Cannabis Provenance by Isotope Ratio Analysis

Understanding the complex details of cannabis along with its many chemical constituents has always been enabled by breakthroughs in biology, chemistry, and pharmacology. ¹ As scientific methods and instrumentation have evolved, so too has cannabis knowledge. Remarkably, recent and exciting progress in cannabis analysis has been facilitated by a rather unexpected source, namely, isotope science. This Perspective explores the impact of this fascinating technology on cannabis research.

What is an isotope? During the formation of the universe by the Big Bang, all elements of the periodic table were created, each with their unique number of differentiating electrons. ² Many elements exist as mixtures of isotopes, which still have the same number of electrons but possess different numbers of atomic core neutrons. Importantly then, isotopes are just elemental forms which vary slightly in their neutron content and therefore their atomic mass unit (amu) weights. For instance, elemental carbon occurs largely in two isotopic forms with amu’s of 12 and 13 (one extra neutron), usually written as ¹²C and ¹³C, respectively. Isotopes can also be either stable or radioactive, but only stable isotopes will be considered here.

All this would be just so much of a dull chemistry lesson were it not for several profound facts, making it extremely relevant to cannabis. First, all elemental isotopes were created in specific and measurable ratios. Usually, the lighter isotope (for instance, ¹²C) is more abundant, and the ratio is conventionally written as ¹³C/¹²C. Secondly, even though isotope ratios were established in the very early universe, here on the Earth a geological force termed “isotope fractionation” has gradually modified those ratios so that they significantly vary across the mantle of our planet. ³ These unique isotope ratios can be precisely measured by the technique of isotope ratio mass spectrometry (IRMS). ⁴ Using IRMS and other methods over the past several decades, much of the Earth’s surface has been geographically surveyed in an ongoing effort to determine its exact spatial isotope ratio (²H/¹H, ¹³C/¹²C, ¹⁵N/¹⁴N, ¹⁸O/¹⁶O, ³⁴S/³²S, etc.) composition. The mapped landscape of these isotope ratios has been termed an “isoscape,” with hundreds of detailed isoscape publications to date in the literature (SciFinder® chemistry database). ⁵ Cannabis grown in the soil of an isoscape will naturally absorb the isotopes present in the soil with their unique ratios. IRMS analysis of the cannabis itself (with its acquired isotope ratio signatures) can then be used to match the cannabis with its original geographic location (geolocation). Importantly, the more cannabis isotope ratios that are IRMS analyzed in a sample (multivariate analysis), the more accurate will be the cannabis geolocation result.

Increasingly, isotope ratio analysis is being utilized to trace the provenance of not only cannabis,^6–8 but also individual cannabis constituents like cannabidiol. ⁹ While IRMS has been employed for most of these isotope ratio studies, an alternative technical method is now suggested here as a future direction for this area. Site-specific natural isotope fractionation-nuclear magnetic resonance (SNIF-NMR®) was developed in the 1980’s at the University of Nantes in France. ¹⁰ The unique power of SNIF-NMR is its ability to determine the precise ratio of hydrogen isotopes (²H/¹H) for every carbon position in a molecule. Although SNIF-NMR has been largely used to authenticate the origin of wine, ¹¹ it may also be valuable in the cannabis area as well.

Clearly, the future applications and growth of isotope ratio analysis in cannabis research will be fascinating to follow.