OCHM Lab Exam 3
What are some possible sources of error for TLC
- Not closing the developing jar (volatile solvent evaporates) - Contamination - Too much eluent in jar - goes above spotting line - Spots put too closely to each other on the spotting line - may run into each other - Spots put too close to edge of TLC plate o Unequal solvent forces for each spot (not enough solvent surrounding spot) o Leads to inaccurate Rf's - TLC plate leaning against filter paper o Eluent on filter paper can seep through sides of TLC plate can interfere with the movement of ascending eluent
What are typical characteristics for solvents used as eluents
- Volatile - Good balance in polarity with solute - ensures spot differentiation - Mixture must be soluble in solvent - Solvent should be able to make concentric rings with sample between origin and solvent front -low viscosity
A mixture of fluorene, fluorenone and fluorenol is examined by TLC and gives the following Rf values: 0.3, 0.5, 0.8. Identify each spot and rationalize the relative Rf values
-0.8 is fluorene, 0.5 is fluorenone, 0.3 is fluorenol. -Fluorenol is an alcohol, so it is more polar than the ketone fluorenone, giving it a lower Rf value.
What order of elution from a column chromatography procedure might you expect for a mixture of anthracene, 2-napthol, benzaldehyde and benzoic acid?
-Anthracene (least polar), benzaldehyde, 2-napthol, benzoic acid (most polar)
Imagine that the TLC of flask 3, following column chromatography to separate fluorene and fluorenone, shows two spots with Rf 0.4 and 0.6. Identify the spots and comment on the success of separation procedure.
-Fluorenone is 0.4, fluorene is 0.6 -Unsuccessful separation
Explain relative Rf values of fluorene and fluorenone.
-Fluorenone is more polar (ketone), has lower Rf value than fluorene (alkene/aromatic) -More attracted to polar stationary phase (silica gel)
In general, how will Rf values change with increasing solvent polarity?
-Incr. solvent polarity = incr. Rf -Rf = (distance traveled by substance/ distance traveled by solvent) -if solvent is more polar, it will interact with the polar Silica gel, and not travel as far. -So incr. solvent polarity = decr. solvent distance = incr. Rf
What entails Running/Monitoring the column?
-Need a lot of clean dry test tubes, Erlenmeyer flasks or small beakers to collect fractions. -Add appropriate vol. of non polar solvent to top of column. This helps carry non polar compounds down the column while the more polar compounds remain near the top of the column. -Analyzed/monitored by TLC to identify compounds that are present; fractions collected and combined based on TLC
Define: polarity, soluble, solvent front, TLC chamber, spotting solvent, developing solvent, adsorbent, Rf, mobile phase
-Polarity = tendency of a molecule to be attracted to electric charges as a result of uneven electron sharing between atoms of different electronegativities -Soluble = Whether a substance can be dissolved in a liquid. -Solvent front = distance up a TLC plate that the developing solvent travels. --used in determining the Rf value. -TLC chamber = medium in which the TLC is performed. --Capped jar with developing solvent and TLC plate standing straight up to prevent curved travel -Spotting solvent = solvent used to dissolve the compound to be analyzed. --Often ethyl acetate or methylene chloride -Developing solvent = solvent that the TLC is placed into inside the chamber, creates the solvent front -Adsorbent = what holds onto the analyte, usually a solid --a thin, uniform layer of a dry, finely powdered material applied to a suitable support -Rf = Retention factor - The ratio of the distance traveled by the spot over the distance traveled by the solvent front. -Mobile phase = Phase that moves up the plate via capillary action; contains developing solvent with samples
How should one handle a TLC plate and what could interfere with TLC analysis?
-Use clean gloves and forceps and only handle at the edges. - Oils from skin and chemicals from gloves will interfere/contaminate the TLC plate.
In a two-step process, 1-pentene can be converted to 1-pentanol, which can then be oxidized to pentanoic acid. If a mixture of all three compounds was separated by column chromatography, what would the order of elution be for the mixture? Explain how you made this prediction.
1-Pentene will be eluted first because it is the least polar. Next, 1-pentanol will be eluted because it is slightly polar and the -OH bond leads to hydrogen bonding. Last will be the pentanoic acid because it is the most polar and O-H and C=O which may lead to H bonding. (Draw the 3 compounds in order of elution on seperate paper.)
What are the 4 steps of TLC analysis?
1. A dilute (1-3%) solution of the mixture to be analyzed is prepared then deposited/spotted onto the TLC plate. 2. Place the TLC plate in a Developing Chamber that contains a small amount of the solvent chosen as the Mobile Phase. 3. Allow Mobile phase to travel up the plate to separate the mixture into its components. 4. Remove plate from the chamber and allow to dry; then visualize spots on plate under UV light or iodine gas chamber.
Before carrying out column chromatography to separate larger amounts of compounds, it is good lab practice to carry out TLC analysis on a small sample to determine whether or not the chosen solvent system is the appropriate one. What three factors should you consider in analyzing TLC results?
1. Different Rf values for all components 2. No component travels with the solvent front 3. No component stays at the origin
Define the terms eluent, mobile phase, stationary phase, adsorbent
1. Eluent = The solvent that the TLC is placed into inside the chamber. -AKA developing solvent; creates the solvent front, is the mobile phase that rises via cap. action. -In column chromatography, the eluent does the same thing, but moves because of gravity 2. Mobile phase (column) = solvent systems that passes through the stationary phase -Phase that moves up the plate via capillary action; contains developing solvent with samples 3. Stationary phase = contains analyte/absorbent system, remains in column/on TLC plate -Phase that doesn't move during TLC -In column chromatography, its the tightly packed dry component of the column. -normally consists of a finely divided adsorbent used in the form of a thin layer on a supporting material. -The mobile phase (a solution containing the sample) flows over or through the stationary phase to effect the separation. 4. Adsorbent = what holds onto the analyte, usually a solid -a thin, uniform layer of a dry, finely powdered material applied to a suitable support
1. How does TLC reveal purity of a given compound? 2. How does TLC provide information about the identity of a given compound? 3. How can TLC be used to judge completeness of reaction?
1. If you see multiple spots in a lane then it's not pure! 2. TLC provides Rf values so that the Rf values of the crude compound can be compared with the Rf of a known compound. it does NOT tell us polarity but uses polarity to separate compounds. 3. If the reaction did not complete then you'll be able to see a spot/spots from the starting materials (multiple spots)
Define the following terms and give an example of each: 1. Mobile Phase 2. Stationary Phase
1. Mobile Phase: The phase in chromatography that desorbs the compounds and travels DOWN the column and UP the plate.. Usually less polar than stationary phase. Example- 20:80 mixture of methylene chloride/hexanes 2. Stationary Phase: The phase in chromatography that reabsorbs the compound and is immobile. It is more polar than the mobile phase. Example- Silica
List the proper disposal of each of the following after an experimental procedure: 1. A reaction mixture of 2-hexanone and 2-hexanol 2. A solution of saturated sodium chloride which was used to help isolate the product 3. A sillica column which was used to seperate a reaction mixture of 2-hexanone and 2-hexanol
1. NHO 2. Dilute w/ base then D 3. Dry it out then TB
Before separating a mixture on a column, its good practice to carry out TLC on the mixture using the same solvent system. What 3 things should be checked on the TLC plate before carrying out the separation on a column?
1. No component travels with the solvent front 2. No component stays at the origin 3. There is differences in Rf values to differentiate between compounds.
Name 2 things you can do to make the spots on a TLC plate more visible.
1. Place fluorescent TLC plate under a Short UV light 2. Put plate in an iodine gas chamber to darken the spots
How does one spot a TLC plate?
1. Place index finger over one end of spotting capillary tube and dip it into the solution of the mixture to be analyzed. 2. Lightly touch the tip of the capillary to the correct spot labeled on the origin line on the TLC plate. Should be 1-2 mm. 3. Allow the first spot to evaporate, then apply a 2nd spot on top of the 1st one and allow to evaporate again. 4. Repeat 7 times to ensure there is a sufficient amount of the mixture on the plate; without the spot becoming too large.
List the proper disposal category for the following: 1. Dry magnesium sulfate that is free of organic residue. 2. 18M H2SO4 from a nitration reaction. 3. A mixture of hexane and ethyl acetate from column chromatography.
1. TB 2.Dilute w/ base; D 3. NHO
1. What was the adsorbent for the TLC portion? 2. What was the absorbent for the column chromatography portion?
1. TLC: silica gel (SiO2) 2. CC: alumina (Al2O3)
Rf of starting material A is 0.2 in 100% pentane 1. Draw TLC plate 2. What change would you make to the solvent and why? 3. Draw the TLC plate that would result from your changes
1. The TLC plate has a spot is a fifth of the way up the solvent front 2. Make solvent more polar to help pull the starting material up the plate. 3. The spot is higher up the plate, and the solvent is lower
Benzoic acid, methyl benzoate and benzene are spotted on the same TLC plate, in lanes 1, 2 and 3 respectively. 1. Draw that TLC plate 2. explain the position the relative Rf values
1. The benzoic acid would have the lowest Rf value, followed by methyl benzoate, followed by benzene 2. Due to decreasing polarity as you go from benzoic acid to benzene.
Rfs of starting material A and product B are both 0.8 in 100% ethyl acetate 1. Draw TLC plate 2. Based on TLC are A and B the same compound? Explain 3. Would you make a change to the solvent? If yes, what change? If not, why not?
1. Two spots traveled same distance (and too far) 2. Can't know if same compound because the spots went too far up the plate to yield information about the products -TLC Rfs should preferably be between 0.2-0.3 and 0.75 3. change should be made to the solvent to make it less polar and reduce the Rf values -The addition of an alkane like hexane would be perfect for reducing the polarity and allow us to read the plates effectively.
Before spotting a TLC plate, it is important to mark the origin. Should this be done with a pen or pencil? Explain why your choice is correct.
A pencil because the ink from the pen contains organic compounds and will wash out into the stationary phase in the developing chamber and contaminate the compounds you are wanting to separate. Also, you will not only be separating out your desired mixture but also the various dyes in in the ink.
For each pair of compounds, predict which would have the higher Rf. Assume that you are using silica plates, and a solvent of medium polarity. For each case explain your answer. A. 1-chlorobutane and 1-butanol B. Salicyclic acid and methyl salicylate C. Toluene and p-nitrotoluene D. Octanal and octanoic acid
A. 1-chlorobutane because it is less polar B. Methyl salicylate because it is less polar C. Toluene because it is less polar D. Octanal because is it less polar
You have just finished a TLC and column chromatography procedure. How would you dispose of the following items: A. A reaction mixture consisting of benzoic acid and methyl benzoate which was separated on a column using hexane and ethyl acetate as the mobile phase B. Alumina which has been thoroughly dried and is free of organic residues C. Sulfuric acid which was used as a catalyst in the reaction D. Methylene chloride from the TLC analysis
A. NHO B. Trash bin C. Neutralize and pour down the drain D. HO
Rank the following types of solutes in order of Rf value: A: carboxylic acid, aldehyde, alcohol, aromatic ether B: Long chain conjugated alkene, ketone, ester, phenol
A: carboxylic acid (most polar) < alcohol < aldehyde < aromatic ether B: phenol (most polar) < ketone < ester < alkene
Rank the following organic solvents in order of polarity: A: hexane, ethyl acetate, acetone, methanol B: diethyl ether, ethanol, acetic acid, toluene
A: hexane < ethyl acetate < acetone < methanol B: toluene < diethyl ether < ethanol < acetic acid
Absorbent materials and specific uses (what kind of adsorbents would be better for what kind of substances)
Absorbent = stationary phase = material of TLC plate Alumina: - More active - Use for nonpolar substances will adsorb (relatively "unreactive") nonpolar molecules more - If used for polar compounds, polar compounds will stay at bottom of TLC plate very low Rf values Silica: - Less reactive - Use for polar substances will not adsorb (relatively "reactive") poplar molecules as much - If used for nonpolar compounds, nonpolar compounds will run to top of TLC plate very high Rf values
In carrying out TLC analysis, it is good practice to put a piece of filter paper in the developing chamber before developing the plate. Why is this done and why is it helpful in TLC analysis?
Adding the piece of filter paper ensures a saturated environment for the plate to develop in. This allows for more consistent and reproducible results.
Once an adsorbent is packed into a column, sand is added, and the eluting solvent drop is never permitted to drop below the top of the adsorbent. Explain why each of these is important in successfully carrying out column chromatography
Adsorbent is packed into the column slowly to avoid bubbles. It is the stationary phase and thus aids in separation of the components. Sand is added so that the mixture being separated is not disturbed when more solvent is added to the column. The solvent drop is not allowed to go below the top of the adsorbent because it will cause veins and bubbles to appear which would result in poor separation.
Compare/Contrast TLC and CC
BOTH: -Are examples of solid-liquid chromatography; Where the SOLID is the STATIONARY phase and the LIQUID is the MOBILE phase. -Use an absorbent such as silica/alumina for the stationary phase. -Use organic solvents for the mobile phase. CONTRAST: TLC: -Compounds are separated as they move UP the plate with the Mobile Phase. -Used to separate and identify active ingredients. -Uses small sample of mixture to determine what chromatographic conditions will allow for the best separation of the components of the mixture. CC: -Compounds are separated as they travel DOWN a column with the Mobile Phase. -Used to separate and isolate the components of a reaction mixture. -Uses a Larger Sample of mixture. - Components isolated via evaporation of the solvent
Why must the column never be allowed to run dry?
Bubbles and channels will form if it runs dry, so no partitioning between the mobile and stationary phases can take place because there is no stationary phase present. IF the compounds cannot partition between the two phases, they cannot separate.
Explain how collecting too large of fractions would effect separation.
Collecting a large fraction could result in the collection of more than one compound. i.e. compounds cannot separate/isolate
What is capillary action and when it is observed (all instances) in the TLC experiment?
Definition: ability of a liquid to fill spaces between solid particles in opposition to gravitational force Occurrences in TLC: - When spotting TLC plate: solvent flows into spotting capillary via cap action - During plate developing step: solvent travels up TLC plate via capillary action
Describe the procedure used to pack a chromatography column beginning from empty column. Include all steps until the point that you begin adding solvent to separate the components of the mixture.
Dry pack method works best. 1. Add pea-sized plug of cotton to bottom of column. 2. Clamp the column vertically ( important to achieve good separation); make sure the stopcock is closed. 3. Add the nonpolar solvent; fill to 1/2-2/3 fullIn the hood. 4. Using Powder funnel, SLOWLY add the absorbent (silica) (Stationary phase) while tapping on the column with a pen. this prevents cracks and bubbles in the column. 5. Drain excess solvent to JUST ABOVE top of adsorbent. Important for preventing the column from running dry resulting in bubbles and channels developing. 6. Carefully add the entire VERY CONCENTRATED mixture you wish to separate to the top of the column. 7. Allow column to drain via stopcock until solution hits the top of the absorbent, then close stopcock. 8. Add thin layer of sand to top of column. This makes it easier to add more solvent to the column without disturbing the top of the column
A column chromatography procedure calls for collecting fractions with a volume of 2.0 mL. Instead, a student collects 5 mL fractions. How might this students ability to separate compounds be affected by this change? Use diagrams of TLC to illustrate what might happen.
He may collect more than one compound in his fractions and this will lead to poor separation. (make illustrations)
A mixture of m-toluic acid and ethyl benzoate is to be separated using a column chromatography, the reaction mixture needs to be as concentrated as possible when it is added to the column. How would the ability to separate these two compounds be affected if the sample was more dilute?
If it were more dilute, the sample wouldn't hit the top of the column at the same time and poor separation would result.
In carrying out TLC analysis, it is important to keep the solvent level below the origin. Why is this so important?
If the solvent reached the origin on the plate it could wash away the spots on the plate; causing faulty results.
What are the effects of using a too polar or too nonpolar solvent?
If use too polar solvent mixture components run to TOP of TLC plate high Rf values If use too nonpolar solvent mixture components will stay at BOTTOM of TLC plate low Rf values
What is Aliquat-336? Why is it used?
It is a phase-transfer catalyst. Used b/c in the presence of fluorene, fluorene can be deprotonated by hydroxide anion to form a carbanion. The carbanion then can attack dioxygen in the atmosphere to form a hydroperoxide which then loses water to form the ketone, fluorenone.
Partition Coefficient (K)
K= (Affinity of A for mobile phase)/(Affinity of A for stationary phase) K values for compounds must be sufficiently different to successful separation
Mobile phase
LIQUID -Determines how compounds partition between the 2 phases (ex: strongest affinity for nonpolar compounds? Can be either a non polar or polar solvent: -Polar:ethyl acetate or ethanol -Nonpolar: hexane or ether Best Choice is often a mixture of both polar and non-polar solvent. Usually determined via trial/error.
Arrange the following compounds in the order you would predict them to elute from a chromatography column using a solvent of medium polarity (fastest first): 1-octene octanol hexanoic acid
Most Nonpolar is first; In order of Least polar to Most polar 1-Octene, Octanol, Hexanoic acid
Explain how the word "Partitioning" pertains to chromatography and how it effects the order of elution from a chromatography column.
Partitioning means to separate or split. It pertains to chromatography because you are trying to separate/split the components of a mixture by their distinctive attraction to the mobile phase and the stationary phase. -Compounds that are of low polarity will partition more into the Mobile phase and will travel with the solvent ( "fast runners") -Polar compounds and those capable of hydrogen bonding will partition more into the Stationary phase which will hold those compounds up ( "slow runners") explanation: -compound is placed on stationary phase -mobile phase passed through the stationary phase -mobile phase solubilizes the components -mobile phase carries the individual components at a certain distance through the stationary phase depending on their attraction to both of the phases
Characteristics of solvents used with respect to samples
Polar solvent will carry polar substances up the TLC plate. Nonpolar solvent will carry nonpolar substances up the TLC plate. If mixture components stay at bottom of TLC plate, add more of POLAR solvent to pull up the substances. If mixture components travel to top of TLC plate, add more of NONPOLAR solvent to stunt their travel
Depending on reaction conditions, the benzyl alcohol can produce partially oxidized to the corresponding aldehyde, or completely oxidize to a carboxylic acid, and in most cases the reaction mixture contains all 3 compounds. The reaction mixture is to be analyzed using TLC and will be separated using column chromatography. 1. Sketch a TLC plate showing the analysis of the reaction mixture, predicting which will have the relative order of Rfs. 2. Predict which compound will elute from the column first and explain your reasoning. In both cases assume the stationary phase is silica and the mobile phase is of intermediate polarity.
Polarity: Acid (3rd) > Alcohol (2nd) > Aldehyde (1st) 2. the aldehyde will elute first because it is the LEAST polar. The alcohol will elute 2nd, and the acid will elute last because it is the MOST polar.
Intermolecular attractive forces
Results in affinity of a molecule for the mobile or stationary phase in chromatography. Stationary phase: Shows greatest attraction for POLAR compounds and is capable of forming hydrogen bonds with other compounds that are capable of forming Hydrogen bonds as well. Attractive Forces: -London Forces: weakest, important for non polar molecules including hydrocarbons and ethers. -Dipole attractions: important in molecules that have polar functional groups (alcohols, esters, carboxylic acids) -Hydrogen bonding: strongest, important for compounds that have O-H and N-H bonds.
What would a TLC plate used to separate a 2-component mixture look like after development? Label: Origin, Solvent Front, Component Spots, Rf
Rf= (Distance component traveled from Origin)/(Distance Solvent Front traveled from Origin)
Stationary phase
SOLID Network solids: -Alumina(Al2O3) -Silica(SiO2) BOTH: are polar, have partially negative charges on oxygen and two lone pairs; strongest affinity for most polar compounds
Chromatography
Separation of compounds is based on the distribution of compounds between 2 phases; the STATIONARY phase and the MOBILE phase. These 2 phases can be solid and liquid, solid-gas, liquid-liquid, or liquid-gas. STATIONARY PHASE: Solid MOBILE PHASE: Liquid
Column Chromatography
Solid-liquid chromatography; Compounds separate as they travel DOWN the column with the mobile phase; Elution of least polar first Apparatus: Small plastic funnel, Chromatography column, small adapter with a frit, and a stopcock.
Precautions to take when spotting, developing and visualizing - how to ensure experiment proceeds efficiently, what may occur if precautions not adhered to and why
Spotting Precautions: 1. Lightly touch capillary tube to TLC plate (or adsorbent will flake off and stop flow of fluent or cause fluent to run crookedly) 2. Draw spotting line lightly in pencil 3. USE PENCIL, NOT PEN. (pen will chromatograph) 4. Make spots small and compact and not too concentrated (streaking will occur) 5. allow solvent to dry completely between applications to the same spot (spots will become too large and run in to each other) 6. Don't put spots too close together on spotting line (may bleed into each other) 7. Don't put spots too close to edge of TLC plate (leads to inaccurate Rfs b/c none enough solvent/adsorbent surrounding spot) Developing Precautions: 1. Make sure solvent is BELOW spotting line (otherwise samples will dissolve in solvent) 2. Hold plate by the sides (oils from fingers sometimes smear a TLC plate) 3. Make sure bottom of TLC plate is level with bottom of developing chamber (solvent will rise up unevenly) 4. Refrain from moving/bumping jar while mixture is eluting (solvent will rise up unevenly) 5. Do not let solvent run past solvent front (5 mm line) (Rf value will be too high) 6. Make sure that TLC plate is not touching sides of filter paper (fluent will be adsorbed by the adsorbent and interfere with the ascending eluent) Visualizing Precautions: 1. Hold UV lamp so that light is pointing straight down onto table 2. Dry TLC plate by placing it in hood before touching it for visualization step 3. Immediately circle exposed spots with pencil after visualizing with UV light
Explain what is meant by the term "TLC is used to measure the progress of the reaction."
TLC plates show progress of reaction because as the reaction progresses and more product starts to form from the reactants, the product spots on the TLC plates should increase in intensity while the reactant spots should decrease in intensity as more of it is converted to product, therefore telling you where you are at in the progress of your reaction
In preparing a chromatography column, the column was tapped as the adsorbent was added. What was the purpose of tapping the column, and how might separations be effected if this wasn't done?
Tapping the column was done so all the holes got filled with the adsorbent and no spaces would be present. If holes or gaps are present, compound will get in them and therefore produce less accurate results.
Sam carries out a column chromatography procedure and the solvent level is allowed to drop below the top of the column. What will happen to the column and how will this affect their ability to separate the components of the mixture?
The column will have bubbles if the solvent drops below the top of the column. This results in gaps in the mobile and stationary phases and a poor separation overall.
Explain the purpose of placing a filter paper in the developing jar before developing the TLC plate.
The filter paper absorbs the mixture, thus making the environment inside the jar saturated. This allows for better results and the ability for the experiment to be reproducible.
When a mixture of compounds is separated using column chromatography, the reaction mixture must be added as concentrated as possible. How would your ability to separate these two compounds be affected if you did not do this?
The less concentrated the reaction mixture is the harder it will be to separate the two compounds. This is because the added solvent interferes with the separation process. It would be much harder to separate and the results would be less accurate if the mixture is not as concentrated as possible.
Two lab workers have each attempted a synthesis of the same organic compound from identical starting materials, but using different experimental procedures. At the end of the procedure, they check the results using TLC analysis with a solvent mixture of 60:40 hexane-ethyl acetate. The Rfs are identical. Does this prove the compounds are the same? How, using TLC only, could additional proof be collected to prove the compounds are identical?
This does not necessarily prove that the compounds are the same. To have more proof that they are the same, one would need to run the system again but with a different solvent mixture/system to see if you get different Rf values or if they are still the same.
Why must at least one fraction have no compound?
To confirm both compounds have eluted. To confirm elusion of one compound, clear for the next; clean breaks
Explain how using too strong adsorbent would effect separation.
Using too strong of an absorbent would result in a poor/if any separation of compounds. All compounds would bond intermolecularly to the stationary phase.
A CC procedure calls for collecting fractions with a vol. of 2.5 mL. Instead, the student collected 5 mL fractions. How might this students ability to separate compounds be affected by this change? Use Diagrams to illustrate what might happen.
When the student collected 5mL fractions instead of 2.5mL fractions, he was collecting too large of a fraction resulting in both poor readings and poor separation on the TLC plate. When you collect too large of a fraction, the non-polar and polar compounds will mix, thus leading to mixed results on the TLC plate (Fig.2). Keeping the vol. of fraction that you collect constant with what the experiment tells you to collect will lead to a better and more accurate separation and results.
You have just prepared an orange compound and the compound is to be purified by column chromatography. Initially you begin to collect fractions using methylene chloride as the eluting solvent, but after 500 mL of solvent has passed through the column, the orange band has not moved down the column appreciably. What can be done to speed up the progress of the orange band down the column?
You can blow air on the top to force the solvent through faster, making sure the solvent is never lower than the top of the silica. You can also switch to a more polar solvent.
Why is it important to keep the solvent level in the TLC developing jar below the level of the sample spots?
compounds will capillary action out the bottom if touching the solvent at the bottom.
TLC spotting solvent
methylene chloride; highly volatile, dissolves many compounds
Solid-Liquid chromatography
molecules are continuously adsorbing onto the stationary phase, desorbing into the mobile phase. As the mobile phase travels over the stationary phase, it will carry dissolved molecules with it. Molecules that show little attraction for the stationary phase (solid) will travel with the liquid phase. A molecule that absorbs strongly to the stationary phase will not travel as fast.
What is spotting troubleshooting?
original spot too large, component spots likely to tail
Define: Ratio to Front (Rf)
ratio of the distance the component spot has traveled to the solvent front; distance of component spot/distance of solvent front
