In U.S. states, Canada, and other countries where medicinal or adult recreational cannabis has been legalized, regulatory entities require a panel of chemical and biological tests to assure quality and safety of the products prior to retail sales. Of the required assays, residual pesticide identification and quantification is arguably the most challenging. The reason for this is the complexity of the cannabis genome that synthesizes phytocannabinoids, terpenes, polyphenols, lipids, and a host of other endogenous chemicals. It is not unusual for today’s selectively bred and cloned cannabis to contain 20–30% ∆9-tetrahydrocannabinol (THC) and other cannabinoids such as cannabidiol (CBD), and 1–3% terpenoids by dry weight. These chemicals alone constitute hundreds of milligrams per gram of sample. In contrast, residual pesticides are typically measured in the 10–1000 ng/g (ppb) range. Pesticide analysis in this matrix requires tandem quadrupole mass spectrometry (MS/MS) because of its mitigation of chemical noise through MS/MS processes. Notwithstanding the power of MS/MS, there are many cases where isobaric interferences effect quantitative results and therefore selectivity becomes as important as sensitivity. In this study, we used liquid chromatography and gas chromatography quadrupole time-of-flight mass spectrometry (LC-qTOF and GC-qTOF, respectively), and gas chromatography tandem mass spectrometry (GC–MS/MS) to evaluate the selectivity of a model pesticide commonly found in regulatory target lists. The LC-qTOF system used negative ion-atmospheric pressure chemical ionization (NI-APCI), and the GC–MS systems used electron ionization (EI). Through this work, we demonstrated that the GC–MS precursor ion and product ion pairs are highly specific derivatives of the parent molecule while the NI-APCI precursor ion is a nonspecific chemical species created in situ through a complex ionization mechanism. In this latter case, all precursor ion and product ion pairs are not selective for the intact analyte molecule.
Materials and Methods
Chemicals and Standards
PCNB and high purity acetonitrile were obtained from Sigma. Ground cannabis flower was obtained from the University of Mississippi Marijuana Project with permission from the U.S. Drug Enforcement Administration (DEA).
Agilent 1290/6545 LC-QTOF, 7890/7250 GC-QTOF, and 7890/7010 GC–MS/MS systems were used for all analyses. Both QTOF systems were operated in high-resolution-accurate mass (HRAM) mode for MS and MS/MS experiments.
The LC-QTOF system was configured with an APCI source in negative ionization mode, and a 3.0 mm x 100 mm, 2.7 µm semiporous particle size phenylhexyl column. The mobile phases were: A) 2 mM ammonium formate and 0.1% formic acid (v/v) in LC–MS grade water and B) 2 mM ammonium formate and 0.1% formic acid (v/v) in LC–MS grade methanol. The GC-QTOF was configured with a split–splitless inlet, a 30 m DB-35MS UI column, and helium carrier gas. The GC–MS/MS system was configured with a multimode inlet (MMI) operated in cold splitless mode, two HP-5MS UI columns connected through a purged union, and helium carrier gas. Both GC mass spectrometers were operated in electron ionization (EI) mode at 70 eV.
For the LC-QTOF and GC-QTOF experiments, 1 µg/mL PCNB in acetonitrile was injected into the systems, and both MS and MS/MS data were collected. For the GC–MS/MS experiments, dry cannabis flower containing 20–30% THC by dry weight was spiked at five calibration levels over the range of 8–120 ppb in vial (200 ppb [0.2 μg/g] to 6250 ppb [6.25 μg/g] in matrix). Extracts were prepared by solid-phase extraction (SampliQ C18 EC SPE Cartridge, Agilent) with acetonitrile to a final dilution factor of 25:1. The extraction process is given in Figure 2.
- D. Tran, et al. Document number: RUO-MKT-02-7607-A. 2018, AB Sciex, Framingham, MA.
- C.N. McEwena and B.S. Larsen, J .Am. Soc. Mass Spectrom. 20, 1518–1521 (2009).
- I. Dzidic, D.I. Carroll, R.N. Stillwell, and E.C. Horning, Anal. Chem. 47(8), 1308–1312 (1975).
Matthew Curtis, Eric Fausett, Wendi A. Hale, Ron Honnold, Jessica Westland, Peter J. Stone, and Jeffery S. Hollis are with Agilent Technologies in Santa Clara, California. Anthony Macherone is with Agilent Technologies and The Johns Hopkins University School of Medicine in Baltimore, Maryland. Direct correspondence to: [email protected]
How to Cite This Article
M. Curtis, E. Fausett, W.A. Hale, R. Honnold, J. Westland, P.J. Stone, J.S. Hollis, and A. Macherone, Cannabis Science and Technology 2(5), 56-60, 70 (2019).