Chromatography is an analytical science in which a complex mixture is separated into its individual constituents as the constituents can be identified and quantified. Since its advent in the nineteen sixties, gas chromatography has caused a major revolution in organic chemistry. More recently, there has been developed apparatus for high pressure liquid chromatography in which a liquid sample is introduced into a chromatographic column and then, instead of being vaporized, is kept in the liquid phase during the separation process. Liquid chromatography offers numerous advantages over gas chromatography. Among them, a liquid chromatographic column can be operated at ambient temperature. Also, separation of the components of the sample mixture in the liquid phase facilitates a degree of control not easily available with gas chromatograph columns. Some compounds which tend to be broken down or suffer unwanted molecular reorganization in a gas chromatographic column can be separated quite readily in a liquid chromatographic column. For example, many organic compounds, because of their polarity, high molecular weight or thermal instability, are not amenable to gas chromatographic techniques but are well suited to high pressure liquid chromatographic analysis. On the other hand, state of the art detectors interface quite readily with gas chromatographs but liquid chromatography tends to be limited by poor sensitivity of the compatible detectors. Substantial effort has been devoted to improvement of the most widely used detectors; refractive index and ultra violet adsorbance. Other techniques have been explored including micro absorption, polarographic, and conductivity detectors.
The early Nobel prize winning work of A. J. P. Martin and R. L. M. Synge Biochem. J. 35, 91, 1358 (1941) set forth the basic liquid chromatographic techniques used in systems today. However, practical application for these techniques have been severely limited by avaliable detectors. As a result, liquid chromatographic analysis is generally a lengthy procedure, often taking hours and even days. The availability of high pressure pumps (in excess of 5,000 psi) permits the use of long, narrow bore (e.g. 1 mm) columns having small diameter packing particles. Use of such small diameter columns minimizes the time required for liquid chromatographic analysis but the level of sensitivity is still below that associated with gas chromatographic systems.
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