Chromatography

Chromatography is not a spectroscopic technique — it does not probe molecular structure directly. Instead, it is a separation technique that divides a mixture into its individual components. However, when combined with spectroscopic methods (particularly mass spectrometry in GC-MS and HPLC-MS), chromatography becomes an extraordinarily powerful identification tool.

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The Principle of Chromatography

All forms of chromatography work on the same basic principle. A mixture is dissolved in a mobile phase (a liquid or gas) and passed through or over a stationary phase (a solid or a liquid adsorbed onto a solid). The components of the mixture interact differently with the stationary phase:

• Components with a stronger affinity for the stationary phase move more slowly.
• Components with a weaker affinity for the stationary phase (and stronger affinity for the mobile phase) move more quickly.

The result is that different components travel at different rates and become separated.

The separation depends on partition (the distribution of a substance between two phases). Each component has a different partition coefficient — the ratio of its concentration in the stationary phase to its concentration in the mobile phase. The greater the difference in partition coefficients between components, the better the separation.