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.
This work illustrates that NI-APCI LC–MS/MS uses nonspecific precursor ion and product ion pairs (MRM). Moreover, according to reference 1, a four-point quadratic calibration curve yields a regression coefficient (r2) <0.97 which is not permissible under California statute that requires a quadratic calibration curve to have at least six calibration levels with r2 >0.99.
In EI GC–MS/MS, every precursor ion is a direct fragment of PCNB. The MRM transitions are more than 3.5 times more selective when compared to NI-APCI LC–MS/MS. When measured in cannabis matrix, these specific MRM transitions are linear and more than 10-fold more sensitive than reported in NI-APCI LC–MS/MS methodologies. The take away point is that specificity is just as important as sensitivity and in this example of a model compound such as PCNB, the NI-APCI LC–MS/MS transitions are not selective for PCNB and fail to meet California regulatory requirements. Conversely, GC–MS/MS in EI affords the accurate, precise, and robust identification and quantification of PCNB and other pesticides not amenable to ESI in complex cannabis matrices that meets and exceeds regulatory statute.
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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).