Introduction

Chromatography, although primarily a separation technique, is mostly employed in chemical analysis. Nevertheless, to a limited extent, it is also used for preparative purposes, particularly for the isolation of relatively small amounts of materials that have comparatively high intrinsic value. Chromatography is probably the most powerful and versatile technique available to the modern analyst. In a single step process it can separate a mixture into its individual components and simultaneously provide an quantitative estimate of each constituent. Samples may be gaseous, liquid or solid in nature and can range in complexity from a simple blend of two entantiomers to a multi component mixture containing widely differing chemical species. Furthermore, the analysis can be carried out, at one extreme, on a very costly and complex instrument, and at the other, on a simple, inexpensive thin layer plate.

The first scientist to recognize chromatography as an efficient method of separation was the Russian botanist Tswett (1), who used a simple form of liquid-solid chromatography to separate a number of plant pigments. The colored bands he produced on the adsorbent bed evoked the term chromatography for this type of separation (color writing). Although color has little to do with modern chromatography, the name has persisted and, despite its irrelevance, is still used for all separation techniques that employ the essential requisites for a chromatographic separation, viz. a mobile phase and a stationary phase.

The technique, as described by Tswett was largely ignored for a along time and it was not until the late 1930s and early 1940s that Martin and Synge(2) introduced liquid-liquid chromatography by supporting the stationary phase, in this case water, on silica in a packed bed and used it to separate some acetyl amino acids. In their paper, they recommended replacing the liquid mobile phase by a suitable gas, as the transfer of sample between the two phases would be faster, and thus provide more efficient separations. In this manner, the concept of gas chromatography was created but again, little notice was taken of the suggestion and it was left to Martin himself and A. T. James to bring the concept to practical reality nearly a decade later. In the same publication in 1941, the essential requirements for HPLC (High Performance Liquid Chromatography) were unambiguously defined,

"Thus, the smallest HETP (the highest efficiency) should be obtainable by using very small particles and a high pressure difference across the column".

Despite his recommendations, however, it was nearly four decades before this concept were taken seriously and the predicted high efficiency liquid chromatography columns became a reality. By the mid 1960s the development of all aspects of chromatography were virtually complete and since then, despite the plethora of publications that have appeared on the subject, the vast majority has dealt with applications of the technique and only a minority with fundamental aspects of the subject and novel instrumentation concepts.

Today, chromatography is an extremely versatile technique; it can separate gases, and volatile substances by GC, in-volatile chemicals and materials of extremely high molecular weight (including biopolymers) by LC and if necessary very inexpensively by TLC. All three techniques, (GC), (LC) and TLC have common features that classify them as chromatography systems.