Tuesday, 2 July 2013

What is Chromatography?

Chromatography in broad terms is the separation of components in a mixture by distribution of the components between two phases – one that is static i.e. the stationary phase and one that moves i.e. the mobile phase. As the mobile phase moves, the components of the mixture move at different rates on the stationary phase as shown in Figure 1.1. Chromatography can be carried out on an apparatus that is planar or a column.

Figure 1:1 Theory of column chromatography separations

When chromatography is performed on a flat surface such as a sheet e.g. paper or plate which could be glass, plastic or metal, it is called Planar Chromatography. The separated components are characterised by distance travelled on the plate relative to the distance travelled by the mobile phase (eluent) i.e. the retention factor, Rf.  There are two types of planar chromatography:
  • Paper Chromatography is when the stationary phase is a special grade cellulose paper simply referred to as chromatography paper, which could be unmodified or modified with other chemicals.
  • Thin-Layer Chromatography (TLC) is when the stationary phase material usually silica or alumina has been coated on to a plastic, glass or metal plate.
When chromatography is performed in a column as previously shown in figure 1.1, which again could be glass, metal or plastic, it is called Column Chromatography (also referred to as Elution Chromatography) because separated components actually get eluted from the column. The separated components are characterised by the time it takes to travel through the column i.e. the retention time, tR. Each separated component will have its own unique tR for a given set of experimental conditions.
Column chromatography techniques vary with different types of stationary phases coupled to different types of mobile phases. Classification purely on the basis of the physical nature of the mobile phase gives techniques such as:
  • Gas Chromatography (GC), in which the mobile phase is a gas, typically hydrogen or helium.
  • Supercritical Fluid Chromatography (SFC) implies that the mobile phase is in a supercritical state i.e. neither gas nor liquid.
  • Liquid Chromatography (LC) obviously means that the mobile phase is a liquid. Note that we have liquid-planar chromatography e.g. TLC as well as liquid-column chromatography techniques. 
An excerpt from book - HPLC Fundamentals by Bunmi Adeyoju