gas+chromatography


 * Gas chromatography** is an analytical technique used to separate mixtures of volatile substances.

A typical gas chromatograph is made up of a few simple components:
 * an injection port
 * a column
 * an oven
 * a pump
 * a detector



The concept is that the sample will enter the column and evaporate, then be carried through the column (the **stationary phase**) by the "carrier gas" (i.e. **mobile phase**). The more polar or larger the molecules in the sample, the slower they will travel. This allows them to separate by polarity and/or mass.

Once they travel through the column tube, they will encounter the detector. The time at which it takes a sample to travel from the injection point to the detector is called the **retention time**.

The slower a sample travels, the larger its retention time will be. Be aware that this is opposite of thin-layer chromatography, where more polar samples will have smaller //retention factors//.

Here is a typical output from a gas chromatograph:



Some advantages of gas chromatography include:
 * small sample size (microliter volumes) to conserve sample
 * high sensitivity to small sample amounts
 * can use different carrier gases
 * can use different column lengths or types
 * can output to other analytical detectors, such as mass spectroscopy

Of course, a GC is pretty expensive, so that can be a disadvantage.

Watch this video on how a mixture of compounds is separated inside a GC column. media type="youtube" key="q0pM-k0SvOQ" height="315" width="560"