<p>Angiotensin I (ANGI) plays an important role in regulating blood pressure and maintaining homeostasis; the accurate analysis of its levels is beneficial in the clinical diagnosis and treatment of diseases. Despite the existence of several detection techniques for ANGI, a “yardstick” to evaluate the results of different detection techniques is still lacking. To ensure the reliability of ANGI tests along with the accuracy and comparability of the analytical results, natural and isotope-labeled ANGI certified reference materials were characterized, and their purities were assessed by mass balance and quantitative nuclear magnetic resonance methods. The moisture content of the samples was determined using the Karl Fischer method, anion content was measured via ion chromatography, and inorganic elements were detected via inductively coupled plasma mass spectrometry. In the quantitative nuclear magnetic resonance method, maleic acid was used as a quantitative internal standard, and the final values of natural and isotope-labeled ANGI were determined on the basis of their quantitative peaks. The final purity was verified by isotope dilution mass spectrometry. The natural and labeled ANGI purities were (0.8511 ± 0.041) g/g and (0.8696 ± 0.042) g/g, respectively. Furthermore, the abundance of double-labeled ANGI—which refers to the labeling rate of ¹³C and ¹⁵N in labeled ANGI—was determined via high-resolution liquid-mass spectrometry, affording 98.98% ¹³C and 99.43% ¹⁵N. This study establishes high-purity natural and isotope-labeled ANGI reference materials, ensuring accurate, comparable measurements for clinical diagnostics and biomarker research. It also supports method validation, quality control, and interlaboratory comparisons through advanced analytical techniques.</p> Graphical Abstract <p></p>