Flashcards in tlc quiz Deck (81):
what are two uses for tlc experiment?
1. to tell if a compound if its pure
2. to see what the compounds are in the sample by comparing it
What does the TLC observe the sepration of?
biomolecules in biochemistry
What do the tlc identify in forensic chemistry?
illicit drugs or dyes in fibers
What is the stationary phase in the tlc experiment?
the adsorbent material in plates
What are two plate materials in experimetn?
silica and allumina
Why is calcium sulfate added?
it is a binder to hold the adsorbents onto the plate
What do we use as spotters?
capillary tubes with two open ends
what is contained in the reference solution?
a mixture of acetominophen, aspirin, and caffeine
what is the mobile phase called?
How does the solvent rise up the tlc plate?
Via capillary action which makes liquid fill in gaps between solid particles in opposition to gravitational force
why must the mixture that has to be separated be soluble in the eluent?
The spotted sample will stay on the spotted line
What determines the solubility of the compounds in the mobile phase?
intermolucular interactions between eluent and compounds
WHat will happen to the spots if the eluent is too high a polarity with respect to the samples?
the spots will move to the top of the plate
What will happen to the spots if the eluent is too low of polarity with respect to sample?
they will stay towards bottom of the tlc plate
what do polar compounds need polar eluents to overcome?
to overcome attractions to adsorbent
WHat happens if the lines drawn on tlc plate are not drawn lightly and are flaked?
gaps form and stops flow of eleuent or drives eluent in another direction
How can one avoid streaking spots?
making spots small and compact
What should one do between applying applications to same spot
dry completely due to spot becoming too large
why should sample not contain many components?
the spots will then run together and appear a streak
why can we put sports too close to edge?
they will have inaccurate rf
why should solvent level (ethyl acetate) be below spotting line?
to dissolve samples on spotting line
why should bottom of tlc plate be in level with bottom of developing chamber?
the solvent will not rise up evenly on tlc plate otherwise
why should top of developing chamber be placed on top while developing stage?
so solvent doesnt evaporate
What should you do before addicng a new tlc plate?
add more solvent to correct level
what occurs if the solvent rises pasts 5mm line?
the spots in developmental stage will then be difficult to evaluate and the rf will be too high
why should filter paper be in the dev. chamber?
to saturate the chamber with the solvent vapors
why should the plate not be leaning against the filter paper?
the eluent on the filter paper will be absorbed by the adsorbent and it will interfere with the ascending eluent
what is UV light used in visualizing plates?
to observe for conjugated comp
Where do pure compounds show up?
one developed spot directly above where it was spotted on spotting line
Is the less polar Rf, higher or lower Rf?
What has to be constant for the Rf to be constant?
adsorbent and solvent
Do two samples of same compounds have to have same Rfs run from same eluent?
WHat is column chromatography used for?
separation and purification of both solids and liquids for microscale experiments
When does separation begin with in comparison to tlc for column chromo.
with a low polarity solvent that allows compounds to adsorb to the stationary phase. The polarity increases to desorb compounds and allow the compounds to move with mobile phase.
what is one limitation of both types of chromotography?
you cannot use them with volatile compounds that have boiling points of lower than 150 degrees celcius
WHy is TLC is typically used to determine the correct adsorbent and solvent for column chromatography?
TLC and column chromatography use some absorbents or TLC and column chromatography both use silica and alumina
TLC can be used to monitor column chromatography. What specifically does the TLC do to achieve this purpose?
Analyze fractions of column chromatography
What advantage is obtained when a TLC is used to determine the number of components in a mixture?
It is sensitive, fast simple, and inexpensive analytical technique
Which use of TLC allows for one to determine the optimum time to halt a reaction?
monitoring progress of a rxn
You spot a sample that contains a mixture of 3 compounds of varying polarities on a TLC plate and develop it. Assuming that all other factors (adsorbent, technique) were appropriate, what kind of solvent would cause all the spots to congregate in the following manner when visualized under UV light?
spots congregated at top of plate
- solvent with too high a polarity in respect to the mixtures
spots congregated at bottom of plate
-a solvent with too low a polarity in respect to the mixtures
You spot two very nonpolar compounds on a TLC plate and develop it in order to distinguish the two compounds. Which adsorbent would most likely produce the result where all spots go all the way to the top of the plate when visualized under UV light if all other factors (solvent, technique) are accounted for?
A solvent too high in polarity?
You spot two very polar compounds on a TLC plate and develop it in order to distinguish the two compounds. Which adsorbent would most likely produce the result where all spots stay at the bottom of the plate when visualized under UV light if all other factors (solvent, technique) are accounted for?
A solvent that is too low in polarity?
During the development stage of the TLC experiment, a student accidentally left the top off the developing chamber and the eluent evaporated. Why is this a problem?
Because the developing chamber wouldn’t be saturated with vapors and the TLC plate would become ineffective and messy
A student develops a TLC plate and realizes during the visualization stage that the eluent had dissolved the samples during the development stage. How could the student have prevented this situation?
•By making sure the eluent is below the spotting line
During a TLC experiment, a student used only 0.5 mL of solvent to dissolve a solid unknown instead of the necessary 1.5 mL before spotting the unknown on the TLC plate. What is the likely observation on the developed plate when it is visualized?
There would be too much sample during spotting which would cause poor separation of the compound and it would be smeared
Why is it important that solvents for TLC (eluents) have low boiling points?
So they can saturate the developing chamber with their vapors effectively
Why is it important that solvents for TLC (eluents) have low viscosities?
so they move up faster
o Why should you never use ink to mark a TLC plate?
• It will chromatograph like any other organic compound and give flawed results
o Why is it acceptable to always use a pencil to mark the TLC plate?
• Because the graphite (carbon) is inert
o What advantage is obtained when a TLC is used to determine the number of components in a mixture?
• Aids in planning further analytical and separation steps
o What is the reason a TLC would be taken after one carries out a distillation?
• To examine effectiveness of purification
o One of the uses of TLC is to monitor the progress of a reaction. What kind of info does TLC provide in this instance?
• When to halt rxn and the effect of changing temp, concentrations and solvent w/o having to isolate product
o Why can TLC be use to determine the appropriate conditions for a column chromatographic separation?
• Adsorbents used for TLC same as adsorbents for column chromatography
o One of the uses of TLC is to determine the number of components in a mixtures. What does this help in achieving?
• Aids in planning further analytical and separation step
o How do you ensure that you make your spots on the spotting line of the TLC plate small and compact?
• By touching the capillary tube lightly on the TLC
o A student accidentally lets the eluent run all the way to the top of the TLC plate…
• The spot in the visualization phase will be difficult to evaluate or the Rf will be too high
o During the development stage of the TLC experiment, a student accidentally left the top off the developing chamber and the eluent evaporated…
• This causes the development to be messy and ineffective
o List 2 instances that you have to be concerned about not flaking off the absorbent of the TLC plate
• Marking the tlc plate for spotting and spotting the plate
o You notice that the tlc plate you are using has some absorbent flaked off. What may be the problem that might occur if you proceed to use that plat for your experiment?
• The solvent will not ride up the plate effectively
o What might happen if you use silica as your absorbent to distinguish between a set of very nonpolar solutes?
• Samples travel all the way too the top of the plate or will travel with the solvent front
o A student develops a TLC plate w/o placing filter paper in the developing chamber, why would this omission hinder the rising of the solvent up the TLC plate?
• Developing chamber would not be saturated with solvent vapors
o Why is it important to remove the tlc plate from the developing chamber as soon as the solvent reaches the solvent front?
• Rf will be too high or spots at visualization stage will be difficult to evaluate
o List 2 precautions you take while spotting
• Spots must be small and compact and sample must not be too concentrated
o Why should one not stick the capillary tube directly into the reagent bottle
• To avoid contamination
o What precaution does one have to take when marking a tlc plate?
• Not to flake off the absorbent
o What might happen if you use silica as your absorbent to distinguish between a set of very polar solutes
• Samples will not migrate very far from spotting line
o You can use 2 different visualization methods to visualize colorless substances, list which method is used for what kind of colorless compound
• Uv- conjugated and iodine for most compounds
o What is important that the top of the chromotograph jar be kept on while developing a tlc plate?
• So solvent does not evaporate
o Why is it important not to dissolve the unknowns in more than 1 ml of methylene chloride?
• Samples will be too dilute to be spotted
o Why should bores of capillaries be small
• To avoid smearing of spots
o What are typical characteristics for solvents used as eluents
• Low boiling points that allow them to be easily evaporated and low viscosities that allow them to migrate rapidly
o Characteristics of solvents that allow them to migrate rapidly
• Low boiling point, low viscosity, high fluidity
o Adsorbent materials and specific uses (what kinds of adsorbents would be better for what kind of substances)
• Alumina- when anhydrous, is more active – it will adsorb substances more strong; adsorbent of choice when the separation involves relatively nonpolar substances such as hydrocarbons, alkyl halides, ethers, aldehydes, and ketones
• Silica- less active- used to separate more polar substrates such as alcohols, carboxylic acids, and amines
This is the order of solute migration
Alkanes > alkyl halides > alkenes > dienes > aromatic hydrocarbons > aromatic halides > ethers > esters > ketones > aldehydes > amines > alcohols > phenols> carboxylic acids > sulfonic acids
WHat are the six uses of tlc
• To determine the number of components in a mixture
• To determine the identity of two substances
• If two substances spotted on the TLC plate give spots in identical locations, they may be identical. If the spots are not the same, the substances cannot be the same
• To monitor the progress of a reaction
• By sampling a reaction from time to time, you can watch reactants disappear and reappear using TLC. The optimum time to talk the reaction can be determined, and the effect of changing such variables as temperature, concentrations, and solvents can be followed without having to isolate the product
• To determine the effectiveness of a purification
• To determine the appropriate conditions for column chromatography
• Column chromatography is used to separate and purify larger quantities of material. The correct adsorbent and solvent used to carryout the chromatography can be determined rapidly by TLC (which would be unsatisfactory for the large quantities of material)
• To monitor column chromatography
• As the column chromatography is carried out and the solvent is collected in small flasks, the fractions can be analyzed by TLC to determine which ones have the desired components of mixture
What are the effects of using too polar or too nonpolar a solvent in respect to sample?
• Too polar- all spots will ove to top of TLC plate
• Too nonpolar- all spots will stay towards the bottom of the TLC plate
o Why would a more polar compound move slowly and travel the shortest distance on a TLC plate?
• Because they will adsorb more strongly to the polar stationary phase
o Why is it important to use a separate spotter for each of your samples?
• Because overlapping of spots will make the identification of separated components difficult
o Besides making the sample spot small, why is it important that the bore of the spotting capillaries be small?
• Because the liquid that is drawn into them should not be able to flow out to form a drop
o Why is it suggested that tweezers be used to place the TLC plate in the developing chamber?
• Because oils from your fingers can sometimes smear or ruin a TLC plate