Planar Chromatography Paper and Thin-Layer

Because of the different interactions involved in partition and adsorption processes, they may be applied to different separation problems. Partition processes are the most effective for the separation of small molecules, especially those in homologous series. Partition chromatography has been widely used for the separation and identification of amino acids, carbohydrates, and fatty acids. Adsorption techniques, represented by ion-exchange chromatography, are most effective when applied to the separation of macromolecules including proteins and nucleic acids. 

In the rest of the chapter, various chromatographic methods will be discussed. You should recognize that no single chromatographic technique relies solely on adsorption or partition effects. Therefore, little emphasis will be placed on a classification of the techniques instead, theoretical and practical aspects will be discussed. 

Because of the similarities in the theory and practice of these two procedures, they will be considered together. Both are examples of partition chromatography. In paper chromatography, the cellulose support is extensively hydrated, so distribution of the solutes occurs between the immobilized water (stationary phase) and the mobile developing solvent. The initial stationary liquid phase in thin-layer chromatography (TLC) is the solvent used to prepare the thin layer of adsorbent. However, as developing solvent molecules move through the stationary phase, polar solvent molecules may bind to the immobilized support and become the stationary phase. 

---

There are two popular LC techniques in which the stationary bed is supported on a planar surface rather than in a column paper chromatography (PC) and thin-layer chromatography (TLC). PC preceded TLC by some 10 to 15 years, and a large number of excellent separations were devised for it. But beginning about 1956, it was found that TLC could also be used for most of these separations and that it was faster, more reproducible, more versatile, and more convenient. As a result, most laboratories have abandoned the use of PC with its large cumbersome glass chambers. Those who have not, continue to use PC because they feel that the original PC methods are superior or because of the lower cost of PC. 

The identification of the target colorant is done directly by UV-Vis spectrometry (if the colorant is unique) or by a previous separation from a mixture. The most available techniques are planar chromatography (paper chromatography and thin layer chromatography) and, especially, high performance liquid chromatography... 

Paper chromatography and thin-layer chromatography (TLC) constitute the planar methods mentioned above. Paper chromatography makes use of a sheet of paper having the consistency of filter paper (cellulose) for the stationary phase. Since such paper is hydrophilic, the stationary phase is actually a thin film of water unintentionally adsorbed on the surface of the paper. Thus, paper chromatography represents a form of partition chromatography only. The mobile phase is always a liquid. 

Planar chromatographic methods include thin-layer chromatography (TLC) and paper chromatography (PC). Each makes use of a flat, relatively thin layer of material that is either self-supporting or is coated on a glass, plastic, or metal surface. The mobile phase moves through the stationary phase by capillary action, sometimes assisted by gravity or an electrical potential. 

Thin-layer chromatography (TLC) is a type of liquid chromatography in which the stationary pease is in the form of a thin layer on a flat surface rather than packed into a tube (column). It is a member of a family cf techniques that include some types of electrophoresis and paper chromatography, more generally referred to as planar chromatccraphy. Since we will not discuss electrophoresis in this section, and since TLC has virtually superseded paper chromategr pby in most analytical... 

Background and principles Thin-layer chromatography is the other most commonly used form of planar chromatography and uses a very similar experimental approach to paper chromatography. The principal difference is that this technique relies on the separation of biomolecules from a mixture on the basis of partition and/or adsorption. There is a distinct difference between the process of adsorption and a/isorption, and they are not interchangeable terms Whereas molecules that are a/isorbed are taken up into , those that are adsorbed stick to a surface. So, in thin-layer chromatography, the mobile phase is adsorbed (sticks to) and subsequently moves along the stationary phase. The stationary phase consists of an adsorbent (sticky) layer on a flat plate or sheet. The most commonly encountered adsorbent layers comprise silica gel, alumina (not aluminium) or cellulose, while popular solvents include hexane, acetone and alcohol. 

Planar arrangement of the chromatographic stationary phase was used by paper chromatography throughout the 1950s and 1960s. However, today s planar chromatc raphy is mainly thin-layer chromatography. 

Currently, most planar chromatography is based on the thin-layer technique, which is faster, has better resolution, and is more sensitive than its paper counterpart. This section is devoted to thin-layer methods. Capillary electrochromatography is described in Section 33D. 

Complementary to paper chromatography, but used more frequently because of the wide availabihty of stationary phases, is thin layer chromatography (TLC). It is simple, fast, reproducible and can achieve high resolution. It is usually performed on a square plate or on strips. A variation of this type of planar chromatography is carried out on a rotating circular plate using an instrument called a Chromatotron. 

Nowadays thin layer chromatography (TLC) has replaced paper chromatography. TLC is now frequently used for the planar separation of anthraquinones, the most frequently used adsorbent being silica [4], Almost every research group has developed their own solvent system and, in Table (2) some examples of eluents are given for the separation of anthraquinones in Rubia tinctorum extracts. Preparative TLC has been applied for the purification of anthraquinones from Rubia tinctorum [78]. 

Thin layer chromatography (TLC) and paper chromatography (PC), which together comprise flat-bed or planar chromatography, are the easiest of all chromatographic methods to perform, and they require only simple and inexpensive equipment. Many of the techniques and principles of TLC and PC are the same. They both readily provide qualitative analytical information, and, with optimization of techniques and materials, can give quantitative data as well. 

---