Solvent choice

http://blog.restek.com/?p=3018

Medical Marijuana Solvent Extraction Efficiency – Potency Determinations with GC-FID
August 3rd, 2011 by Jack Cochran


Amanda Rigdon and I recently investigated the extraction efficiency of various solvents for medical marijuana potency determinations (although technically we’re not working with medical marijuana; instead we use seized illicit marijuana and did the work under the auspices of the Penn State University Police Department with Randy Hoffman, an Evidence Technician there).

We used an Agilent GC-FID with split injection on a SkyTM 4mm Precision Liner with Wool and a 15m x 0.25mm x 0.25µm Rxi-5Sil MS. A cannabinoids standard containing cannabidiol, delta-9-THC, and cannabinol was used for calibration and quantification. The last eluting compound of interest, cannabinol, had a retention time of about 4.85 min.

One consideration for extraction efficiency in marijuana potency determination is that THCA is the main source for THC determined with GC. (THCA, or delta-9- tetrahydrocannabinolcarboxylic acid, decarboxylates during smoking, or other heating, including in a hot GC inlet, to delta-9-THC.) Solvent polarity index (higher value means more polar solvent) was used to assure a range of tested solvents from non-polar (hexane) to highly polar (acetonitrile and methanol) as we tried to achieve maximum extraction efficiency of THCA from marijuana. Approximately 0.2g ground samples (except in the case of acetonitrile where we ran out of marijuana and only used just over 0.1g) were extracted with 40mL solvent in a precleaned VOA vial.

As seen in the bar graph, methanol was the most efficient extraction solvent for delta-9-THCA (analyzed as THC). Generally, the delta-9-THCA extraction efficiency correlated with solvent polarity, although not dramatically so. We had hoped that the non-polar solvent, hexane, would be equally efficient as the polar solvents for THCA extraction for one very important reason that can be seen in the photograph of the extracted samples, and that is, reduced extraction of chlorophyll by the non-polar solvent. Chlorophyll, a non-volatile substance providing the green color to the extracts quickly degrades GC inlet and column performance and could lead to erroneous potency determinations over time without timely GC inlet maintenance.

More later on the LC analysis of THCA from my colleague Amanda…







Hexane brings across the least chlorophyll because it is non-polar. Boils at 68-69 °C, 341.6-342.2 K, 155-156 °F

Rotary evaporation is most often and conveniently applied to separate "low boiling" solvents such a n-hexane or ethyl acetate from compounds which are solid at room temperature and pressure. However, careful application also allows removal of a solvent from a sample containing a liquid compound if there is minimal co-evaporation (azeotropic behavior), and a sufficient difference in boiling points at the chosen temperature and reduced pressure.

Solvent	Chemical formula	δD Dispersion	δP Polar	δH Hydrogen bonding
Non-polar solvents
Hexane	CH3-CH2-CH2-CH2-CH2-CH3	14.9	0.0	0.0
Benzene	C6H6	18.4	0.0	2.0
Toluene	C6H5-CH3	18.0	1.4	2.0
Diethyl ether	CH3CH2-O-CH2-CH3	14.5	2.9	4.6
Chloroform	CHCl3	17.8	3.1	5.7
1,4-Dioxane	/-CH2-CH2-O-CH2-CH2-O-\	17.5	1.8	9.0

Boiling point

68-69 °C, 341.6-342.2 K, 155-156 °F