Organic compounds most commonly appear colorless on the white background of a TLC plate, which means that after running a TLC, chemists often cannot simply see where compounds are located. The compounds have to be 'visualized' after elution, which means to temporarily convert them into something visible.

Visualization methods can be either non-destructive (compound is unchanged after the process) or destructive (compound is converted into something new after the process. Viewing a TLC plate under ultraviolet light is non-destructive, while using a chemical stain is destructive.

Visualization Summary

Below is a summary of various visualization techniques, and the functional groups that generally react with each. A more detailed discussion of each technique is provided later in this section.

UV Light: For aromatics + conjugated systems	Iodine: Visualizes ~half the time. Strongly reacts with aromatics	p-Anisaldehyde: For many aldehydes, ketones, and alcohols	Vanillin: For many aldehydes, ketones, and alcohols
Permanganate: For alkenes, alkynes, or oxidizable groups (aldehydes, alcohols)	Phosphomolybdic Acid (PMA): For alcohols, phenols, alkenes, and many carbonyl compounds	Iron(III) Chloride: For phenols	Bromocresol Green: For acidic compounds

Table 2.4: Summary table for TLC visualization methods.

Thin Layer Chromatography Benefits and Drawbacks. Finally, let’s explore some of the benefits and drawbacks of using the thin layer chromatography procedure. Advantages of TLC. These include: The separated spots of TLC can be further visualized without any trouble. This chromatography process is cost-effective as compared to other methods. Drawing Elements of different types can be selected from the Drawing Elements tool palette. Includes the Arc tools. TLC Plate Draw TLC plates with. Preparing and Spotting the TLC Plate a. The powdered side of the silica gel plate (n ot the shiny, plastic side ) is the side that is spotted. Lightly draw a pencil line about 1 cm from the end of a plate. Use a pencil, not a pen. Be careful not to scratch the surface of the plate too deeply. Mark three spots along this line.

Obtain two TLC plates with the dimensions of 5 cm x 6.7 cm. Draw a line in pencil, not pen, 1 cm from the bottom along the short (5 cm) side of each plate. Be careful not to disturb the silica gel as you draw these lines! Use the small capillary tube provided to spot four spots, one of each color, along the line drawn on each of the. Solvent polarity on Rf. Look over the photos showing TLC on the course website. (1.) TLC analysis of analgesics. The powdered side of the silica gel plate, not the shiny, plastic side is the side that is spotted. Lightly draw a pencil line about 1 cm from the end of a plate. Use a pencil, not a pen,.

Ultraviolet Absorption

The most common non-destructive visualization method for TLC plates is ultraviolet (UV) light. A UV lamp can be used to shine either short-waved (left( 254 : text{nm} right)) or long-waved (left( 365 : text{nm} right)) ultraviolet light on a TLC plate with the touch of a button. Most commercially bought TLC plates contain a fluorescent material (e.g. zinc sulfide) in the silica or alumina, so the background of the plate will appear green when viewing with short-waved UV light. If a compound absorbs (254 : text{nm}) UV light, it will appear dark, as the compound prevents the fluorescent material from receiving the UV light.

This method is so quick and easy that it is often the first visualization method tried. It is most useful for visualizing aromatic compounds and highly conjugated systems, as these strongly absorb UV. Most other functional groups do not absorb UV light at the wavelengths used and will not appear dark under the UV lamp even though they are still there. It doesn't hurt to try UV after performing TLC with all compounds just in case. Since the compounds remain unchanged after viewing with UV light, a further visualization technique can be used afterwards on the same plate.

Procedure for UV visualization of TLC plate:

Use a UV lamp to look at your developed TLC plate by pressing the short-waved button. Depending on what is available at your institution, you might tilt the UV lamp at an angle in a darkened area of the room (Figure 2.31a), or you might look at the plate inside a box designed to protect eyes from UV damage (Figure 2.31b).
Safety note: Care should be taken to never look directly at the UV source, and to minimize exposure to eyes.
The plate background will appear green under short-waved UV light, and UV-active compounds will appear dark (Figure 2.31c). Use a pencil to lightly circle spots, as they will disappear when the UV light is removed.
Some compounds themselves fluoresce (Figure 2.32), appearing a variety of colors when exposed to either short- or long-waved UV light (bright purple or blue is the most common). Record these types of observations in your notebook if you see them, as they are rare, and are therefore an excellent identification tool. They are most common with highly conjugated compounds.

The TLC plate can be further visualized with some other method if desired (iodine or a chemical stain).

Iodine

A commonly used semi-destructive visualization method is to expose a developed TLC plate to iodine (left( ce{I_2} right)) vapor. An 'iodine chamber' can be created by adding a few iodine crystals to a TLC chamber, or by adding a few iodine crystals to a chamber containing a portion of powdered silica or alumina (Figure 2.33a). When a developed TLC plate is placed in the chamber and capped, the iodine sublimes and reacts with the compounds on the plate, forming yellow-brown spots (Figure 2.33d). The coloration occurs because iodine forms colored complexes with many organic compounds. This stain will work with approximately half the compounds you may encounter.

This method is considered 'semi-destructive' because complexation is reversible, and the iodine will eventually evaporate from the TLC plate, leaving the original compound behind. When the coloration fades, it is theoretically possible to use another visualization technique on the TLC plate, although it's possible the compound may have also evaporated by that time.

Procedure for visualization of TLC plate with iodine:

If not already prepared, make an 'iodine chamber' (Figure 2.33a): in a fume hood, place a few centimeters of powdered silica or alumina in a screw-capped TLC chamber and add a few crystals of solid iodine (safety note:silica and alumina are lung irritants, and iodine vapor is considered an irritant and toxic). A beaker and watch glass will not work in this context as the iodine vapors will escape. Let the silica or alumina and iodine sit together for a while with periodic swirling, and eventually the powder will become orange from adsorbing the iodine vapor.
In a fume hood, place the developed TLC plate in the iodine chamber with forceps (Figure 2.33b) and close the lid. Gently shake the chamber to bury the TLC plate in the iodine-stained silica or alumina (Figure 2.33c) until the spots become colored (Figure 2.33d). Many spots will appear yellow-brown almost immediately, and may darken with extended time. For many compounds, it takes less than 10 seconds to develop a plate, but some compounds require 10 minutes or longer. (Note: alcohols, carboxylic acids, and alkyl halides often do not stain.)
Promptly record appropriate observations of the TLC in your notebook, or circle the spots with a pencil, as the colors will soon fade as the iodine evaporates from the plate. Further visualization may be attempted after the color fades.

Chemical Stains

There are a variety of destructive visualization methods that can turn colorless compounds on a TLC plate into colored spots. A plate is either sprayed with or dipped in a reagent that undergoes a chemical reaction with a compound on the TLC plate to convert it into a colored compound, enabling the spot to be seen with the naked eye. Since a chemical reaction is occurring in the process, it is common to gently heat a plate after exposure to the reagent to speed up the reaction, although this may be unnecessary with some stains. Not every compound can be visualized with every reagent if they do not react toge

Tlc Plate Drawing Tool