Quantitative Nuclear Magnetic Resonance (qNMR) Spectroscopy: A Comprehensive Review of Principles, Methodologies, and Multidisciplinary Applications

Quantitative Nuclear Magnetic Resonance (qNMR) spectroscopy has undergone a transformative evolution from a relative analytical tool to a well-established primary method for absolute quantification. Its fundamental principle—the direct proportionality between signal intensity and the number of nuclei—provides unique advantages over chromatographic techniques, including the absence of need for identical reference standards, inherent structural elucidation capability, and non-destructive analysis. This comprehensive review systematically details the theoretical underpinnings, critical experimental parameters, and rigorous validation protocols essential for reliable qNMR. We extensively survey its pivotal applications across diverse fields: pharmaceutical analysis (purity assignment of drug substances and certified reference materials), metabolomics (absolute metabolite quantification), natural products and food science (authentication and standardization), polymer chemistry (end-group analysis, composition), and process control. Emerging technological frontiers are critically examined, including advanced pure-shift and ultrafast 2D NMR methods, hyperpolarization techniques (SABRE, DNP), low-field benchtop NMR, and AI-driven data processing. With over 50 key references, this review serves as both a foundational guide and a state-of-the-art assessment, positioning qNMR as an indispensable, robust, and versatile quantitative analytical platform in modern science.