Help
Options
Didn't know it?
click below
click below
Knew it?
click below
click below
Don't Know
Remaining cards (0)
Know
retry
shuffle
restart
0:00
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
TLC
| Question | Answer |
|---|---|
| How are you supposed to hold the TLC plate? | You are supposed to hold the TLC plate with tweezers or at the edges — avoiding touching the top or cradling it in your hand. Your finger oils could tamper with the results. |
| Why should you not use ink to mark the TLC plate? | You should not use ink to mark the plate, because pencil graphite (carbon) is inert. If ink is used to mark the plate, it will chromatograph just as any other organic compound, interfering with the samples and giving flawed results. |
| What is the purpose of putting filter paper in the chromatography jar? Why is it important that the jar should be capped while developing the TLC plates? | It saturates interior vapor with solvent for better separation and movement. The jar should also be capped so that nothing evaporates, separation occurs with ease, no impurities become involved, and nothing tampers with the distance moved/rf values. |
| A student ran TLC on a sample and after developing the plate only one spot is seen at the starting point. The student concluded the sample contains only one pure compound. Is his conclusion justified? Explain why his conclusion is correct or wrong? | He's wrong, because this is not an indication of purity; it’s more likely that the compound didn’t move because it’s not soluble or it’s too polar and stuck to the stationary phase. The student should change the TLC conditions and try again. |
| Who made chromatography? | Milkail S. Tsweet developed it to separate plant pigments. “Chroma” means color and “graphein” means write. |
| Chromatography: | the separation of compounds that employ that system with two phases: a stationary phase and a mobile phase. |
| Stationary Phase: | the solid surface the sample sticks to. In TLC, this is usually ground alumina or silica particles coated as a thin layer on a glass slide. Therefore, the more polar molecules to be separated, the stronger the attraction to the stationary phase. |
| Mobile Phase: | the solvent or solvent mixture that moves up the plate by capillary action. It is usually liquid (e.g., a mix of ethyl acetate and hexane). It dissolves the sample and carries it up the plate. |
| Affinity Comparison: | a component with a high affinity for the mobile phase moves relatively quickly through the chromatographic system, whereas one with a high affinity for the solid phase moves more slowly. |
| Thin Layer Chromatography: | a method of separation that includes a finely divided adsorbent solid/liquid (stationary phase) spread over a glass plate and liquid as a mobile phase to move along the plate. |
| The factors that affect the separation depends on: | the nature of the stationary phase (adsorbent), the polarity of the mobile phase, the rate of elution (flow) of the mobile phase through the stationary phase. |
| Nonpolar Vs. Polar Molecules: | the more polar the molecule, the more likely it is to stick to the stationary phase and not move. Meanwhile, the more nonpolar the molecule is, the more likely it is to move far and quickly. |
| Alumina vs. Silica Gel: | alumina is stronger/active stationary phase that increases the binding interaction towards the polar compound. Use silica gel when working with more polar molecules. Use alumina if compound is more nonpolar or if silica doesn’t give good separation. |
| Decreasing Polarity of Compounds (From Highest to Lowest): | acids > phenols > alcohols > aldehydes > ketones > ester > ethers > conjugated diamines and aromatic hydrocarbons > unsaturated hydrocarbons > saturated hydrocarbons. |
| Mobile Phase (from most to least polarity): | acetic acid > water > ethanol > acetone > ethyl acetate > methyl chloride > toluene > cyclohexane |
| What happens when one uses an eluent (mobile phase) with too high a polarity with respect to the samples? | all components may elute (move) very quickly. The eluent competes strongly with the stationary phase for binding to the compounds, displacing them too easily. |
| What happens if one uses an eluent (mobile phase) with too low a polarity with respect to the samples? | The samples may barely move (or not move at all). The eluent is not strong enough to break the interactions between the samples and the polar stationary phase (e.g., silica gel). |
| If the mixture’s components do not move very far… | Add a polar solvent such as ether or ethyl acetate to the hexanes for competition. If they run with the solvent front, increase the proportion of the nonpolar solvent because the mobile phase is probably too polar and competing with the stationary. |
| What happens during developing? | Different components in the mixture travel at different rates (partitioning between the mobile phase and the stationary phase) along with the solvent, thus separating into different components. |
| Rf: | distance traveled by the substance / distance traveled by the solvent. Measure from the baseline (where you spotted the sample) to the center of the spot. Measure from the baseline to the solvent front (the farthest point the solvent reached). |
| Free: | dissolved in the liquid or gaseous mobile phase. |
| Adsorbed: | sticking to the surface of the solid stationary phase. |
| K Value: | partition coefficient, measure of the distribution of molecules between the mobile phase and the stationary phase and is similar to the distribution coefficient for liquid/liquid extraction. |
| Why is silica gel a good stationary phase? | Its extended covalent network creates a very polar surface, as does its electropositive characteristics. |
| What happens if a prepared sample is too concentrated? | Streaking can occur on the plate, leading to the overlap of two or more compounds and the skewing of results. |
| How should you spot the plate? | You should spot lightly so that the silica gel isn’t scratched off. You should also make sure that your spots are as small as possible; if the spot is large, then the two or more spots might overlap, leading to improper separation. |
| The best Rf values are between: | 0.3 and 0.7. If two spots travel the same distance or have the same Rf values, they might be the same compound, but this is not always the case. |
| Adsorption: | the strength of the attraction between the compounds and the stationary phase. |
| Separation: | the measure of the elution or migration rate of the compounds. |
| Lycopene: | red pigment of the tomato. |
| B-carotene: | yellow pigment of the carrot, an isomer of lycopene. |
| The three major phases of the experiment are… | spotting (where you lightly tap the open capillary against the paper), development (waiting for the solvent to move) and visualizing (taking note of the results using UV light). |
| If the developing liquid is higher than the applied spot in a thin layer chromatography experiment… | the sample will wash away, dissolve or simply disappear, making it extremely difficult to gain accurate results. If there are results, they will likely be meshed together or not move far. |
| When the TLC plate has been allowed to remain inside the developing chamber too long… | the solvent might run all the way to the top and begin to diffuse. This would make it very difficult to assess the Rf values, because you cannot gain accurate distance measurements. The spots you see could be smudged or blurry. |
| The order of the following in regards to increasing Rf with acetic acid, acetaldehyde, 2-octanone, decane, and 1-butanol would be: | acetic acid < 1-butanol < acetaldehyde < 2-octanone < decane. |
| The result of applying too much compound to a TLC plate would be… | smudged spots or an overlap between different solvents, making it difficult to tell which is which and how far they traveled. It can also affect the separation with solvents struggling to move down the TLC plate. |
| What will be the appearance of a TLC plate if a solvent of too low polarity is used for development? A solvent of too high polarity? | The spots would be very close to the beginning with low polarity and too close to the end with high polarity. |
| Why might it be very difficult to visualize the separation of cis- and trans-2-butene by TLC? | Cis- and trans-2-butene are very difficult to separate by TLC because they have nearly identical polarity, lack polar functional groups, and interact weakly with the polar stationary phase, resulting in similar Rf values. |
| What error is introduced into the determination of an Rf value if the top is left off the developing chamber? | The solvent can evaporate, making the solvent front moving too slowly or fast, causing uneven development of the plate. |
| In what order (top to bottom) would you expect to find naphthalene, butyric acid, and phenyl acetate on a silica gel TLC plate developed with dichloromethane? | Naphthalene, phenyl acetate, butyric acid → because this is also the order of least to polar to most! |
| Caffeine, aspirin, and acetaminophen in order to most to least polar: | Caffeine > acetaminophen > aspirin. Caffeine has four nitrogens and two carbonyl oxygens. Acetaminophen has an amide and alcohol group. Aspirin has a carboxylic acid and ester group but the H-bonding isn’t strong there. |
| Column chromatography vs TLC: | the former uses a column to separate and collect large amounts of compounds, while TLC (Thin-Layer Chromatography) is used to monitor reactions or check purity with small sample amounts. |
Created by:
smurtab