If the TLC plate runs samples which are too concentrated,
the spots will be streaked and/or run together
. If this happens, you will have to start over with a more dilute sample to spot and run on a TLC plate.
What is the result of applying too much sample in chromatography?
Applying too much sample may result in
large, diffuse bands of chemical moving up the plate
, making it difficult to accurately measure the distance the chemical has been transported.
What happens if the spots are made too large when preparing a TLC plate for development?
What happens if spots are made too large when preparing a TLC plate for development? Why must the spots be above the level of development solvent in chamber?
They will dissolve in the development solvent if they were submerged
.
What would be the result of applying too large of a spot of this mixture to the TLC plate?
Too large of a spot would result in
not being able to see how far the substance moved
. Since it only moves 0.5cm it could easily be covered up by a large spot. It could also lead to long streaks and bleeding into surrounding mixtures being tested.
When spotting a sample on a TLC plate the spot should be as small as possible?
3. Draw a small amount of the dissolved mixture up into the tip of a TLC spotter and briefly touch the tip of the spotter onto the baseline of the TLC plate in the appropriate position to deposit a small amount of the sample in the form of a “spot.” Make the spot as small as possible (
typically 2 to 3 mm
).
What should the spots placed at least 5mm from the edge of the plate?
The lanes should not be placed too close to the edge (keep at least 5mm away from each edge), as it is not unusual for
solvent
to travel slightly “fast” at the edge where capillary action of the solvent is greater (Figure 2.24).
What does a high Rf value mean?
What does a high RF value indicate? A high Rf (Ie 0.92) would refer to a
substance that is very non-polar
. Ie that substance moved a 92% of the entire distance the solvent traveled. … IE that substance was only able to move 10% of the entire distance the solvent traveled.
How long should you run a TLC plate for?
Question: How long should you run a TLC plate for? Until the solvent front is about half way up. Until the solvent front is at the top of the plate.
2 minutes
.
How much time approximately did it take to run each TLC?
TLC is a sensitive technique – microgram (0.000001 g) quantities can be analyzed by TLC – and it takes little time for an analysis
(about 5-10 minutes)
. TLC consists of three steps – spotting, development, and visualization. Photographs of each step are shown on the course website.
Why do TLC spots have to be small?
Spots are applied to the plate using very thin glass pipettes. The capillary should be thin enough to apply a neat spot, but
not so thin as to prevent the uptake of an adequate quantity of analyte
. Here is a popular method of producing TLC pipettes.
Why is Rf less than 1?
By definition, Rf values are always less than 1. An Rf value of 1 or too close to it means that the spot and the solvent front travel close together and
is therefore unreliable
. This happens when the eluting solvent is too polar for the sample.
Why silica gel is used in TLC?
Silica gel is by far the most widely used
adsorbent
and remains the dominant stationary phase for TLC. … The surface of silica gel with the highest concentration of geminal and associated silanols is favored most for the chromatography of basic compounds because these silanols are less acidic.
What happens if you touch the front of a TLC plate with your finger s )?
What happens if you touch the front of a TLC plate with your finger(s)? After placing the TLC plate in the eluent,
the eluent will travel up the TLC plate till it encounters the spot (compound)
.
Do polar compounds move slower up TLC plate?
The stronger a compound is bound to the adsorbent ,
the slower it moves up the TLC plate
. Non-polar compounds move up the plate most rapidly (higher Rf value), whereas polar substances travel up the TLC plate slowly or not at all (lower Rf value).
What are the limitations of TLC?
Disadvantages of TLC include
application to only nonvolatile compounds, limited resolution capability (separation numbers or peak capacities of 10–50)
, and the absence of fully automated systems, although the individual steps of the technique can be automated.
What are two common methods of visualizing the spots of molecules on TLC plates?
Visualizing the plate:
There are two common methods available for visualizing the spots on the tlc plate. If the compounds under study contain a chromophore (ie an aromatic compound),
the plate may be placed under an ultraviolet lamp
. The spots will show as dark circles on a fluorescent background.
