A solvent plays an important role in resolution of sample constituents on the filter paper. For the purpose use can be made of a single pure solvent, undiluted solvent with water or a mixture of miscible solvents. The greater the distance between the resolved spots (measured by Rf values) better is the solvent mixture combination.
Solubility plays a critical role. Ideally the constituents should have small but definite solubility in the solvent combination. If the solubility is very high the constituent will move with the solvent upto the solvent front and on the other hand low solubility will result in hardly any movement from the point of application.
Each sample constituent has a polarity different from other constituents. Solvent polarity can be modified by changing the proportion of solvent mixtures. Increase in solvent polarity will improve the spatial movement along the filter paper.
Criteria for selection of a working solvent system
- Solvents should not be toxic or carcinogenic. In such cases adequate handling care should be exercised.
- The solvent mixture composition should not change with time
- Solvent constituents should not chemically react with any of the sample constituents
- Solvents should not interfere with the detection of spots on the developed paper
- Differences in Rf values of any two components should be at least 0.05 for differentiation between two closely spaced spots.
Commonly used solvent combinations
In general solvents can be made by saturating an organic solvent such as n – butanol with water. Several solvent combinations have lesser amounts of water so that polar compounds move slowly. Modifications can be further made by adding an acid, base or a complexing agent. Some suitable solvent combinations for different categories of polar compounds are suggested:
Strongly polar compounds – ethyl acetate: butanol: acetic acid: water in 80:10:5:5 proportion
Polar compounds – 10% methanol or less in dichloromethane
Strongly basic compounds – 10% ammonium hydroxide in methanol and make 1 – 10% mixture of this in dichloromethane