Medical Marijuana Potency Testing

Medical Cannabis Potency Testing using the SRI 8610C GC

SRI Gas Chromatograph ( GC ) configuration 8610-0090 is perfect for testing the potency of Medical Cannabis. While most GCs require high pres-sure gas cylinders such as hydro-gen and helium to operate, this GC operates on air only. The air is supplied by a built-in “whisper quiet” air compressor. The air is dried ( water removed ) by the SRI gas stream dryer ( part# 8670-5850 ) which passes the air stream through a Nafion tube which is im-mersed in dessicant beads. This GC configuration also in-cludes a 12 vial sample heater which maintains the extracted samples at 50C for better reproducibility.

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The GC includes SRI’s Thermionic Ionization Detector ( TID ) which is able to measure the cannabinoid molecules using the principles of surface ionization using air as a reaction gas. The cannabinoid molecules CBD THC CBN Are separated by a 15 meter metal capillary column which is heated to 200 degrees centigrade in the col-umn oven. The entire GC plugs into any Win-dows computer using a USB ca-ble.



SRI’s PeakSimple software is in-cluded with the GC. PeakSimple software collects the GC data and generates a calibrated result which can be printed or transferred to other programs such as Excel or Word. The chromatogram hard-copy printout at right shows the three peaks CBD, THC and CBN which were injected to calibrate the GC. An actual canna-bis sample is shown at right. The THC content is calculated to be 12.35%, while the CBD and CBN were very low. This is typi-cal of most real world medical cannabis sam-ples.



Set the column oven tem-perature to 200 degrees isothermal for 5 minutes. Set the Integration parameters as shown



Prepare the calibration standard by ordering the individual com-pounds from a chromatography supplier like Restek ( restek.com ) 800-356-1688 The standards are available at a concentration of 1000ng/ul in Methanol. No license is required to purchase. The Restek part numbers are: CBD #34011 THC #34067 CBN #34010 Break the glass ampoule and transfer the contents into a 2ml septum vial ( Restek #21154 and #24495 ). Restek provides one free vial with each standard. You will end up with three vials: CBD at 1000ng/ul THC at 1000ng/ul CBN at 1000ng/ul We will refer to these as the “primary standards”



Use the 100ul syringe, which is in-cluded with the SRI GC,( Restek#24863 ) to transfer 100ul of each 1000ng/ul ( primary )standard into another 2ml vial. We will refer to this as the “working standard”. You will end up with 300ul of work-ing standard containing 333ng/ul each of the three compounds ( CBD,THC and CBN ). Rinse the syringe first then: Use the 10ul syringe delivered with the GC ( SRI #8670-9550 ) to with-draw 3-4ul of the working stan-dard. Puncture the septum rather than open the vial to avoid letting the methanol solvent evaporate each time the vial is opened. Pump the plunger several times to get rid of air bubbles.



With 3-4ul of liquid in the syringe, hold the needle vertically or at least slanted upwards so any air bubbles will rise towards the nee-dle. With air bubbles removed, push the plunger to the 1ul mark. It is important to be as precise as pos-sible. Wipe the needle with your fingers or a tissue to remove any liquid from the outside of the needle. Pull the plunger back to the 3ul mark and note the amount of liq-uid. It should be 1.6-1.8 ul be-cause the needle also contains .6-8ul and this adds to the 1ul you measured with the plunger. Leave the plunger at the 3ul mark.



With the plunger still at the 3ul mark, place the needle up against the septum ( but not poking through it yet ). Press the Start Run button. Insert the syringe all the way through the septum as far as it will go. Immediately depress the plunger. Twist the syringe one half turn ( to wipe off any liquid on the tip of the needle ) and then with-draw the syringe.



You should see three peaks. Identify the three peaks so that each peak is defined by a “retention window”. See the Peak-Simple tutorial describing the proc-ess of creating retention windows. Calibrate each peak by creating a calibration curve. See the Peak-Simple tutorial describing this process. In the calibration curve enter the amount of standard you just in-jected. This will be 333 ( for 333ng/ul ). Type this number in the top left cell of the spreadsheet in the calibration curve. Then click the Accept New button to transfer the peak’s area into the top row 2nd column. Save the curve under some name.



Navigate to the View/Results screen to see the report. The answers will be in the units of nanograms per microliter ( ng/ul ) because that is the unit we calibrated in. Click the Integration button at the bottom of the screen. The Stan-dard weight and sample weight fields will be set to 1.000. Change the Standard weight to the number 4.00 and the Sample weight to the number 100.00. This will convert the 333ng/ul an-swer to 13.32 percent which is what most people want to know, the percentage of THC in the cannabis. You are now calibrated and ready to inject real cannabis samples.



Remove the cap from the 40ml vial and place it on the balance. The balance should be capable of reading 1 milligram ( .001 gram ). A balance like this can be pur-chased brand new for less than $300 on E-bay. With the 40 ml vial on the balance tare the reading ( make the read-ing 0.000 ). Then carefully add 100 milligrams of manicured can-nabis. Drop the bits of cannabis into the vial slowly until the reading is close to 100 milligrams. Don’t worry if you are slightly un-der or above 100. In the photo at right, the reading is 98 milligrams which is close enough. You will enter the reading in the sample weight field in PeakSimple soft-ware which will mathematically correct the calcu-lated answer to compensate for weights slightly above or below 100.



Remove the 40ml vial from the bal-ance and fill to the neck of the vial with 91% IPA. You can use 70% IPA, methanol, ethanol or other solvents, but drug store IPA is the least expensive and works equally well. Non-polar solvents like hex-ane are not recommended be-cause they do not extract the THC acid as well as polar solvents. Shake the vial for a few seconds and then let it sit for about 20 min-utes. Use the 10ul syringe which comes with the GC to inject 1ul of the ex-tract as shown previously with the calibration standard. It is impor-tant to be very precise with the sy-ringe since the overall accuracy of the test depends on this.



A real cannabis sample will look something like the chroma-togram at right. There will be one big peak ( THC ) and a much smaller for CBD. CBN may or may not be detected. There may be other peaks which are not CBD, THC or CBN. These other peaks are cannabinoids for which there are no calibration standard available at this time. The Results screen will show the concentration of all peaks detected based on the calibration we have previously done. Print the chromatogram and results for a hard-copy record of the analy-sis.


There may be other un-

identified peaks

THC

Enter the actual sample weight from

the balance to correct for

more or less than exactly 100 milli-

grams of cannabis in the vial

Medical Marijuana Potency Testing

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