Thursday 11 April 2013

Putting yourself out (there) with TLC

No, not tender loving care, but rather Thin Layer Chromatography, which is what we spent an hour or more doing this morning.  Usually 'we' means that a class of students do the work while I watch over their shoulders and hope that no-one sets fire to themselves.  But here I had a task.  TLC is a technique for characterising and identifying small chemicals like (today) aspirin, paracetamol and caffeine.  You dissolve the chems and put a spot of the solution at one end of a 'stationary phase' such as blotting paper and then add a solvent as a 'mobile phase'.  The solvent drags the chemicals along as it soaks up the stationary phase and, as different chemicals have different affinities for the two phases, you can separate out a mixture of chemicals.  You can, for a domestic example, show that black ball-point ink is a mix of green, red and blue dyes.

For today, blotting paper was not required and we used a very thin layer of silica gel stuck to a sheet of stiffish plastic. It was my task to cut the TLC sheets into strips small enough to fit in a small glass bucket. These strips were dotted with the chemicals (and an unknown mixture) and dunked in a bath of ethyl-acetate/chloroform/formic acid aka the mobile phase.

I made an embarrassing hames of my first cut using some wonky paper scissors, but the experiment worked very nicely for most of the kids (this is my 1st Year Chemistry for Physiologists class).  There was some (real - because unexpected) science to learn as well because a couple of the results showed not a single clear migrated spot for each chemical but a clatter of 2 or three spots: one stronger than the others.  It seems that the device (a thin glass capillary tube) used to make the spots had, in some cases, been used before and the residual chemicals had redissolved and run up the TLC sheet along with our drugs.

Making a bags of Scissor Skills 101 made me think about what folks did for amusement before we could blog or control the antics of Lara Croft and so I told the class:

I grew up in a sibship of three, with an older brother and a twin sister who was (it's the XX chromosomes, lads) effectively two years older than me all through the years that mattered.  On rainy afternoons in the 1960s, when school was out, we'd either tip out a biscuit tin full of stamps and start ordering them into bins and classes (country, colour, butterflies, Malaysian tigers, find all the triangles) OR we'd do scrap-books.  Somehow it was decided that, whereas I could be trusted with glue (no Pritt-sticks then), scissors were too dangerous and one of the others would have to cut whatever I wanted from the pile of old magazines.  So we worked away, each on his or her parallel project.

Then attention was drawn to the fact that ALL my pictures were of food - cornflakes packets, bananas, small tins of soup, large tins of soup, plates of soup (with projecting spoon - tricky for the cutting out), a potato.  I was immensely embarrassed to have my subconscious laid bare, in this fashion on the table-cloth, denied it with blushes and ordered the cutting out of three birds.  Years later, I found the scrap book and it was clear that, after The Birds Episode, I quickly reverted to blancmanges, Hovis loaves and more . . .  soup.

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