How does column chromatography work?
Submitted by benny
Replies
Submitted by Matt
That's kind of a broad question, so here's an overview:
Say you have an impure mixture of three different molecules, A, B, and C. How can you separate them?
One way is to pass them through a column, which is sort of like a garden hose that's "sticky" on the inside.
It's not really sticky, but to some molecules it is- the inside of the garden hose is polar, so polar molecules will "stick" to the hose, and take longer to pass through.
Because the three molecules don't all stick to the inside of the hose to the same degree, they will exit the garden hose at different times. Say, the molecule A exits after 1 minute or 1 gallon of water passed through, molecule B exits after 3 minutes or 3 gallons of water have passed through, and molecule C exits after 7 minutes or 7 gallons of water have passed through.
This is analogous to what happens in column chromatography:
garden hose = the column
sticky inside of the garden hose = the stationary phase of the column, usually silica gel
water passing through the hose = the mobile phase, usually a mixture of ethyl acetate and hexanes
That's the general idea.
If 10 gallons of water passed through the garden hose in total, you wouldn't only use one bucket to collect it all. You'd use 100 smaller buckets (test tubes). You would then use TLC to determine which buckets contain which molecules (say, molecule A in buckets 10-15, molecule B in 30-40, and molecule C in buckets 70-80).
Finally, you would combine and evaporate the buckets to collect the pure molecule. For example, pour buckets (test tubes) 10-15 into a round-bottom flask and then use a rotary evaporator to be left with pure molecule A.
Here's a more detailed explanation of column chromatography:
www.mendelset.com/articles/691/column-chromatography-isolation-caffeine-tea