Standard drying for TLC

[tcay584]

Hello,
I am running an ID test by TLC using a USP standard. The standard label says to dry it, and I can understand that for quantitative analysis. However, all I need is Rf value comparison. I hate to waste 3 hours drying a standard for this test if I don't need to, but can't find anywhere in the USP where it says that it is alright to do this. Comments? References? Anything????

[JGK]

I would suspect that drying is performed to eliminate the possibility of the water content of your sample causing a localised retention effect in the thin layer media which may affect your results.

[tom jupille]

Dry it *before* or *after* development?

Drying after development shouldn't make much difference for qualitative analysis unless the presence of water affects your visualization chemistry.

Drying before development is very important for normal-phase (silica gel) TLC because the activity of silica can vary tremendously based on water content. The fact that you are comparing the Rf to that of a standard on the same plate (I assume!) helps somewhat, but the variation can be great enough that you lose selectivity (and thus specificity).

[HW Mueller]

Dry the plate or dry the standard?

If you are talking about drying the standard to weigh it accuratly then you may in deed not need this step if you want to get only the Rf. But, I have noticed that if you apply a sample in water and insert the plate into a hydrophobic mobile phase before the water is evaporated, the m. p. moves around the application spot. The shape and Rf was then different than when the plate was inserted after the spot was dry. Apparently one needs to make sure that the application spots are handled the same.

Regarding drying of plates after development: In many applications it is also important to do this to stop diffusion of m. p. The diffusion causes changes in the chromatogram. Very often I see spots above the solvent front when using fast diffusing solvents, even though I use a hot air blower immediatly after development.

[tcay584]

Sorry, I meant dry the standard. The plate gets dried afterwards prior to development. The test is an ID test of an injectable formulation compared to a USP standard (on the same plate, of course). The formulation has no interfering substances. I'm just trying to say "Yup, they didn't mess up the label or ship us the wrong stuff". The application spot is also dried prior to placement in the development tank.