Wednesday, December 14, 2016

Chromatography - Paper Chromatography

We have seen in our previous post that any chromatographic technique consists of a stationary phase and a mobile phase. In the case of paper chromatography, which is one of the type of plane chromatography, paper acts as a stationary phase while the mobile phase generally consists of a mixture of water and an organic liquid. Lets understand the principle and types of paper chromatography.

The sample is spotted on the filter paper at the base line or the spotting line. This filter paper is then placed in the suitable solvent. When the solvent rises up via the capillary action, the individual components in the sample get separated. Different components travel at different rates and the most soluble substance move further on the filter paper than the least soluble substance. The solvent travels the farthest on the filter paper and it leaves the line called as ‘solvent front’.

The Retention Factor or Rf Value:
Okay, the components got separated, but now, the question is, how to identify the components of the mixture? The answer to this is by calculating the retention factor or the Rf value. The distance traveled by a particular component is constant relative to the solvent as long as all other factors (like, the type of paper and the exact composition of the solvent) are kept constant .

Rf is the distance moved by the component divided by the distance traveled up the paper by the solvent as can be seen in the adjacent diagram.

For example, if the component traveled 5 cm (distance 'a' in the adjacent diagram) and the solvent traveled 15 cm (distance 'b' in the adjacent diagram), then the Rf value of that particular component will be 5/15 or (a/b) i.e., (distance traveled by paper/distance traveled by the solvent) = 0.3.

Nature of Paper:
The paper used in paper chromatography consists of a highly purified cellulose (homopolysaccharide of glucose). The paper exhibits weak ion exchange adsorptive properties. In recent times, various forms of modified paper are available wherein the paper has been impregnated with alumina, silica gel, ion exchange resins, etc. To get excellent results, the exact type of paper to be used is important. Generally, Whatmann filter papers are used for paper chromatography. Just to get an idea, this is the composition of Whatmann filter paper no.1:
a-cellulose – 98-99%b-cellulose – 0.3-1%Pentosans – 0.4-0.8%Ash – 0.07-0.1%Ether soluble matter – 0.015 – 0.1%

Types of Paper Chromatography:
1. Ascending Paper Chromatography
The filter paper is put into an airtight tank containing the solvent (solvent tank). The paper is supported on a frame or on the thread with the help of the clips. This is carried out in an air-tight container lined with paper and saturated with solvent so that there is a constant atmosphere and the separation takes place in constant temperature. Here, the solvent will ascend into the paper via the capillary action i.e., the solvent will travel in the upward direction of the chromatographic paper. The substance which is the most polar will be at the bottom of the tank while the least polar substance will be on the top end of the tank.

2. Descending Paper Chromatography
In descending type paper chromatography, the upper end of the filter paper is put into the solvent mixture contained in a narrow trough. This paper is mounted near the top of the container. This develops along with gravity which makes this technique little quick and easy. In contrast to the ascending technique, the substance which is the most polar will be at the top of the tank while the least polar substance will be on the bottom of the tank.

3. Radial Paper Chromatography
In this type of chromatography, there is a circular paper. The sample is placed at the center of the paper. This paper is tied parallel (in a horizontal way) to the petri-plate which contains the solvent (As can be seen in the 'side view' diagram). There is a wick of the paper and this wick is dipped inside the solvent. As a result, the solvent rises through the wick via capillary action and the components get separated in a circular zone.

4. Two-dimensional Chromatography
In this technique, a square or a rectangular paper is used. The first step is to perform an ascending type where the sample is placed at the bottom corner (say, right corner) of the paper saturated with and dipped in the solvent. After few hours, the paper is turned at a right angle, 90degree, clockwise, to the direction of first run with a different solvent. 

This was all about different techniques of paper chromatography. In the next post, we will discuss about anohter method of plane chromatography which is thin layer chromatogrpahy.