For a long time, the cannabis market has been largely unregulated. So far, 36 states have legalized cannabis for medicinal use, and 18 have allowed recreational use. Canada and Uruguay have also legalized cannabis for medicinal and recreational use. Other countries have decriminalized possession of cannabis.
With increasingly more people ingesting cannabis, consumer safety has become a pertinent issue that needs to be addressed. The only way to ensure safety is through vigorous and consistent testing of all cannabis products before they reach dispensaries and ultimately consumers.
Cannabis plants are susceptible to pests and diseases, and many cultivators opt to use commercial pesticides to mitigate the risk. Some illicit growers use pesticides ruthlessly, and this results in high pesticide residue on mature cannabis. An example is carbofuran; despite it being banned in the US, it was present in 78% of unauthorized grow sites in 2017. 
It is crucial to test all cannabis to prevent exposing consumers to life-threatening health risks. However, cannabis testing is not without its challenges.
Challenges in Cannabis Testing
The standard methods for testing pesticide residue on cannabis are gas and liquid chromatography with tandem mass spectrometers (GC/MS/MS and LC/MS/MS). These methods are super sensitive and selective but will only identify pesticides that are on the target list. Pesticides and contaminants that are not on the list could be missed. Highly toxic carbofuran is not detected by a GC/MS and LC/MS analysis. This exposes consumers to carbofuran poisoning, which causes blurred vision, breathing difficulties, incontinence, abdominal cramps, increased salivation, and increased blood pressure.
Screening for Cannabis Pesticides Using GC/Q-TOF
A new method for testing pesticides and other contaminants in cannabis has been described in a 2020 paper.  This is a gas chromatography method that is coupled with a quadrupole time-of-flight spectrometer (GC/Q-TOF).
Samples were first processed using solid phase extraction with acetonitrile to remove as much of the matrix as possible without degrading any pesticides present. Next, the samples were diluted with 50/50 hexane/acetone for a final dilution factor of 125:1.
A commercially available Personal Compound Database and Library (PCDL) was used for the GC. The PCDL had mass spectra plus locked retention times for a total of 1,020 compounds. Two approaches were used to find suspect compounds: Find-by-Fragments (FbF) and Unknowns Analysis (UA). While most entries were pesticides, other environmental pollutants such as nitrosamines, fire retardants, and phthalates were included. A smaller subset PCDL that eliminated rare environmental contaminants as pesticides were also developed.
In total, 21 confiscated cannabis samples were analyzed. Ten of these samples had no detectable pesticides, two samples had two pesticides each, and eight samples revealed one pesticide. One sample contained seven pesticides including p,p′-DDE, hexazinone, DEET, atrazine, mevinphos, fenarimol, and dieldrin.
In two samples, carbaryl and malathion were detected in amounts 10 times higher than what is set for food in the US and almost 4,000 times higher than what is allowed in Canada for dried cannabis. 
This research highlights two main points: some of these confiscated samples measured by the University of Mississippi contained health risks that regulated cannabis should not, further bolstering the idea of access to legalized, safer cannabis; the second point highlights the importance of our 3rd party analytical testing labs, and the diligence needed to track down potential health risks before products reach consumers.
1. Thompson CM, Gabriel MW, Purcell KL. An ever-changing ecological battlefield: marijuana cultivation and toxicant use in western forests. Wildlife Prof. 2017;11(3):42-46. [journal impact factor = N/A; times cited = 4]
2. Wylie PL, Westland J, Wang M, Radwan M, Majumdar CG, ElSohly MA. Screening for more than 1,000 pesticides and environmental contaminants in Cannabis by GC/Q-TOF. Med Cannabis Cannabinoids. 2020;3:14-24. [journal impact factor = N/A; times cited = 3]