internal standard-correction (ISC), followed by an SI-traceable quantitative nuclear magnetic resonance (qNMR) analysis of the reference solution (SI = International System of Units). Digitoxin, a widely used cardiac glycoside in the treatment of heart failure and arrhythmias, was used as a model compound. By employing a mobile phase of supercritical CO₂ modified with 24%-26% methanol, digitoxin with a purity exceeding 99.87% was isolated and collected. A national CRM of ethyl <i>para</i>-hydroxybenzoate was used as the IS for digitoxin. The SFC-ISC reference solution can be prepared in 15 min without the need for precise weighing, simply by reconstitution of the SFC collected high-purity analyte in deuterated methanol containing IS. The feasibility and reliability of the SFC-ISC-qNMR method were exhibited through the quantitative determination of digitoxin in chemical mixtures. The purity of a commercially available digitoxin reagent was determined to be 94.17% ± 0.15%, which was consistent with high-resolution qNMR analysis. The contents of spiked digitoxin in a digoxin analytical standard were measured at 1.06%, 1.37%, 2.39%, and 4.43%, respectively, showing high consistency with the gravimetric value with a bias less than 0.13%. In contrast, direct qNMR with deconvolution and HPLC analysis failed to provide accurate measurements. These results highlight the applicability of the SFC-ISC-qNMR method for drug evaluation and research, demonstrating its significant potential for the purity assessment of APIs.