Question: An unknown sample was spotted on a silica gel TLC plate and developed in hexane. The Rf value was calculated to be 0.35. Which of the following statements is true if ethyl acetate was used as the developing solvent instead of hexane?
Answer: The compound would travel further, therefore the Rf value would increase
I don't understand what they mean by "compound". Do they mean the sample would travel further? We don't know if the sample is polar or nonpolar to begin with though. If the sample was nonpolar, wouldn't it travel further if the solvent was hexane because it is nonpolar as well?
Plastic, metal or glass plates are available in general, for TLC. These plates have a coating of silica gel or alumina. This coating is called the “sorbent” and serves as the stationary phase. The sorbent is held tightly to the plate with a binder (plaster, hydrated calcium sulfate). A number of solvents can be used to develop a TLC plate. A mixture of varying ratios of hexanes and ethyl acetate works well in most cases. Other solvents include dichloromethane, ethyl ether, a mixture of methanol with dichloromethane (methanol being 10% or less), etc. The developing solvent used, serves as the mobile phase. In normal phase chromatography (which is more commonly used), the stationary phase is more polar than the mobile phase. There are cases where the polari ties are switched or reversed, which is called reverse phase chromatography, which, for example, is used in separation of DNA or RNA molecules. Organic Chemistry: Techniques and Transformations. Therefore, when developing a plate, the more polar components stick more to the silica gel and the relatively less polar components move further along with the developing solvent. This results in a distribution of spots on the TLC plate, which enables one to determine the order of polarities and hence the identities of the samples spotted. A helpful way of determining the polarities is the use of Rf values. The Rf value is the ratio of the distance moved by the solute to the distance moved by the solvent. The optimal value for Rf is 0.2 - 0.4. The choice of a TLC developing solvent is driven by the goal to achieve optimal R f value, in addition to good separation between spots. In order to help visualize the spots applied to a TLC plate, usually, fluorescent indicators are added to the plate (there are plates without these indicators, but we will use the plates with UV indicators). This indicator absorbs and emits UV light, which appears green. The presence of a UV active compound on the plate prevents the UV light from reaching those parts of the plate. This results in a darker coloration on the plate.
Hope this helps.
Dr. Romano