5.1 CHROMATOGRAPHIC TECHNIQUES

Chromatographic techniques have probably the widest and most versatile applications in analytical chemistry. They include a number of distinct separation techniques differring from each other. In a broader terminology chromatography is a technique for separating a sample into various fractions, and then measuring or identifying the fractions in some manner. The components to be separated are distributed between two mutually immiscible phases. The heart of any chromatography is the stationary phase, which is sometimes a solid but most commonly a liquid. The stationary phase is attached to a support, a solid inert material. The sample, often in vapor form or dissolved in a solvent, is moved across or through the stationary phase. It is pushed along by a liquid or a gas the mobile phase. As the mobile phase moves through the stationary phase, the sample components undergo a series of exchanges (partitions) many times between the two phases. What is exploited are differences in the chemical and physical properties of the components in the sample. These differences govern the rate of movement (called migration) of the individual components. When a sample component has emerged from the outlet of a chromatograph, it is said to have been eluted. Components emerge from the system ideally as gaussian-shaped peaks and in the order of increasing interaction with the stationary phase. Separation is obtained when one component is retarded sufficiently to prevent overlap with the peak of an adjacent neighbor.