Liquid Column Chromatography:
A thorough discussion on the classification and general principles of chromatography has been presented. While classifying the different chromatographic techniques, the main criteria used were the nature of the mobile phase. It was pointed out that a large number of diversifications are available in the case of liquid chromatography. These are majorly because of shape of the support (column and two dimensional), nature of support (simple and bonded) and the mechanism (adsorption, partition, ion exchange and sieving) responsible for separations. In this unit, it is proposed to discuss liquid column chromatography. Normally, in the liquid column chromatography, the various mechanisms cited above should be involved but in the common parlance of chromatography, the technique includes only two mechanisms, adsorption and partition. Therefore, the discussion in this unit will confine to liquid- solid adsorption and liquid- liquid partition chromatography. In this course, separate units have been assigned for ion exchange and gel sieving chromatography. Therefore, we will be discussing only liquid-solid adsorption chromatography (LSC) and liquid- liquid partition chromatography (LLC). The situation is more or less similar to gas chromatography where we have GSC and GLC.
If we compare the two important types of chromatography, viz gas and liquid chromatography, some of the benefits of liquid chromatography become extremely apparent. The tremendous ability of gas chromatography to separate and analyze complex mixtures is widely appreciated but the drawback of this technique is that only 20% of known organic compounds can be handled satisfactorily by gas chromatography. Liquid chromatography, instead, is not limited through sample volatility or thermal stability. Therefore, liquid chromatography is ideally suitable for the separation of macromolecules, ionic species, labile material products and a huge variety of other high molecular weight compounds. Liquid chromatography also enjoys certain another benefit over gas chromatography in view of the fact in which very difficult separations is frequently more readily achieved through liquid chromatography than through gas chromatography. The other benefits is about the sample recovery. Separated fractions are simply collected and recovery is quantitative. The recovery of separated elements in gas chromatography is also possible but is generally less convenient and less quantitative. It might be significant here to point out that gas chromatography, in common, is faster than liquid chromatography.