Gas Chromotography and Mass Spec

Chromotography is a technique used to separate out a mixture of chemicals. There are two essential parts of chromatography. The first is a mobile phase, the gas or liquid that moves over the surface of the stationary phase. The stationary phase is the phase that stays in place during the separation. An easy way to think of chromatography is as a race. All the chemicals start out at the same position, however due to their different "athletic ability" or ability to travel over the stationary solid, they begin to spread out. As the liquid or gas moves over the solid phase, some of the molecules get pulled into the stationary phase for a short amount of time before being let back into the liquid or gas phase. Each of the different chemicals in the mixture experience this interaction with the stationary phase in different ways. When the analytes pass through the column they are separated by two factors, volatility, the tendency of a compound to vaporize and polarity, tendency of a molecule to be attracted or repelled by electrical charges. More volatile and less polar samples travel through the column quickly. The below video is an awesome animation of what happens in chromatography and shows you what I mean by "racing." Skip to 32 seconds in.

 

In gas chromatography, we start with a liquid sample that is vaporized before it is transferred into the column. The column is a very thin capillary tube anywhere from 15-60 meters long, that is coiled inside the oven of the instrument. The oven is heated before the sample is injected and is usually programmed to raise the temperature a certain amount every so often. The reason for this is the less volatile chemicals need an extra push to make it through the column in a reasonable amount of time.  As the compounds elute, they are quantified by the detector. Retention time is the time it took for an analyte to elute from the column. A chromatogram is the plot of the intensity and the retention time. For a specific column and program, each analyte will have a comparable retention time which can be used to identify the compounds in the sample.

When paired with mass spectrometry, this can be an extremely powerful tool in analytical chemistry. A mass spectrometer ionizes molecules, or breaks them apart into smaller pieces. The ions, small charged pieces of molecules, are then accelerated so they have the same kinetic energy and then deflected by a magnetic field. The lighter a chunk of a molecule is, the more it is deflected. These ions are then detected by the machine. Each organic molecule has a different mass spectra due to different fragmentation, which allows the ability to identify them in unknown samples.

When paired with gas chromatography, we can obtain mass spectras for each analyte that gets separated. It is then possible to definitively identify each compound in a sample. Although mass spectra seem like they would be tough to analyze, in analytical chemistry it is fairly simple. It only takes a fraction of a second to do a library search to match the spectra of the unknown with a known spectra. Then it's as simple as matching and confirming peaks.

GCMS spectra of Cannabinoids (Tetrahydrocannabinol, Cannabidiol, Cannabinol), the active ingredients in marijuana. 

Above is a chromatogram and spectra I obtained of a cannabinoid sample using GCMS. There are three major peaks, being tetrahydrocannabinol, cannabidiol, and cannabinol, which as we can see separated successfully. The correlating mass spectra for the middle peak at 8.152 is below it. Looks crazy right? It really isn't so bad. Below are the sample mass spectra and the mass spectra from the library. All of the major peaks we expect to see for THC are there, so it is a positive match. See, not so bad.

Mass spectra of the middle peak (8.152) above the library search that matched the spectra. A match for delta-9-tetrahydrocannabinol or THC

Hopefully you're leaving with a better understanding of one way chemists identify chemicals and if I'm lucky, maybe you'll even agree with me on how awesome this technology is.