Abstract: An accurate analytical method based on ultra-high performance liquid chromatography-triple quadrupole tandem mass spectrometry (UPLC-MS/MS) was established for the quantification of vanillin in dairy products. Additionally, a gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) method was developed for the traceability and identification of vanillin. The sample pretreatment conditions and the UPLC-MS/MS parameters were optimized, and the methods were validated systematically. The UPLC-MS/MS method showed a limit of detection (LOD) of 0.3 μg/kg and a limit of quantification (LOQ) of 1.0 μg/kg, with recoveries at different spiked levels ranging from 97.30% to 100.80%, demonstrating excellent sensitivity and accuracy. Matrix effect verification indicated that the pretreatment process did not induce significant isotope fractionation, confirming that the GC-C-IRMS method was stable and reliable. The established methods were applied to the analysis of 26 commercial dairy samples. The UPLC-MS/MS method enabled accurate quantification of trace vanillin, with positive results primarily found in formula milk ((87.47 ± 0.91) mg/kg). Traceability analysis by GC-C-IRMS confirmed that the vanillin detected in the positive samples with high vanillin content was synthetic. While UPLC-MS/MS provided high-sensitivity quantification, it could not distinguish the sources of the analyte. The complementarity of these two techniques highlights the significance of combining “precise quantification” with “stable isotope traceability” in differentiating between natural and synthetic vanillin in dairy products. This study achieves the deep integration of vanillin quantification and traceability, providing a rigorous technical tool for the scientific determination of vanillin sources.

Key words: vanillin; dairy products; ultra-high performance liquid chromatography-triple quadrupole tandem mass spectrometry; gas chromatography-combustion-isotope ratio mass spectrometry; source identification