Screening for More than 1,000 Pesticides and Environmental Contaminants in Cannabis by GC/Q-TOF

Abstract

A method has been developed to screen cannabis extracts for more than 1,000 pesticides and environmental pollutants using a gas chromatograph coupled to a high-resolution accurate mass quadrupole time-of-flight mass spectrometer (GC/Q-TOF). An extraction procedure was developed using acetonitrile with solid phase extraction cleanup. Before analysis, extracts were diluted 125:1 with solvent. Two data mining approaches were used together with a retention-time-locked Personal Compound Database and Library (PCDL) containing high-resolution accurate mass spectra for pesticides and other environmental pollutants. (1) A Find-by-Fragments (FbF) software tool extracts several characteristic exact mass ions within a small retention time window where the compound elutes. For each compound in the PCDL, the software evaluates the peak shape and retention time of each ion as well as the monoisotopic exact mass, ion ratios, and other factors to decide if the compound is present or not. (2) A separate approach used Unknowns Analysis (UA) software with a peak-finding algorithm called SureMass to deconvolute peaks in the chromatogram. The accurate mass spectra were searched against the PCDL using spectral matching and retention time as filters. A subset PCDL was generated containing only pesticides that are most likely to be found on foods in the US. With about 250 compounds in the smaller PCDL, there were fewer hits for non-pesticides, and data review was much faster. Organically grown cannabis was used for method development. Twenty-one confiscated cannabis samples were analyzed and ten were found to have no detectable pesticides. The remaining 11 samples had at least one pesticide and one sample had seven detectable residues. Quantitative analysis was run on the confiscated samples for a subset of the pesticides found by screening. Two cannabis samples had residues of carbaryl and malathion that were estimated to be about 10 times greater than the highest US Environmental Protection Agency tolerance set for food and about 4,000 times greater than the Canadian maximum residue limits for dried cannabis flower.

Keywords: Cannabis; Extraction method; Gas chromatography; High-resolution accurate mass; Marijuana; Personal Compound Database and Library; Pesticides; Quadrupole time-of-flight mass spectrometer.

Fig. 1

GC/Q-TOF scan chromatograms for 2 different samples of confiscated cannabis flower (#3185 and #3653) showing the complexity of the extracts after SPE cleanup and 125:1 dilution with solvent.

Fig. 2

a A portion of the P&EP PCDL showing some of the content available for each entry. b The accurate mass EI spectrum of chlorpyrifos from the PCDL.

Fig. 3

List showing 12 of the 21 qualified targets found by the Find-by-Fragments algorithm. Additional information for myclobutanil is shown in Figures 4 and 5. The table shows among other things the molecular formula (a), the compound name (b), the difference in the measured mass of the molecular ion and the theoretical value in ppm (c), an indication that the compound was qualified (met the user set criteria) or other information about the compound (d), the difference between the measured retention time and the PCDL value (in min) (e), a score (0–100) for the relative abundance for the extracted ions (f), the number of qualified ions for the compound (out of 6 in this case) (g), and the average coelution score (0–100) for all qualified ions (h).

Fig. 4

a Overlaid chromatograms of the 6 ions extracted for myclobutanil. b A coelution plot showing the abundance of each ion over the full peak width. The EIC responses are normalized to the reference ion response. If each EIC had the identical relative peak shape, the plot would show a horizontal straight line. c The isotope pattern of the molecular ion (black vertical lines) compared to the theoretical pattern (red boxes).

Fig. 5

FbF compound identification results showing, among other things the compound's name, molecular formula, measured and theoretical mass for the molecular ion, the difference in those masses (in ppm), the measured and PCDL retention times, and the difference between the two (in min) (a), the coelution score (0–100) for each extracted ion (relative to the reference ion) (b), the difference between the measured mass and theoretical mass of each extracted ion (in ppm) (c), and the relative abundance of the six ions in the PCDL spectrum (d).

Fig. 6

Identification of chlorpyrifos pesticide residue in a sample of confiscated cannabis (#3658) using Unknowns Analysis software. The subset Pesticides PCDL was used for retention time and library matching. The spectrum on the bottom of the mirror plot is the PCDL spectrum compared to the component spectrum (top). SureMass was used as the peak finding algorithm.