Tools of the Trade: Chromatography

So…where were we? Oh yeah, Chrom-a, chrom-a, ah ah, Chromatography! (Yeah, I just made a Lady Gaga reference) When most chemicals are made, the crude reaction mixture typically contains some unwanted side products, unreacted starting material as well as many other chemicals involved in the reaction such as catalysts. In order to be used, these chemicals will have to be separated and purified from this crude mixture. This is where chromatography comes into play. Chromatography simply is a fancy-pants chemistry term for the act of separating a complex chemical mixture into its individual components, typically based on the polarity of the chemicals.

The polarity of a molecule is somewhat analogous to that of a regular metal magnet. Certain elements, like fluorine and oxygen, are greedy and do not like to share their electrons with other atoms (this is what chemists refer to as electronegativity). This means that the electrons in covalent bonds (bonds made by two atoms sharing electrons) tend to lie on the more electronegative atom. The uneven sharing causes a slight negative charge on the more electronegative atom (one pole of the “magnet”) and a slight positive charge on the other atom (the other pole of the “magnet”). In a broad sense, polar molecules tend to stick to other polar molecules and likewise for nonpolar molecules. This phenomenon is why oil (very nonpolar) and vinegar (polar) do not mix together.

Schematic of a Gas Chromatograph

All chromatography consists of a stationary phase and a mobile phase. The molecules that we are trying to separate stick to the stationary phase. Since each molecule has a specific polarity, they interact differently with the stationary phase. The variations in interactions cause different molecules in the mixture to move at different speeds through the system, which causes them to separate. The mobile phase serves to move all the molecules through the system.

 In the case of the gas chromatograph (GC), which was mentioned in my earlier post on mass spectroscopy, the mobile phase is a gas such as nitrogen, and the stationary phase is a tiny tube, called a column, lined with a thin polymer film. The GCMS instrument is extremely useful because the GC separates the mixture into individual components that can then be analyzed by the mass spectrometer. If the complex solution were put directly into the MS without separation, we would not be able to definitively determine what compounds were in the sample.

Source:

http://en.wikipedia.org/wiki/Chromatography