pertaining to paper chromatography- How will u compute for d' Rf values if d solvent exceeded d solvent front?

Answer 1

The solvent front is measured as the space between the spotting points and the farthest point reached by the solvent. If you pre-marked a solvent front line on the paper, and the solvent exceeded that point, you should use the actual point at which the solvent stopped. The pre-marked point is only a guideline and an actual value should always be used, even if it is a millimeter off.

Answer 2

You know, you paid for the whole keyboard, so those extra few letters won't kill you (particularly if you want advice from us old foggies beyond the IM generation). :-)

Depends on your chamber. The reason that the spots move is because the solvent is moving. Once the solvent hits the end of the paper it....

a) stops moving (wicking). Your Rf value is then just analyte spot divided by the distance from the starting point to the edge of the paper.

or

b) evaporates from the top edge. Serious bummer. In effect, the paper is getting longer except that its invisible paper. This is the principle behind column chromotagraphy and an old (I don't know if its used anymore with the advent of cheap HPLC) prep-scale paper chromatography method (solvent is poured down the paper rather than wicked upwards).

I usually go with option a and I'll let you figure out the reasons why option b can be ignored (legitimately).

Answer 3

that can't happen. How can a spot move faster than the solvent? It can't