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Usually in preparative chromatography one needs to know when the desired substance to be separated is coming down the pike. Often this required some sophisticated instrumentation. But, it occurred to me that another way would be to two find dyes that (1) won't combine with anything in the mixture to be separated, (2) have Rf values that bracket the Rf value of the desired compound w/o including any others. Then one can simply visually inspect the chromo column (assuming it is glass)to tell where the product is and when it is about to come out. So, if this has been done then really my question is: is a table of dyes, and what kinds of functional groups they are safe with, and their Rf values.

In answer to comments below: I know a TLC is relatively fast. I was thinking of preparative chromatography.

Even approximate Rf values of dyes, run with maybe 2-3 different, pure solvents, in dry silica, would be useful. One could then guess which dyes would be useful, run test TCLs to find dyes that are either just above or below the desired substance X on the TLC plate.

Ideally the dye spots would not overlap the X spot at all so no separation would be required after.

Then in the preparative step I don't need to auto-sample or use a UV detector: just glance over at the column (thinking of low pressure flash chromo in a glass column) to see if the first dye is approaching the bottom - while doing other stuff. When the first dye just runs out, then I change the collection flask until the post-X dye starts showing up. Sounds easy! But I know there would be complications.

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    $\begingroup$ I think you could spend a lot of time trying to find suitable dyes. Running a tlc of your fractions really does not take very long. $\endgroup$
    – Waylander
    Dec 3, 2021 at 7:38
  • $\begingroup$ Rf values are not tabulated. They vary strongly with the solvent (e.g. you Cyclo:EE mixture ratio, it's water content), and the specific column material, including the age/storage conditions of the material. Even your packing. $\endgroup$
    – Karl
    Dec 3, 2021 at 7:50
  • $\begingroup$ And you'd need a big cupboard of dyes, not all of which are very stable. $\endgroup$
    – Karl
    Dec 3, 2021 at 7:52
  • $\begingroup$ If you have easy separation problems, this might work. Often enough, separation problems are difficult, and you would collect many fractions and analyze them afterwards anyway, e.g. by thin layer chromatography. So you lose your desired advantage. $\endgroup$
    – Snijderfrey
    Dec 3, 2021 at 9:02
  • $\begingroup$ Assuming everything going smoothly and no interference occurs you can perhaps do that. But you need a dye which can be removed easily, say by decanting active coal or filtering through it. Alternatively you might find the perfect scenario in which the dye just signal the incoming target. It depends on the complexity of the crude and the easy of separation. Principles apart. I did upvote the answer by Farook. $\endgroup$
    – Alchimista
    Dec 3, 2021 at 10:13

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the idea is good for academic purposes and good for a lecture hall demo. One may also try a UV lamp, if your compound fluoresces in the column. Caution UV is very harmful for the eyes (protective goggles)!

As far as I know, no this is not the practice because preparative chromatography in pharmaceutical industry is automated beyond imagination. The sample collection in vials etc. are actuated and triggered by UV-Vis or mass spectrometry detectors.

The basic purpose of chromatography is to keep the mixture as simple as possible rather than adding more compounds.

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    $\begingroup$ I the UV lamp idea as it essentially broadens the dyes I could use to fluorescent ones. Pyrex is opaque to shortwave UV, I think. But long wave would work. $\endgroup$
    – stephen penguin
    Dec 4, 2021 at 2:27

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