<Li> The plate is visualized . As some plates are pre-coated with a phosphor such as zinc sulfide, allowing many compounds to be visualized by using ultraviolet light; dark spots appear where the compounds block the UV light from striking the plate . Alternatively, plates can be sprayed or immersed in chemicals after elution . Various visualising agents react with the spots to produce visible results . </Li> <P> Different compounds in the sample mixture travel at different rates due to the differences in their attraction to the stationary phase and because of differences in solubility in the solvent . By changing the solvent, or perhaps using a mixture, the separation of components (measured by the R value) can be adjusted . Also, the separation achieved with a TLC plate can be used to estimate the separation of a flash chromatography column . (A compound elutes from a column when the amount of solvent collected is equal to 1 / R .) Chemists often use TLC to develop a protocol for separation by chromatography and they use TLC to determine which fractions contain the desired compounds . </P> <P> Separation of compounds is based on the competition of the solute and the mobile phase for binding sites on the stationary phase . For instance, if normal - phase silica gel is used as the stationary phase, it can be considered polar . Given two compounds that differ in polarity, the more polar compound has a stronger interaction with the silica and is, therefore, better able to displace the mobile phase from the available binding sites . As a consequence, the less polar compound moves higher up the plate (resulting in a higher R value). If the mobile phase is changed to a more polar solvent or mixture of solvents, it becomes better at binding to the polar plate and therefore displacing solutes from it, so all compounds on the TLC plate will move higher up the plate . It is commonly said that "strong" solvents (eluents) push the analyzed compounds up the plate, whereas "weak" eluents barely move them . The order of strength / weakness depends on the coating (stationary phase) of the TLC plate . For silica gel - coated TLC plates, the eluent strength increases in the following order: perfluoroalkane (weakest), hexane, pentane, carbon tetrachloride, benzene / toluene, dichloromethane, diethyl ether, ethyl acetate, acetonitrile, acetone, 2 - propanol / n - butanol, water, methanol, triethylamine, acetic acid, formic acid (strongest). For C18 - coated plates the order is reverse . In other words, when the stationary phase is polar and the mobile phase is non-polar, the method is normal - phase as opposed to reverse - phase . This means that if a mixture of ethyl acetate and hexane as the mobile phase is used, adding more ethyl acetate results in higher R values for all compounds on the TLC plate . Changing the polarity of the mobile phase will normally not result in reversed order of running of the compounds on the TLC plate . An eluotropic series can be used as a guide in selecting a mobile phase . If a reversed order of running of the compounds is desired, an apolar stationary phase should be used, such as C18 - functionalized silica . </P> <P> As the chemicals being separated may be colorless, several methods exist to visualize the spots: </P>

What are the nonpolar and polar phases in tlc
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