As it seems impossible to enter a reply into an already existing forum I´m obliged to start a new thread:
O.K. Ma_Huang here goes the answer to your question:
A petrol ether extract (soak 1hr at room temp., pour off, repeat) of not too old hemp will consist mostly out of cannabinoid acids, such as THC-acid and CBD-acid, depending on the strain one is working with.
These cannabinoid acids can be brought into aqueous solution as alkali salts by washing the petrol-ether extract with dilute KOH for instance (0.1molar KOH with some Na2SO3 thrown in just in case this prevents some oxidation).
Acidification of the aqueous extract and extraction with petrol ether will yield (after washing excess acid away) upon careful evaporation on a water bath (60° centigrade is possible) in the vacuum an oily to glass-like residue, which consists, depending on the strain of hemp one is working with and the quality of the material (seedless?) up to ca 90% out of THC-acid(s).
This will give THC of similar purity upon decarboxylation (heat up to 150° centigrade under vacuum – careful! Foaming! Product sensitive to oxidation).
The initial petrol ether extract of the hemp will (after it has been extensively washed with dilute alkali, excess alkali been removed with dilute citric acid and finally washed with water) yield the neutral cannabinoids, as far as they have been present in the plant material, along with other lipophilic materials.
CBN will still be largely in this "neutral" fraction as the high lipophilicity of cannabinoids hardly allows the purely phenolic materials to form any water-soluble salts on treatment with dilute alkali.
Grass that has been sitting in the dark at room-temperature for 2 years may have about ½ of its original THC-acid content decarboxylated. About 1/3 of the THC may have oxidized to CBN in the same time.
Choice of solvent for extracting hemp:
More polar organic solvents will also extract cannabinoids but also more non-cannabinoids.
The aqueous extraction of cannabinoid acids with dilute alkali works only really well with hemp extracts in non-polar solvents.
Be aware that THC is extremely sensitive to oxidation when it is dissolved in petrol ether or other lipophilic solvents (an exception maybe the solution in fats/vegetable oil, which is quite stable) .
Work fast and avoid high temperature!
Anecdotal reports about bioassays of oxidized material show this material to cause bad headache.
An alcohol extract of hemp will be much more stable and the danger of unwanted oxidation of THC to CBN is greatly reduced, but you will end up with a far less pure product.