During my chemistry lab course, I was told by my instructor to make 2 small cuts on the bottom corners of the TLC (Thin-Layer Chromatography) plate. Does this make the solvent rise uniformly on the plate? Is this a standard practice in labs? I could not find any information online. In this question the OP has made the cuts as well, but this does not seem to make the solvent rise uniformly.
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asked Apr 9, 2019 at 6:48
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$\begingroup$ My guess is so you will remember which is the bottom of the plate. I cannot see how it would improve flow in any significant way. $\endgroup$– Buck Thorn ♦Commented Apr 9, 2019 at 7:16
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$\begingroup$ Never heard of this. Certainly not std practice in any lab I've worked in. $\endgroup$– WaylanderCommented Apr 9, 2019 at 7:27
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$\begingroup$ Re: comments, It’s quite a personal thing. Plenty of people I know do it, plenty don’t. $\endgroup$– orthocresolCommented Apr 9, 2019 at 8:42
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$\begingroup$ @orthicresol I know plenty of people that don't operate correctly. This is totally another story. One can get decent results, nevertheless. I know lot of people using ematocrito capillaries to deposite on the plate. It does not make it handbook practice, nor it should recommended. Of course preparing ad hoc capillaries is tedious and requires a higher than usual flame temperature, so many go with far too big capillaries. Etc. $\endgroup$– AlchimistaCommented Apr 9, 2019 at 9:06
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$\begingroup$ @Alchimista What are ematocrito capillaries? Googling seems of no use. I use glass capillaries with a smaller diameter than melting point capillaries. $\endgroup$– Apoorv PotnisCommented Apr 9, 2019 at 19:04
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1 Answer
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Yes, your question contains the answer. Think about capillarity.
Without the little cuts the eluant would be "sucked" and flow from both the bottom and the left ( and right) edges of the thin plate, resulting in a tilted flow that
tends to move the elute faster along the vertical edges while clumping the spots to the center;
decrease the resolution at the given conditions by deforming and enlarging the spots.
Think of it as a weird bad executed two dimensionally-developed plate, if it helps.
Note that beside resolution, the cuts should be there to guarantee a levelled horizontal and straight flow front, as requisite to obtain reliable and useful ratio front values.
(Good practice: draw the real appearance of the developed plate or even post it to the lab journal, so that a future user get a feeling of the separation whatever it happened. The latter not recommended if a product spots out to be irritant/allergenic).
Left: correct - Right: not horizontal flow frontline due to capillarity at vertical edges.
As OP mentions an example with cutted corners but not levelled flow, obviously cutting the corners but plunging the plate in too much eluant is useless. The idea is to have eluant sucked up just by the bottom edge.
answered Apr 9, 2019 at 7:34
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$\begingroup$ Well I've seen the light now! Never heard of this trick. Surprised it would matter. Is it considered standard practice? $\endgroup$– Buck Thorn ♦Commented Apr 9, 2019 at 8:09
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2$\begingroup$ It should be standard practice! Still, a not perfect plate might be of some help, of course. Sometimes you do the same over and over for instance. But a Rf value shouldn't take into accoubt how far from center a compound/mixture was dropped. $\endgroup$ Commented Apr 9, 2019 at 8:11
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$\begingroup$ Would help if TLC plates were sold like this. Cutting your own plates is a good way to have a reportable H&S incident. $\endgroup$ Commented Apr 9, 2019 at 8:30
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$\begingroup$ Scissors marketing perhaps :) @Waylander. More seriously, normally TLC prepared plates come in a rather large size, or at least these are cheaper, and then one cuts them in the meaningful size. $\endgroup$ Commented Apr 9, 2019 at 8:31