The “Green Rush” of cannabis and hemp continues to increase because of the medicinal and health benefits of these two plants. The two major beneficial compound classes are the cannabinoids and terpenes. This article explores the various techniques for conducting analysis of these compounds, including: integrated versus modular high performance liquid chromatography (HPLC), HPLC versus ultrahigh-pressure liquid chromatography (UHPLC), UHPLC-ultraviolet (UV) versus UHPLC-photodiode array (PDA) detectors, HPLC-UV versus quadrupole liquid chromatography mass spectrometry (LC–MS) versus triple quadrupole mass spectrometry (LC–MS/MS), quadrupole time-of-flight mass spectrometers (QTOF-MS) versus matrix-assisted laser desorption or ionization time-of-flight mass spectrometers (MALDI-TOF-MS), LC versus gas chromatography (GC) systems, and sample preparation for LC- and GC-based methods.
Generally, QC labs using HPLC-UV or PDA do not use internal standards; however, some states, such as New York, require an internal standard (Norgestrel) as well as a surrogate (4-pentylphenyl 4-methylbenzoate). With LC–MS/MS, internal standards are usually a requirement. Also, the most popular LC–MS/MS sample introduction method is electrospray ionization (ESI), but for cannabinoid analysis, atmospheric pressure chemical ionization (APCI) is required. Finally, contract laboratories may charge in the $50 range for cannabinoid analysis, so those incentivized by money are not likely to tie up the very expensive LC–MS/MS system, unless required to do so. LC–MS/MS would be more likely used for a higher billing pesticide analysis, typically in the $225 range per sample, and for simultaneous mycotoxin and aflatoxin analysis, which yields another $75, for a total of $300 per sample.
QTOF-MS Versus MALDI-TOF-MS
Quadrupole time-of-flight mass spectrometers (QTOF-MS) and matrix-assisted laser desorption and ionization time-of-flight mass spectrometers (MALDI-TOF-MS) are referred to as high-resolution mass spectrometers (HRMS). Mass spectrometry measures the mass (m) to charge (z) ratio (m/z) in a sample. High-speed LC–MS and LC–MS/MS measures the nominal mass of a compound, while HRMS measures the exact mass to several decimal places, providing more confidence for the analysis of targeted and untargeted compounds. QTOF also has the advantage of chromatographic separation by UHPLC. A QTOF-MS system costs in the range of $400K.
MALDI-TOF-MS is a broad term encompassing a range of instrumentation types, including ion trap, benchtop and floor linear models, linear reflectron, and TOF-TOF, costing in the $150–$400K range depending on the specifications. MALDI-TOF does not utilize the chromatographic separation power of the previously discussed techniques, so high resolution is very important.
Cannabis consists of more than 500 compounds with over 140 cannabinoids and over 200 terpenes. It also contains hydrocarbons, sugars, nitrogenous compounds, fatty acids, flavonoids, amino acids, aldehydes, ketones, esters, steroids, protein, elements, pigments, and vitamins. High resolution is important for targeted and untargeted compounds.
LC Versus Gas Chromatography Systems
As described above, LC may be the gold standard for cannabis analysis. However, gas chromatography (GC) systems have been reported to have better sensitivity and higher throughput than LC systems. But, because of the heated injection port and column, GC methods can only provide “total ∆9-THC” as the ∆9-THCA is converted to the ∆9-THC. The same is true for any of the acidic cannabinoids, such as CBDA and CBGA, which will be converted to the neutral forms for CBD and CBG, respectively. It should be noted that ∆8-THC and THCV are
not converted to ∆9-THC.
Figure 7 shows a chromatogram of Shimadzu’s 11-cannabinoid premix standard, which has only eight peaks because the three acidic components of THCA, CBDA, and CBGA have been converted to the neutral forms. Many analysts find this unacceptable, since the cannabinoid concentrations in the original sample are missing some information. It should be noted that adding a headspace sampler to a GC coupled to a flame ionzation detector (FID) or GC–MS allows its utilization for terpene and residual solvents analysis in the cannabis and hemp industries and may still be required in many instances.
Cardenia and colleagues (2) reported on the derivatization of the acids to stabilize the structure and increase volatility for analysis by GC–MS. The authors reported the analysis of 10 cannabinoids, including the three acidic cannabinoids. The derivatization step may be automated with the use of the proper autosampler.
Professor Kevin Schug and his colleagues at the University of Texas, Arlington (3–5) have coupled a GC and GC–MS to a vacuum ultraviolet (VUV) detector from VUV Analytics. The VUV region of the spectrum is below 200 nm, and the working region of the authors was 120–240 nm, well below the HPLC-PDA region of 190–400 nm discussed earlier, which provided additional information. The electrons are excited from one energy level to a higher level such as –* (sigma to sigma star transition) for alkanes, n–* (n to pi star transition) for O, N, S and halogens, and –* (pi to pi star transition) for unsaturated alkenes. In their publication, they showed GCVUV and GC–MS-VUV could analyze the neutral and acidic cannabinoids, but the later compounds would require derivatization.
As a bonus, the authors showed they could measure terpene isomers using the GC–MS-VUV system, measuring terpene alcohols, terpenes with single, double, and triple bonds, oxygenated terpenes with double bonds, and ethers, ketones, and aromatic terpenes (3–5).
Sample Preparation for LC- and GC-Based Methods
As cannabis and hemp are analyzed for cannabinoids, sample preparation depends on the sample type. As described above, LC systems have been the gold standard. Sample preparation for analysis of concentrates is quite simple, typically only requiring a dilution in solvent. For flower, the sample preparation may involve extraction with one solvent and dilution with another solvent. Homogeneous foods such as gummies may also involve a solvent extraction with solvent dilution, which are relatively easy procedures.
The difficulty arises when the foods are nonhomogenous, such as energy bars that may contain chocolate, almonds, peanut butter, and caramel. Different foods can require different sample preparation methods depending on the components of salts, sugars, sweeteners, fats, natural products, coloring, additives, preservatives, cholesterol, fiber, and so on. For LC, sample preparation would usually require a multiple step process: grinding into a homogeneous mixture, dissolving in a solvent, and then solid phase extraction (SPE). The SPE steps then consist of:
- conditioning a cartridge with a solvent, followed by
- conditioning with water,
- applying the sample to collect the target compounds on the cartridge while the rest goes to waste,
- washing the cartridge with water to remove impurities, and
- extracting the target compounds by washing the cartridge with solvent.
This is very time consuming and varies depending on the matrix.
A pyrolyzer (PY), used for thermal desorption, can be added as the sample introduction device to a GC–MS. PY-GC–MS can be used to characterize complex materials at trace levels, often without any sample pretreatment. Because of the direct sample introduction and the chromatographic separation, it is possible to analyze very small amounts and collect detailed, unique information, which would generally require significant sample preparation.
Sample preparation could be as simple as weighing the nonhomogeneous food sample into a sample cup and having the autosampler drop the cup into an oven between 100–300 °C, which is high enough to vaporize the cannabinoids but low enough to leave behind higher boiling interfering compounds. Depending on the matrix, the sample may require grinding and, possibly, a liquid extraction step, but sample preparation steps are minimized compared to LC and LC–MS sample preparation steps.
HPLC-based systems have been the gold standard for cannabinoid analysis because they offer a variety of selections between HPLC and UHPLC, integrated or modular formats, photometric detectors such as UV-vis and PDA or DAD, and mass spectrometry detectors such as MS, MS/MS, and QTOF-LC–MS, with each component having advantages and disadvantages. GC and GC–MS-based methods are improving by reducing sample preparation with thermal desorption sample introduction for cannabinoid and terpene analysis utilizing VUV detection.
Future experiments should involve combining PY-GC–MS with GC–MS-VUV for faster sample preparation analysis of edibles with three forms of confirmation with retention time, mass spectral identification, absorption spectral identification for cannabinoids and terpenes, and faster chromatography.
- ICH Harmonised Tripartite Guideline prepared within the Third International Conference on Harmonisation of Technical Requirements for the Registration of Pharmaceuticals for Human Use (ICH), Text on Validation of Analytical Procedures (1994).
- V. Cardenia, et al., J. Food Drug Anal. 26(4), 1283–1292 (2018).
- A. Leghissa, Z. Hildenbrand, and K.A. Schug, J. Sep. Sci. 41(1), 398–415 (2018).
- A. Leghissa, J. Smuts, Q. Changling, Z. Hildenbrand, and K.A. Schug, Sep. Sci. plus. C1, 37–42 (2018).
- C. Qiu, J. Smuts, and K.A. Schug, J. Sep. Sci. 40, 869–877 (2017).
About the Authors
Bob Clifford, PhD, is the General Manager of Marketing at Shimadzu Scientific Instruments in Columbia, Maryland. Craig Young is the HPLC Product Manager at Shimadzu Scientific Instruments. Alan Owens is a Senior GCMS Product Specialist at Shimadzu Scientific Instruments. Jim Mott, PhD, is a Field Tech Support Supervisor at Shimadzu Scientific Instruments. Rachel Lieberman, PhD, is the Forensics Marketing Manager at Shimadzu Scientific Instruments. Direct correspondence to: [email protected].
How to Cite This Article
B. Clifford, C. Young, A. Owens, J. Mott, and R. Lieberman, Cannabis Science and Technology 3(1), 34-42 (2020).