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.
Results and Discussion
The ionization mechanisms of NI-APCI are (2):
- Electron capture: M + e- → M.-
- Dissociative electron capture: M + e- → F1- + F2.
- Proton abstraction: M + OH- → (M-H)- + H2O
- Anion adduction: M + A- → MA-
Chlorinated nitrobenzene compounds like PCNB have been shown to form phenoxide ions under negative atmospheric pressure ionization conditions (3). Therein, the predominant ionization mechanism for the formation of (M – Cl + O)- was described as electron capture of O2 to form superoxide O2- followed by an in-situ reaction with the neutral molecule and dissociative electron capture to form the ionized species. The general reaction scheme was proposed as:
O2- + M → (MO2)- → (M – Cl + O)- + OCl.
More specifically, for PCNB this is illustrated in Reaction Scheme 1.
A secondary ionization mechanism is also observed that forms pentachlorobenzyl phenoxide. The putative secondary ionization mechanism is: Reaction scheme 1 followed by dissociative electron capture (NO2-) and anion adduction (Cl-). This is illustrated in Reaction Scheme 2.
Although the molecular species formed in Reaction Scheme 1 can occur at both the ortho and the para positions of PCNB, Reaction Scheme 3 suggests the ortho position is favored based on the collision-induced dissociation (CID) species formed through the MS/MS processes.
- 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).