Biochemscientist - 21-6-2012 at 20:12
Hi all,
I recently made my first attempt at making some homemade TLC plates by spreading a paste of aluminum oxide & calcium sulfate on some glass
microscope slides, as demonstrated by Robert Bruce Thompson on youtube: http://www.youtube.com/watch?v=pNDQkM3jasA
I allowed the plates to thoroughly dry in the sun for several hours, and then activated the plates by baking in the oven for 30 minutes.
Unfortunately, the plates seemed to perform very poorly when tested with pigment extracts of spinach and tomato paste. The pigments separated very
poorly, often making a smear across the plate instead of several clear distinct spots (even when using a single spot of dilute pigment extract). I
tried varying the concentration of the pigment extract, spotting multiple times or spotting only once, and I also tried changing the eluent, but
still had poor results. Even paper chromatography seemed give better results and separations than these homemade TLC plates.
Is there something I am doing wrong, or do alumina-based plates tend to perform poorly in general? Would silicon-based plates work any better?
Zan Divine - 22-6-2012 at 05:59
In my entire working lifetime as a synthetic chemist I used alumina plates as seldom as possible. Essentially, I used them only when 2-dimensional TLC
suggested the substrate was unstable to silica gel or the compound was just too polar to move well on SiO2.
The two are supposed to compliment one another. Silica gel is usually considered to be acidic. Alumina is typically neutral or basic. Some highly
polar compounds are very difficult to move on silica gel. When I couldn't even get a compound to move well with n-BuOH-HOAc-H2O on silica gel, it
would easily move on one of the aluminas.
Our lab section had 1 box of alumina plates which sat for years. In contrast we consumed a box or two per month of silica gel plates. I ran a handful
of alumina wet columns or dry columns (where you slice up the column rather than elute) over 20 years at **** and hundreds of silica columns.
Silica gel plates are FAR superior to alumina for the vast majority of organic chemistry. Separation is better and spots stay smaller and more well
defined.
P.S. That streaking thing...welcome to the world of alumina plates!
[Edited on 22-6-2012 by Zan Divine]
S.C. Wack - 22-6-2012 at 11:43
"a paste of aluminum oxide & calcium sulfate" and "baking in the oven for 30 minutes" is a little vague from here. Maybe different Al2O3,
different amounts of water and CaSO4, and different heating would be different?