[mir]

Is it possible to isolate TLC fractions like cutting out the fraction from the TLC-plate and dissolve it before running GC-MS?

And one more question, how do you spot out a band rather than a spot on a TLC plate (which would be more appropriate for the method outlined above: Higher concentration)? What genious device are they using?

(My guess: Probably a piece of thin glass with a straight edge - probably a coverglass from microscope-objectiveglass set, which is dipped into the mixture of interests, and gently pressed on the TLC plate.)

[russellm72]

Hi,

We rarely have to use this process and it's called plate scraping.

You can apply your sample to a TLC plate using a piece of cotton wool attached to a pipette and paint it on. After running the plate and identifying your bands simply scrape them off carefully into a suitable vessel using a microspatula or something similiar. Then you can place the material on a sinter and wash the product off with a suitable solvent. Beware of dissolved silica in your sample especially if you have a high MeOH content. In that case passing the fraction down a plug of silica may help using a less polar solute.

R.

[mir]

You say rarely. But why is it like that? You will get a direct link from TLC-data to structure. And importantly, a way to determine the retention order on GC based on the TLC data.

[russellm72]

Trust me its not much used perhaps more with preparative chromatography when separating complex mixtures from plant extractions. We dont use it preperatively well maybe once a year when we use paper chromatography for some sugars.

Normally we use HPLC when purifying substances but sometimes have to do plate scrapping as an alternative form of counting. Remember I am a Carbon-14 chemist and sometimes the Japanese prefer this technique.

R.

[movies]

We use this technique all the time for small scale purification.  We call it prep. TLC.  Usually we use a whole or half plate (20 cm x 20 cm) and apply our compound in a line about 1 cm from one edge, as you described.  The thickness of your plate will determine how much sample you can load.  For the analytical thickness plate (0.5 mm SiO₂, I think) you can load up to about 20 mg of product.  It's a nice technique because you can often get better separation than you can with a flash column and without the cost of a prep. HPLC.  Also, flash chromatography can get a little bit iffy on very small scale; you tend to lose your stuff if the fractions you collect are too dilute.

There are several methods for loading your sample onto a prep plate.  The "painting" method described above is one, but I prefer taking the end of a glass Pasteur pipet and bending the glass to about a 60 degree angle and then pullling the glass at the end to a fairly sharp point.  Then you can use this to transfer a solution of your sample in ~250-500 microliters of dichloromethane (in a small glass vial) onto the prep plate by draging the sharpish tip of your spotter along the plate.  Capillary action should do all the work for you.  It's a bit tough to describe the technique in words, but once you get the hang of it you can load a plate pretty quickly.

One other point: once you scrape the band that you want off of your plate, it is sometimes preferrable to stir the silica gel rapidly with a suitable solvent (usually dichloromethane or EtOAc) for about 20-30 mins and then pass the slurry through a filter (cotton plug, usually).  This is preferrable for relatively polar compounds that might stick to the SiO₂ more than usual.

[Swimfan092]

Is it possible to isolate TLC fractions like cutting out the fraction from the TLC-plate and dissolve it before running GC-MS?

And one more question, how do you spot out a band rather than a spot on a TLC plate (which would be more appropriate for the method outlined above: Higher concentration)? What genious device are they using?

(My guess: Probably a piece of thin glass with a straight edge - probably a coverglass from microscope-objectiveglass set, which is dipped into the mixture of interests, and gently pressed on the TLC plate.)

I use TLC a lot as a presumptive test for the presence of compounds prior to using HPLC and Mass spectrometry. If there is no compound, then there's no point wasting time and money, right?

To make bands on a plate you can use a capillary tube and simply spot two or three spots next to each other along the same plane (helps to have a light source to see where your spotting).

I've scraped TLC fractions before, then centrifuged them with an appropriate solvent and kept the supernatant once filtered. I've used this technique before considering a LC-MS method. Just ran the filtered supernatant through the mass spec straight away. Its a quick, easy and relatively low cost way to detect the presence of ions without using lengthy chromatography methods if that's what you want out of it. 

[kriggy]

Sure but if you use TLC and see that there is a compound/s then you can run the sample on HPLC. The retention will be reversed. It works for simple mixtures rather well. Im not sure if its good idea to scrap the silica because unless you are doing prep TLC then the amount of sample will be quite small.

There are also commercial prep TLC plates, they can separate (as I was told) up to 5grams of compound if you use 20x20cm plate

[critzz]

Sometimes I use this technique if I find an interesting side product on TLC and scrape it off to analyze is with Mass spectrometry (you only need really little for a sample). If you scrape enough you can even have enough for a NMR sample.
You can also do a LC-MS-run if it's available.