Equavilency between Mass Balance and qNMR metholologies

Currently, many front-line applied scientists are concerned about whether HPLC is comparable to the qNMR method in the quality control (QC) scenario. The HPLC method is a secondary analytical method for quantitative analysis, belonging to part of the mass balance method. The two types of analytical methods (HPLC vs qNMR) are clearly thought to be incommensurable.

The mass balance method is one of the most commonly used methods of measuring the purity of substances. Specifically, it applies a suite of analytical methods that includes multiple analytical techniques that complement each other. For example, HPLC separates analytes and provides purity measurement of a compound (some are organic) using a suitable detector. Thermogravimetric analysis (TGA) or gas chromatography assesses volatile impurities such as organic solvents. Karl Fischer directly analyzes moisture in the target analyte. The ash content of combustion samples (e.g., inorganic salts) measures the non-volatile residues. Through a series of measurements, it enables assignment of all impurities in the test sample. Weighing the value of the test sample and subtracting the sum of all impurities allows one to calculate the mass fraction (mg/mg; often representing purity or potency, %) of the target analyte in the test sample.

Based on 1H qNMR investigations performed at BIPM, mass balance approaches and qNMR methods using internal calibration methodology can achieve an equivalent level of precision and accuracy for purity assignment of an organic analyte. This single qNMR method directly allows for the precise and accurate measurements of the sample mass fraction, without need for impurity assignments. Compared to mass balance, the qNMR method enables a rapid purity determination for organic chemicals.

Reference/Book Chapter: Quantitative NMR in Qulality Control
in Quality Control of Chinese Medicines-Strategies and Methods