The History And Development Of Preparative HPLC
Chromatography can be defined as the separation of mixtures by distribution between two or more immiscible phases. Some of these immiscible phases can be gas-liquid, gas-solid, liquid-liquid, liquid-solid, gas-liquid-solid and liquid-liquid-solid. Strictly speaking, a simple liquid-liquid extraction is in fact a chromatographic process. Similarly, distillation is a chromatographic process that involves separation of liquids by condensation of their respective vapours at different points in a column.
Most will remember the school science project of placing an ink blot in the centre of a filter paper and following this by dripping methylated spirits on to the ink. Watching in fascination as concentric circles of various pigments develop is probably the first and sometimes last experience of a chromatographic separation many will encounter. Like too many of our observations the essence of this experiment is to demonstrate that black ink is made up of several different pigments and the underlying process, in this case chromatography is dismissed with blatant disregard.
Fortunately for us, some very clever scientists have seen the ‘wood for the trees’ and have taken these simple observations and developed them into complex highly efficient methods of purification.
The invention of chromatography was rightly accredited to Mikhail Tswett in 1903 for his detailed study of the selective adsorption of leaf pigments on various adsorbents, though somewhat unwittingly, the first demonstrations of preparative chromatography probably stem back to ‘bleaching’ of paraffin by passage through a carbon bed in the 1860’s.
The first column based separations performed in a true industrial setting can be more truly demonstrated by the purification of petroleum on Fuller’s earth in the 1920’s. The 1950’s marked the development of simulated moving bed (SMB) chromatography for the separation of sucrose and fructose in the sugar industry. However, these separations are limited low to medium pressure chromatography since the columns could be packed and operated in place. The high pressure generated by the small particles used as stationary phases in HPLC dictates the use of specialist hardware. The columns are generally machined from a solid ingot in order to avoid the flaws that can be observed in welded columns. The weight of the thick walled columns normally limits the scale at which columns can be manually handled so it is unusual to find pre-packed columns greater than 10cm diameter. Scaling beyond this requires fixed hardware and it can be said that the first true high pressure based preparative chromatographic separations did not arrive until the 1980’s following the invention of dynamic axial compression (DAC) based columns.