Quantitation of Cannabinoids in Dried Ground Hemp by Mid-Infrared Spectroscopy

December 16, 2019
Abstract / Synopsis: 

A novel, small, portable, general purpose quantitative mid-infrared (IR) spectrometer has been invented and applied to the analysis of dried, ground hemp. The unit was calibrated using cannabinoid concentrations determined by high performance liquid chromatography (HPLC) at a state licensed laboratory, and mid-IR spectra measured on the same samples. The analyzer was validated using the leave-one-out cross validation method. Mid-IR calibration models for delta-9-tetrahydrocannabinol (Δ9-THC), tetrahydrocannabinolic acid (THCA), total THC, cannabidiolic acid (CBDA), cannabidiol (CBD), total CBD, cannabigerolic acid (CBGA), and cannabichromene (CBC) were constructed. The accuracy for the determination of total THC, which is important in determining the legality of a hemp crop, is ±0.04 weight percent, more than sufficient for compliance testing. The analyzer requires little sample preparation, features push button operation, produces results in 2 min, and at a cost of $0/sample. Hemp is a naturally variable material, so obtaining representative data on a grow requires averaging results from many samples. The speed and ease-of-use of mid-IR spectroscopy makes this feasible, as opposed to chromatography where typically only one or a few samples from a grow are analyzed so representative data are not obtained. Applications of this analyzer for hemp farmers, hemp extractors, state regulators, and law enforcement are discussed.

Because of the 2018 Farm Bill, growing industrial hemp in the United States is legal if the sample contains not more than 0.3 dry weight percent (wt.%) total tetrahydrocannabinol (THC) (1,2). There is a need then to test hemp samples to insure they comply with the new law. In the past, cannabis and hemp have been analyzed for cannabinoid content via high performance liquid chromatography (HPLC) (3,4) or gas chromatography (GC) (5). However, chromatography suffers from several problems. Samples must be weighed, ground, extracted, vortexed, diluted, and filtered before injection (3–5). To perform these many manual sample preparation steps properly takes a skilled analyst several minutes and involves the use of expensive consumables. Also, chromatography runs can take at least 5 min or more (3–5), which when combined with the sample preparation time means it takes at least 10 min to analyze one sample. Between the consumables and labor, the cost per sample can be $20 or more. It is hard to imagine laypeople having the time or skill to perform these steps properly, which is why cannabis regulations require highly trained scientists to operate chromatographs (2).

Another concern with chromatography is the lack of representative sampling. Cuttings from adjacent plants in a grow, and even buds from the same plant, can vary by several weight percent in their cannabinoid content. This is illustrated in Table I, which shows the results for five different cannabis strains. The numbers represent weight percent tetrahydrocannabinolic acid (THCA) as measured by HPLC, and the low, medium, and high values represent samples take from different positions on the same plant (3).

Plants from five different strains in Table I are represented. Note that the potency variation across an individual plant can vary by almost 3 wt.%. This means cannabis is a heterogeneous, naturally varying material. The scientifically correct way then to sample a cannabis grow is to collect composite samples from many places in a grow, analyze them, and average the results (6,7). This may mean dozens or even hundreds of samples need to be analyzed, and these analyses must be done over time to insure a hemp grow does not go above the 0.3% total THC limit. Analyzing this many samples by chromatography would be prohibitively expensive. Because of the time, expense, and trouble of chromatographic analyses, typically only one sample from a grow is analyzed. This is by definition not representative (6,7). The danger here is that by using chromatography hemp farmers may be obtaining nonrepresentative data on their grow, misleading them as to its legality and economic value.

Table I

  1. 115th United States Congress, Senate Bill S.2667, ”Hemp Farming Act of 2018.”
  2. https://www.federalregister.gov/documents/2019/10/31/2019-23749/establishment-of-a-domestic-hemp-production-program.
  3. M.W. Giese, M.A. Lewis, L. Giese, and K.M. Smith, J. AOAC Int. 98(6), 1503 (2015).
  4. C. Giroud, CHIMIA Intl. Journal of Chemistry 56, 80 (2002).
  5. T. Ruppel and M. Kuffel, "Cannabis Analysis: Potency Testing Identification and Quantification of THC and CBD by GC/FID and GC/MS," PerkinElmer Application Note (2013).
  6. B.C. Smith, Cannabis Science and Technology 2(1), 14–19 (2019).
  7. P. Atikins, Cannabis Science and Technology 2(2), 26–34 (2019).
  8. B.C. Smith, P. Lessard, and R. Pearson, Cannabis Science and Technology 2(1), 48–53 (2019).
  9. B.C. Smith, Cannabis Science and Technology 2(2), 12–17 (2019).
  10. B.C. Smith, Cannabis Science and Technology 2(3), 10–14 (2019).
  11. B.C. Smith, Fundamentals of Fourier Transform Infrared Spectroscopy, 2nd Edition (CRC Press, Boca Raton, Florida, 2011).
  12. B.C. Smith, Infrared Spectral Interpretation: A Systematic Approach (CRC Press, Boca Raton, Florida, 1999).
  13. B.C. Smith, Quantitative Spectroscopy: Theory and Practice (Elsevier, Boston, Massachusetts, 2002).
  14. B.C. Smith, M. Lewis, and J. Mendez, "Optimization of Cannabis Grows Using Fourier Transform Mid-Infrared Spectroscopy," PerkinElmer Application Note (2016). https://www.perkinelmer.com/lab-solutions/resources/docs/APP_Determination_of_THC_and_CBD_CannabisFlower.pdf.
  15. B.C. Smith, Terpenes and Testing Nov.-Dec., 48 (2017).
  16. B.C. Smith, Terpenes and Testing Jan.-Feb., 32 (2018).
  17. B.C. Smith, Manuscript in preparation.
  18. https://www.mprnews.org/story/2019/06/06/hemp-production-minnesota-farmer-sues.
  19. https://docs.google.com/spreadsheets/d/1x8doatlR6w1W3W6hA0hlu67qwe9uOSfYoHe3vjcYs6Y/edit#gid=855723386, courtesy Toni Anthony, Key to Life Inc.
  20. https://cannabisindustryjournal.com/tag/big-sky-scientific/.
  21. https://www.youtube.com/watch?v=DhQlp-cwQh0&t=3s.

About the Author

Brian C. Smith, PhD, is Founder, CEO, and Chief Technical Officer of Big Sur Scientific in Capitola, California.  Direct correspondence to: [email protected]

How to Cite this Article

B.C. Smith, Cannabis Science and Technology 2(6), 28-33 (2019).