Thin Layer ChromatographyTable of contents
Thin layer chromatography (TLC) is a chromatographic technique used to separate the components of a mixture using a thin stationary phase supported by an inert backing. It may be performed on the analytical scale as a means of monitoring the progress of a reaction, or on the preparative scale to purify small amounts of a compound. TLC is an analytical tool widely used because of its simplicity, relative low cost, high sensitivity, and speed of separation.TLC functions on the same principle as all chromatography: a compound will have different affinities for the mobile and stationary phases, and this affects the speed at which it migrates. The goal of TLC is to obtain well defined, well separated spots.
Retention FactorAfter a separation is complete, individual compounds appear as spots separated vertically. Each spot has a retention factor (Rf) which is equal to the distance migrated over the total distance covered by the solvent. The Rf formula is \[ R_f= \dfrac{\text{distance traveled by sample}}{\text{distance traveled by solvent}} \] The \( R_f\) value can be used to identify compounds due to their uniqueness to each compound. When comparing two different compounds under the same conditions, the compound with the larger \( R_f\) value is less polar because it does not stick to the stationary phase as long as the polar compound, which would have a lower \( R_f\) value. \( R_f\) values and reproducibility can be affected by a number of different factors such as layer thickness, moisture on the TLC plate, vessel saturation, temperature, depth of mobile phase, nature of the TLC plate, sample size, and solvent parameters. These effects normally cause an increase in \( R_f\) values. However, in the case of layer thickness, the \( R_f\) value would decrease because the mobile phase moves slower up the plate. If it is desired to express positions relative to the position of another substance, x, the \( R_x\) (relative retention value) can be calculated: \[ R_x= \dfrac{\text{distance of compound from origin}}{\text{distance of compound x from origin}} \] \( R_x\) can be greater than 1. ApparatusPlatesAs stated earlier, TLC plates (also known as chromatoplates) can be prepared in the lab, but are most commonly purchased. Silica gel and alumina are among the most common stationary phases, but others are available as well. Many plates incorporate a compound which fluoresces under short-wave UV (254 nm). The backing of TLC plates is often composed of glass, aluminum, or plastic. Glass plates are chemically inert and best withstand reactive stains and heat, but are brittle and can be difficult to cut. Aluminum and plastic plates can be cut with scissors, but aluminum may not withstand strongly acidic or oxidizing stains, and plastic does not withstand the high heat required to develop many stains. Aluminum and plastic plates are also flexible, which may result in flaking of the stationary phase. Never under any circumstances touch the face of a TLC plate with your fingers as contamination from skin oils or residues on gloves can obscure results. Instead, always handle them by the edges, or with forceps. The properties of your sample should be considered when selecting the stationary phase. As shown below in Table 1, silica gel can be exclusively used for amino acids and hydrocarbons. It is also important to note that silica gel is acidic. Therefore, silica gel offers poor separation of basic samples and can cause a deterioration of acid-labile molecules. This would be true for alumina plates in acidic solutions as well. It is important to note that there are differences between silica gel and alumina. Alumina is basic and it will not separate sample sizes as large as silica gel would at a given layer thickness. Also, alumina is more chemically reactive than silica gel and as a result, would require more care of compounds and compound classes. This care would avoid decomposition and rearrangement of the sample.
Chromatographic Columns is a good reference to learn more about the different types of columns and stationary phases. SolventProper solvent selection is perhaps the most important aspect of TLC, and determining the best solvent may require a degree of trial and error. As with plate selection, keep in mind the chemical properties of the analytes. A common starting solvent is 1:1 hexane:ethyl acetate. Varying the ratio can have a pronounced effect of Rf. Rf values range from 0 to 1--0 means that the solvent polarity is very low and 1 means the solvent polarity is very high. When performing your experiment, you do not want your values to be 0 or 1 because your components that you are separating have different polarities. If the value is 0, you need to increase your solvent polarity because the sample is not moving and sticking to the stationary phase. If the value is 1, you need to decrease your solvent polarity because the compound was not able to separate. If you know that one component of a mixture is insoluble in a given solvent, but another component is freely soluble in it, it often gives good separations. How fast the compounds travel up the plate depends on two things:
You should be able to determine which by looking at the Rf value. Acids, bases, and strongly polar compounds often produce streaks rather than spots in neutral solvents. Streaks make it difficult to calculate an Rf and may occlude other spots. Adding a few percent of acetic or formic acid to the solvent can correct streaking with acids. Similarly for bases, adding a few percent triethylamine can improve results. For polar compounds adding a few percent methanol can also improve results. The volatility of solvents should also be considered when chemical stains are to be used. Any solvent left on the plate may react with the stain and conceal spots. Many solvents can be removed by allowing them to sit on the bench for a few minutes, but very nonvolatile solvents may require time in a vacuum chamber. Volatile solvents should only be used once. If the mobile phase is used repeatedly, results will not be consistent or reproducible. Useful Solvent Mixtures
Pipettes
Spotting and DevelopingDeveloping a TLC plate requires a developing chamber or vessel. This can be as simple as a wide-mouth jar, but more specialized pieces of glassware to accommodate large plates are available. The chamber should contain enough solvent to just cover the bottom. It should also contain a piece of filter paper, or other absorbent material to saturate the atmosphere with solvent vapors. Finally, it should have a lid or other covering to minimize evaporation.
An example is shown below: Figure 1: TLC Sequence
Below is a TLC plate under UV light: Figure 2: TLC plate under UV light.
VisualizingIf fluorescent plates are used, a number of compounds can be seen by illuminating the plate with short-wave UV. Quenching causes dark spots on the surface of the plate. These dark patches should be circled with a pencil. For compounds which are not UV active, a number of chemical stains can be used. These can be very general, or they can be specific for a particular molecule or functional group. Iodine is among the most common stains. Plates are placed in a jar containing iodine crystals, or covered in silica gel with iodine dispersed throughout, for approximately one minute. Most organic compounds will be temporarily stained brown. Some popular general use stains are Permanganate, ceric ammonium molybdate (CAM), and p-anisaldehyde. These can be kept in jars which plates are dipped into, or in spray bottles. To develop a plate with permanganate, spray or dip the plate and heat it with a heat-gun. Hold the plate face up 10 to 20 cm above the heat gun until the bulk water evaporates. Then move the plate to 5 to 10 cm above the heat gun and heat it until white/yellow/brown spots appear. Overheating will turn the entire plate brown, obscuring the spots. If glass plates are used it is often easier to see spots through the backing because it is harder to overheat. CAM and p-anisaldehyde stained plates are developed similarly. Overheating CAM stained plates turns everything blue. Attached is a pdf of common stains for use in TLC, what the stains are good for and how to make them. This was taken from the Dalton research group at the university of washington. http://depts.washington.edu/eooptic/ Advantages of TLCTLC is very simple to use and inexpensive. Undergraduates can be taught this technique and apply its similar principles to other chromatographic techniques. There are little materials needed for TLC (chamber,watch glass, capillary, plate, solvent, pencil, and UV-light). Therefore, once the best solvent is found, it can be applied to other techniques such as High performance liquid chromatography. More than 1 compound can be separated on a TLC plate as long as the mobile phase is preferred for each compound. The solvents for the TLC plate can be changed easily and it is possible to use several different solvents depending on your desired results. As stated earlier, TLC can be used to ensure purity of a compound. It is very easy to check the purity using a UV-light. Identification of most compounds can be done simply by checking Rf literature values. You can modify the chromatography conditions easily to increase the optimization for resolution of a specific component. Disadvantages of TLCTLC plates do not have long stationary phases. Therefore, the length of separation is limited compared to other chromatographic techniques. Also, the detection limit is a lot higher. If you would need a lower detection limit, one would have to use other chromatographic techniques. Common Problems in TLCThere are common problems in TLC that should be avoided. Normally, these problems can be solved or avoided if taught proper techniques. Over-large SpotsSpotting sizes of your sample should be not be larger than 1-2 mm in diameter. The component spots will never be larger than or smaller than your sample origin spot. If you have an over-large spot, this could cause overlapping of other component spots with similar Rf values on your TLC plate. If overlapping occurs, it would prove difficult to resolve the different components. Uneven Advance of Solvent FrontUneven advance of the mobile phase is a common problem encountered in TLC. Consequences would be inaccurate Rf values due to the uneven advance of sample origin spots. This uneven advance can be caused by a few factors listed below.
Rarely, water is used as a solvent because it produces an uneven curve front which is mainly accounted for by its surface tension. StreakingIf the sample spot is too concentrated, the substance will travel up the stationary phase as a streak rather than a single separated spot. In other words, the solvent can not handle the concentrated sample and in result, moves as much of the substance as it can up the stationary phase. The substance that it can not move is left behind. This can be eliminated by diluting the sample solution. To ensure that you have enough solution, use a short-wave UV light to see if the spot is visible (normally purple in color), as stated earlier. SpottingThe sample should be above the solvent level. If the solvent level covers the sample, the sample spot will be washed off into the solvent before it travels up the TLC plate. An example is shown below: ExamplesBelow are TLC plates to help show how samples separate. These images are chromatograms of Figure 3: Chromatogram of TLC plate
black ink taken from http://en.wikipedia.org/wiki/File:TLC_black_ink.jpg Figure 4: Chromatogram of 10 essential oils
10 essential oils taken from http://en.wikipedia.org/wiki/File:TL...ntial-Oils.jpg Figures 5-11:Chromatograph of green leaves (7 stages)
extract of green leaves in its 7 stages of development taken from http://en.wikipedia.org/wiki/Thin_la...chromatography Outside Links
References
Outside Links
Problems and SolutionsFigure 3: TLC plate under UV light with values for distance traveled of solvent and components.
Given: #1=1.4 cm #2= 1.5 cm #3= 3.1 cm #4= 3.6 cm
Using only the given information and the above figure, answer the problems listed below.
Answers
Contributors
This page viewed 136192 times
 UC Davis ChemWiki by University of California, Davis is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. TweetPermissions beyond the scope of this license may be available at copyright@ucdavis.edu. Terms of Use Powered by Mindtouch Core 2010
You must login to post a comment.
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
