Finn McCool was an Irish giant. Legend has it that he volleyed vast chunks of the coastline at a rival giant, named Benandonner, who slung taunts across the aqueous expanse from Scotland. Separated by the sea, Finn embodied a real mudslinger in his efforts to construct a footpath to bridge the rival giants as each attempted to dole out lessons with fists. And so, this is the story of two juggernauts.
There are instrumental juggernauts in analytical chemistry, two of which are named Gas and Liquid Chromatography (GC and LC, respectively). Like the emptying of a pillowcase on Halloween, chromatography supplies a means of separating out a mixture to see what you’ve got. Chromatography operates using similar principles as performing a chemical extraction, like the subdivision of cannabinoids and terpenes from cannabis, or alpha acids and terpenes from hops. Extraction, after all, is a form of separation.
Analogous to the extractor’s refinement of crude oil to more purified and distinguished phytochemical constituents, chromatography segregates extracted mixtures into singular chemical constituents. To perform the separation, the analyte is passed over a column, typically filled with solid particles like silica. This is called the stationary phase. The analyte’s affinity for the stationary phase dictates specifically when it will disentangle itself from the column (its retention time). Molecules will often interact with the column for differing periods of time, enabling the separation as they are ejected to either their imminent demise in a waste vessel, or marched onwards to collide with the detector. The chromatographer can choreograph things like the temperature of the GC oven, or the level of organic solvent used as a mobile phase in LC, to help persuade molecules to disengage from their interaction with the column.
So really, it’s like any other social function, where some people float by without even a hello, and others don’t know when to get lost.
As the name implies, GC measures gases, and so, an ideal prerequisite for selecting this technique is knowing the volatility of a specific molecule, or how well it converts into the gas phase. If the molecule is a large, non-volatile compound like a carbohydrate, GC may not be the best technique.
In GC, the column is often made of fused silica (glass beads), with a polymer coating. The sample is vaporized, either through heating in the sample vial, where a needle draws an aliquot of the headspace (region of a vial above the liquid) containing gases, or a liquid solution can be injected into the instrument for subsequent heating. Thus, if a labile molecule like tetrahydrocannabinolic acid (THCA) is injected, the temperature used inside the GC can decarboxylate THCA, forming THC. There are methods available for chemically modifying labile target molecules, however. While this adds an extra step to the sample preparation, it enables the parsing of THCA and THC using GC.
Another option is to use liquid chromatography, or LC. In LC, a molecule’s volatility is not of concern. Rather, a target molecule (the analyte) is dissolved in a solvent like water, methanol, or acetonitrile, and is then that solution is passed over the silica column.A standard column in LC is called a C18 column, since there are 18-carbon octadecylsilyl groups chemically bound to the silica. As in GC, analyte constituents will interact with the column to different levels of affinity. The column may or may not be heated.
Often, GC gets the nod if the sample chemistry is favorable for generating gaseous mixtures. Thus, GC is the workhorse of cannabis labs for measurements of residual solvents in cannabis products, or quantification of terpenes that express a sample’s aromatic bouquet. Potency is traditionally evaluated using LC. Thus, both instruments are fundamental to the cannabis lab. And like a good slice and a pint, or Agent Orange and MötleyCrüe, some things just synergize when combined. With LC and GC, that instrumentally powerful tandem involves mass spectrometry, which we’ll talk about some other day.
(In case you were wondering, Finn didn’t dispel his rival. Oh, he tried, but even with giants, there are apparently gradients. Benandonner, you see, was a bit of a towering skyscraper. So, like many tales of ill-fated valor, Finn retreated, and was subsequently dolled up like a baby, by his wife, whose clever thinking saved her husband from the likely beating the thundering Benandonner was bringing. For Benandonner mistook father for child, and thought, “if the son is this big, what’s his pop gonna look like”?)