Up in Smoke: The Naked Truth for LC–MS/MS and GC–MS/MS Technologies for the Analysis of Certain Pesticides in Cannabis Flower

October 25, 2019
Abstract / Synopsis: 

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.

The legalization of medicinal and recreational marijuana in some U.S. states has resulted in legislation mandating safety and quality testing prior to retail sale. These regulations include analytical chemistry and biological assays to identify and quantify cannabinoids such as tetrahydrocannabinol (THC), residual pesticides, mycotoxins, heavy metals, residual manufacturing solvents, terpenes, and microbial contaminates. In the U.S., marijuana (as defined by a Cannabis spp. THC concentration >0.3% [dry wt/wt]) remains an illegal, Schedule I narcotic at the federal level. Therefore, there is no federal guidance for proper safety testing of the consumer products, and this has resulted in each U.S. state defining unique chemical target lists and action levels. This is not the case in Canada where the federal government has defined the action levels for a target list of 96 pesticides.

In the U.S., the lack of interstate harmony has created many challenges for cannabis testing laboratories. Most conspicuously are the various state residual pesticide lists that can range from a couple dozen with relatively high action levels to more than 60 with relatively low action levels. The Venn diagram in Figure 1 illustrates the varied nature of four U.S. state pesticide lists.

Figure 1

Further complicating the issue is the inclusion of pesticides not amenable to electrospray ionization (ESI): the most common liquid chromatography–tandem mass spectrometry (LC–MS/MS) ionization source for this analysis. For example, Canada, California, Nevada, Pennsylvania, Florida, and other U.S. states have included pentachloronitrobenzen (PCNB), captan, cis/trans-chlordane, chlorfenapyr, or methyl parathion in their respective target lists. These compounds are known “bad actors” when analyzed via LC–MS/MS using ESI. To overcome this issue for compounds like PCNB, the relevant literature space has proposed using negative ion-atmospheric pressure chemical ionization (NI-APCI) LC–MS/MS. For example, the analysis of PCNB using NI-APCI LC–MS/MS has been shown elsewhere with a precursor ion of 275.5 m/z and a mechanism of ionization reported as the loss of HCl followed by formation of an ammonium ion adduct (1). There are two intrinsic problems with this description: First, the empirical formula for PCNB is C6Cl5NO2 thus there is no hydrogen atom to lose as H-Cl, and second the formation of a positively charged ammonium ion in negative ionization mode is highly unlikely. Even if it were correct, the above description defeats a primary function of soft ionization: the formation of a specific ion representing the intact molecule. Please note, these authors do not debate the presence of a detectable MS/MS transition for PCNB in NI-APCI LC–MS/MS, but the lack of specificity and nonlinear response that has been shown in the literature are problematic for the accurate, precise, and robust identification and quantification of PCNB in complex cannabis matrices.

  1. D. Tran, et al. Document number: RUO-MKT-02-7607-A. 2018, AB Sciex, Framingham, MA.
  2. C.N. McEwena and B.S. Larsen, J .Am. Soc. Mass Spectrom. 20, 1518–1521 (2009).
  3. 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).