Question

Why is UV-light used for the “reading” of the TLC-plate?

Answer 1

generally uv light will be absorbed by molecule have double bonds (and multiple ones at that).

UV light will be from 100nm to 400 nm and visible light will be 400-800 nm.

If a compound absorb above 400 then those compound will get excited and after some time it will start to deexcite, if this deexcitation energy comes above 400 nm then we can see our naked eye. So problem is for compound dexcite below 400 nm. In such case We have to shine compound using uv light to see its nature.

Now TLC is thinlayer chromatography. Chromatography is used to separate mixtures of substances into their components. All forms of chromatography work on the same principle.

Generally chemist will use thinlayer of silica or alumina as a stationary phase, and solvent as mobile phase.

when we are giving spots in TLC and dip in solvent vertically, then solvent will start to rise through spot . So according to our nature of compound spot will rise. If a compound is not absorbing light from visible region we cant see those compound through naked eye.

In such occation we have to shine this TLC with UV light and observe, If compound absorb uv light then we will be able to see that spot.

The electrons in these double bonds can jump to different states, and when they return they will emit light (and thus be seen).