Fragment-Specific Stable Carbon Isotope Analysis for Screening the Authenticity of Red Yeast Rice Products

Monacolin K (MK) in red yeast rice (RYR) is structurally identical to pharmaceutical lovastatin in its lactone form, making their origins difficult to distinguish. This study examined the δ13C value of hydrolysis-derived 2-methylbutyric acid (2-MBA) as a fragment-specific screening marker using liquid chromatography–isotope ratio mass spectrometry (LC-IRMS). Within-day repeatability was 0.19‰ for RYR, interlaboratory standard deviations were 0.18–0.19‰, and a two-component expanded uncertainty estimate was ±0.58‰ (k = 2). Hydrolysis-derived 2-MBA ranged from −32.73‰ to −26.68‰ in 45 authentic RYR samples and from −10.17‰ to −6.87‰ in 23 pharmaceutical lovastatin batches. Controlled blends showed a linear relationship between δ13C and blend ratio (R2 = 0.9997). A one-sided tolerance limit at 95% confidence and 99% population coverage gave an investigational screening threshold of −26.47‰ for the sampled Chinese authentic RYR population. Three of 20 commercial products exceeded this threshold and were classified as isotopically inconsistent with that population. Overall, this study developed and analytically evaluated a fragment-specific LC-IRMS approach for screening source-related isotope differences between RYR-derived MK and pharmaceutical lovastatin under the investigated conditions, providing a potential analytical tool for RYR authenticity assessment.