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Liquid Chromatography

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Liquid chromatography; another useful topic in understanding the foundation and principles of chromatography. So far, we have learnt about the basics and applications of chromatography on our special article series. What we meant when we told you that chromatography is a complex analytical separation and identification technique is that it consists of numerous interrelated and integrated concepts. These concepts originate from the classification of chromatography into its different types.

What is liquid chromatography

Chromatography can be classified on the basis of the physical states of the stationary phase and the mobile phase. Liquid chromatography is the term used when classifying a chromatographic procedure on the basis of mobile phase. As its name suggests, a liquid mobile phase is used while performing liquid chromatography. In contrast to that, the stationary phase employed in liquid chromatography could either be solid or liquid in nature.

Historical Significance of liquid chromatography

The first chromatographic technique ever applied in the early 20th century by the Russian botanist M. Tsvet was also a liquid chromatographic process. The first Nobel prize regarding chromatography was jointly awarded to Archer John Porter Martin and Richard Laurence Millington Syne for the invention of partition chromatography, again a liquid chromatographic protocol.

Types of liquid chromatography

There are different sub-types of a liquid chromatographic process as shown in the figure below:

Solid-liquid chromatography

A solid stationary phase against a liquid mobile phase gives the liquid chromatographic process its name: solid-liquid chromatography. This type of chromatography could either be normal phase or reverse phased. In case of normal phase, the solid stationary phase is more polar than the liquid mobile phase components and vice versa in reverse phase chromatography.

In both the cases, the solute molecules from the analyte mixture get adsorbed onto the solid stationary phase. Different kinds of intermolecular forces of attraction are involved for the retention/adsorption of solute molecules onto the solid surface.

Based on the type of retention force, the solid-liquid chromatography can be specifically named. In accordance with this concept, ion-exchange chromatography, size/molecular exclusion chromatography and affinity chromatography are all solid-liquid types of liquid chromatography. The solid stationary phase could either be held on a planar surface or packed into a column. Thin-layer chromatography (TLC) for instance, is a type of solid-liquid chromatography. It operates on the adsorption mechanism; it is always performed in a normal phase mode and the stationary phase in this case is the silica gel or alumina coated on an inert surface.

All the other types of solid-liquid chromatography mentioned above are performed using a solid stationary phase packed into a column.

Liquid chromatography with stationary phase held on a plane. Image by jove.com
         Liquid chromatography with stationary phase packed in a column.  
Video source: reddit.com                                                                                                

Liquid-liquid chromatography

Liquid-liquid chromatography is the second main type of liquid chromatography. In this type, in addition to a liquid mobile phase, the stationary phase used is also liquid in nature. The liquid based stationary phase is coated onto a solid support.

Paper chromatography is the most popular and widely used example of liquid-liquid chromatography. While performing paper chromatography, water is the stationary phase which is trapped in the cellulose layers of a chromatographic paper, thus a planar chromatographic medium. Separation of analyte components occur on the basis of partitioning between the two phases thus it is a partition type of chromatography.

High performance liquid chromatography (HPLC) however is the most versatile liquid chromatographic technique. It falls both under solid-liquid and liquid-liquid types of chromatography based on the stationary phase used. Adsorption of solute components onto the stationary phase occurs in case of the former while separation takes place on the basis of partitioning in later. Similarly, HPLC can be performed in both normal as well as in reverse phase modes. Whatsoever, in all the cases, the stationary phase is compactly packed in a high-definition column as you can learn in our article, exclusively dedicated for understanding HPLC.

Conclusion

We can responsibly conclude that liquid chromatography is an umbrella term which includes all the major types of chromatographic techniques except gas chromatography. It is therefore an integral part for studying chromatography and this article will act as a starting point for you while learning the diverse topic i.e., chromatography.

For more valuable information on this topic, you may like the following sources:

  1. Difference between liquid chromatography and gas chromatography
  2. A video tutorial on liquid-liquid chromatography
  3. The use of liquid chromatography in life science applications
  4. Pharmaceutical applications of liquid chromatography

References  

1. Calderon, L. d. A. (2016). Chromatography: The Most Versatile Method of Chemical Analysis.

2. Danielson, N. D. (2003). Liquid Chromatography. Encyclopedia of Physical Science and Technology (Third Edition). R. A. Meyers. New York, Academic Press: 673-700.

3. M.Younas (2017). Organic Spectroscopy and Chromatography.

Liquid Chromatography

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