Monday, November 20, 2006

Let's talk about TLCs Part 1 - Vanillin Stain

I've lost count of the number of times I've searched the web for TLC stain information long ago. I have a feeling that this is a reoccurring issue for many synthetic organic chemists so we've decided to start a "Let's talk about TLCs"-series on Curly Arrow. With time this site will hopefully be the only place you'll ever have to visit for information on TLC stains and related stuff. Not only will we present the recipe for any particular stain but there will also be some comments regarding what it's good for detecting and some pretty pictures so you can see what it looks like after the stain treatment. Let's start the series with a classic: The Vanillin Stain. This is my favourite all purpose stain and after a quick look under the UV lamp this is my next stop. It virtually always works well to give spots in a wide variety of colours which can be handy for spotting what your after when you have multiple spots.
Vanillin Stain Recipe:
100 ml container
6 g Vanillin
1.5 ml Conc. sulfuric acid
95 ml 96% Ethanol
Add the vanillin to your container followed by ethanol to give a clear solution. Carefully add the sulfuric acid. The final product is a clear colourless solution. However, after some dipping of plates it will quickly become a clear yellow solution (see picture above).
How to use the vanillin stain: Run your TLC and let it dry. If you want to check for UV activity you should do so now and not later. Dip the TLC plate in your vanillin stain and heat it using a heat gun (gently at first so that you don't spray the stuff all over your hood) until coloured spots appear. Remember to put the lid back on tightly otherwise it'll dry out.
To the left there's a picture of a TLC plate that I developed today using the vanillin stain.
Alternatively, you can use an atomiser and spray the TLC plate with the stain. This prevents the stain from turning yellow and makes it last a lot longer since you are using much less per treatment. D!

21 comments:

Anonymous said...

Here's a good one from OrgPrepDaily

http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Anonymous said...

smells nice too. you should do "PIP"-potassium iodoplatinate. excellent stain for nitrogen, better than ninhydrin, imo.

milkshake said...

A guy I knew in Prague dropped by in the lab right after a major oral exam, still wearing his best suit. When he turned his back to his fellow students, he was dusted with the vanilin powder we had there for making TLC stains.

The next day the guy described the experience - he had tickets for theater the same night and as he was sitting with his parents and watching this long and boring performance, he noticed that many people around him were sniffing and looking very puzzled.

Daniel Sejer said...

Didn't know about the PIP-stain. I'm actually messing around with amines these days so I may give that a go. Thanks for the tip. D!

Anonymous said...

It works like potassium permanganate...heat for faster results, but less contrast, leave for slow results, but more contrast. Nice for Weinreb amides.

Daniel Sejer said...

You wouldn't happen to have the PIP-stain recipe handy? I'd like to give it ago one of the coming days. D!

Anonymous said...

potassium iodoplatinate stain (PIP)

1 g hexachloroplatinate (alfa)
20 g KI
592 mL H2O
54 mL conc. HCl

dissolve overnight at 0 C. store stock at 0 C, stain decomposes over time.

Jordan said...

I used PIP a lot in grad school. Good for nitrogen heterocycles.

pmgb said...

you opened my eyes, the vinillin stain in my recently joined lab is dark purple, that means it is decomposed or something...need to prepare fresh

Silje said...

I foolishly let someone in my lab borrow my vanillin stain for a short while, and now its dark purple! Is it ruined? do i need to make a new batch?

Daniel Sejer said...

Silje, stains usually change colour with use and become nasty looking. The simple answer to your question is try to use it and see if it works. It's probably fine. D!

Anonymous said...

We're using a vanillin reagent (1 g vanillin, 50 milliliters ethanol, 0.3 milliliters sulfuric acid) in our lab. We make a fresh solution daily, but recently have been running into an issue. The solution has been turning a light purple moments after addition of the acid, which is confusing us.

So far to fix it we've tried brand new flasks, a different sulfuric acid source and a different vanillin source. The ethanol was working before, we've been pouring it from the bottle into a beaker rather than sticking a pipette in, so it shouldn't be that. We've borrowed automatic pipetters from other labs and it doesn't fix the issue. Any suggestions?

Greta moraes said...

I have used this reagent a lot. One way of keeping the vanillin stain in good condition for a long time is not to add the sulphuric acid to it. I normally spray the tlc plate with a 5%H2SO4 in ethanol and then spray the the plate with 1% vanillin in Ethanol and get the same beautiful results. If you heat the plate slowly, different compounds stain quite differently.

peach said...

Vanillin as a stain, interesting.

Why do you cut your plates with the V shape at the start point.

I'm a TLC noob and the MIT guides don't mention it.

John

Daniel Sejer said...

@John, cutting corners is useful when you use aluminium (or plastic) TLC sheets. Occasionally you will find that the solvent front doesn't move up in a straight line. Cutting corners more or less eliminates this problem. D!

Anonymous said...

Molecular iodine also works in some cases. Just put the TLC in a jar with a decent amount of iodine, seal it up, and wait a few minutes.

mariasnake said...

Anyone know a respected published source for the vanillin stain?

Daniel said...

I'm feeling a little confused right now. I made vanillin stain previously using your recipe, and obtained a light yellow solution. I decided to try another recipe this time, using 0.5g vanillin in 85ml methanol, 10ml acetic acid, and 5ml conc. sulfuric acid. The rational behind the AcOH is that its supposed to make the stain last longer or make it more sensitive or something. However, this time the stain came out light purple, almost like a very faint lavender colour (reminds me of the background of p-anisaldehyde stained plates). While it may seem rational that I should expect a different result for a different recipe, a colleague I used to work with used to use an acetic acid based recipe (which I recall is similar/same as this)and I'm sure his stain came out the usual yellow colour.

So is there any rational behind this? No doubt it probably stains fine, I've yet to test it but I'll let you know when I do. I'm just interested why the colour is different this time.

Anonymous said...

Regarding the purple impurity in vanillin stains, I once observed that after making a batch. The only thing I had done differently to previous batches is rinse the container out with acetone prior to use. I presume that residual acetone reacted with either the sulfuric acid or the vanillin itself and contaminated the entire batch.

Rinsing the container with ethanol avoided that issue. The recipe given here is a success, but I would like to clarify that 95% ethanol is key. Do not use absolute ethanol, make sure the 5% water is there. Also, wrap the container (just use an old pasta sauce jar) with aluminium foil as this will extend the longevity of the stain. I find the stain does not degrade in quality and every last drop can be put to use.

Stefanie Wiese said...

Hey there,
I'm acutally also working with vanillin stains and when it changes colour it should be prepared fresh since I know.
But when you keep the solution in the fridge it will be usable for at least 3-4 weeks (my experience - my solution is in the fridge for that time and is still good looking).
You should definetly keep it in dark glas and ALSO put some alufoil arround it.

Question: I'm actually working with Iminodiacetic acids and there is a method to stain them with vanillin but WITHOUT heating and it actually works. When an Iminodiacetic acid-derivate is put into the stainsolution the spot on the DC-plate turns orange/yellow without heating.
Can somebody tell me what might happens there? All I can imagine is that the vanilin is oxidized by the product and turns yellow/orange. Does vanillin turn orange when oxidized?

Lina said...

Today I've made fresh vanillin TLC stain and it gave me a light-yellow coloured solution.

Recipe: 15 g of vanillin, 250 ml of ethanol, 2,5 ml of conc. sulfuric acid.