MadSci Network: Chemistry

Re: how does an analyst select a mobile phase?

Date: Thu Apr 18 19:39:17 2002
Posted By: Sojo Luis E., Staff, Reseach and Development, Axelson Biopharma Research
Area of science: Chemistry
ID: 1017892602.Ch

Selecting the appropriate mobile phase in thin layer chromatography (TCL) 
requires a bit of luck and experience.  In most cases it is done by trial and 
error.  The main thing to keep in mind is that you want to be able to separate 
your mixture before any of the spots reaches the end of the plate.  The 
criterion for this is that the Rf values between 0.2 and 0.5.  If you remember, 
the Rf value is the ratio of the distance traveled by the compound (spot) to 
the distance traveled by the solvent front.  If the Rf value is 1, it means 
your spot (compound) traveled the same distance as your solvent front.  

Most TLC plates are made of silica or alumina. These are polar phases.  
The mobile phases used in TCL range in polarities from that of hexane (low 
polarity) to that of water (the most polar one).  So a good starting point 
is to use solvent of medium polarity (See Table below) for starters, then 
adjust your polarity up or down by adding a miscible solvent of higher or 
lower polarity as needed.

If your are separating the pigments in ink, you will have to adjust the 
polarity of the mobile phase until you get the maximum number of spots.  
Remember that some solvents are not miscible with each other.  For 
examples hexane does not mix with water, but it does with toluene and 

Good luck

Polarity Table

Ethyl acetate
Diethyl Ether
Hexanes			        LOWEST POLARITY

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