Sometimes it just takes a little THC TLC
April 10th, 2009 by excimer
Preperatory TLC is awesome. It’s like running a column but, like, way easier.
Especially when the plate GLOWS.[1]
And even more so when you can’t separate the glowy impurity from what you actually want.
[1] The pics were taken on my phone, so that’s why they don’t look great.
Don’t forget to wash the plates in clean hexanes first! For some reason, ours always has grease on them… dunno why. Maybe we just buy cheap prep plates.
because greasy dirty people TOUCH THEM. gross.
You’re in chemistry graduate school – what did you expect?
If a few people can’t remember to take showers on a regular basis, how are they going to remember that no one actually wants to purify their fingerprint components on prep plates? (Oh, and the same people also couldn’t remember that 1) thionyl chloride should not be a long-term resident in the rotovap receiving flask and 2) other people might want to use the instrumentation once in a while – planning multihour runs whose sole purpose is to use buffer and time is not going to be popular with the other denizens). Just sayin’.
As people who research the contact angle between water and metals will tell you, any truly clean surface exposed to atmosphere is liable to pick up an organic layer faster than you’d think.
Yeah, as one of my materials science profs liked to say, “organic chemists have no idea how dirty things really are.”
silica plates absorb vapors of phtalate plasticisers from air. You probably have lots of cheap vinyl floor cover in your department and you store your TLC plates in open box. (Another culprit can be the mist from oil pumps).
Hmmm… our floors are concrete and and TLC plates are stored in a closed box in a drawer. It might be the wax they use on the floor? I dunno.
At least on the bottom one, it looks like you have good separation – even on the top one, it looks like most of the material in the two bands is separated. Can you just run it again, or are the bands broader than it looks here?
Those pictures were taken after three runs. There’s no separation for the bottom bands (which is what I was really looking to separate) after the first run.
At what point do we break down and admit that an expensive camera is a basic piece of lab equipment and insist upon having one available at all times between the pipettes and the kimwipes? (probably because it would get knocked over alot, but that’s besides the point.)
“But… we really need to show people glowy shit. Someone won a Nobel-frikken-Prize for glowy shit. You didn’t need to repair that hood anyway. We just need to remember to keep the alarm turned off.”
Hah! Way to knock the GFP guy.
I agree entirely. I bought a cheapish (~$100) digital camera for myself and I’ve used it in the lab to document reaction color changes, colors of products, TLCs and the like.
When the world gets really wired, the UV lamp for your TLCs will come with a built-in digital camera that will e-mail the pics to your e-notebook.
and cost $3000.
But for you, academic researcher, it will be only $2,800!
Back when I was doing molecular biology, we had a really nice gel-viewing box like that–it even had its own little printer…
And it’ll be MS compatible, which means script kiddies from Russia will be sending Goatse pictures to your gel printer about ten minutes after it’s turned on.
“Dude, that’s not my protein.”
Most silica or alumina or magnesia (Fluorisil) plates are somewhat deactivated in manufacture and shipping and storage. They pick up all kinds of polars, including water. It was standard practice in the days when YLC was still done by chromatographers who published techniques, that you wash the playes with a volatile solvent. Hexane or pentane were good, DCM was even better because it desorbed more. Then you air dried the plates to get most of the solvent off, activated the plates in a vac oven at moderate to high temperature to drive off water. You uniformly activated plates by using a constant humidity chamber. Retention times were longer (with more resolution) and more uniform than without activation.
Wholly dip a silica gel test TLC plate in Et3N, then modestly heat dry to remove volatiles. You now have a strange pseudo-reversed phase zwitterionic thingie. Elute with CCl4, hexane, or toluene. Always distill hexanes for TLC – mineral oil.
Likewise dope the silica gel or alumina test plate with (trace) picric acid and use a more polar eluent (that doesn’t move the picric acid). Dropping your stuff down as a bulk picrate and purifying that can have unintended consequences.
if the bad stuff glows, react its excited state. UMPOLUNG!
iPhone is a pretty cheap camera, relatively speaking. Think I could write one into a grant?