Stable Isotope Ratios of Marijuana. I. Carbon and Nitrogen Stable Isotopes Describe Growth Conditions*

Abstract: There remains significant uncertainty in illicit marijuana cultivation. We analyzed the δ¹³C and δ¹⁵N of 508 domestic samples from known U.S.A. counties, 31 seized from a single location, 5 samples grown in Mexico and Colombia, and 10 northwest border seizures. For a subset, inflorescences and leaves were analyzed separately. These data revealed a strong correspondence, with inflorescences having slightly higher δ¹³C and δ¹⁵N values than leaves. A framework for interpreting these results is introduced and evaluated. Samples identified as outdoor‐grown by δ¹³C were generally recorded as such by the Drug Enforcement Administration (DEA). DEA‐classified indoor‐grown samples had the most negative δ¹³C values, consistent with indoor cultivation, although many were also in the outdoor‐grown domain. δ¹⁵N indicated a wide range of fertilizers across the dataset. Samples seized at the single location suggested multiple sources. Northwest border δ¹³C values suggested indoor growth, whereas for the Mexican and Colombian samples they indicated outdoor growth.