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Removing Purple Pigments from Cannabis Prior to Analysis

By Alexander Beadle
Published: Aug 02, 2018   
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In order for cannabis testing laboratories to ensure accurate and reliable results, it is necessary to remove unwanted matrix components from the plant material prior to analysis. Compared to many staple agricultural crops, cannabis has an extremely complex chemical makeup, consisting of over 500 unique compounds, making good sample preparation essential but challenging. 

In terms of mass, a third of cannabis seed, half of the cannabis flower, and nearly the entirety of cannabis extract consists of essential oils such as terpenes and cannabinoids. These compounds are responsible for many of the properties of cannabis, such as its flavor or aroma, or its psychotropic and medicinal effects. The amount of each terpene or cannabinoid present varies between cannabis strains and with some cannabis strain databases containing over 2,000 entries the reported biodiversity of cannabis is astounding. 

Many of the compounds that could hamper analysis are removed during the normal process of making a cannabis extract. Pigments such as chlorophyll are not always removed, and so separate removal methods have had to be developed to account for these pigments. In addition to chlorophyll, which causes green pigmentation, several cannabis strains also contain compounds that can cause the cannabis flowers to have a purple hue. 

QuEChERS for cannabis sample clean up

United Chemical Technologies (UCT). recently published proof of a sample preparation technique for mycotoxin and pesticide analysis that is also capable of removing chlorophyll as well as other hues of pigmentation. The group took samples from four different cannabis strains (Purple Drink, Grand Daddy, Tahoe OG, and Agent Orange) which were then hydrated in deionized water to improve their extraction efficiency. After the addition of acetonitrile, the samples were homogenized with a SPEX Geno/Grinder. QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) extraction salts were added into the sample which was then centrifuged.

After centrifugation, fractions of the solution were transferred into centrifugation tubes containing different blends of dispersive solid phase extraction (dSPE) salts. The tubes were then vortexed and centrifuged. These final samples were visually analyzed to determine which dSPE blends removed the most pigmentation. Recoveries of 48 pesticides and four mycotoxins for the two best pigment removing blends were further analyzed using liquid chromatography coupled to tandem mass spectroscopy (LC-MS/MS).

Seven different blends of the dSPE salts were evaluated during the study:

A. 150 mg MgSO4, 50 mg PSA, 50 mg C18, 50 mg Chlorofiltr®

B. 150 mg MgSO4, 50 mg C18, 50 mg Chlorofiltr®

C. 150 mg MgSO4, 50 mg PSA

D. 150 mg MgSO4, 25 mg C18

E. 150 mg MgSO4, 50 mg PSA, 50 mg C18

F. 150 mg MgSO4, 25 mg PSA, 7.5 mg GCB

G. 150 mg MgSO4, 50 mg PSA, 50 mg C18, 50 mg GCB

Based on visual analysis, blends A, F, and G were determined to be the best for the removal of chlorophyll across all four cannabis strains, and the removal of further pigmentation in the two strains containing purple pigment, Purple Drink and Grand Daddy. The two blends chosen for the further LC-MS/MS analysis were A and F, as the large quantities of GCB present in blend G have previously been shown to be detrimental to the recovery of pesticides.

Comparison of the pesticide and mycotoxin recoveries of blend A, containing UCT’s novel Chlorofiltr®, versus blend F, containing small amounts of GCB, revealed that blend A was the most effective at recovery. Blend A was found to have an average recovery of 75.6%, compared to blend F’s average of 67.6%. 

As a result of this, UTC’s study concluded that the most effective way to remove green and purple pigmentation from cannabis plants during sample preparation is through a blend of dSPE salts containing MgSO4, PSA, C18, and Chlorofiltr®. This blend also maximized the recovery of pesticides and mycotoxins, which is important as the accurate analysis of pesticides present on cannabis samples is a huge public health issue in every jurisdiction that permits cannabis use. 


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