Posted on Oct. 18, 2013
For all TLC plates that are run, you should first check the plate under UV then in the vanillin dip. Some samples may show up in both methods, usually though, the samples will only show up in one or the other of the "developing" methods. If there is a mixture of the two on the plate, you should indicate on the drawing if the spots are UV or vanillin sensitive,. Sometimes the vanillin colour can be useful: if you see two spots at the same Rf and they show different colours in the vanillin dip or if one is only UV sensitive and the other seen in both methods, the spots are of two different components.
For liguid samples, a drop of the liquid dissolved in a mL of acetone will usually give good results. Spotting samples once or at most twice should give good sample sizes. Extra spottings may be needed for some samples if they are difficult to detect. Solid sample concentrations are a bit more tricky....it depends on the "packing" of the solid, some are "fluffier - with air pockets" and therefore less dense and will form less concentrated solutions but usually a small amount of solid in 0.25 mL of acetone will usually work.....but solids are more trial and error until you get a "feel" for the amounts.
Some samples (both starting materials and products) will not be detectable under either of these "developing" methods and in that case, we will not use any other methods and the sample will not be able to be commented on for TLC purity.
When spotting TLC plates, always spot the starting materials for the reaction (unless otherwise specified not to, due to toxicity, reactivity or some other factor) and the crude product (or products). If the sample is purified, spot the purified sample as well. If the crude sample is recrystallized by Craig tube, the mother liquor from the purification should be kept and spotted on the plate as well.
If the solvent system is the same for two or more of the steps of a reaction, both steps can be run on the same plate. If you wish to break up the TLC into separate plates for each step, that is acceptable as well, but the spotting rules outlined in the previous paragraph have to be followed on each plate.
When drawing the plates in your lab note book:
Plates run for each step should be with that portion of the notes, e.g. plate one and comments in step one, plate two and comments in step two, etc.. If two or more steps are combined on one plate, as long as it is in the correct experimental write up section of the book, it is acceptable, e.g. if steps one, two and three are done on one plate, the drawing and all comments can be in either the step one, two or three write up sections.
When drawing the plate(s) in your lab submission forms:
If plates are run for each step, the plate drawing and comments, etc. should be drawn in each appropriate space in the form.
If multiple steps are run on the one plate, it only necessary to draw the plate in one of the spaces on the forms in one of the step write ups. The comments from the samples on the plate (and Rf values, etc.) must be broken up into the corresponding step forms. Do not completely repeat the information on each TLC area and do not put all the information only with the plate drawing.
As an example....In the unused plate spaces you can write in refer to step 3 for drawing of plate....this is acceptable. For example if you have a three step reaction and all samples are run on the one plate....start 1, prod 1, prod 2 start 3 (reacted with prod 2) prod 3 and purified 3....the one plate could be drawn in step 3 with the TLC space in steps one and two containing... for plate see step 3.
- By the empty plate box in step one there should be comments on start 1 and prod 1, Rf's and reaction comments.
- By the empty plate box in step two there should be comments on prod 1 (starting for step 2) and prod 2, Rf's and reaction comments.
- By the plate drawing in step three there should be comments on start 3, prod 2 (starting for step 3) prod 3 and purified prod 3, Rf's and reaction comments.
We will look for the plate drawing but we will NOT look for the comments, if they are not in the appropriate section, no marks.
Refer to the sample writeup in the outline for information on what is required for the data from the plate.