[christina]

1.Is TLC a qualitative or quantitative analysis? Is there any way that it can be both? Explain.

~I say it can be both since it can be used to go and determine what exactly is in a sample of mixed compounds which would be qualitative analysis.

It can also be quantitative since it can be used to monitor a reaction and thus you can see about how much is produced since the substance you are testing with can later be tested (compound still in container after reaction) and the purity and ammount of substance can be determined from it.

I don't know if the 2nd part is correct since I don't know if they are only refering to the TLC plates themselves or it can also refer to the compound in the container that can be tested since the TLC plates themselves cannot determine how much sample there is since it only determines what is in the sample. I also don't know if monitoring of the progress of a reaction is considered quantitative.

2. Name 5 things that may occur to cause poor results in TLC analysis.

~not sure but...

1.) If the spot is too large the different samples would mix and they would not be distinguishable from each other during UV analysis and would create 1 spotty smudge.
solution- to place moderately sized spotted samples with the samples being spaced properly apart so that they don't mix during the analysis

2.) The solvent goes to the top of the plate which would cause the samples to mix since they wouldn't travel anymore since the plate is saturated.

solution: take the TLC plate out when the solvent is 1-2cm from top of plate.

3.) During the monitoring of a reaction and spotting the TLC plate if the timing when the sample must be taken is off, there could be more than the actual ammount of product in that spotting sample.

solution: keep track of the time when spotting is to be done carefully

4.)Allowing the filter paper to touch the side of the TLC plate. This leads to problems since the solvent on the filter paper would come into contact with the TLC plate causing the solvent to get the plate to get moistened above the sample thus when the sample being moved from the bottom of the plate up gets into contact with that spot of moisture it would be less effective in moving the sample above that spot since it is moist already.

(I have no idea if that is correct above)

solution: don't allow the filterpaper with solvent to come into contact with the sides of the TLC plate

5.)If you allow the plate to dry considerably before you go and draw the solvent from that could lead to a larger than actual Rf value.

solution: quickly go and draw the line of the solvent front before the actual solvent front disappears from drying.


Is this and the above correct?

[CT101]

I think your answer for TLC being qualitative and quantitative is fine. You might want to add another sentence stating exactly how TLC is used for quant analysis....for example scraping the bands, derivitzing and then analysis by GC.

You have good reasons for TLC problems, but I would remove #4. My experience with the sides of the filter paper getting wet was the spots becoming "curved" (think of a smily face), which resulted in uneven spots.

Here are some other reasons for poor TLC results

1. Not optimizing the mobile phase. If the mobile phase is too polar or too non polar for your sample, the separation will be poor.

2. Choosing the wrong kind of plate. For some applications, silica gel plates are all you need. For others (like lipid separation) a plate with a gypsum binder is needed.

3. Not activating the plate prior to use.

[christina]

thank you