Gas Chromatography

Operational Theory

The gas chromatography is a device that permits us to separate and identify volatile liquids. Separation is accomplished by passing the sample mixture through a fused silica micro capillary whose interior has been coated with a non-polar material known as the stationary phase (see diagram at right). As the original sample mixture is pushed through the capillary (with compressed He gas), the various sample molecules begin to interact with the stationary phase to an extent that depends on a particular molecule's polarity. Non-polar sample molecules interact strongly with the stationary phase (solid dots ) while relatively polar molecules (circles ) are unaffected. Thus non-polar molecules are held back while polar molecules proceed rapidly through the column. The effect is much like what happens during a long-distance marathon race. All runners begin at the same time but as the race proceeds, runners of various speeds and abilities sort out into packs with the fastest runners out in front and the slowest further back. Also note that the non-polar molecule's strong interaction with the stationary phase is an example of "like dissolves like".

The capillary column gas chromatography diagramed at left consists of a sample injection port, the column/oven assembly, and the detector hydrogen flame. 1.0 µ L samples are injected onto the column using a 10 µ Hamilton syringe. Compressed He gas carries the sample through the 25 m long capillary column in about 3-4 minutes. Separated sample components emerge one by one at the end of the column where they are burned/ionized by a hydrogen/air flame whose electrical conductivity consequently goes up. These changes are monitored and displayed on a graph (see integrator description below). The column oven serves to keep the capillary and sample mixture at a constant temperature above the boiling points of any of the sample components.