## 3 Overview of 31P qNMR for Phosphorus Compounds
### 31P Quantitative NMR Method  
31P nuclear magnetic resonance spectroscopy is a valuable analytical technique for examining phosphorus-containing compounds. The 31P nucleus has a natural abundance of 100% and a high gyromagnetic ratio, resulting in excellent NMR sensitivity. Additionally, 31P NMR exhibits a broad chemical shift range, allowing for better separation and identification of different phosphorus environments [9]. These features make 31P NMR particularly beneficial for quantitative analysis. Typically, 31P NMR produces sharp signals, often corresponding to a single phosphorus atom within a molecule, facilitating precise quantification [31; 32; 33]. Compared to 11 NMR, 31P NMR can often avoid complex resonance interference, leading to clearer spectra [20]. Moreover, 31P NMR does not require expensive deuterated solvents, simplifying the sample preparation process [34].  
31P qNMR is a highly effective technique for analyzing phosphorus-containing compounds. It has demonstrated the capability to measure trace amounts of these molecules in vivo [35]. Studies have shown that the accuracy and precision of 31P qNMR are comparable to those of chromatographic methods, making it a reliable tool for both pharmaceutical and food analysis [36; 37; 38]. Many researchers have improved and optimized 31P quantitative NMR, developed new methods, and refined the use of 31P nuclei for NMR quantification.  
1. Research on 31P qNMR standard substances. [PERSON] et al. [39] investigated 31P qNMR internal standards with metrological traceability, such as triphenyl phosphate and phosphoacetic acid, and their practical applications as shown in Figure 5. The study assessed the purity of tris(2-chloroethyl) phosphate using different traceability schemes and solvent systems. The findings demonstrated consistent results within measurement uncertainty ranges, highlighting the reliability of the certification approach. The application of these certified reference materials was further exemplified through measurements of tris(2-chloroethyl) phosphate. Additionally, researchers have used the 31P nucleus for quantitative NMR, testing various phosphorus-containing standards based on solubility and application needs. Internal standards, such as triphenyl phosphate and sodium phosphate, or external standards like phosphoric acid, are commonly employed to prevent reactions with analytes [40].
2. Development of Internal Standards for Coaxial Insertion. [PERSON] et al. [41] developed an absolute quantification method for 31P NMR using coaxial nuclear magnetic resonance tube inserts containing internal standards. This approach physically separates the reference material from the test sample, avoiding chemical interactions and mixing errors. The method demonstrated precision and accuracy exceeding 1% for purity determinations and detected impurities at concentrations as low as 25 \(\upmu\)g/mL. Compared to traditional chromatographic methods, it eliminates the need for extensive sample preparation and reference standards, providing significant cost-effectiveness and operational simplicity.
3. Integration of 31P qNMR with Other Analytical Techniques. [PERSON] et al. [42] studied the interaction between two extractants in deuterated chloroform using 1H, 13C, and 31P nuclear magnetic resonance spectra, along with NOESY experiments. [PERSON] et al. [43] applied quantitative NMR techniques, including 27 Al, 31P, and 1H magic-angle spinning NMR, together with temperature-programmed desorption to study the impact of various treatment processes on materials. NMR spectroscopy played a key role in this study, offering detailed characterization of transformations in aluminum, phosphorus, and acid sites. The study revealed the mechanisms underlying phosphorus modification and its partial reversibility, offering critical insights for advancing the understanding and optimization of zeolite-based catalysts in industrial applications.