What will be the result of the following errors in a gravity ...

[Haya94]

What will be the result of the following errors in a gravity chromatography process?
  (a) Holes or air bubbles are present in the column packing.
  (b) The column is loaded with too much sample.
  (c) The column is eluted first with 5 diethyl ether in hexanes, then the eluting solvent is
  immediately switched to 50 diethyl ether in hexanes.
  (d) The column is allowed to go dry before it is completely developed.

Question 2

List the following compounds in order of expected elution from a chromatography column
  packed with silica gel, using a petroleum etherdiethyl ether solvent system.

 

Question 3

A highly polar compound is moving through a gravity chromatography column too slowly.
  What can the chromatographer do to increase its rate of movement?

[ghepp]

Answer to Question 1

All of the above lead to poor separation of the components:
(a) The samples would trail and diffuse around the holes in the packing, causing the samples
to run together rather than separate. Sometimes holes in the packing cause the column to
run very slow, if at all.

(b) The separation would be poor. A column packing can only adsorb a set amount of com
pound: the rest would be sluffed off and flush down the column and come off in the first
fractions.
(c) Switching quickly to a solvent with a high percentage of diethyl ether causes the ether to
heat and vaporize, causing holes in the column and thus poor separation of the com
pounds.
(d) The packing is very delicate. If it dries out, it is no longer able to adsorb compounds.

Answer to Question 2

From first to last: (b) an aromatic hydrocarbon, (d) a ketone, (a) a phenol, (c) a carboxylic acid.
In practice, (a) and (c) would both elute very slowly and would probably be difficult to sepa
rate.

Answer to Question 3

The chromatographer should gradually increase the polarity of the solvent mixture by adding
a small amount of a more polar solvent.

[Haya94]

Wow, this really help

[juliaf]

Excellent