SolutionsChromatographic Methods
1. Classify
State whether each technique is planar or column chromatography and what its mobile phase is: a) thin-layer chromatography; b) gas chromatography; c) HPLC; d) paper chromatography.
- Planar; liquid mobile phase (a solvent).
- Column; gaseous mobile phase (helium or nitrogen).
- Column; liquid mobile phase.
- Planar; liquid mobile phase.
2. Thin layer on silica gel
On a silica gel plate, a compound moves 3.0 cm while the solvent front moves 7.5 cm. Calculate its Rf. If an alcohol and a hydrocarbon are eluted on the same plate, which one moves further?
Rf = 3.0 / 7.5 = 0.40.
The hydrocarbon moves further: silica gel is very polar and adsorbs the alcohol more strongly, since the alcohol forms hydrogen bonds with the OH groups of the surface.
3. The big ones come out first
Why do large molecules come out before small ones in size-exclusion chromatography?
Because they do not fit into the pores of the stationary phase and only travel through the space between the particles; small molecules enter the pores, travel a longer path and take longer to come out.