TLC tends to produce more useful chromatograms than paper chromatography, which show greater separation of the components in the mixture - and are therefore easier to analyse. The distance a sample travels can depend on the size or the polarity of the molecules involved.
Larger molecules take longer to move up the chromatography paper or TLC plate, whereas smaller molecules are more mobile. Likewise, the polarity of the molecules can affect how far the spots travel, depending on the type of solvent used. How do polar molecules differ from nonpolar molecules? How do polar molecules form hydrogen bonds? How does electronegativity impact polarity of molecules? Why are polar molecules said to have dipoles?
Why are polar molecules hydrophilic? Question 37d How can I calculate the polarity of a solvent? How does polarity relate to electronegativity?
See all questions in Polarity of Molecules. Impact of this question views around the world. Silica gel is used to separate a wide variety of compounds such as hydrocarbons, alcohols, ketones, esters, acids, azo compounds, and amines.
Alumina is also used extensively, and comes in three forms: acidic,. Acidic alumina is used for separating acidic materials such as carboxylic acids and amino acids. Basic alumina is used to separate amines, while neutral alumina can be used to separate non acidic and non basic compounds. Likewise, cellulose, starch and sugars are used to separate natural products, and magnesium silicate is used in the separation of acetylated sugars, steroids and essential oils.
In the diagram above, cotton is added to prevent the sand and silica from running out the bottom. The sand is added on top to create a level surface for the silica to rest on. Next, the silica is added, followed by another layer of sand. This layer of sand protects the silica underneath from being jostled by the addition of sample and solvent later on. It is important to keep silica and all adsorbents both level and moist while running a column.
You want your adsorbent to be as uniform as possible all the way around. If the surface is not level, compounds traveling down one side of the adsorbent will get through faster than those traveling down the other side, which can lead to overlapping bands think of the side with less silica as giving its compounds an unfair head start, which might allow slower-moving, more polar components to travel as fast as faster-moving, less polar components traveling down the other side.
Letting the column go dry will create cracks in the adsorbent, which act as similar shortcuts. A column may be packed either 'wet' by pouring a solvent-adsorbent slurry into the tube or 'dry' by filling it with dry adsorbent.
If it is packed dry, it must still be kept wet once solvent has been added. The mixture to be purified is then dissolved in a small amount of the appropriate solvent and added carefully to the top of the solid adsorbent.
It is added carefully to ensure that the packing is not disturbed. The column is developed by adding more solvent to the top, then collecting the fractions of eluent the compound-containing solution that come out the bottom. For 'flash' column chromatography, moderate air pressure is used to push the solvent through the column.
The success of the separation and the contents of the fractions can be determined by spotting the fractions along with the initial mixture on TLC. A column may be developed with a single solvent or a solvent gradient a solvent system which gradually increases in polarity.
For example, a column may be developed first with a low polarity solvent such as hexane, and as fractions are collected the developing solvent is changed to , , and hexane-methylene chloride. A polarity gradient is used for mixtures of compounds with very different polarities. Solvents: A common non-polar solvent for both thin-layer and chromatography is hexane. It can be used with a variety of polar solvents.
The following solvents are listed in approximate order of increasing polarity: cyclohexane, petroleum ether, pentane, carbon tetrachloride, benzene, toluene, chloroform, ethyl ether, ethyl acetate, ethanol, acetone, acetic acid, and methanol. The higher the percentage of polar solvent, the faster compounds will elute.
Elution sequence: An approximate elution sequence, based broadly on polarity, for compounds from fastest to the slowest is hydrocarbons, olefins, ethers, halocarbons, aromatics, ketones, aldehydes, esters, alcohols, amines and acids. Note that the more polar the solvent, the faster compounds elute, regardless of the compounds polarity. This means changing the solvent polarity cannot change the order compounds elute from a TLC or column.
This may seem non intuitive, as it would seem that a polar solvent would move a polar compound farther than a nonpolar compound. To help visualize this concept, consider that solvents will compete with compounds for sites on the stationary phase. A less polar solvent will not compete well, allowing the compounds to remain bound to the stationary phase, resulting in slow elution. A polar solvent will compete well with molecules and will occupy sites on the stationary phase.
It may also be helpful to remember that alumina and silica are much more polar than any organic solvent. Therefore, the stationary phase will always be more polar than the mobile. Gas Chromatography. As in other types of chromatography, the analytes exist in equilibrium between the stationary and mobile phases.
The analytes can be 'stuck' on the adsorbent as a liquid, or moving with the carrier gas as a vapor. GC is somewhat different from the other two methods explored in this experiment in that here the boiling point is the primary property on which the separation depends. However, you will see that if two compounds have similar boiling points but very different polarities, they can be separated by polarity via GC. The gas chromatograph contains a long 6 ft.
This is non-polar like thin layer chromatography and column chromatography. An inert gas, helium in our lab, is passed through the column at a controlled flow rate and serves as the mobile phase. A small amount about one microliter of a liquid sample is injected into the tube, and compounds are detected as they emerge from the outlet.
The detector response is plotted vs. The time it takes an analyte to emerge from the column is called the retention time, RT , and is analogous to R f for TLC. The detector response is proportional to the amount of compound passing through it, so the area under a peak is roughly proportional to the total amount of compound in the sample.
Hence, the ratio of areas in a single chromatogram is approximately equal to the ratio of compounds in the mixture. In the old days these areas were measured by cutting out and weighing the paper for each peak! Our GCs are computer controlled and automatically calculate the retention time and area for each peak. Basic Theory. It is easiest to imagine a GC as a miniature distillation.
A small amount of a mixture of liquids is injected into one end of a long capillary tube. They all heat at the same rate, until the temperature rises to the boiling point of the lowest-boiling liquid in the mixture. It becomes a vapor and is carried along by the helium carrier gas towards the detector. As it travels, the second-lowest-boiling component may boil and begin traveling down the column as well, behind the first fraction.
When the first component reaches the detector, a peak is recorded. If the separation was good, there should be as many peaks as components in the mixture.
Because the column is polar, components will not travel straight through but will be slowed down more or less based on their own polarity. More polar compounds will adsorb on the stationary phase and travel more slowly, leading to longer retention times.
However, this effect is generally minor. So long as your liquids have boiling points that differ by ten degrees or so or more, you are likely to see your components come out in order of boiling point. If, however, you have liquids that boil within a few degrees of one another, polarity may come into play. Rather than choosing a solvent as is done for TLC and column chromatography, one chooses an oven temperature for GC. The oven temperature is analogous to the polarity of the. A high temperature leads to short RT and little separation because all compounds are vaporized and they move at the same rate as the mobile phase.
A very low temperature leads to long or nearly infinite RT since the compounds remain adsorbed on the solid phase. In addition, diffusion causes the peaks to spread out as the RT increases, so compounds that are retained in the column for a long time give broad, ill-defined peaks.
The temperature of the injection port and detector are controlled separately from the oven temperature. The injection port must be hotter than the oven to insure that the entire sample goes into the column rather than condensing in the injector. The temperature of the detector is also set higher than that of the column so that compounds do not condense in it. Gas Flow.
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