if you are using the method for just identification method and the material is in powder form then what i think the best method is to dissolve the powder in mg/ml concentration in any solvent. and then apply the spot.

mostly very less amount is preferable for TLC method.

Dear sir

I want to known in the respect of preparative TLC . I choose one solvent system for extract with the help of TLC and when i did preparative TLC there are two more spot in same solvent system.It may be because there we use concentrated form of extract or anything else.

what is your objective of your preparative TLC.???are you trying to isolate any compound?

if yes then higher the concentration more will be the desired product at the respective RF value.

the appearance of two more spots is due to increased concentration as the impurities or undesirable compounds increase with increase in concentration.

try increasing the concentration on TLC as u used on prep TLC and then verify the result (presence of additional spots).

all the best:)

If you are doing a normal TLC then a diluted solution spotted in such a way that all of it concentrates at the same point in the best method. Here not the solution but the sample application process becomes important. The way u apply and concentrate the spot at exactly the same place during multiple applications when u apply. Because to apply in a very sharp point u need to apply it again and again with simultaneous drying . Then it will give a great separation.

In case of Prep TLC, entire scenario changes. Here you are opting for separation of a compound, so you need to apply sample in a higher amount and you should use the most concentrated solution that can pass through your capillary tube or the TLC sample applicator. IN such cases, use a broad plate and apply the sample in a linear pattern form one end till the other i.e. at the origin. Then concentrate the spot by repeated applications. Run the plate in the solvent system, and then separate the whole band that u desire by scrapping at the particular Rf (which u know already from your TLC data for the sample applied)

Concentration must be such that you see your spots while you do not overload your chromatograohically active layer. In some cases you need to see impurities, so you use a more concentrated sample; sometimes your main spot is so easy to detect that minute amounts of a dilute solution will result in a good plate.-

When you use diluted solution for TLC some minor compounds or impurities will not appear so when you made Preparative TLC there were 2 additional spots only appear with highly concentrated solution so I prefer to use concentrated solution for TLC

lower the concentration of the loaded compound better is the separation. If the concentration of the loaded compound has been increased, it effect the separation as tailing has been occurred in the different spots due to increased concentration and RF values have not been calculated correctly. In general to get better separation and clear Rf values lower concentration (infect appropriate concentration for preparatory TLC that have to be evaluated for particular extract and solvent system) of the samples have been recommended.